Temperature and CO2 feedback loop ‘weaker than thought’ - but won’t reduce predicted temp rises

28/1/2010 BBC Runaway CO2 rise ‘could be lower’The most alarming forecasts of natural systems amplifying the human-induced
greenhouse effect may be too high, according to a new report.
The study in Nature confirms that as the planet warms, oceans and forests will
absorb proportionally less CO2.
It says this will increase the effects of man-made warming - but much less than
recent research has suggested.
The authors warn, though, that their research will not reduce projections of
future temperature rises.
Further, they say their concern about man-made climate change remains high.
The research, from a team of scientists in Switzerland and Germany, attempts to
settle one of the great debates in climate science about exactly how the Earth’s
natural carbon cycle will exacerbate any man-made warming.
Positive, negative
Some climate sceptics have argued that a warmer world will increase the land
available for vegetation, which will in turn absorb CO2 and temper further
warming. This is known as a negative feedback loop - the Earth acting to keep
itself in balance.
But the Nature research concludes that any negative feedback will be swamped by
positive feedback in which extra CO2 is released from the oceans and from
already-forested areas.
The oceans are the world’s great store of CO2, but the warmer they become, the
less CO2 they can absorb. And forests dried out by increased temperatures tend
to decay and release CO2 from their trees and soils.
Commenting in Nature on the new research, Hugues Goosse from the Université
Catholique de Louvain in Belgium said: “In a warmer climate, we should not
expect pleasant surprises in the form of more efficient uptake of carbon by
oceans and land… that would limit the amplitude of future climate change”.
The IPCC’s fourth assessment report had a broad range of estimates as to how far
natural systems would contribute to a spiral of warming. The Nature paper
narrows that range to the lower end of previous estimates.

The report’s lead author, David Frank from the Swiss Federal Institute for
Forest, Snow and Landscape Research, told BBC News that many of the calculations
for the IPCC assessment report did not include an integrated carbon cycle.
He said that if the results his paper were widely accepted, the overall effect
on climate projections would be neutral.
“It might lead to a downward mean revision of those (climate) models which
already include the carbon cycle, but an upward revision in those which do not
include the carbon cycle.
“That’ll probably even itself out to signify no real change in the temperature
projections overall,” he said.
The team’s calculations are based on a probabilistic analysis of climate
variation between the years 1050 and 1800 - that is, before the Industrial
Revolution introduced fossil carbon into the atmosphere.
Using 200,000 data points, the study - believed by Nature to be the most
comprehensive of its kind so far - compared the Antarctic ice core record of
trapped CO2 bubbles with so-called proxy data like tree rings, which are used to
estimate temperature changes.
The most likely value among their estimates suggests that for every degree
Celsius of warming, natural ecosystems tend to release an extra 7.7 parts per
million of CO2 to the atmosphere (the full range of their estimate was between
1.7 and 21.4 parts per million).
This stands in sharp contrast to the recent estimates of positive feedback
models, which suggest a release of 40 parts per million per degree; the team say
with 95% certainty that value is an overestimate.
The paper will surely not be the last word in this difficult area of research,
with multiple uncertainties over data sources.
“I think that the magnitude of the warming amplification given by the carbon
cycle is a live issue that will not suddenly be sorted by another paper trying
to fit to palaeo-data,” Professor Brian Hoskins, a climate expert from Imperial
College London, told BBC News.
Professor Tim Lenton from the University of East Anglia said: “It looks
intriguing and comforting if they are right. The immediate problem I can see is
that past variations in CO2 and temperature over the last millennium were very
small, and this group are assuming that the relationship they derive from these
very small variations can be extrapolated to the much larger variations in
temperature we expect this century.

“We have plenty of reason to believe that the shape of the relationship may
change (be nonlinear) when we ‘hit the system harder’. So, I don’t think they
can rule out that the positive feedback from the carbon cycle could become
stronger in a significantly warmer climate.”

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