Describing the Response of Clouds to Changing Climate The Need for Multiple Indices of Climate Change

Although there is no question that clouds must have changed as a result of forced climate change and because of the impositions of anthropogenic aerosol on the atmosphere, major questions and uncertainties remain in terms of the details: How have clouds changed? How much have they changed? How will they change in the future? The simplest climate models of the 1960s, as well as the zero-dimensional model of Arrhenius (1896), projected increased

Table 1.3 Indices of climate change.

Index of change

Symbol, unit

Global mean surface temperature Ocean heat content

Change in regional-scale surface temperature

Change in global- or regional-scale atmospheric water content

Total greenhouse absorption

Global or regional mean radiative forcing

Global or regional mean precipitation

Atmospheric GHG concentration or concentration change

Ocean pH

Global or regional mean albedo Sea level change

Global or regional change in solar irradiance at the surface Change in cloud cover, type of cloud, height of cloud, etc.

A T Joules A T

BH2O, g m2 BH2O region

anthropogenic water vapor as a result of global warming. Yet the simplicity of this phenomenon (attributable to consideration of the accurately known and strong dependence of water vapor pressure on temperature change) belies the complexity of responses via a multitude of cloud processes and feedbacks. Indeed, temperature change is a misleadingly simple index of the known and suspected changes in clouds, cloud processes, and cloud functions.

Temperature change is the only "gold standard" index of forced climate change and natural variability. However, because this parameter does not capture the essence of changes in clouds and cloud functions (e.g., their role in planetary albedo or the amount, location, and timing of precipitation), we suggest that other indices can and should be used to describe more fully and quantify the consequences of change in the atmosphere caused by human activity (see Table 1.3). Of these, several indices pertain to the known or suspected changes of clouds in the perturbed climate system. In Table 1.3, we include regional-scale variables because regional changes are generally more important to society than global mean changes and because regional-scale changes in cloud-related parameters may, in some cases, be easier to detect and attribute than global-scale changes.

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