COLUMN-Study opens new cracks in scientific front on climate change: Gerard Wynn

Source: Reuters - Fri, 21 Jun 2013 12:06 GMT
Author: Reuters
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A silhouetted airplane flies past a 'super moon' over New York on June 23, 2013. REUTERS/Zoran Milich
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By Gerard Wynn

LONDON, June 21 (Reuters) - A major recent study has put the cat among the pigeons on climate change, challenging the size of the problem in the near-term and the role of a recent slowdown in warming.

The paper, published in the journal Nature Geoscience in May, involved scientists from 14 institutions and calculated that more extreme climate change was now less likely, after taking into account slower warming in the past decade.

The notion of changing a view as a result of a single decade of observations is somewhat controversial, given that natural patterns can span several decades.

And the paper applied a method for estimating future climate change which is known to produce lower warming estimates than a corrsponding measure used by the U.N. climate panel, the Intergovernmental Panel on Climate Change (IPCC) and the International Energy Agency (IEA).

Either way, the big picture is the same: the Earth is warming up because of manmade greenhouse gas emissions.

But the more uncertain the science is over the scale of warming, the less political will there may be.

This is at a time when U.N. climate negotiations have floundered, the U.S. congress is reluctant to act, and arguably sceptics playing to the consumer cost of low-carbon energy have gained more traction at the grassroots level than environmental groups.


The science, for its part, reflects a vastly complicated climate system and the number of variables at play, although there is some hope that new research plus observed warming may resolve some uncertainties by the end of the decade.

The new paper's key conclusion, at least politically, is that it forecasts the long-term impact of a doubling of CO2 levels on the earth at 2 degrees of warming.

Actual warming may be more, depending on the net effect of other greenhouse gases and pollution.

The estimate of 2 degrees is lower than the IPCC estimate by a whole degree and, interestingly enough, precisely in line with the maximum safety limit that every climate conference for the past five years has put on warming.

A common benchmark that scientists use to quantify the climate problem is the amount of warming expected from a doubling of carbon dioxide levels in the atmosphere.

That includes two related measures.

One is the short-term warming at the moment CO2 levels reach double their baseline levels, called the transient climate response (TCR).

The other is the "equilibrium climate sensitivity" (ECS), or temperature change after the Earth has fully responded centuries later to the same doubling in CO2.

There are several ways to estimate them.

One approach uses complex computer models to imitate the planet's entire climate system using general circulation models based on our knowledge of atmospheric and ocean physics.

The models are calibrated to try and reproduce the observed climate over the temperature record of the past 150 years, and then simulate future climate change as a result of rising CO2.

An alternative approach is based on an estimate of the Earth's "energy budget".

It starts with the observed temperature change at the Earth's surface, and then takes into account other factors which may brake surface warming, such as the uptake of heat by the deep oceans.

It, too, uses computer models to estimate other influences such as from manmade pollution, which can cool the Earth.

A computer simulation approach may capture vital unseen influences on the Earth's climate and so be more nuanced.

By incorporating recent temperature observations, the energy budget approach is more up to date.

It is hard to say which is more accurate.


Last month's paper - "Energy budget constraints on climate response" - used an energy budget approach, and found that by focusing on the most recent decade of temperature observations it could rule out more extreme warming in their calculated ranges for ECS and TCR.

The authors accounted not only for lower surface temperature in the last decade, but also a rapid build up of heat in the deep oceans.

"Recent observations suggest the expected rate of warming in response to rising greenhouse gas levels ... is likely to lie within the range of current climate models, but not at the high end of this range," said lead author Alexander Otto.

The finding that the top end of the temperature range can now be eliminated is perhaps not especially surprising.

First, warming has slowed compared with the 1980s and 1990s.

Second, it is to be expected that extreme warming values could be revised downwards: upper estimates are open-ended and speculative while lower estimates are limited by warming that we know has already happened, at 0.8 degrees Celsius since the Industrial Revolution.


The more interesting result is that the paper using the energy budget approach estimated less warming than a computer simulation relying less on current observation.

The different results have something to do with, first, the uncertain impact of natural variation in driving climate changes, and second, the uncertain role of changing cloud cover; heat uptake in the deep oceans; and manmade pollution which can cool the Earth.

Scientists take comfort at least that the two methods produce results in the same range.

The short-term TCR is more relevant - because this is warming that we will see this century, and it makes less heroic assumptions about how people will keep a lid on further emissions in future centuries.

At present emissions rates, CO2 levels will reach double pre-industrial levels at some point in the second half of the century.

At that point warming should therefore reach the level estimated using the TCR measure, plus any extra warming from other greenhouse gases.

The recent article has a best estimate for TCR of about 1.3 degrees versus 1.8 degrees Celsius using a computer simulation approach. ("Global warming under old and new scenarios using IPCC climate sensitivity range estimates", February 2012)

Repeated communiques from U.N. climate conferences have warned that 2 degrees warming compared with pre-industrial temperatures is manageable, but no more.

Scientists are saying that we are on track to reach that level of warming this century, or perhaps not.

Climate change is clearly serious, given hugely costly and possibly irreversible risks such as sea level rise and deadly droughts and floods.

And neither scientific approach accounts for the possibility of climate tipping points, such as the sudden release of greenhouse gases from the sea bed as a result of warming oceans.

But the science shows a complicated problem which is hard to pin down, and is therefore struggling to engage voters. And the more cracks appear in the consensus that climate change demands urgent action, the less political will there will be to do more. (Reporting by Gerard Wynn; editing by Patrick Graham)


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