23-27 April 2018







Concept of the Week: Climate Sensitivity

Climate sensitivity is a relatively new and powerful concept in climate science. It is a measure of how responsive the temperature of Earth's climate system is to a change in radiative forcing due to increases in atmospheric carbon dioxide, an important greenhouse gas, combined with the contributions of feedbacks within the system. Specifically, the term is defined as how much the global mean surface temperature will increase if there is a doubling of atmospheric greenhouse gases (in terms of equivalent CO2), once the planet has had a chance to settle into a new equilibrium after the increase occurs. In other words, it's an assessment of how Earth's climate will respond to that doubling.

According to NASA climate scientist James Hansen, the concept of climate sensitivity has its origins in a request made by President Jimmy Carter in 1979 for the National Academy of Sciences (NAS) to report on the potential impact on climate of the increasing atmospheric concentration of carbon dioxide. Jule G. Charney (1917-1981) of the Massachusetts Institute of Technology (MIT) led the Academy investigation team. He designed a now classic experiment where computer models of Earth's climate system had the atmospheric concentration of CO2 doubled while all other variables (except temperature) were held constant.

The addition of CO2 makes the atmosphere more opaque for outgoing infrared radiation (heat), warming the lower atmosphere and cooling the upper atmosphere. Applying basic radiation laws, Charney found that doubling the atmospheric CO2 concentration would reduce the net radiative flux (from Earth to space) at the tropopause by a global average of about 4 watts per square meter (W/m2). How much warmer would Earth's surface become as a consequence of this enhanced greenhouse effect? According to the Stefan-Boltzmann law, the radiation emitted by an object is directly proportional to the fourth power of the object's absolute temperature. To reestablish radiative equilibrium following a doubling of atmospheric CO2, Earth must radiate to space an additional 4 W/m2, brought about by a global warming of 1.2 Celsius degrees (or 0.3 Celsius degrees per W/m2).

Charney's initial experiment accounted for the effect of a forcing agent (i.e., atmospheric CO2) on global climate but not the influence of feedbacks. As noted in the Concept of the Week for Week 2, forcing agents and mechanisms drive climate change, while feedbacks determine the magnitude of climate change. Hence, Charney's "no-feedback" experiment significantly underestimates the amount of global warming likely to accompany a doubling of atmospheric CO2. With inclusion of feedbacks, the 1979 Academy study indicated that global warming could range from 2 to 3.5 Celsius degrees. The most recent IPCC report (AR4) estimates the magnitude of warming with feedbacks incorporated as 3 Celsius degrees with a range of uncertainty of 2 to 4.5 Celsius degrees. This greater sensitivity depends primarily on all the different feedbacks, both positive and negative, that either amplify or diminish the greenhouse effect. The three primary feedbacks involve clouds, sea ice, and water vapor.

In summary, climate sensitivity is usually expressed in terms of the equilibrium change in global mean annual surface temperature caused by an increment in downward infrared radiative flux that would result from sustained doubling of atmospheric CO2 concentration compared to its pre-industrial level (taken to be 280 ppmv).

Historical Events:

Return to ECS RealTime Climate Portal

Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.