Climate Feedback

One of the more irritating aspects, if you will, of global change is that air pollution has so far prevented the planet from warming more rapidly than it actually did. Clean air is of course a good thing. But reducing pollution might expose an as of yet ‘masked’ portion of global warming.

This could have a dramatic affect on the Amazon rainforest. A team led by Peter Cox of the University of Exeter, UK, reports in a paper in this week’s Nature that reductions in aerosol pollution will tremendously increase the risk of severe drought in the Amazon region. Here is an editor’s summary of the paper.

Although it accounts for nearly a quarter of the world's fresh water, drought is not unknown in Amazonia.

In the dry season, from July to October, rainfall in the region is linked to sea surface temperatures (SST) in the tropical Atlantic. In years with a pronounced temperature gradient - warming of the tropical Atlantic north of the equator relative to the south – the normal’ position of high and low atmospheric pressure systems can shift, delaying or suppressing the onset of the South American monsoon.

The effect has been observed in 2005, when large parts of the Amazon region were hit by the worst drought in decades. See a Nature news story by Mike Hopkin here (subscription required) and a New York Times story here about the devastating event.

Cox thinks that the 2005 drought was a harbinger of things to come. Their “simulations for the 21st century show a strong tendency for the SST conditions associated with the 2005 drought to become much more common, owing to continuing reductions in reflective aerosol pollution in the Northern Hemisphere.”

Droughts like in 2005 will happen every two years by 2025, and in nine out of ten years by 2060, the model suggests.

How robust is this dire prediction? The Amazonian climate, for reasons not quite understood, is notoriously difficult to simulate. But the Hadley Centre’s climate model which was used for this study has previously reproduced features of the regional climate with greater accuracy than other models.

In Mike Hopkin's words, “the ultimate fear is that the Amazon forest - often touted as an invaluable piece of armour against climate change - could become part of the problem rather than a key element of the solution. Droughts make it more likely that it will become a net source of greenhouse gases to the atmosphere, rather than mopping them up.”

Quirin Schiermeier

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

Posted by Quirin Schiermeier on May 08, 2008
Categories: Biosphere feedbacks, Climate Science, Climate prediction, Extreme Events, Quirin Schiermeier, Topics | Permalink | Comments (3) | 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)

Signs of a very different 21st-century climate are already showing up, and not just in the melting arctic. A new review in Nature Geoscience highlights reports that the boundaries of the tropics, defined by temperature, rainfall, wind, and ozone patterns, have shifted poleward by at least 2 degrees latitude in the last 25 years. According to climate models, that's as far as the tropical belt was supposed to creep by the end of this century. Five different methods used to measure the tropics all show more or less breakneck rates of expansion -- one gives as much as 4.8 degrees in 25 years.

Continue reading "Tropics expanding fast " »

Posted by Anna Barnett on December 04, 2007
Categories: Anna Barnett, Climate Science, Extreme Events | Permalink | Comments (1) | TrackBacks (0)

Olive Heffernan

This week on Nature Reports Climate Change, Amanda Leigh Haag looks at how climate change is increasingly becoming an issue of national security, raising the alarm on issues of border control and immigration policy globally.

The feature details how regions likely to bear the brunt of climate impacts are already beginning to look to neighbouring states for potential resettlement deals, while less vulnerable nations are considering the likely spillover of large-scale migration from areas impacted by severe drought or flooding.

This raises some interesting issues, such as whether adaptation should focus on protecting the rights of people to live in their home, rather than offering relocation programmes, and whether these scenarios are inevitable without drastic measures to prevent further warming….but more on that shortly.

Posted by Olive Heffernan on September 26, 2007
Categories: Adaptation, Extreme Events, Features, In the News, Olive Heffernan | Permalink | Comments (10) | TrackBacks (0)

Olive Heffernan

Much like the rest of Britain, I’m beginning to wonder if it’s ever going to stop raining. And despite feeling slightly miffed at an appalling excuse for a summer, I realise I'm lucky to be based in slightly soggy London, given that large areas of the country are currently besieged by some of the worst flooding in recent British history.

Calling it a ‘21st century catastrophe’, Michael McCarthy at the Independent writes that “Britain is suffering from a wholly new type of civil emergency: a disaster caused by 21st-century weather,” which has left more than a third of a million people without drinking water, nearly 50,000 people without power, thousands more people homeless and caused more than £2bn worth of damage so far.

Britain is not alone in experiencing extremely heavy rainfall. As reported on MSNBC, “parts of China had the heaviest rainfall since records began, killing more than 700 so far this year. Hundreds of thousands have been displaced by flash floods in southern Pakistan.”

While these single events cannot be attributed to climate change, many are questioning if the flash flooding is a sign of what is in store for the future. And scientists have some of the answers.

In a paper coming out in Nature this Thursday, Francis Zwiers of the Canadian Centre for Climate Modelling and Analysis in Toronto and colleagues present the first evidence that human-generated greenhouse gas emissions have altered rainfall patterns in the 20th Century. In the region between 40 and 70 degrees North, covering northern Europe, Russia and parts of North America, rainfall increased by 62 millimetres per century between 1925 and 1999. Zwiers and colleagues say that 50-85% of this increase can be attributed to human activity. For further discussion and comments on the paper, there’s a news story by my colleague Daniel Cressy on News@Nature. And it’s also been picked up by the BBC.

And a recent paper published in Science in June suggests that global warming may result in even more rainfall worldwide than is currently evident in climate model simulations. Frank J. Wentz of Remote Sensing Systems in Santa Rosa, California and co-workers compared global satellite data from 1987 to 2006 and found that rainfall increased at the same rate as atmospheric water vapour per degree Celsius of surface warming. Climate models had projected a dampened response of rainfall to global warming owing to a decrease in surface winds, but Wentz and colleagues found that surface winds have in fact become stronger, leading to heavier rainfall (more on this in Nature Reports Climate Change soon).

Coming back to Britain… the situation is likely to worsen over the next 24 hours. Eight severe warnings have been issued covering the rivers Thames, Severn and Ouse, in particular for towns such as Gloucester, Tewkesbury, Oxford, Abingdon, Reading and Bedford. Fifty other flood warnings are in place across England and Wales.

To see the areas generally most at risk of flooding in England and Wales, visit the Environment Agency’s flood map online, where you view flood risk by postcode. For an up to date interactive on the current situation, the Guardian has quite a snazzy interactive highlighting areas most at risk.

Olive Heffernan
News Editor
Nature Reports Climate Change

Posted by Olive Heffernan on July 24, 2007
Categories: Climate Science, Extreme Events, In the News, Olive Heffernan | Permalink | Comments (3) | TrackBacks (0)