Central to the current conceptual understanding of climate sensitivity is a box-model of the Earth’s energy balance, in which radiative forcing and a set of feedback mechanisms together determine climate sensitivity. The sources of this uncertainty is of general interest, and a number of approaches to the problem have been developed in the past. The amount by which the Earth system must change its temperature in order to obtain energy balance is known as the climate sensitivity with respect to a certain forcing.Ĭlimate sensitivity is an uncertain quantity, and in summarizing what we know from observations and climate models, the IPCC Fourth Assessment Report states a likely range of 2 to 4.5 K with respect to a doubling of atmospheric CO 2 from pre-industrial concentrations (Solomon et al. The resulting accumulation of heat in the Earth system will eventually increase the temperature, thereby increasing the emitted infrared radiation such that a new balance is approached. the radiated energy is no longer sufficient to offset the energy absorbed. An increase in the atmospheric carbon dioxide (CO 2), a greenhouse gas, reduces the emitted infrared radiation, yielding a positive energy imbalance at the top of the atmosphere (TOA), i.e. This system changes its climate in response to, e.g., altered atmospheric composition or shifts in the solar input, which offset the energy balance. The Earth’s climate system tends towards a state of balance between the absorbed fraction of the incoming solar radiation and the emitted terrestrial radiation. Our results highlight the importance of treating the coupling between clouds, water vapor and temperature in a deepening troposphere.
Negative synergies surround the surface albedo feedback, as associated cloud and water vapor changes dampen the anticipated climate change induced by retreating snow and ice. By subsequently combining feedback mechanisms we find a positive synergy acting between the water vapor feedback and the cloud feedback that is, the combined cloud and water vapor feedback is greater than the sum of its parts. Analysis suggests that cloud-induced warming in the upper tropical troposphere, consistent with rising convective cloud anvils in a warming climate enhances the negative lapse-rate feedback, thereby offsetting some of the warming that would otherwise be attributable to this positive cloud feedback. We find a close correspondence between forcing, feedback and partial surface temperature response for the water vapor and surface albedo feedbacks, while the cloud feedback is inefficient in inducing surface temperature change. In the studied model water vapor feedback stands for about half the temperature change, CO 2-forcing about one third, while cloud and surface albedo feedback contributions are relatively small. The method is shown to yield a near-perfect decomposition of change into partial temperature contributions pertaining to forcing and each of the feedbacks.
Here we test these assumptions by systematically controlling, or locking, the radiative feedbacks in a state-of-the-art climate model. The climate system is frequently interpreted in terms of a simple energy balance model, in which it is assumed that individual feedback mechanisms are additive and act independently. Earth’s climate sensitivity to radiative forcing induced by a doubling of the atmospheric CO 2 is determined by feedback mechanisms, including changes in atmospheric water vapor, clouds and surface albedo, that act to either amplify or dampen the response.
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