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 (4) | 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)

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)

Over at News@nature, Mike Hopkin reports from the Ecological Society of America's meeting in San Jose on research into tropical forest growth rates. Looking at plots in Panama and Malaysia, the researchers found that increases in mean daily minimum temperature over a couple of decades correlated with decreases in growth rates. They associate this with lower net photosynthetic activity.

The team, led by Harvard's Ken Feeley, suggests that if this sort of effect were repeated in bigger rainforests (most of which have only experienced marginal warming to date, as I understand it) then what are now stable stores of carbon would become net sources as theworld heats up. This is obviously a considerably less optimistic scenario than the possibility that carbon-dioxide fertilisation would make them sinks. It would presumably make the net effect of the increase in soil respiration that Peter Cox and others always stress (Nature paper from 2000) an even worse problem.

It's not a dead cert that the change is due to temperature -- the paper (published in Ecology Letters) seems to suggest that increased cloudiness could be playing a role. And there could be internal botanical changes too -- maybe the lianas are doing more damage? But all in all it doesn't sound good.

Mike is blogging the conference on the newsblog.

Posted by Oliver Morton on August 10, 2007
Categories: Biosphere feedbacks, Climate Science, Oliver Morton | Permalink | Comments (0) | TrackBacks (0)

Further to the post and subsequent discussion on Sunshades, which grew out of this article on geoengineering, I thought I'd point to the new paper by Damon Matthews and Ken Caldeira in PNAS (Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0700419104). It's an interesting paper that has some fascinating insights into the links between climate and the carbon cycle, and I think contains some pretty bad news for would-be geoengineers.

The paper uses a University of Victoria inermediate complexity GCM along with a land cover and carbon cycle model (the Hadley Centre MOSES2 and TRIFFID -- which is pretty much the best acronym in the business) to track climate from 1900 to 2100, using historical data up to the end of the twentieth century and the IPCC A2 carbon-dioxide emissions scenario from then on. Left to itself this gives a temperature increase of 3.5 centigrade over the 200 years. They then compared this baseline to alternative scenarios in which geoengineering strategies were turned on and off at various times. The geoengineering effect -- think of it as a layer of sulphates in the stratosphere, though the model wasn't that specific -- was calibrated to reduce the incoming sunlight in such a way as to counteract the radiative forcing of the carbon dioxide at any given time.

They found that the geoengineering could reduce the change in temperature in the model to something pretty negligible, though with some latitude-dependent effects; in the geoengineered world the poles warm a little compared to 1900 while the tropics cool a little. It also appeared that you could get back to 1900 temperatures even if you started the geoengineering well into the twentyfirst century, as long as you did enough of it.

Various reports of this work have highlighted a fairly obvious subsequent finding: if you stop the geoengineering while having done nothing about carbon emissions you can get some truly horrendously quick warming; your protection vanishes almost instantaneously and the potential warming you have stored up by allowing carbon-dioxide levels to rise suddenly all appears at once. Though it's nice to have some figures on this, it hardly comes as a surprise. Stephen Schneider has been going on about the fact that once you start you can't stop for decades, and in Tom Wigley's Science article (Science 314 pp. 452 - 454 (2006) DOI: 10.1126/science.1131728) last year, which explored the possibility of using a brief period of geoengineering to buy time in which to develop and field the technology needed for radical emissions reduction, there was a nasty looking blip in the warming rate at the point where the geoengineering was turned off. But it's still a sobering thought. While geoengineering through something like sulphate in the stratosphere is "reversible", in that if it starts having nasty effects you can just turn it off and the sulphate will fall out in a few years, that doesn't just leave you with the status quo ante -- it leaves you facing a far faster rate of warming that ypu have ever seen, and the adaptation challenges that go along with that.

There's an extra wrinkle in this paper, too; in the geoengineered world, you get increased carbon-dioxide uptake by the biosphere through the carbon-dioxide fertilisation effect on plants, but no offsetting increase in the carbon dioxide given off by soil respiration, which is taken to be temperature dependent. Turn the geoengineering off and the resultant warming drives up soil respiration in a positive feedback, releasing yet more carbon dioxide and pushing temperatures yet higher. It's a good example of the links between climate and the carbon cycle and the ways they can mess you up. Not as good an example, though, as that offered by the precipitation outlook, which seems to me the most startling result here.

Continue reading "More on geoengineering" »

Posted by Oliver Morton on June 10, 2007
Categories: Biosphere feedbacks, Climate Science, Oliver Morton | Permalink | Comments (1) | TrackBacks (0)

This week in Nature we have a news story on an attempt to follow up Frank Keppler's work on methane produced aerobically by green plants which we published early last year (news story | paper). The Keppler piece, which suggested that methane emissions from green plants were a significant but previously unappreciated factor in global methane emissions, caused quit a lot of fuss, understandably, in the media -- since methane is a greenhouse gas which, over short time horizons, is about 75 times more powerful than carbon dioxide -- and quite a lot of befuddlement among plant scientists. If it were true, it would have significant implications for the way that people model methane production, and the levels of production that one might predict in a warming world. The debate rumbled on last year (another news report, this time by my colleague Quirin).

The new work that Tom Dueck and colleagues have published in New Phytologist (paper), though , finds no methane emissions from plants at all.

Obviously, not necessarily the last word. As Mike Hopkin reports:

Both groups have criticized the other's choice of experimental method. Dueck says that Keppler's group kept plants in sealed plastic containers instead of flow chambers, and exposed them to sources of stress such as bright sunlight and high temperature, which could have produced methane as an artefact. Keppler retorts that the use of ¹³C is an artificial piece of chemical trickery with unknown effects on plant metabolism, and also argues that methane production can vary by up to three orders of magnitude between species.

What Mike doesn't mention, because an evil news editor (me) wouldn't give him the space, is that various people in the community have pointed to an interesting contrast between the way plant scientists responded to the discovery of isoprene emissions and the Keppler work. With isoprene people said oh that's interesting, replicated, and got on with it. This work has had a far frostier welcome.

On isoprene, this is as good a place as any to mention an interesting perspective by Manuel Lerdau in Science a few weeks ago on a possible isoprene-ozone positive feedback (paper). Isoprene within leaves protects the plants that produce it against ozone. But when isoprene gets out into the air, as it will, it can react with nitrogen oxides to make ozone. Only some species produce isoprene, and so these isoprene-producing plants both protect themselves against ozone and, in Nox-rich environments, increase the ozone stress on their non-isoprene-producing neighbours.

If this effect is real, it might have significant effects on forest composition over the next century.

One last thing to note on the Keppler story: it led to Carl Zimmer saying something nice about us, and that is always a good thing. As of course is Carl.

Posted by Oliver Morton on May 04, 2007
Categories: Biosphere feedbacks, Oliver Morton | Permalink | Comments (8) | TrackBacks (0)