Climate Feedback

Daniel Cressey; cross-posted from The Great Beyond

The world’s carbon dioxide ‘sinks’ are not able to keep up with the amount of the greenhouse gas being produced, according to a paper published in Nature Geoscience.

Reviewing the recent literature Corinne Le Quéré, of the University of East Anglia, and colleagues report that between 1959 and 2008 43% of each year’s carbon dioxide emissions have remained in the atmosphere with the rest being absorbed by land and ocean sinks. However in the last 50 years they suggest that the fraction remaining in the atmosphere has increased from about 40% to 45%.

They also found that a 29% rise in carbon emissions between 2000 and 2008 can be attributed to a large extent to burning coal and the growth of the so-called ‘emerging economies’.

“The Earth’s carbon sinks are complex and there are some gaps in our understanding, particularly in our ability to link human-induced CO2 emissions to atmospheric CO2 concentrations on a year-to-year basis,” says Le Quéré (press release). “But, if we can reduce the uncertainty about the carbon sinks, our data could be used to verify the effectiveness of climate mitigations policies.”

Continue reading "That (carbon) sinking feeling" »

Posted by Olive Heffernan on November 18, 2009
Categories: Biosphere feedbacks, Climate Science, Daniel Cressey, Ocean science | Permalink | Comments (0) | TrackBacks (0)

The Institute of Marine Engineering, Science and Technology – an international body that traditionally has represented marine industry and more recently, scientists too – today released its position statement on climate change.

The institute has been somewhat slower than many scientific bodies to release such a statement, perhaps given that much of its member base is in shipping, oil and gas. I was involved in helping to ensure the scientific accuracy of the statement, and I joined a panel discussion at the institute this morning, together with oceanographer Ralph Rayner of the London School of Economics (and various other institutes), Colin Summerhayes (executive director of the Scientific Committee on Antarctic Research), commercial oceanographer Mark Calverley (who instigated IMarEST’s observer status with the IPCC), Ian Leggett (formerly with Shell, now the head of Metocean Engineering for Europe) and Malcolm Newell (marine engineer and former consultant for Shell, Golar, Exxon Mobil, among others).

I was prepared for a certain amount of scepticism from the audience, which may seem surprising in this day and age (or maybe not, given the recent news coverage). But reassuringly this morning’s discussion suggested that views in the industry are now aligned with the scientific evidence. Without exception, members were keen to discuss the practicalities of how to reduce emissions from shipping, and how to move to a low carbon economy.

The institute now has the task of putting together detailed synopses on the science, impacts, mitigation, and adaptation, with specific relevance to the marine sector. I’ll update as and when those reports come out.

Olive Heffernan

Posted by Olive Heffernan on October 13, 2009
Categories: Business, Communicating Climate Change, Energy, Ocean science, Olive Heffernan, Technology | Permalink | Comments (1) | TrackBacks (0)

Immersing yourself in the impacts of extreme climate change can give rise to a certain amount of gallows humor. Conversation among my dinner companions last night turned to whether this week's 4 Degrees and Beyond conference or March's Copenhagen Climate Congress provided "more apocalypse for your conference fee". The far more serious question, of course, is how much upheaval and human suffering would come with the substantial warming that delegates here are contemplating. Some interesting talks today looked at the facets of sea level rise and population displacement.

Stephan Rahmstorf of the Potsdam Institute for Climate Impacts Research had an intriguing update on a 2007 Science paper in which he'd come up with a new method for projecting sea level rise. Rahmstorf throws out any attempt to use complex climate models on this controversial problem and instead starts from scratch, with observed relationships between temperature rises and sea level rises. Now, working with Martin Vermeer of the University of Technology in Helsinki, he's upgraded the extremely simple equation he used in 2007 to an only slightly less simple equation - one that takes account of the rate of warming and the amount of water that humans sequester in reservoirs. They use this to reconstruct a remarkably faithful record the last millennium's sea levels.

Turning it to IPCC temperature projections, the team finds that sea level rise by 2100 could range, depending on the emissions scenario, from 0.75 to 1.9 metres, and a 4-degree world would likely see 0.98 to 1.3 metres of rise this century. A caveat: the recent sea-rise data that inspired and calibrated this equation and the past data that it explains don't include the full effects of melting ice sheets that could lie ahead. That means 0.75 to 1.9 metres may be a conservative estimate.

But if we're concerned with how creeping seas affect coastal populations, the amount of rise is not the only matter to settle. This was a key argument made by Francois Gemennes of the Institute for Sustainable Development and International Relations in Paris. Gemennes' talk was based on a study of environmentally induced migration that I covered earlier this year. When it comes to climate refugees, he says, the numbers that are often tossed around - such as 200 million new migrants by 2050 - are based on the assumption that greater climate impacts will push more people around. What they aren't based on is empirical data about how populations respond to environmental change. But according to the recent EACH-FOR project - the first global-scale survey of environmental migrants - the size of impacts isn't the crucial variable at all: migration largely depends on policies making it possible for people to react to impacts by migrating. In particular, Gemennes argues, the poorest and most vulnerable will not be able to migrate unless they are given resources and exit routes - if we don't encourage migration as an adaptation strategy, they'll be trapped in the frying pan (or flooding delta).

The next event of the conference is a panel discussion on "4 degrees of climate change: alarmist or realist?", which I'll be tracking over on Twitter. Follow @annabarnett.

Anna Barnett

Posted by Anna Barnett on September 29, 2009
Categories: 4 Degrees and Beyond, Adaptation, Anna Barnett, Ocean science, Society | Permalink | Comments (0) | TrackBacks (0)

As warming starts to shake up marine food webs, ecologists say it may give an unexpected boost to some fisheries - but also make them more precarious.

This is one of the implications of a new experimental study in PloS Biology that takes a panoramic point of view. Rather than tackling complex food webs species by species, the authors look at how warming affects growth and metabolism across the board within the broad groups of organisms at the base of the web.

Mary O’Connor of the University of North Carolina, Chapel Hill, and colleagues turned up temperatures in outdoor containers holding phytoplankton, the ocean’s primary producers, and bacteria and zooplankton, the smallest consumers. Warming of two to six degrees drove up the productivity of phytoplankton, as expected. But consumers increased their growth even more. Zooplankton retain only about 10% of the biomass they eat, so total biomass declined as the hungry hordes munched on the phytoplankton.

Zooplankton are fish food. O’Connor tells New Scientist, "The effect could be translated up the food chain" to a gain in fisheries, but “that top-heavy food web structure could be less stable, and crash all together." The group found that the consumer boom was much greater when nutrients were added, so they suggest that food webs in nutrient-poor waters - such as the ocean surface - may be more resilient to climate change.

The study is timely: NOAA’s National Climatic Data Center recently announced that world ocean temperatures were the hottest on record last month. The temperatures beat the 20th-century average by nearly 0.6 degrees Celsius. According to AP, meteorologists are attributing the record high to a combination of global warming trends, an El Nino phase just getting started, and other natural variation. Apparently, an unusual and unexplained weather pattern this summer is concentrating warmth over the ocean while land surfaces stay cooler.

And this won’t be a brief blip in sea temperatures:

Breaking heat records in water is more ominous as a sign of global warming than breaking temperature marks on land, because water takes longer to heat up and does not cool off as easily as land.

"This warm water we're seeing doesn't just disappear next year; it'll be around for a long time," said climate scientist Andrew Weaver of the University of Victoria in British Columbia.

Anna Barnett

Image: Mysis zooplankton / Uwe Kils, Creative Commons license

Posted by Anna Barnett on August 25, 2009
Categories: Anna Barnett, Biodiversity and Ecology, In the News, Ocean science | Permalink | Comments (1) | TrackBacks (0)

In the latest salvo of the scientific debate over future sea level rise, a new report counters claims that rapidly swelling seas will soak estimates published by the UN climate planel in 2007.

A major “it’s worse than we thought” story out of March’s Copenhagen Climate Congress, for example, was that sea level could climb more than a metre by 2100 - seemingly far worse than the rise of up to 59 centimetres indicated in the 2007 report from the Intergivernmental Panel on Climate Change (IPCC). This was in fact something of a straw-man comparison, since the IPCC total explicitly excluded the impacts of accelerated glacier melt, and the new studies were attempting to add these impacts in.

But the latest study suggests that even considering glacier effects, the 2100 rise is likely to be well under a metre. A trio of researchers - Mark Siddall of Columbia University in New York, Thomas Stocker of the University of Bern (current co-chair of IPCC Working Group I) and Peter Clark of Oregon State University - used a new method that looks to the past to inform this future projection.

Continue reading "Sea level rise: not so fast" »

Posted by Anna Barnett on July 28, 2009
Categories: Climate prediction, In the News, Ocean science, Palaeoclimate | Permalink | Comments (2) | TrackBacks (0)

Blankets of low clouds shield and cool the Earth’s surface - but in a warming climate, will this safety blanket thicken, or will it deteriorate? That question has bugged climatologists for decades. A paper published in Science today (subscription) now offers convincing evidence that warming leads to fewer clouds, and thus exposure to more warming.

The positive feedback effect was observed in the northeast Pacific Ocean, where the relationship between short-term local meteorological conditions and cloud cover is fairly well understood. In contrast, scientists hadn’t previously said much about how this relationship plays out on a decadal scale - in part because any long-term data set would be viewed askance. Satellite cloud records go back only 25 years, and their accuracy can be wrecked by instrument drift and data gaps. Another type of cloud data, eyeball observations made from ships, is considered suspiciously subjective.

But in the new study, Amy Clement of the University of Miami and colleagues looked at both types of measurements from the northeast Pacific and found they were remarkably consistent. Clement says in a press release,

"The agreement we found between the surface-based observations and the satellite data was almost shocking. These are subtle changes that take place over decades. It is extremely encouraging that a satellite passing miles above the earth would document the same thing as sailors looking up at a cloudy sky from the deck of a ship."

The data showed that over the climatic ups and downs since the 1950s, periods of warming have dispersed the clouds. The authors then compared 18 global climate models to see if they could match the observations. Of these, just two showed the correct relationship between temperature, circulation and clouds - and only one of the pair, from the UK’s Hadley Centre, also responds to greenhouse gas forcing in a manner that matches the average response of the 18 models.

The Hadley model also happens to be one with highly sophisticated methods for simulating clouds, so it may point the way to incorporating the group’s discovery into future models.

Anna Barnett

Image: Jason Pratt, Creative Commons License

Posted by Anna Barnett on July 23, 2009
Categories: Anna Barnett, Atmospheric Science, Ocean science | Permalink | Comments (0) | TrackBacks (0)

Some glacial periods in the Earth’s more recent geological past have been cooler and more severe than others, despite very similar greenhouse gas concentrations and orbital parameters. What is it that decouples global temperature from carbon dioxide levels and the solar heat?

Changes in ocean circulation, particularly in the climatically crucial North Atlantic region, are the most likely candidate. A paper in Nature (subscription required) now suggests that some of these changes originate more than 10,000 kilometres away in the subtropical Indian Ocean.

Edouard Bard and Rosalind Rickaby analysed an 800,000-year record of sea surface temperature and ocean productivity from an ocean sediment core retrieved off the southeastern coast of South Africa (Editor's summary). This is the region where a portion of the warm and salty water carried southwards by the Agulhas current, the Indian Ocean equivalent to the Gulf Stream, leaks into the South Atlantic. The inflow compensates for the export of cold Atlantic deep water to other ocean basins. More importantly, it fuels the Atlantic overturning circulation which carries warm tropical surface water towards the poles, and cold deep water back towards the equator.

The strength of this heat conveyor depends on the position of ocean fronts, boundaries between water masses of different temperature and salinity, which are known to intermittently shift northwards and southwards.

Bard and Rickaby suggest that the Agulhas current between Madagascar and the African coast has almost come to a halt during times when the subtropical front in the Indian Ocean migrated northwards by up to 1,000 kilometres. Isotopic data from the sediment record suggest this has happened at least twice, namely during glacial stadials around 340,000 and 420,000 years ago. The closure of the Agulhas ‘valve’ might explain why these glacial periods have been severely colder than most others before and thereafter.

Continue reading "Indian Ocean: Gatekeeper to climate extremes?" »

Posted by Anna Barnett on July 15, 2009
Categories: Ocean science, Palaeoclimate, Quirin Schiermeier | Permalink | Comments (0) | TrackBacks (0)

As the Earth has alternated between glacial and inter-glacial periods, the steep climatic ups and downs have gone hand in hand with changes in atmospheric carbon dioxide levels. But where was the CO2 going to and coming from? Scientists have pointed to the ocean – currently a vast sponge for the greenhouse gas.

But a talk at the AGU Chapman Conference today by palaeoclimatologist Lowell Stott of the University of Southern California suggests a radically different reservoir: pools of liquid carbon dioxide trapped in seafloor hydrothermal vents.

These pools were spotted in the mid 2000s unleashing bubbles of liquid CO2 from the Okinawa trough in the Pacific Ocean (see the video here).

The CO2 pools form when one oceanic plate buckles under another and carbonates in the sediment break down under the intense heat. Perforations around underwater volcanic vents can allow CO2 droplets to escape and bubble up to the surface, but where the seawater is cold enough it effectively freezes the CO2 into a solid, or hydrate, form that acts as a lid. NOAA has a further explanation and diagram.

Stott points out that the carbon isotope signatures in some mid-latitude ocean sediment don’t tally with the conventional view of carbon entering the ocean system via photosynthesizing algae. The chemistry of the sea-vent carbon is a much better match, he argues. What's more, unpublished work by Stott and colleagues shows that past changes in deep sea temperatures around the vents would have been sufficient to destabilize hydrate caps and thus modulate the vents' release of CO2 in time with the rising and falling atmospheric CO2 concentrations.

Continue reading "AGU Chapman: Could seafloor vents control atmospheric CO2?" »

Posted by Anna Barnett on June 17, 2009
Categories: AGU Chapman conference on Abrupt Climate Change, Anna Barnett, Ocean science, Palaeoclimate | Permalink | Comments (0) | TrackBacks (0)

The system of surface and deep currents which make up the Atlantic Ocean circulation, a powerful heat conveyor belt and a poster child for abrupt climate change, is a more complex affair than straightforward textbook diagrams suggest.

Atlantic meridional overturning circulation is driven by cold water that sinks in the seas off Greenland and returns towards the equator at depth. How exactly this machinery works, and where its components sit, is interesting for a variety of reasons – not least in that it helps oceanographers look at the right spots for possible system failures.

That’s why a discovery, reported in Nature today [subscription], is worthy of note. Amy Bower of the Woods Hole Oceanographic Institution in Cape Cod, Massachusetts, and colleagues report that on its way back to the tropics cold Atlantic water takes a different pathway than previously assumed.

Bower and colleagues monitored the trajectories of drifting floats released from 2003 to 2006 into the southward-flowing Deep Western Boundary Current off Canada’s east coast. To their surprise, most floats soon drifted towards the high sea instead of strictly following the boundary current which has been thought to be the main pathway linking the Labrador Sea with the subtropical North Atlantic. Virtual floats ‘released’ in a three-dimensional ocean model at similar positions displayed the same preference for internal ocean pathways as did their material counterparts.

A secret interior ocean path? There you go, modelers!

Quirin Schiermeier

Posted by Olive Heffernan on May 13, 2009
Categories: Journal Club, Ocean science, Quirin Schiermeier | Permalink | Comments (0) | TrackBacks (0)

In a Mexican eco park, of all places, scientists have found compelling evidence that sea-levels can rise – nah, jump – at scary rates during warm climates such as ours.

That the global sea-level can rise by almost half a metre per decade when huge glaciers melt towards the end of an ice age has already been known. But a paper in Nature today (Editor’s summary) suggests that a similar jump has occurred at the close of the sea-level ‘highstand’ during the warm period, the Eemian Interglacial.

The team, led by Paul Blanchon of the National Autonomous University of Mexico in Cancun, analyzed the age structure of exceptionally well-exposed coral reefs at Xcaret, a popular theme park on the northeast Yucatán peninsula. Because no earthquakes have occurred in the more recent geological history of the region, the peninsula is an ideal location to study sea-level behaviour.

The team found that at the end of the last interglacial many reefs were flooded and replaced by new reefs on higher ground. Age and layering of the corals indicate that a rapid 2-3 metres jump in sea-level occurred around 121,000 years ago, possibly within less than one century.

Only swift and substantial melting of Greenland and Antarctic ice sheets can explain the extreme rate at which the seal level rose to its highstand some 4-6 metres above today’s sea-level.

The implications for our warming planet are clear. As modern temperatures approach those at the height of the Eemian Interglacial, the rate of seal-level rise could soon – perhaps very soon - shift gear, from modest to catastrophic.

The spectacular break-up of the Wilkins Ice shelf off the Antarctic Peninsula is a reminder that we are getting closer to the point where things could get really nasty.

“Given the dramatic disintegration of ice shelves and discovery of rapid ice loss from both the Antarctic and Greenland ice sheets, the potential for sustained rapid ice loss and catastrophic sea-level rise in the near future is confirmed by our discovery of sea-level instability at the close of the last interglacial,” the authors conclude.

Quirin Schiermeier

Posted by Quirin Schiermeier on April 16, 2009
Categories: Ocean science, Palaeoclimate, Quirin Schiermeier, Topics | Permalink | Comments (1) | TrackBacks (0)

Cross posted from The Great Beyond

A controversial experiment which poured iron into the Southern Ocean has also poured cold water on the idea that such ‘ocean fertilization’ can mitigate against climate change.

The Lohafex project was investigating suggestions that carbon dioxide can be removed from the atmosphere by promoting algal blooms with iron. Despite protests from some groups, researchers aboard the Polarstern research vessel carried out their experiment this month.

However, the Alfred-Wegener institute, which was backing Lohafex, says “only a modest amount of carbon sank out of the surface layer by the end of the experiment. Hence, the transfer of CO2 from the atmosphere to the ocean to compensate the deficit caused by the LOHAFEX bloom was minor compared to earlier ocean iron fertilization experiments.”

Continue reading "Results cast doubt on potential ‘climate fix’" »

Posted by Olive Heffernan on March 24, 2009
Categories: Daniel Cressey, Features, In the News, Ocean science, Technology | Permalink | Comments (2) | TrackBacks (0)

During the last ice age, the northern and southern polar regions danced between warm and cold periods, but the two were out of step with each other. When the Arctic cooled, the Antarctic warmed and vice versa. To explain this polar offset, paleoclimate researchers have proposed that the answer lies in the oceanic conveyor-belt system of currents that transport heat around the globe.

A new paper by Stephen Barker of Cardiff University and his colleagues (subscription) offers evidence to support this model. That team describes a detailed record of the end of the ice age, as witnessed by plankton and geochemical proxies from a site in the South Atlantic Ocean at 41 degrees S. Those indicate that the changes there happened abruptly, just as they did in the North. Past studies of records from Antarctica had suggested that the transitions were gradual in the far South, which potentially presented a problem for the conveyor-belt model, says Jeffrey Severinghaus in a News and Views this week. The new data, however, suggests that in the mid-latitudes, the southern Atlantic see-sawed from cold to warm and back quite quickly, matching predictions made by the conveyor model.

It also points toward a mechanism for how the ice age ended. The trigger would have been the periodic Milankovitch changes in Earth’s orbit, which increased the amount of sunlight hitting the northern high latitudes during summer starting around 22,000 years ago. The extra energy started melting the northern ice sheets, thus dumping considerable amounts of freshwater into the North Atlantic. That, in turn, slowed the Atlantic Meridional Overturning Circulation (AMOC)—the conveyor belt that carries cold water from the North Atlantic into the deep ocean and eventually to Antarctica. With an anemic AMOC, the Southern Ocean warmed up and released carbon dioxide stored in the deep ocean, thereby turning up the earth’s thermostat enough to melt the northern ice sheets, suggest Barker and colleagues.

Severinghaus goes even farther. If the slow-down in AMOC did cause a massive discharge of carbon dioxide from the southern Ocean, that should give us pause, he says. Models predict that AMOC should weaken in response to global warming, which could release enough carbon dioxide to finish off some of the ice sheets left standing from the last glacial period, he suggests. Moreover, current climate projections do not take into account this positive feedback, which could amplify the warming effects of the pollution we put into the atmosphere.

Posted by Richard Monastersky on February 26, 2009
Categories: Ocean science | Permalink | Comments (0) | TrackBacks (0)

I may need to start a file for ‘ocean impacts we hadn’t thought of’. First there was the projection that the seas will get noisier as a result of ocean acidification, which whale conservation groups were running with at a UN conference in December. Now researchers report in PNAS that ocean acidification may make fish larvae lose the sense of smell they use to find a home.

Most coastal marine species are swept out to sea during their larval stage and have to find their way to a habitat they can settle down in. Orange clownfish - yes, that’s the famously lost fish from Finding Nemo - must get back to reefs, often ending up in the same ones where they hatched. Philip Munday and Danielle Dixson of James Cook University in Australia have been studying olfactory cues the clownfish may follow.

But they and their colleagues report that clownfish larvae reared in aquariums at pHs of 7.8 and 7.6 don’t respond to smell tests the same way as control fish. These pHs are low compared to the 8.15 the fish live with today, but a business-as-usual rise in carbon dioxide emissions could take the ocean to 7.8 by 2100 and 7.6 in the following century.

Larvae reared at pH 7.8 favour the pungent smell of a swamp tree that clownfish normally avoid. They also fail to discriminate between the smells of their parents and other adults, suggesting that acid-addled Nemos might end up inbreeding more often. The pH-7.6-reared fish don’t swim toward any of the tested scents, suggesting they've stopped smelling altogether.

It’s not just clownfish that smell their way home. If acid damages fish's olfactory capabilities the researchers say, many other coastal marine species could be affected.

One question the paper doesn’t take up is whether an ocean pH of 7.6 would leave any reefs for clownfish to come back to. The danger to reefs, and other better-known impacts of acidification, have meanwhile been highlighted in a declaration by 155 ocean scientists from 26 countries - and they want action.

Continue reading "Ocean acidification disorients fish, riles up scientists" »

Posted by Anna Barnett on February 03, 2009
Categories: Anna Barnett, Biodiversity and Ecology, In the News, Ocean science, Politics | Permalink | Comments (0) | TrackBacks (0)

A couple of news items from the ocean fertilization front:

The Indo-German LOHAFEX experiment in the Southern Ocean, suspended two weeks ago, can be conducted as planned. Independent reviews sought by the German science ministry concluded that the experiment is in agreement with environmental standards and international law. On Tuesday, the team on board the German Polarstern started dumping its cargo of 20 tonnes of iron sulphate. The ship will stay around the area for around six weeks, giving the scientists’ enough time to observe the growth and decay of an ‘artificial’ algal bloom.

Picture: RV Polarstern (Alfred Wegener Institute)

It’ll be interesting how their observations, in particular concerning the rate and efficiency of carbon export to the deep ocean, will relate with data reported in a Nature paper this week on natural iron fertilization.

The CROZEX study was conducted in 2004 and 2005 near the Crozet archipelago at the northern boundary of the Southern Ocean. Raymond Pollard of the National Oceanography Centre Southampton and his team found that natural iron fertilization, by dust supplied from the Crozet Islands, increases biological production and the amount of organic carbon taken down into the deep ocean.

That’s not particularly surprising. But what’s amazing is this: The amount of carbon sequestered to 200 metres depth, while 18 times greater than that during an artificially induced bloom (like LOHAFEX), was a stunning 77 times smaller than the amount that had previously been determined during a natural bloom in the nearby Kerguelen region. What’s more, carbon flux at 3,000 metres, where carbon dioxide sucked up at the surface would be safely locked away for centuries, was just 3% of that at 100 metres. Check out this week's Nature podcast and the paper here (subscription)

“CROZEX carbon sequestration for a given iron supply (…) falls 15-20 times short of some geo-engineering estimates,” the authors conclude. This, you’ve guessed it, has “significant implications for proposals to mitigate the effects of climate change through purposeful addition of iron the ocean.”

It has indeed. The Nature news story here makes the point that the findings, if they hold up, could actually be the final blow to such proposals. The notion that putting a little iron into the oceans here and there will suck up most of the surplus atmospheric carbon dioxide is pretty much dead, so it seems. Alas, it was just too good to be true.

Quirin Schiermeier

Posted by Quirin Schiermeier on January 29, 2009
Categories: Climate Science, GHG emissions, Mitigation, Ocean science, Quirin Schiermeier, Topics | Permalink | Comments (2) | TrackBacks (0)

Carbon dioxide emitted today - and the warming it causes - could stick around for centuries or millennia, reported Mason Inman over on Nature Reports Climate Change not long ago. New research (subscription) published online in Nature Geoscience this week looks at the impacts of CO2 emissions on the global ocean over a timescale even longer and less imaginable.

Because warm water holds less oxygen than cold water, oceans are expected to lose some of the dissolved gas as a consequence of climate change. This is already happening in certain tropical regions. Using a low-complexity model of the Earth system, Gary Shaffer of the University of Copenhagen and colleagues now find that the full effects of ocean warming and deoxygenation could lag thousands of years behind changes in the atmosphere - and that oxygen levels may not fully recover for the next 100,000 years.

Their projections are based on two IPCC emissions scenarios: A2, a high-emissions world, where carbon dioxide emissions climb through this century, and B1, a more moderate scenario, where emissions peak by around 2050. In both cases, Shaffer et al. assume emissions rapidly fall to zero after 2100. The researchers also try two levels of ‘climate sensitivity’ - the amount of global warming expected for a doubling of CO2 concentration.

Have a look at their figure to see just how long CO2 and its effects are found to linger under various combinations of these variables. The worst-case result is that mean ocean oxygen concentration falls to a low of about 68% of pre-industrial levels in the next few millennia, while low-oxygen ‘dead zones’ - which don't support fish or many other marine animals such as crabs and clams - spread nearly six-fold to cover 12.8% of the sea surface area. Their best case is a low that represents 89% of pre-industrial oxygen levels, with dead zones covering 5.2% of the sea surface.

"Such expansion would lead to increased frequency and severity of fish and shellfish mortality events, for example off the west coasts of the continents like off Oregon and Chile", says Shaffer. "The future of the ocean as a large food reserve would be more uncertain."

The number of these underwater deserts has reportedly been doubling each decade since the 1960s, fueled not just by warming but by agricultural runoff - which is expected to increase with flooding in some regions (see this 2008 review in Science; subscription). Other studies have suggested that rapid dead zone expansion this century will be driven by high levels of marine carbon. As Shaffer et al's models don't seem to include these effects, their estimates could run low.

Continue reading "The ocean's next 100,000 years" »

Posted by Anna Barnett on January 27, 2009
Categories: Anna Barnett, Ocean science | Permalink | Comments (1) | TrackBacks (0)

Stimulating algal growth by adding iron to nutrient-poor ocean regions is one of several geo-engineering methods that could possibly mitigate greenhouse warming. But given widespread worries about possibly harmful side-effects on marine life, large-scale ocean ‘fertilization’ is currently not considered advisable.

Predictably, environmental groups have therefore jumped on an iron fertilization experiment which an international team of oceanographers is set to conduct over the next two months in the Southern Ocean near the island of South Georgia. Critics claim that LOHAFEX violates the moratorium on ocean fertilization activities which the United Nations had agreed upon last year. The Nature news story here has more details.

The somewhat ambivalent wording of the legally binding UN Convention on Biological Diversity adds to the controversy. ‘Small-scale’ scientific experiments in ‘coastal waters’ are exempted from the moratorium, it reads. But ‘small-scale’ is a relative term, and where exactly coastal waters give way to the open ocean remains also undefined.

Picture: The Polarstern (Alfred Wegener Institute)

The team on board the German ‘Polarstern’, who plan to spread 20 tonnes of iron sulphate over less than 20 by 20 kilometres-large patch of ocean surface in the Scotia Sea, hope that the study will provide new insight into how ocean ecosystems respond to fertilization – the very data, hence, that are needed to assess whether or not larger-scale future activities might be justified. But opponents counter that such doing already qualifies as an activity banned by UN law. Pressure groups have launched a signature campaign aimed at stopping the Polarstern crew, which will reach its destination by the end of the week, from dumping its load.

A number of companies, such as the now defunct Planktos Inc., had in the past hoped to commercialize ocean fertilization for the carbon credit market. Scientists and institutes participating in LOHAFEX stress that the experiment has no commercial background whatsoever.

UPDATE:
The Indo-German ocean fertilization experiment, LOHAFEX, has been suspended. The German science ministry, in response to environmental concerns, has asked the Alfred Wegener Institute (AWI) in Bremerhaven that an additional independent assessment be conducted before the planned activities can commence.

Meanwhile, the Polarstern, scheduled to reach the planned study region in the Scotia Sea by the end of the week, will continue its journey as planned. On arrival, the 48 scientists on board will start doing preparatory work, but the team will have to await permission from the ministry before they can dump any nutrients into the ocean. AWI has today commissioned two undisclosed institutions to carry out the required extra assessment. It hopes the reports will be delivered within ten days.

Quirin Schiermeier

Posted by Quirin Schiermeier on January 14, 2009
Categories: Biosphere feedbacks, Business, Carbon markets, In the News, Mitigation, Ocean science, Politics, Quirin Schiermeier, Research, Technology | Permalink | Comments (3) | TrackBacks (0)

At a conference this week, marine scientist Andrew Lenton of the Université Pierre et Marie Curie presented a new model that sketches out a beautiful causal chain: from the ozone hole over Antarctica, to rising southern winds, to stronger Southern Ocean currents, to more deep-sea stored carbon arriving at the sea surface.

The simulation, which Lenton reported at a meeting of the CARBOOCEAN research consortium in Dourdan, France, is the first coupled carbon-climate model to account for what biogeochemists have recently seen in the Antarctic waters. As I noted last month, observations (Science, subscription required) suggest the Southern Ocean's considerable carbon dioxide sink isn't soaking up as much of the gas as climate modellers expected, perhaps because there's already too much dissolved in surface waters. That means more climate-warming carbon accumulates in the atmosphere.

By bringing in stratospheric ozone damage, which earlier studies had excluded, the model of Lenton and his colleagues manages to reproduce the recent disappointing sink - a step toward resetting future projections.

I reported this story for Nature News this week, and it posed two problems. One, I didn't get to go to France - I heard it all here at my desk. Two, an issue flagged up in my earlier post also dogs Lenton's research. One of the links in the chain - between speedy winds and more powerful Southern Ocean circulation - isn't supported by oceanographic data (Nature Geoscience, subscription required).

Continue reading "The latest on the Southern Ocean sink" »

Posted by Anna Barnett on December 11, 2008
Categories: Anna Barnett, Conferences and Meetings, Ocean science | Permalink | Comments (0) | TrackBacks (0)

In what could be good news for the Earth’s ability to mitigate warming, scientists have reported that deep convection has resumed in the North Atlantic after more than a decade of little activity.

Also known as overturning, deep convection transports cold water to depths of more than 1000m, bringing carbon dioxide with it and keeping the greenhouse gas out the atmosphere for centuries. It's also partly reponsible for maintaining the global climate system, as we know it - at high latitudes, deep convection forms a mass of cold water that drives the Atlantic oceanic conveyor belt and carries warm water northwards.

In the past decade, however, the process of deep convection has been sluggish, causing some to speculate that warming of surface waters due to climate change is already taking its toll on ocean circulation.

Its recent return has now been independently reported by two groups of scientists; the first team, led by Kjetil Våge of the Woods Hole Oceanographic Institution in Massachusetts, publish their data in the latest issue of Nature Geoscience. Igor Yashayaev and John Loder of the Bedford Institute of Oceanography in Nova Scotia, Canada, have a forthcoming paper in Geophysical Research Letters.

Continue reading "There’s life in the cold sink yet " »

Posted by Olive Heffernan on December 02, 2008
Categories: Climate Science, Features, In the News, Ocean science, Olive Heffernan | Permalink | Comments (0) | TrackBacks (0)

The Southern Ocean’s carbon-sponging capacity has been getting a scientific rethink lately, and a paper in Nature Geoscience this week (subscription required) offers new info on what assistance we can expect from these frigid waters. Unfortunately, the wire report on the paper garbles its bottom line.

According to one long-term prediction, the Earth’s oceans - our greatest natural ally in any war on climate change - will soak up 70 to 80% of the entire industrial era's anthropogenic carbon dioxide emissions over the next several centuries . The Southern Ocean accounts for some 15% of that storage. Last year, however, a widely reported study (Science, subscription required) led by the University of East Anglia’s Corinne Le Quéré concluded that the Southern Ocean’s ability to absorb the gas was weakening.

The new paper, by Claus Boening of IFM-GEOMAR and colleagues, has garnered a Reuters article that unsurprisingly doesn't go into detail about past studies. But the summary from Reuters wrongly implies an about-face on worries over a weakening ocean sink. The story leads with:

The Southern Ocean has proved more resilient to global warming than previously thought and remains a major store of mankind's planet-warming carbon dioxide, a study has found.

and it later continues:

The analysis shows the Southern Ocean has maintained its ability to soak up excess carbon despite changes to currents and wind speeds.

Actually, though, the Boening et al. paper doesn’t evaluate carbon-soaking ability - current and wind speeds are what it's all about. In contrast, storage capacity is the primary concern of Le Quéré et al. So the direct contradiction between today's story, Southern Ocean changing but still major CO2 sink, and the 2007 Reuters report on Le Quéré et al., Southern Ocean saturated with carbon dioxide, looks like an oversimplification.

I called Boening to clarify. “We don’t have anything to directly challenge that conclusion [of Le Quéré et al.],” he says. “We are just challenging the scenario behind that conclusion, namely changing circulation patterns.”

Continue reading "Antarctic currents aren't a-changing" »

Posted by Anna Barnett on November 24, 2008
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A paper this week in Nature (subscription) sheds new light on the causes of pronounced greenhouse-gas and climate fluctuations during glacial times.

The last ice-age, which covered the period from around 110,000 to 10,000 years before now, is famed for a series of climate swings known as Dansgaard-Oeschger events.

Scientists have found evidence in Greenland ice cores for abrupt warming episodes of up to 15 degrees Celsius within few decades, followed by a more gradual cooling. These glacial warm and cold periods swung back and forth between the poles in a kind of thermal seesaw effect, whereby Antarctic temperatures rose when Greenland temperatures dropped, and vice versa.

It has long been assumed that Dansgaard-Oeschger events were triggered by changes in Atlantic ocean-circulation. The new modelling study by Andreas Schmittner and Eric Galbraith now adds new evidence to the idea. Weakening Atlantic meridional overturning circulation, the heat conveyor which carries warm water surface water northwards and cold deep-water back south, is indeed the primary physical mechanism driving glacial climate fluctuations, they conclude. Here’s an editor’s summary.

Schmittner and Galbraith carried out simulations with a coupled model of glacial climate and (simplified) biogeochemical cycles. When they manipulated the Atlantic circulation – artificially ‘switching’ it off and on in their model, that is - the model nicely reproduced the temperature changes typical for Dansgaard-Oeschger events. Remarkably, the model also reproduced reasonably well the ice-core-derived changes in atmospheric concentrations of the greenhouse-gases carbon dioxide and nitrous oxide which accompany such events.

Continue reading "Glacial climate swings: It’s in the ocean" »

Posted by Quirin Schiermeier on November 20, 2008
Categories: Climate Science, Ocean science, Palaeoclimate, Quirin Schiermeier, Topics | Permalink | Comments (0) | TrackBacks (0)

Cross-posted from Daniel Cressey on The Great Beyond

Cyclones appear to be responsible for a large amount of organic carbon tied up in ocean sediments.

In a paper published in Nature Geoscience, Robert Hilton and colleagues report on the impact of cyclone-induced floods on carbon in the LiWu River in Taiwan. They found that between 77 and 92% of non-fossil carbon eroded from the LiWu catchment area was moved during floods linked to cyclones.

As increased sea surface temperatures from global warming could increase the intensity of cyclones, this could create negative feedback, with bigger cyclones locking up more organic carbon in sediments. Sadly this is not going to stop global warming.

Continue reading "Cyclones’ carbon capturing" »

Posted by Anna Barnett on October 20, 2008
Categories: Atmospheric Science, Climate Science, Daniel Cressey, Earth science, Hurricanes, Ocean science | Permalink | Comments (0) | TrackBacks (0)

If carbon dioxide is trump, then methane is the joker in the greenhouse game. The flammable gas (CH4) is produced in wetlands, landfills and in the guts of cattle and sheep, and it is stored in vast amounts in so-called clathrates, or gas hydrates, in the ocean floor.

The latter stuff has always kindled imagination. In the 1930s, dumbfounded Russian sailors who had lit dynamite for navigational purposes in the Siberian Arctic reported that the air around them started to burn. Had they set on fire methane released from clathrate reservoirs? Perhaps.

Less likely is that methane bubbling up from the ocean floor can makes the water so foamy that ships floating above sink like a rock. File this under Bermuda triangle myths.

But catastrophic methane bursts do seem to be linked with anomalous warming episodes in the Earth’s past, such as the one that occurred at the Palaeocene-Eocene Thermal Maximum around 55 million years ago. Dissociating clathrates may well have been the culprit then.

What had uncorked the bottle is unclear. But in any case, reports this week of methane emissions from sub-sea permafrost beneath the Siberian shelf, and from the seabed off Svalbard, sound alarming.

Geologists assume there are large methane hydrate reservoirs in both regions. Are they beginning to destabilize? Have we lit a time bomb? Is global warning getting out of control?

Media reports this week imply all this, some more and some less cautiously. It’s a wonderful story of course: Weird things happening in the Arctic, strange tales from the bottom of the ocean, Apocalypse Soon! It’s new, it’s exciting, it’s scary - no wonder journalists love it.

But wait a minute. The methane system discovered off Svalbard has probably been active for thousands of years, it’s only that no-one has ever looked for it.

The methane emissions detected in the Laptev Sea are also not a new phenomenon. Russian scientists have observed methane plumes there since the mid-1990s when they began to regularly visit the remote and inaccessible region. It does seem that there are many more, and possibly more vigorous, emission hotspots than was previously thought. But observations are still few; it’s not too much of a surprise that the harder they look the more they will find. I have tried to put the recent discoveries in context in my news story here.

That’s not to say that rising methane emissions, and thawing permafrost, are no concerns. They are, and their sources and causes need to be studied carefully. Long-overlooked methane emissions from living plants, as were just recently confirmed, are proof enough for how poorly methane cycles are actually understood.

But not only in matters climate change there’s a danger of confusing people by media coverage that alternates between alarmism and appeasement. Andrew Revkin of the New York Times has appositely termed the effect a journalistic whiplash for the public.

Science, although intrinsically a never-ending process, will every so often generate journalistic scoops - and sometimes journalistic kitsch. The methane story is exciting, but inflationary use of ‘dramatic’, 'alarming' etc in science stories produces only cheap thrills.

Quirin Schiermeier

Posted by Quirin Schiermeier on September 28, 2008
Categories: Recent contributors, Biosphere feedbacks, GHG emissions, In the News, Ocean science, Palaeoclimate, Quirin Schiermeier | Permalink | Comments (2) | TrackBacks (0)

As residents of New Orleans prepare to return home and breathe a sigh of relief that Hurricane Gustav was less damaging than feared, new research published today in Nature [subscription] suggests that the strongest tropical cyclones will pick up speed in the coming decades.

Weighing in on the long-running and at times very stormy debate over whether and how warmer seas will affect the intensity and frequency of hurricanes is a team led by climatologist James Elsner of Florida State University.

Using a 25-year archive of satellite data, Elsner and colleagues derive wind speeds for tropical cyclones over the globe. They find that the maximum wind speeds reached by the strongest tropical cyclones increased from 1981-2006 in most ocean basins, with the greatest changes in the North Atlantic and Northern Indian Oceans.

There was no trend in the intensity of cyclones occuring over the South Pacific, however, and the upward trend observed over a couple of ocean regions was not statistically significant. The researchers also found no increase in either the frequency or average intensity of tropical cyclones over the globe.

The approach taken by Elsner and colleagues – looking at whether the most severe cyclones will hit a higher speed limit during their lifetime – is both novel and socially relevant, simply because the most severe storms do the most damage if they make landfall. Once tropical cyclones reach speeds of over 74mph, they are officially classified as hurricanes.

The real bone of contention within the scientific community has been whether hurricanes will become more intense and more frequent as a result of human-induced climate change. Elsner and colleagues steer well clear of linking the trend to global warming though - they can’t attribute cause as their study doesn’t investigate other factors such as cyclone origin and duration, proximity to land, El Niño conditions and solar activity.

Continue reading "As Gustav subsides, new study says strongest cyclones will pick up speed" »

Posted by Olive Heffernan on September 03, 2008
Categories: Atmospheric Science, Extreme Events, Features, In the News, Journal Club, Ocean science, Olive Heffernan, Society | Permalink | Comments (1) | TrackBacks (0)

The North Atlantic Ocean may still be an active storehouse for atmospheric carbon dioxide, said scientists at the European Geosciences Union here in Vienna yesterday.

Following evidence published last year showing that both the Southern Ocean and North Atlantic Ocean have weakened as carbon sinks in the past two decades, the new results suggest that the trend has recently reversed in the North Atlantic.

Scientists have feared that the weakening trend could be a long-term impact of global warming and that it could be typical of the ocean as a whole, which absorbs an estimated 25 per cent of anthropogenic carbon dioxide emissions yearly. If the ocean switches from a storehouse to a source of the greenhouse gas, this would jeopardise efforts to stabilise atmospheric greenhouse gas levels.

Speaking at a press conference at the EGU assembly yesterday, Ute Schuster from the University of East Anglia in Norwich, UK and Christoph Heinze at the University of Bergen, Norway, presented the results of a yearly analysis of carbon dioxide fluxes across the North Atlantic Ocean.

Previously, Schuster and colleagues showed that carbon uptake by the North Atlantic had halved between the mid-1990s and the early 21st century. But further analysis of the data on a year-by-year basis has shown that the uptake of carbon dioxide in the region has been increasing since 2002 and showed an even greater increase, relative to the early 2000s, in 2005.

The researchers caution that the results are preliminary and are not yet published. The coverage was poor in 2006 and they have not yet finished the analyses for 2007, but they say that the results so far indicate that the trend in weakening of the North Atlantic carbon sink is not linear.

The reasons for this variation are unclear. “I personally think we can’t say with confidence that the trend [in weakening sinks] is attributable to [anthropogenic] climate change”, says Schuster. Surface circulation in the North Atlantic has changed in recent years, she says, but these changes could be due to natural climate variability. Specifically, the North Atlantic Oscillation, a large-scale atmospheric pattern that has important impacts on European climate, could be influencing the rate of carbon dioxide uptake.

Continue reading "EGU: North Atlantic Ocean may regain status as carbon sink" »

Posted by Olive Heffernan on April 17, 2008
Categories: Biosphere feedbacks, Climate Science, Conferences and Meetings, EGU 2008, Ocean science, Olive Heffernan | Permalink | Comments (0) | TrackBacks (0)

It’s been quite a while since the Gulf Stream was last on the Nature cover. This week the old highlight is back.

Now that’s a topic which has caused an awful lot of confusion before. “How global warming will cause the next ice age”, stuff like that. So just to be clear: the Gulf Stream is the mostly wind-driven upper limb of the Atlantic meridional overturning circulation, which ceaselessly transports warm surface water from the Caribbean to middle and high latitudes on the other side of the Atlantic. Yes, ceaselessly. As long as the Earth keeps rotating there’s really nothing in the world (not even global warming) that could bring it to a halt.

It is common knowledge - and true - that the British Isles and Scandinavia enjoy a much warmer climate than Newfoundland or Labrador thanks to the Gulf Stream. But its climatic influence goes far beyond that, a US-Japanese team report in a paper in Nature this week.

They detected the Gulf Stream’s signature in the entire lower atmosphere - namely in air and cloud temperatures, rain bands, pressure fields and wind convergence - above its meandering cross-Atlantic course, and far inland in Europe.

That the influence of the Gulf Stream might penetrate deeply into the atmosphere has been previously assumed. Firm evidence that this is indeed the case, and vehemently so, comes from the combination of satellite observations, operational weather analysis and atmospheric circulation models which the team utilised for their study.

Very likely the Gulf Stream’s direct local effects on the atmosphere are tele-connected, via planetary atmospheric waves, with weather conditions in far-away regions. How frequent and pronounced these remote responses might be is not at all clear. But it seems at least as if Gulf Stream-driven atmospheric dynamics over the North Atlantic have a marked influence on hemisphere-wide climatology.

This, you’ve guessed it, adds another piece to the climate change puzzle. Come what may, the Gulf Stream will not ‘run dry’. But its strength does vary, and a possible weakening of the Atlantic overturning circulation, to which it belongs, is unlikely to leave the Gulf Stream unaffected.

A new ice age will not come over Europe because of that, but storm tracks and rainfall patterns could be affected in rather unpredictable ways.

Quirin Schiermeier

You can vote or comment on the importance of the new Gulf Stream paper in the Journal Club of Nature Reports Climate Change.

Posted by Olive Heffernan on March 12, 2008
Categories: Extreme Events, Features, Journal Club, Ocean science, Quirin Schiermeier | Permalink | Comments (5) | TrackBacks (0)

If you like coral reefs you should enjoy them while you can, they won’t be around for long. Global warming and the ocean acidification that comes with it will decimate reefs by the end of this century, according to a new review article in Science.

“The impact of climate change on coral reefs is much closer than we appreciated. It's just around the corner,” says study author Ove Hoegh-Guldberg, of the University of Queensland. “... The warmer and more acidic oceans caused by the rise of carbon dioxide from the burning of fossil fuels threaten to destroy coral reef ecosystems, exposing people to flooding, coastal erosion and the loss of food and income from reef-based fisheries and tourism. And this is happening just when many nations are hoping that these industries would allow them to alleviate their impoverished state.” (Reuters ... Telegraph)

Although there is no new original research here, when all the numbers are brought together they are pretty frightening. Atmospheric carbon dioxide will exceed 500 parts per million sometime between 2050 and 2100. This will drive global temperatures up at least 2°C, “values that significantly exceed those of at least the past 420,000 years during which most extant marine organisms evolved” says the paper.

It adds ominously, “Under these conditions, reefs will become rapidly eroding rubble banks such as those seen in some inshore regions of the Great Barrier Reef, where dense populations of corals have vanished over the past 50 to 100 years.”

The appearance of a massive swarm of jellyfish, and their subsequent decimation of an Irish salmon farm, are this week being blamed on global warming.

Stock worth £1million were suffocated in their cages by the swarm, which is estimated to have covered 25 square kilometres of sea and been up to 10 metres thick. The fish farm's director said “It was unprecedented, absolutely amazing. The sea was red with these jelly fish and there was nothing we could do about, it, absolutely nothing.” says Northern Salmon Company managing director, John Russell (Telegraph). The full story is on The Great Beyond.

This isn't the first time climate change has been linked to jellyfish outbreaks. Last summer, the same jellyfish (Pelagia notiluca) was spotted in unusually high concentrations in the Mediterranean, again prompting speculation about impacts of sea temperature rise (New Scientist). Reuters reported that a volunteer campaign had removed eight tons of jellyfish from the Spanish coastline. Both reports mention temperature and decline in predators as causes.

A recent study linked increasing populations of jellyfish in the North Sea to climate change and predicted that more jellyfish would appear over the next 100 years.

If the two events are truly linked the UK's salmon industry may have to be added to the list of climate change victims.

Image via Wikipedia

Daniel Cressey and Anna Barnett

Posted by Anna Barnett on November 23, 2007
Categories: Anna Barnett, Daniel Cressey, In the News, Ocean science | Permalink | Comments (1) | TrackBacks (0)

An interesting contribution from the comments thread on Lovelock and Rapley propose cure for global warming by Peter Williams which I thought I'd hoist up here for wider circulation. -- Oliver

Lovelock and Rapley (Nature, 449,403, 2007) put forward the idea that by pumping up nutrient rich deep oceanic water, the subsequent stimulation of planktonic photosynthetic production would give rise to a very significant drawdown atmospheric CO2. The concept is flawed scientifically on two accounts. Planktonic photosynthesis results in the assimilation of inorganic nitrogen and CO2 in a ratio which has a modal value in the region of 6.6 – the so-called Redfield ratio. A fraction of the organic particles that arise as a consequence of photosynthetic production, sink into the deeper parts of the ocean. The C/N ratio of these particles is somewhat higher than the Redfield ratio, as there is some fast decomposition of the nitrogen (and phosphorus) rich organic components before the particles reach deep water. The particles are eventually decomposed in the deeps, with the production in inorganic nutrients, along with CO2. If this water, now enriched in inorganic nitrogen (and phosphorus), were brought to the surface, it would indeed stimulate planktonic photosynthesis and result in the assimilation of CO2. However, the upwelled water is not only enriched in inorganic nitrogen but also CO2 produced at the same time, the latter being slightly in excess of the Redfield requirement due to the elevated C/N ratio of the settling particles. Thus, rather than drawing down atmospheric CO2 from the atmosphere, there would be export of CO2. The situation in fact would be worse, as the upwelled water would need to warm up (otherwise it would simply sink back again) this would reduce the solubility of CO2, resulting in further export of oceanic CO2 into the atmosphere.

Further, from the engineering point of view the concept is infeasible – to lift up a 10m diameter column of dense (cold) to the surface would require a net lift of a number of tonnes and would almost certainly collapse a flexible tube or would cause a ribbed tube to concertina.

Even if the engineering problems could be solved, and the system made cost effective, both of which seem very doubtful, the proposal would have the reverse effect of that claimed.

Posted by Oliver Morton on October 08, 2007
Categories: Biosphere feedbacks, Ocean science | Permalink | Comments (4) | TrackBacks (0)

Posted by Olive Heffernan on behalf of Kevin Trenberth

The 2007 hurricane season is about to get officially underway. Never mind that nature has already provided the first named storm in the North Atlantic: Andrea. Several forecasts suggest that the 2007 season in the North Atlantic will be well above average. Sea surface temperatures (SSTs) are above normal and atmospheric conditions look likely to be favorable for tropical storm activity.

In 2005, the record breaking year in the North Atlantic, record high SSTs in the critical region from 10 to 20 degrees N in the North Atlantic provided ample fuel for the 28 named storms (normal about 15, previous record 21 in 1933) and 15 hurricanes (normal about 6, previous record 12). Atmospheric conditions were favorable with weak wind shear (that otherwise tends to tear a vortex apart) and the absence of stable layers that prevent convection from developing. In contrast in 2006, SSTs were much lower and closer to the long term normal, and the atmospheric conditions were not favorable, as a developing El Niño in the Pacific created an atmospheric circulation that increased the wind shear in the Atlantic. This year, in 2007, there is no El Niño in the Pacific, and SSTs and the upper ocean heat content are more favorable for Atlantic storms.

There seems to be general agreement on these points, yet the whole issue of Atlantic hurricanes is mired in controversy over the role of global warming. It is not a disagreement that SSTs are higher but rather whether the warming is due to natural processes such as the Atlantic Multi-decadal Oscillation or global warming. To the public, the result is the same for now. To me this is obvious: global warming is “unequivocal” to quote the recent IPCC Working Group I report and global SSTs have increased about 0.6 degrees C. In the last half century this warming is associated with increased greenhouse gases in the atmosphere. It is not possible that the Atlantic has escaped from this warming.

The latest shot fired on this issue was by Chris Landsea in Eos on the long term Atlantic hurricane record. The Atlantic has the best record of tropical storms owing in part to aircraft surveillance since 1944. It is widely recognized that the older records are less reliable as the reports depend on shipping or landfall of the storms. Landsea makes this case quite well, but he goes further. He assumes that the percentage of landfalling storms should be constant over time and this provides a basis for adjusting the older records and increasing the numbers of storms by 2.2 per year from 1900 to 1965, when the satellite era began. The critical assumption is that this fraction should remain the same, yet it is abundantly clear that SSTs have increased and so other things have not remained the same. Given the dependence of hurricanes on high SSTs above about 26 degrees C, shouldn’t increased SSTs increase the scope for storms to develop farther to the east over what were colder waters and for the tracks to become longer? An example may be Vince in October 2005, which was the first tropical storm to make landfall in Spain and Portugal. Indeed shouldn’t one expect a decreasing fraction of landfalling storms in a warming climate?

Continue reading "Global climate change and hurricanes" »

Posted by Olive Heffernan on May 16, 2007
Categories: Climate Science, Kevin Trenberth, Ocean science | Permalink | Comments (3) | TrackBacks (0)