Water Vapor Feedback and Global Warming

Populations left behind during climate-driven range shifts can persist in enclaves of benign environmental conditions within an inhospitable regional climate. Such climate relicts exist in numerous plant and animal species worldwide, yet our knowledge of them is fragmented and lacks a general framework. Here we synthesize the empirical evidence considering (a) relict habitats, (b) abiotic and biotic constraints on population dynamics, (c) mechanisms promoting population persistence, and (d) uncertainties concerning their future prospects. We identify three major types of climate relicts: (a) those constrained primarily by climate-driven abiotic factors, (b) those restricted to areas that are inaccessible to antagonistic species for climatic reasons, and (c) those requiring a host or mutualistic species that is itself limited by climate. Understanding the formation and functioning of climate relicts is essential for their conservation and for our understanding of the response of species and populations to climate change.

In this review, we show how climate affects species, communities, and ecosystems, and why many responses from the species to the biome level originate from the interaction between the species’ ecological niche and changes in the environmental regime in both space and time. We describe a theory that allows us to understand and predict how marine species react to climate-induced changes in ecological conditions, how communities form and are reconfigured, and so how biodiversity is arranged and may respond to climate change. Our study shows that the responses of species to climate change are therefore intelligible—that is, they have a strong deterministic component and can be predicted.

Water vapor is the dominant greenhouse gas, the most important gaseous source of infrared opacity in the atmosphere. As the concentrations of other greenhouse gases, particularly carbon dioxide, increase because of human activity, it is centrally important to predict how the water vapor distribution will be affected. To the extent that water vapor concentrations increase in a warmer world, the climatic effects of the other greenhouse gases will be amplified. Models of the Earth’s climate indicate that this is an important positive feedback that increases the sensitivity of surface temperatures to carbon dioxide by nearly a factor of two when considered in isolation from other feedbacks, and possibly by as much as a factor of three or more when interactions with other feedbacks are considered. Critics of this consensus have attempted to provide reasons why modeling results are overestimating the strength of this feedback.

Our uncertainty concerning climate sensitivity is disturbing. The range most often quoted for the equilibrium global mean surface temperature response to a doubling of CO2 concentrations in the atmosphere is 1.5°C to 4.5°C. If the Earth lies near the upper bound of this sensitivity range, climate changes in the twenty-first century will be profound. The range in sensitivity is primarily due to differing assumptions about how the Earth’s cloud distribution is maintained; all the models on which these estimates are based possess strong water vapor feedback. If this feedback is, in fact, substantially weaker than predicted in current models, sensitivities in the upper half of this range would be much less likely, a conclusion that would clearly have important policy implications. In this review, we describe the background behind the prevailing view on water vapor feedback and some of the arguments raised by its critics, and attempt to explain why these arguments have not modified the consensus within the climate research community.

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