A climate of hope

If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch, says a recent article in the Economist. Barry Brill looks at recent climate sensitivity estimates and considers the possibility of a downgrade.

The world has been discussing anthropogenic global warming (AGW) for over 20 years, but knows very little about its extent. Is it trivial or dangerous? This crucially depends on the relationship between atmospheric carbon dioxide and global temperatures - known as the 'equilibrium climate sensitivity (ECS)'.

Gareth Morgan's book, _Poles Apart_1, canvasses the views of both sides but eventually comes down on the side of orthodoxy in saying (p248):

"We regard the IPCC range of values for sensitivity to carbon dioxide - between 1.8°C and 4.4°C as plausible, or more plausible, at any rate, than the Sceptic's unreasonably low 1.5°C".

In a lengthy report headlined A Sensitive Matter, a recent issue2 of The Economist captured the policy dilemma:

"If, as conventional wisdom has it, global temperatures could rise by 3°C or more in response to a doubling of emissions, then the correct response would be the one to which most of the world pays lip service: rein in the warming and the greenhouse gases causing it…

If, however, temperatures are likely to rise by only 2°C in response to a doubling of carbon emissions (and if the likelihood of a 6°C increase is trivial), the calculation might change. Perhaps the world should seek to adjust to (rather than stop) the greenhouse-gas splurge. There is no point buying earthquake insurance if you do not live in an earthquake zone. In this case more adaptation rather than more mitigation might be the right policy at the margin."

The phenomenon triggering the article is set out in its first two lines:

"Over the past 15 years air temperatures at the Earth's surface have been flat while greenhouse-gas emissions have continued to soar."

There has been no significant warming since 1996 (or longer according to three of the six recognised archives3). The 20-year warming episode of 1978-98 has been followed by a plateau of almost the same duration. The trend has been slightly downwards since the beginning of this century, but that figure is not yet statistically significant.

The major challenge to the orthodox view has been the failure of the IPCC predictions of previous years. A 2012 IPCC graph showing actuals versus predictions was described in one UK newspaper4 as showing "a spectacular miscalculation".

The correct figure or range for the ECS depends upon the chaotic action of clouds and has long been debated. Its shaky history appears from the Wikipedia entry for "climate sensitivity" (the original source, from Science, is behind a paywall):

"A committee on anthropogenic global warming convened in 1979 by the National Academy of Sciences and chaired by Jule Charney estimated climate sensitivity to be 3°C, plus or minus 1.5°C. Only two sets of models were available; one, due to Syukuro Manabe, exhibited a climate sensitivity of 2°C, the other, due to James E. Hansen, exhibited a climate sensitivity of 4°C."According to Manabe, Charney chose 0.5°C as a not-unreasonable margin of error, subtracted it from Manabe's number, and added it to Hansen's. Thus was born the 1.5°C-to-4.5°C range of likely climate sensitivity that has appeared in every greenhouse assessment since…"

A rash of recent journal papers are now (after 30 years and billions of dollars) proposing to adjust Charney's 3°C best estimate down to 1.5 - 2.5°C or perhaps less. This is hugely significant because warming up to 2.2°C is expected to have net beneficial effects5.

It's pretty clear that the accumulated effect over the whole period until the 2.5°C level is reached (if ever) will be a positive experience. Further, many believe that the present temperature plateau is likely to last for another 15-20 years, removing any serious threat of dangerous AGW during the 21st century. Predictions of future temperatures will always be rife with dissension and uncertainty, but recent science should provide considerable comfort to the worriers.

References

1. Gareth Morgan & John McCrystal 2008: Poles Apart: Beyond the Shouting Who's Right About Climate Change. ISBN 978 1 86979 045 5
2. www.economist.com-news-science-and-technology-21574461-climate-may-be-heating-up-lessresponse-greenhouse-gas-emissions
3. skepticalscience.com-trend.php
4. www.dailymail.co.uk-news-article-2301757-Governments-climate-watchdog-launches-astonishingattack-Mail-Sunday-revealing-global-warming-science-wrong.html
5. wattsupwiththat.com-2013-02-04-new-paper-by-richard-tol-targets-for-global-climate-policy-anoverview