by Luisa Cristini, PhD, University of Hawaii at Manoa.
[Note from the editor: This is the tenth in a series of blog entries that will focus on introductory topics in climate dynamics and modeling, and will be a great insight into the current understanding of the science.]
The concepts of radiative forcing (previously discussed in the 9th part of this series on climate dynamics), climate feedback and climate sensitivity are very useful when considering a general overview of the behavior of the climate system.
In response to a radiative forcing, the variables characterizing the state of the climate system will change, modifying the radiative fluxes reaching the Earth. These modifications involve very complex mechanisms. However, we can gain insights into the response of the climate system by assuming that the changes in the radiative fluxes can be estimated as a function of the changes in global mean surface temperature. These changes depend on the feedbacks of the climate system. A term “stolen” from electronics, a feedback is a process that tends to amplify (positive feedback) or reduce (negative feedback) the response of a system to a perturbation through mechanisms internal to the system itself.
If the perturbation lasts a sufficiently long time, the climate system will eventually reach a new equilibrium and the equilibrium climate sensitivity (ECS) can be measured as the change in the global mean temperature at equilibrium in response to a radiative forcing. The Intergovernmental Panel on Climate Change (IPCC) defines the ECS as the global mean surface temperature change after the climate system has reached a new equilibrium in response to a doubling of the carbon dioxide (CO2) concentration in the atmosphere. It is measured in degrees centigrade (°C) and, according to the most recent estimates (Randall et al., 2007), its value is in the range 2-4.5°C (35.6-40.1°F).
On the other hand, a long adjustment of the climate system to the radiative forcing leads to the definition of the transient climate response (TCR), which is defined by the IPCC as the global average of the annual mean temperature change averaged over 60 to 80 years in an experiment in which the CO2 concentration is increased by 1% per year until the 70th year (by which time it has doubled its initial value). The TCR values derived from models are generally between 1.4 and 2.5°C (Randall et al. 2007). The uncertainty in the TCR is thus smaller than the one on the ECS, because the TCR is more constrained by recent changes in temperature.
Feedbacks in the climate system can be direct physical feedbacks (e.g., those related to atmospheric water vapor, clouds and the cryosphere), but also geochemical, biogeochemical and biogeophysical feedbacks (e.g., changes in the carbonate compensation level, the interaction between plate tectonics, climate and the carbon cycle, the interaction between climate and the terrestrial biosphere). Some of these feedbacks are relatively simple to understand and explain, while others require a deeper knowledge of chemical, biological and geological processes that go far beyond the aim of this article. We encourage the interested reader to dig under the surface in the resources below.
Goosse H., P.Y. Barriat, W. Lefebvre, M.F. Loutre and V. Zunz, (2012). Introduction to climate dynamics and climate modeling. Online textbook available at http://www.climate.be/textbook.
Randall, D.A., R.A. Wood, S. Bony, R. Colman, T. Fichefet, J. Fyfe, V. Kattsov, A. Pitman, J. Shukla, A. Noda, J. Srinivasan, R.J. Stouffer, A. Sumi and K.E. Taylor (2007). Climate models and their evaluation. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.