Doubling the amount of carbon dioxide in the atmosphere won’t produce as much global warming as some previous estimates, says a new study. However, scientists don’t universally accept this result nor do they agree about whether it makes much difference about what we should do about it.

The study, published in the journal Climate Dynamics, finds doubling the amount of carbon dioxide (from pre-industrial levels) in the atmosphere is likely to warm the climate by about 2.4 F (1.3 C)* by the time doubling occurs. The Intergovernmental Panel on Climate Change (IPCC), in its latest assessment report, had estimated the value of this quantity known as the “transient climate response (TCR)” to be 33 percent higher, or 3.2 F (1.8 C)*.

The authors, British scientist Nicholas Lewis and Georgia Tech’s Judith Curry, developed a model using historical measurements of temperature and ocean heat content as well as estimates of the levels of particulate matter in the atmosphere to estimate the warming effect of carbon dioxide.

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In a blog post explaining the study, Lewis stressed their results “are less dependent on global climate models” than other analyses.

Myles Allen, a climate scientist at Oxford University – not involved in the study, called the methodology “sensible enough”, similar to that of a study he was involved in a year ago.

But Allen is not convinced the study’s results are novel but rather fit into an existing consensus. He noted that the uncertainty range* given for TCR in the Lewis and Curry study is very similar to that of the IPCC.

“[I]f we all go into Paris [where the next round of climate policy negotiations will occur] agreed on a TCR range of 1-2.5C, it doesn’t make any difference to any policy decisions whether … the most likely value is 1.8 C (the average of the models used in the latest IPCC projections) or 1.33 C (Lewis and Curry’s “best estimate”),” Allen said.

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“A 25 per cent reduction in TCR would mean the changes we expect between now and 2050 might take until the early 2060s instead … So, even if correct, it is hardly a game-changer,” Allen told the blog Carbon Brief.

While the Lewis and Curry study does not differ much from existing literature on TCR, its estimate for the value known as “equilibrium climate sensitivity (ECS)” is considerably lower.

ECS is a measure of total amount of warming from doubling of carbon dioxide that takes into account the slow-moving processes in the climate system such as the full exchange of heat from the ocean to the atmosphere. Because of these slow processes, the full warming from a carbon dioxide doubling isn’t expected for decades to hundreds of years after doubling occurs.

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The Lewis and Curry study likely** range for ECS is just 2.25 to 4.4 F (1.25 to 2.45 C) compared to IPCC’s toastier range of 2.7 to 8.1 F (1.5 to 4.5 C).

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On her blog, Curry concedes their range is subject to change:

Is this paper the last word on climate sensitivity estimates? No. . . . There remains considerable meta uncertainty in the determination of climate sensitivity, including how the problem is even framed.

The blog And Then There’s Physics lists several reasons why Lewis and Curry’s estimate could be too low, including not fully accounting for the transfer of heat between the ocean and atmosphere.

It further cautions one must use be careful in leaping to conclusions from the results of a single study. Computer modeling studies generally estimate higher values for ECS, as do some studies based on paleoclimate data.

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On the other hand, there are an increasing number of recent studies – using methods similar to Lewis and Curry – that conclude the ECS is towards the low end of IPCC’s range.

“It seems that some folks want climate sensitivity to be on the high side of its historical range,” said Michael Schlesinger, a climate scientist at University of Illinois who has published on climate sensitivity. “But it is not what is wanted but what Mother Nature provides that is important. The fact that recent estimates of climate sensitivity are on the low side of the range is good news as it will allow us to phase out greenhouse gas emission from 2020 to 2100 following the Fair Plan to Safeguard Earth’s Climate.”

The blog Skeptical Science warns that even if the sensitivity is low, significant warming is still likely if emissions are not cut.

It’s … important to remember that climate sensitivity is not the same as total warming.
Instead, it’s what we’ll get every time the carbon dioxide concentration doubles above pre-industrial levels.
If emissions stay as high as they are, that means even a low value of climate sensitivity would see a significant amount of warming by the end of the century

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Footnotes:

* The study, gave an uncertainty range of 1.6 to 4.5 F (0.9 to 2.5 C) for TCR, which is very similar to the IPCC’s range of 1.8 to 4.5F (1 to 2.5 C).
** Likely is defined as 18-83 percentile range.

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