New Thinking About Climate Sensitivity

AGU image

While climate models struggle to include ever more parts of the climate system, ideas based on simpler models are providing insights into the long-term evolution of climate.


SAN FRANCISCO, CA., DEC. 9, 2013 — It’s now generally recognized that climate change is going to be with the inhabitants of Earth for many thousands of years. Even if humans stop emitting greenhouse gases in the next few decades — even if we stopped emitting today — our climate has already been altered, and it is going to keep changing significantly.

Much attention is paid to climate sensitivity — the ultimate amount of warming that will happen with a doubling of atmospheric levels of carbon dioxide — but a more relevant quantity is the “transient climate response” — how much warming will have occurred at the point when CO2‘s level is twice that of pre-industrial times, or 560 parts per million.

At our current emission rates, this is likely to happen by the end of the century. So what is the value of TCR, the transient climate response?

This and more were the subject of an AGU session I went to on Monday afternoon: “Climate Sensitivity and Feedbacks: Advances and New Paradigms.” It took the high road in climate science, away from all the messy, difficult details, to present what can be learned from TCR and ECS (Equilibrium Climate Sensitivity) from some simple models that mostly focus on the world ocean. The ocean’s heat capacity is about 1,000 times that of the atmosphere, and it is the best measure of the kind of energy imbalance created by an enhanced greenhouse effect.

Myles Allen, of Linacre College in the U.K., started the session. He’s often in the media, and respected by everyone, and was a co-author on the Otto et al 2013 paper in Nature Geoscience that got so much attention earlier this year. Its new analysis of “the Pause” influenced the IPCC’s 5th Assessment Report; Allen noted how the IPCC had lowered the lower bound of ECS since the 4th Assessment Report by 0.5°C, so it now stands at 1.5 to 4.5°C, and lowered the upper bound of TCR by an equal amount, so its estimate is now 1.0 to 2.5°C.

Allen has a new paper just out with Thomas Stocker, of the University of Bern, Switzerland, that reveals the consequences of our delays in stark terms: “peak carbon-dioxide-induced warming is increasing at the same rate as carbon dioxide emissions themselves — almost 2% per year — much faster than observed warming.”

His paper’s press release has an example: “So if we were aiming in 2010 to limit warming to two degrees, a delay of only 5 years has already cost us two tenths of a degree if we make the same effort starting in 2015 — that’s equal to the observed warming since the early 1990s.”

Ben Kravitz of Pacific Northwest National Laboratory used a relatively simple “aqua world” — all ocean, coupled to an atmosphere — to derive some interesting numbers. He found that doubling the present day CO2 concentration balances, after the climate stabilizes, a decrease in the energy the Earth gets from the Sun of about 2.2%.

Elisabeth Moyer of the University of Chicago looked at the long tail of climate change — 3,000 and 7,000 years after the climate stabilizes from our input of carbon dioxide. When the session ended I asked her for a summary in her own words.

From the stand point of physics, these long-term, big-picture analyses, based on energy balance considerations, are more convincing than projections for the next few decades from the big climate models. There is a lot of natural variation in the short-term — a decade or three — but they wash out over long periods of time and energy conservations rules supreme.

David Appell

David Appell is a science writer living in Oregon and a regular contributor to Yale Climate Connections. (E-mail: david@yaleclimatemediaforum.org, Twitter: @davidappell)
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