Climate Feedback

As promised, here's the YouTube version of the Nature film on climate change:

Posted by Olive Heffernan on October 19, 2009
Categories: Atmospheric Science, Climate Policy, Climate Science, Communicating Climate Change, Olive Heffernan, UN Conference on Climate Change, Copenhagen | Permalink | Comments (0) | TrackBacks (0)

Despite decades of research, relationships between clouds, aerosols and precipitation are poorly understood, concludes a review article in Nature today.

Before a cloud can produce rain or snow, rain drops or ice particles must form. Atmospheric aerosols, tiny particles of mineral or organic origin, serve as the nuclei for condensation.

But the precise effects of aerosols on cloud formation and radiative forcing remain controversial. In their review article, Bjorn Stevens and Graham Feingold propose that the uncertainties reflect a failure to take into account processes that act to buffer the response of clouds and precipitation to aerosol changes. Worse, existing tools and methodologies for untangling these processes are inadequate, they say.

"If we wish to make significant strides in understanding the interplay among the aerosol, clouds and precipitation, we consider it imperative to launch significant new international initiatives, with comprehensive, coordinated and enduring measurements, targeting specific regimes and coupled to state-of-the-art modelling," the duo concludes.

Among other things, they say, what's lacking is an array of ground-based remote sensors capable of vertically and temporally resolving the aerosols, clouds, precipitation and the meteorological state.

Quirin Schiermeier

Image: Flickr user laffy4k, Creative Common license

Posted by Anna Barnett on September 30, 2009
Categories: Atmospheric Science, Quirin Schiermeier | Permalink | Comments (1) | TrackBacks (0)

Global warming doesn’t just change the weather, it also affects the ozone layer. According to a detailed new modelling study, by 2095 the springtime UV index (UVI) could go up by as much as 20% on the southernmost section of the planet, as altered atmospheric circulation pushes more stratospheric ozone into the Northern Hemisphere. That’s nearly half the UVI increase caused by ozone-eating pollutants in the late twentieth century - but coming from climate change alone.

In a study published in Nature Geoscience this week (subscription), Michaela Hegglin and Theodore Sheperd at the University of Toronto used the Canadian Middle Atomosphere Model, which fully resolves stratospheric circulation, to project ozone changes under the IPCC’s medium-emissions A1B scenario. As climate change unfolded, the model showed increased atmospheric upwelling in the tropics, and what went up in the tropical stratosphere came down disproportionately on Earth’s northern half. As a result, even though the damage done last century by chlorofluorocarbons and other nasties is expected to heal in the next several decades, the ozone layer over the Southern Hemisphere will remain thinner than its pristine state circa 1965.

Continue reading "Sunburnt Southern Hemisphere in 2095" »

Posted by Anna Barnett on September 09, 2009
Categories: Anna Barnett, Atmospheric Science | Permalink | Comments (2) | TrackBacks (0)

Twenty years after the Montreal Protocol came into effect to regulate substances that deplete the ozone layer, the annual ozone hole above Antarctica shows no signs of recovery.

A feature article and editorial in Nature today explain why this is so, and why the Montreal Protocol has been a unique success nonetheless.

As things stand, scientists expect the first signs of recovery of springtime ozone depletion in the polar stratosphere around the year 2065. Outside polar regions, where chemical ozone destruction is less pronounced but potentially harmful to human health, it appears as if ozone levels are beginning to increase.

Globally, the recovery of ozone will occur in a changing atmospheric environment. Greenhouse gases have a cooling effect on the stratosphere, and climate change is likely to also alter atmospheric transport and circulation patterns.

What this means for the ozone layer is not exactly clear. It appears that the changes will in some places delay its recovery, while elsewhere they might lead to a ‘super-recovery’ of ozone.

But not only must models of ozone loss and recovery factor in global warming – abnormally low stratospheric ozone has also a marked effect on climate change here and now. Most strikingly, extreme seasonal ozone depletion over Antarctica seems to explain why the Antarctic Peninsula is warming at an alarming rate while the rest of the continent has actually cooled over the last 30 years.

Quirin Schiermeier

Image: View of the South Pole from NASA's TOMS (Total Ozone Mapping Spectrometer) satellite. Credit: NASA

Posted by Anna Barnett on August 12, 2009
Categories: Atmospheric Science, Climate Policy, Quirin Schiermeier | 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)

The skies over most land areas are not, as previous studies have suggested, becoming cleaner.
Aerosol pollution has in fact increased most everywhere since 1973, a team reports in Science today (Abstract). Only over Europe have skies brightened, they found.

Kaicun Wang of the University of Maryland, and his colleagues, looked at a 35-year record of clear-sky transparency from 3,250 stations around the world. At many stations visibility had notably decreased - the skies have dimmed.

Air pollution is worst In India, China, Africa and South America, where hundreds of millions are breathing air thick with soot and smog. Clean-air technologies are lacking, or have not yet had much effect, in these regions.

Previous studies concluded that the skies are getting cleaner, and that as a result more sunlight is reaching the surface. Atmospheric scientists believe that aerosols and dust have been shielding us from the worst of global warming.

But the overall cooling effect of aerosols has lately been questioned. The connection between different types of aerosol particles, clouds and solar radiation is more complicated than previously thought. Now it appears as if darker skies don’t necessarily mean less warming, and vice versa. My story
over at Nature News has more about these disturbing uncertainties.

Meanwhile, China is trying hard to reduce air pollution in its most populated cities. Strict regulations aimed at curbing pollution from cars have been set up last year in Beijing and Shanghai.

Quirin Schiermeier

Image: Punchstock

Posted by Quirin Schiermeier on March 13, 2009
Categories: Categories, Atmospheric Science, Quirin Schiermeier, Topics, Uncertainty | Permalink | Comments (0) | TrackBacks (0)

When it comes to describing how human activities are altering the Earth’s climate, aerosol emissions can tie your tongue in knots. Airborne pollution particles grouped under the "aerosol" heading come in a wide assortment - and some, as this NRCC article explains, tend to absorb sunlight and heat up the atmosphere, while others are more reflective and cooling. On top of that, it’s been thought to make a difference whether this potpourri of pollutants ends up drifting in clear skies or above clouds. Coming in now are the first experimental data that show just how important the effect of cloudiness is.

A study out this week in Nature Geoscience (subscription) uses a new type of satellite data to look at the smoky haze wafting above the southeast Atlantic, mostly from fires in southern Africa, during July-October of 2006 and 2007. The findings confirm what models had suggested: aerosols over cloudier patches of ocean have a net warming effect, but they switch to cooling over unclouded ocean.

Simply put, a veil of aerosols darkens white clouds, but it lightens the dark surface of the sea, and this difference in reflectivity swaps the outcome. The authors, led by Duli Chand of the University of Washington, estimate that 40% cloud cover is the turning point where the southeastern Atlantic’s aerosol cocktail starts adding to greenhouse warming rather than subtracting from it. Cloud cover in the study averaged 48%, and the spatial distribution of clouds and aerosols also overlapped significantly. The overall warming that this produces is about three times what it would be if clouds and aerosols were floating around independently instead of near each other, the group finds.

Continue reading "Untangling aerosol effects" »

Posted by Anna Barnett on February 23, 2009
Categories: Anna Barnett, Atmospheric Science, Regional climate | Permalink | Comments (0) | TrackBacks (0)

A study published in Science today (subscription) uses carbon-14 measurements to figure out where the black carbon drifting in the haze above South Asia is coming from. That’s a prerequisite to cleaning it up - which, as we’ve reported here, could be a major boon to a very vulnerable region. The light-absorbing compound not only causes cancer (among other ill health effects), but reportedly warms some places as much as greenhouse gases do. Because its lifespan in the atmosphere is far shorter than carbon dioxide’s, these impacts could potentially be reduced quickly - if we knew where to clamp down.

Writing in Science, Örjan Gustaffson of Stockholm University and colleagues call black carbon the "dark horse" in the current climate debate, saying “substantial uncertainties exist about its atmospheric longevity, aerosol mixing state, measurement, and sources.” Black carbon is unusually concentrated in the ‘brown cloud’ over South Asia, they note. But estimates vary wildly on how much of this is from fossil fuel burning and how much from smoky fires of wood, dung and other biomass: the ratio ranges from about 1:10 to 10:1, depending on the technique and study area.

Radiocarbon measurements give a more reliable read on this question than other methods, according to Sönke Szidat of the University of Bern, who discusses the new paper in an accompanying Perspective. The principle is simple: fossil fuels are ancient enough that all their carbon-14 has decayed away, whereas freshly gathered biofuels have plenty of the isotope. And while other chemical clues about the brown cloud can change as it wafts around, carbon-14 stays stable.

The carbon-14 levels tell Gustaffson et al. that around half of the black carbon in the cloud, or more, is from biomass. They took samples at ground stations in the Maldives and atop a West Indian mountain, downwind of the rest of South Asia. The 50% figure, Gustaffson says, comes from a method of isolating black carbon that picks up airborne coal dust as well as combusted carbon in soot; in soot-only samples, about two-thirds was from biomass.

Says Szidat:

The study shows that the importance of biomass burning for local and global BC [black carbon] budgets has been underestimated. This was previously pointed out for urban, rural, and remote areas in Europe, but never were the consequences as severe as for the Asian haze.

Continue reading "Biomass, biomass, burning black" »

Posted by Anna Barnett on January 22, 2009
Categories: Anna Barnett, Atmospheric Science, Regional climate | Permalink | Comments (0) | TrackBacks (0)

The world's most advanced simulation of extreme weather on a warming Earth completed its first run last Friday - though the data won't be fully digested into human-readable format until spring. Yesterday I talked to meteorologist Greg Holland, co-leader of the study, at the Willis insurance company's London office - whose cycle racks, I can report, are tucked away discreetly across the street from its intimidatingly curved and purple-lit lobby.

Willis's research arm funded the work, along with the offshore oil industry, the US Department of Energy and the US National Center for Atmospheric Research (NCAR), where Holland is based. They all want to know how climate change will alter hurricane patterns in the Atlantic. At the request of several US state governors, the project is also looking at rainfall over the Rockies and winds in the Great Plains. Says Holland:

I'm not going to forecast a squall line through New York in 2050. But what we want to do is be able to say: "What are the statistics of squall lines going through New York in 2050?" or "What are the statistics of hurricanes coming into Miami in 2050?"

Continue reading "Forecasting the future of hurricanes" »

Posted by Anna Barnett on December 11, 2008
Categories: Anna Barnett, Atmospheric Science, Climate prediction, Hurricanes, In the News | Permalink | Comments (1) | TrackBacks (0)

The AGU Chapman Conference on water vapour and its role in climate has come to a close, and I have headed back to not so sunny London. In addition to getting scientists out of the lab, the meeting afforded great opportunities for normally independent communities to interact. Pupu platters and Longboard Ales led to a very interesting discussion about the meaning of terms such as mean global precipitation and temperature rise. Are statistics such as these preventing scientists from meaningfully communicating results about climate change? This of course comes back to old faithful argument “if the Earth is getting warmer, why did it snow last week?” Definitely something to think about when preparing press releases or giving interviews.

Continue reading "AGU Chapman Conference on water vapor - the final report" »

Posted by Alicia Newton on November 03, 2008
Categories: AGU Chapman Conference on Atmospheric Water, Alicia Newton, Atmospheric Science | Permalink | Comments (0) | TrackBacks (0)

With a month to go until its official finish, the 2008 Atlantic hurricane season has seen more damage, as measured in dollars, than any other year except the monster 2005 season. Scientists have yet to agree whether human-induced climate change has caused spiking Atlantic hurricane activity since the early 1990s - and while the season has raged on, researchers have continued to go back and forth on whether worse is in store as the ocean keeps warming. Science this week has the latest salvo in the longtime debate: a Perspective (subscription) by Gabriel Vecchi of NOAA, Kyle Swanson of the University of Wisconsin-Milwaukee, and Brian Soden of the University of Miami.

Warming sea surface temperatures (SSTs) in the Atlantic, thought to fuel hurricanes, are correlated with the recent surge in storm activity. The trend is roughly linear, note Vecchi et al., and extending it along with predicted temperature rises implies that by 2100, a hurricane season like 2005’s might be considered mild.

But that’s only true, they caution, when you look at absolute temperatures. Relative warming - the Atlantic heating up even more than other tropical seas - is equally well-correlated. Citing their recent papers in Nature and Geochemistry, Geophysics and Geosystems, and Knutson et al. in Nature Geoscience, the authors make an argument that relative warming is more likely to be the true cause of increasing activity. And relative warming of the Atlantic sea surface, in contrast to absolute warming, doesn’t keep trending upwards in 21st-century climate predictions.

Continue reading "Hurricanes and sea surface temperature: all relative?" »

Posted by Anna Barnett on November 03, 2008
Categories: Anna Barnett, Atmospheric Science, Climate Science, Hurricanes | Permalink | Comments (0) | TrackBacks (0)

I'm here in Kailua Kona for the AGU Chapman conference on atmospheric water vapor and its role in climate. Given the high humidity and afternoon rain, the topic seems quite appropriate.

In the keynote lecture, Brian Soden of the University of Miami gave a great introduction to the role of water vapor in climate change. It seems to be a general consensus that there is a positive feedback between water vapor and climate. There has been an increase in water vapor in the atmosphere over the past 15 years, and Soden reported that model simulations show that greenhouse gas emissions are at least in part to blame.

He suggests that increasing atmospheric water vapor will play havoc with atmospheric circulation. Wet regions will get wetter while arid regions will dry even more. In addition, floods and droughts will become more frequent and more extreme.

Continue reading "AGU Chapman conference: water vapor and climate" »

Posted by Anna Barnett on October 22, 2008
Categories: AGU Chapman Conference on Atmospheric Water, Alicia Newton, Atmospheric Science, Climate Science | 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)

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)

Ozone never ceases to surprise. Scientists report in this issue of Nature that halogen compounds such as bromide and iodine oxide play an unexpectedly large role in chemically breaking down the gas in the troposphere.

Ozone is best known for its presence – or in fact for its seasonal absence - in the upper atmosphere. The stratospheric ozone layer, which protects the earth from harmful ultraviolet radiation, is still being chemically attacked by reactions involving long-lived chlorofluorocarbons (CFCs) banned 20 years ago. But Nature reported last year about a puzzling inconsistency discovered in the established chemical model of stratospheric ozone destruction.

The troposphere is a very different story, though. Near the ground, ozone can affect human health; in high concentrations it is even toxic. Above the ground layer it acts as a greenhouse gas which, when it chemically breaks up, initiates the removal of methane (an even stronger greenhouse gas) and other hydrocarbons from the atmosphere. So ozone loss in the troposphere is a good thing, really.

The gas is typically destroyed in marine regions by chemical reactions involving water vapour. Tropical oceans regions, where levels of solar radiation are high, are thought to be the most important ozone sink.

So what about halogens? Anthropogenically produced CFCs are stable in the troposphere, and are therefore not a reactive halogen source there. But models studies have suggested that bromine and iodine chemistry does contribute to breaking down ozone in the troposphere, potentially changing the global ozone budget by up to 20 %.

The measurements which Katie Read of the University of York and her team have made in the tropical Atlantic Ocean now provide strong confirmation of substantial ‘halogen-mediated’ ozone destruction. An editor's summary and the paper are here.

The team analysed the first eight months of measurements from the new Cape Verde Atmospheric Observatory on the remote island of São Vicente in the tropical Atlantic. They found that average daily ozone loss was around 50 % more than predicted by a state-of-the-art chemistry model that excludes halogen chemistry. Aircraft-borne observations in the area confirmed these findings.

But the team also measured ubiquitous daytime concentrations of iodine and bromine oxides in the layer of the troposphere where most ozone loss occurs. These reactive molecules are emitted by marine algae and liberated from salty sea spray.

In a simple model calculation, the observed halogen concentrations induced just about the extra 50 % ozone loss in the region. The omission of halogen sources in atmospheric models “may lead to significant errors in calculations of global ozone budgets,” the authors write.

What’s special about this study, says Roland von Glasow in an accompanying news and views article here (subscription required), is that the measurements were made in the open ocean where they cannot be influenced by local features that produce halogens.

But even so, the results are significant. Ozone is a powerful oxidizing agent and a key constituent of the troposphere; changes in its concentration will feedback on the lifetime of methane and other hydrocarbons in the air.

It is good to know that tropical ocean regions are a larger-than-thought ozone sink. But the composition of the atmosphere is in such fine balance that its cleansing ability cannot be taken for granted as being permanent. In the face of the new measurements it seems essential to include halogen sources and chemistry in global climate models.

The measurements are likely more representative of the troposphere above the global ocean than previous studies near coastlines. But as Cape Verde is surrounded by biologically highly productive waters one must still be cautious when extrapolating the results, says von Glasow.

Note: RealClimate has an interesting discussion on how the media have spun the story.

Quirin Schiermeier

Picture caption: Model of an ozone molecule. Image created by Ben Mills.

Posted by Quirin Schiermeier on June 26, 2008
Categories: Atmospheric Science, Quirin Schiermeier | Permalink | Comments (0) | TrackBacks (0)

A new study, published online Sunday by Nature Geoscience, presents solid evidence that temperatures in the Earth's lower atmosphere are increasing in line with temperature changes on the ground.

This issue has been hotly disputed in the past, partly owing to the fact that temperatures measured in the troposhere - the portion of the atmopshere stretching from 12 to 16 kilometeres above the Earth's surface - by satellites and weather balloons in the early 1990s didn't mirror the changes on the ground.

This fact was used as evidence against climate change, despite the fact that it has been long known that there were problems with the original data collection and analysis.

In the search for more accurate measurements, two scientients Robert Allen and Steven Sherwood of Yale University, have now developed a novel approach using wind rather than temperature data. Their research shows that the lower atmosphere has indeed warmed since 1970, as projected by most climate models, and in sync with warming on the ground measured using temperature data.

In a related News and Views article, aslo on Nature Geoscience, Peter Thorn of the UK Met Office Hadley Centre, one of the world's premier climate modelling facilities, writes:

This is not simply an interesting academic aside — not knowing where observational problems begin and modelling limitations end undermines our ability to understand and predict global climate change.

For further reading on the topic, check out the post over on Real Climate last week discussing the same issue, and highlighting some upcoming papers in the Journal of Climate.

Olive Heffernan

Posted by Olive Heffernan on May 27, 2008
Categories: Atmospheric Science, Climate Science, Climate prediction, Features, Journal Club, Olive Heffernan | Permalink | Comments (1) | TrackBacks (0)

Isotope chemistry is a bit of an arcane world for the non-initiated. But variants of elements that differ only in the number of neutrons in their nuclei are common tools of the trade of archaeologists, geologists and climate researchers.

In the past 50 years, a much-improved chronology of past climatic events has evolved through analyses of the oxygen isotope record of marine shells and minerals in deep-sea and lake sediments. But information about the Earth’s deep geological past, in particular concerning the chemical composition of the atmosphere, is still hard to get by.

The debut of a new stable mineral-isotope proxy for ancient atmospheric condition is therefore a remarkable event, the editors of a paper in this week’s Nature note in their summary.

When analysing the triple oxygen isotope composition of ancient sulphate deposits, a team of geophysicists led by Huiming Bao of Louisiana State University found that they exhibit variable negative oxygen-17 anomalies over the past 750 million years. They propose that these small anomalies, first noticed a few years ago in a study unrelated to atmospheric chemistry, reflect those of atmospheric oxygen and carbon dioxide in the past.

The new proxy is hardly sensitive enough to record the relatively subtle variations in atmospheric oxygen and carbon dioxide content during the Pleistocene, the Earth’s recent period of repeated glaciations.

It could be useful, though, when evaluating extreme climates much earlier in our planet’s history. For example, oxygen-17 anomalies in barite sulphates display a negative spike – hinting at an extremely high level of atmospheric carbon dioxide - around 635 million years ago, when the Earth was likely recovering from a period of global glaciation in the Early Cambrian. This finding supports the not undisputed ‘snowball’ Earth hypothesis and/or massive methane release in the aftermath of Neoproterozoic glaciation

Quirin Schiermeier

Posted by Quirin Schiermeier on May 22, 2008
Categories: Atmospheric Science, Climate Science, Palaeoclimate, Quirin Schiermeier | Permalink | Comments (0) | TrackBacks (0)

A paper in Nature Geoscience this week (subscription required) serves up an important figure for climate modellers: the size of the greenhouse effect caused by ozone near the Earth's surface, estimated from direct observations.

Whereas 20 years ago the discovery that the stratospheric ozone layer was thinning led to international prohibitions on ozone-eating chemicals, this new study reflects concerns about excess ozone produced nearer to the ground, in the troposphere, by reacting pollutants. Tropospheric ozone is a greenhouse gas - and although carbon dioxide still gets the most column inches, other greenhouse gases such as ozone (see also methane) are drawing more and more scientific attention.

Previously, the best estimates of the radiative forcing from ozone - its planet-warming power - came from simulations. The latest IPCC report used several such models to assess forcing from anthropogenic ozone alone, excluding ozone from natural sources, and came up with a range of values from 0.25 to 0.65 watts per square metre (in comparison, forcing by all anthropogenic greenhouse gases together was estimated at 1.6 watts per square metre). Now, NASA's Aura satellite has collected enough measurements of infrared radiation and ozone thickness in cloudless patches of sky to pin down the combined effects of natural and human-produced ozone. According to these data, the global average forcing in the year 2006 was 0.48 (+/- 0.14) watts per square metre. No surprises, but a palpable step forward in the hardworking and sometimes underappreciated field of Earth observation.

Anna Barnett

Image: Artist's rendering of Aura satellite, NASA

Posted by Anna Barnett on April 24, 2008
Categories: Anna Barnett, Atmospheric Science, Climate Science | Permalink | Comments (1) | TrackBacks (0)

If there was an eye in the sky keeping watch on our greenhouse gas emissions, what carbon crimes would it reveal?

The ability to measure greenhouse gases from space, soon to become a reality, could answer this question.

Currently, it’s virtually impossible to identify the exact source – and destination - of greenhouse gases, a prominent theme at this year’s European Geosciences Union conference in Vienna.

But, according to scientists speaking today at the conference, this is all set to change within the coming year when two major satellites designed to monitor greenhouse gases will be launched into space.

Due to leave Earth on December 15, the first of these is the Orbiting Carbon Observatory (OCO), a US$300 million-or-so innovation of scientists and engineers at NASA’s Jet Propulsion Laboratory in California. The Japanese version, known as the Greenhouse Gases Observing Satellite, or GOSAT, has an anticipated launch date in January or February 2009.

For a detailed low-down on the satellites and how they will work, see Amanda Haag’s news feature in Nature (subscription) last December. Since then, the scientists have mostly been testing and calibrating the instruments to make sure they work once they’re orbiting the Earth.

Within a year or two, if not sooner, they will enable scientists to identify major sources and sinks or carbon, says Charles Miller, one of the Principal Co-ordinators of the OCO mission. The greenhouse gas measurements taken by the instrument, which will orbit the planet 14.5 times per day, will be three times more precise than any trace gas measurements ever taken from space.

All-in-all, the missions represent an unprecedented effort to collect global climate data from space. While this is fascinating from a scientific perspective, it should also have some interesting political implications by enabling the easy identification of climate culprits.

“If one were to imagine a way to monitor or verify [emissions], then this would be the way to go”, says Miller. For instance, it should quash (or raise, depending on who you’re talking to) fears that nations claiming credits for avoiding deforestation under the Kyoto Protocol will be able to divert the problem elsewhere.

Miller says they often joke that the instrument could detect the greenhouse gas emissions of serious carbon heaveyweights from space. But while the new satellites won’t realistically help reporting on individual carbon crimes, it could act as a ‘big brother’ to keep countries in line with their Kyoto commitments. Personally, I’m curious to know the OCO’s own carbon footprint!

That’s all from me from this year’s European Geosciences Union conference in Vienna. Over and out….

Olive Heffernan

Posted by Olive Heffernan on April 18, 2008
Categories: Atmospheric Science, Conferences and Meetings, EGU 2008, Olive Heffernan, Technology | Permalink | Comments (0) | TrackBacks (0)

Kevin Trenberth

A recent paper published by Gaertner et al. in Geophysical Research Letters (GRL) (in press as of 12 July 2007) explores the possibility for tropical cyclones to occur in the Mediterranean area in the future with global warming. It has been featured in the Times. The paper correctly points out that tropical cyclones have recently ventured into some unusual places with Catarina on the east coast of Brazil in March 2004 and Vince making landfall in Spain in 2005. Gaertner et al. use an ensemble of regional climate models to assess new locations of tropical cyclone occurrence. They find an increase in extremes of cyclone intensity over the Mediterranean Sea in regional model climate change scenario simulations. This increase, they claim, is clearly related to tropical cyclone formation, revealing for the first time a risk of tropical cyclone development over the Mediterranean Sea under future climate change conditions.

The regional model framework for this study is one limitation as it may not create the correct large-scale atmospheric circulation across the region. In particular, the Mediterranean climate in summer is one of clear skies and sunny days associated with the overall global monsoon circulation, such that the upward motion and rains in southern Asia are linked to the subsiding air over the Mediterranean that makes for a very stable environment unfavorable for storms. Unless that link is properly simulated (and it may not be, especially in a regional model) the vertical atmospheric temperature and wind structure are unlikely to be right.

The authors are aware of the summer difficulties and so they choose September as the time for the simulations. This has the advantage that the sea surface temperatures (SSTs) are highest then. The sun has been beating down all summer, but it tends to form a shallow layer of warm water and whether there is adequate heat below the surface is also a critical question. As cyclones form they churn up the ocean, bringing cold waters up from below and cool the ocean, creating a cold wake. In this work the SSTs were specified and fixed and this was not allowed to happen.

Moreover, by September, the mid-latitude westerlies have set in and the winds in the upper troposphere are typically over 20 m/s, so the wind shear environment is generally unfavorable for such storms. It is possible that the weather could change enough to relax the winds for just long enough to open a window for tropical cyclones but the odds are not high. This is the extratropics, after all.

All these questions remain. Nonetheless, with higher SSTs it does seem likely that cyclones would be more vigorous. But whether they are truly tropical or not is a key issue. The paper does not comment on water vapor: the Mediterranean Sea is surrounded by land where the air is drier and this can cause storms to run out of moisture and peter out. Nor does it mention rainfalls and flooding: a chief characteristic of tropical cyclones is the heavy rainfalls of several mm per hour. Nonetheless, warm core storms have been noted in the Mediterranean and are colloquially referred to as “Medicanes”.

For further discussion of tropical cyclones and climate change, see my recent article in Scientific American here.

Kevin E. Trenberth
Head of the Climate Analysis
US National Center for Atmospheric Research

Posted by Olive Heffernan on July 24, 2007
Categories: Atmospheric Science, Climate Science, Hurricanes, In the News, Kevin Trenberth | Permalink | Comments (0) | TrackBacks (0)

Posted by Olive Heffernan on behalf of Kevin E. Trenberth

Aerosols in the atmosphere have been hot topics in several recent climate studies but one wonders if the pollution has made not only the atmosphere murkier but also the scientific reasoning?

In March a widely reported study in PNAS by Zhang et al. linked changes in storm tracks over the North Pacific to Asian pollution. In this case effects on radiative forcing by atmospheric aerosols supposedly increased deep convective clouds over the Pacific Ocean in winter, a finding based on long-term satellite cloud measurements (1984–2005). The blame was assigned to the aerosol effect from Asian pollution, which supposedly leads to intensified storms. Unfortunately the authors did not seem to be aware of the major problems in the satellite-based cloud data that give spurious trends owing to changes in satellites and associated instruments. In particular there were major changes in 1994 in all 3 geostationary satellites (GOES West, GOES East and GMS (Japanese)) observing the Pacific Ocean. All are known to be associated with spurious changes in cloud present in the International Satellite Cloud Climatology Project (ISCCP) data. The ISCCP record is simply not reliable for trends. The time series of the paper are perhaps more a measure of the problems with the data than they are of climate change or evidence of effects of aerosols?

Another recent study by Lau and Kim in the February 27 issue of EOS claims that nature foiled the 2006 hurricane season by increasing aerosols over the tropical North Atlantic in the summer of 2006 (see also Wu 2007 for more general analysis). There is no doubt that sea surface temperatures were lower in 2006 than in the record breaking year of 2005 and that aerosols were more plentiful. It is also clear that there were fewer tropical storms in 2006. But is the former the cause of the latter, as inferred in the article? Or is it more likely that the absence of storms led to less rain and thus less washout of aerosols leading to the increase in atmospheric aerosols? Of course, even in the latter case, there may be a feedback, but estimates of the aerosol effects places them much smaller than effects from changes in cloud and sunshine and from evaporative cooling as winds change.

[Note the contrasting findings between the above two studies with one finding more convection and the other finding less, all due to aerosols!]

Now someone else thinks they can use aerosols to change hurricanes, (as reported by Joshua Zaffos in the Rocky Mountain Chronicle in June 2007). Bill Cotton, a professor from Colorado State University, thinks that dumping dust into a hurricane could be used to weaken the storm. The irony in this case is that Cotton is a skeptic on the issue that humans are changing the climate in spite of overwhelming evidence. Human changes in the greenhouse effect amount to about a 1% change in the energy flow through the climate system. But the energy in a hurricane is enormous; it is typically equivalent in energy to a 1 megatonne nuclear bomb going off every 5 seconds, and Cotton thinks he can mess with a hurricane! What if he is actually right and instead it strengthens the storm?

Posted by Olive Heffernan on June 26, 2007
Categories: Atmospheric Science, Climate Science, Kevin Trenberth | Permalink | Comments (2) | TrackBacks (0)