Importance of Small Cloud and ERB Changes and the Difficulties in Measuring Them

Global climate is very sensitive to even slight changes in clouds and radiation, hence requiring very precise measurements of variations in these parameters. For example, an instantaneous doubling of CO2 (100% increase) would produce about a 4 W m-2 reduction in outgoing radiation flux, which is less than 2% of the 235 W m-2 in global mean outgoing radiation. Since the trend in CO2 concentration over the span of a few decades is much smaller than a doubling, the trend in ERB is also commensurately much smaller and thus even more difficult to detect. Such seemingly minor departures from radiative balance, when sustained over decades, could nevertheless produce changes in global temperature and climate sufficient to have substantial effects on human society and natural ecosystems. A second measurement obstacle is that radiation flux, unlike long-lived greenhouse gases, has little spatial coherence and must be measured from space at many locations to obtain a reasonable global average for ERB. This must be done by satellites, which suffer the additional disadvantage of being able to make observations of upwelling radiation intensity from only a single direction and spot on the Earth. Radiative modeling is required to convert this into a hemispheric flux.

Small cloud changes are important because they can exert more leverage over ERB than equivalent changes in greenhouse gases. For example, a 15-20% relative increase in low-level cloud amount is presumed to counteract the radiative forcing caused by a doubling of CO2 (Slingo 1990). Low-level marine stratocumulus and summertime midlatitude frontal clouds have an especially large negative net CRF and impact on ERB because, unlike the case for tropical deep convective clouds, their SWCRF is much greater than their LWCRF (Ramanathan et al. 1989). In contrast, optically thin high-level clouds have a positive net CRF because they strongly reduce LW emission while transmitting SW radiation. Since various cloud types have strikingly different radiative effects, it is not a simple matter to determine the overall global impact of cloud changes; each cloud type and climate regime must be examined in particular. Moreover, alterations of cloud albedo, cloud emissivity, and cloud height can affect ERB even when cloud amount remains the same. Since insolation strongly effects the magnitude of SWCRF, shifts in the latitude or seasonal cycle of clouds can change ERB even if mean cloud amount and cloud properties do not change. To detect long-term variations in their properties and radiative effects, clouds must be monitored with precision everywhere around the globe.

Trends in Observed Cloudiness and Earth s Radiation Budget 21 Cloud Simulations and Climate Sensitivity in Global Climate Models

Over the past two decades, generations of global climate models (GCMs) have been tuned to ERB, CRF, and cloud data to ensure that they represent at least the current effects of clouds as well as possible. Despite this empirical contribution, many studies comparing simulated clouds with observed clouds have found that GCMs poorly represent clouds when evaluated on terms for which they were not explicitly tuned. In fact, GCMs frequently reproduce the observed time-averaged ERB only by compensating for errors (e.g., cloud fraction that is too small and cloud optical thickness that is too large). Observed relationships between clouds and dynamical parameters are especially poorly simulated by GCMs (e.g., Norris and Weaver 2001; Tselioudis and Jakob 2002) even though these are more likely to be relevant to climate change questions than is the production of realistic cloud and radiation climatologies. One of the main reasons for such incorrect and inconsistent cloud and radiation simulations is that GCMs lack sufficient spatial resolution to represent properly the nonlinear small-scale convective, turbulent, and microphysical processes that control cloud properties, which instead must be crudely parameterized.

Climate sensitivity has been conventionally defined as the equilibrium change in global mean surface temperature in response to the radiative forcing caused by a doubling of CO2 in the atmosphere. Recent GCM intercomparisons definitively indicate that low-level marine clouds provide the greatest contribution to the spread in climate sensitivity between models (Bony and Dufresne 2005). This was confirmed in the findings of the recent IPCC Fourth Assessment Report (Randall et al. 2007). Some GCMs suggest that the horizontal extent of low-level clouds over low-latitude oceans will increase with higher global temperature, whereas other GCMs suggest that low-level cloud amount will decrease. Changes in trade cumulus (small cloud fraction and weak negative net CRF) may be more important than changes in marine stratocumulus (large cloud fraction and strong negative net CRF) because the former occur over a much larger area of the ocean. The amount of coverage by trade cumulus or stratocumulus is notoriously difficult to predict in large-scale models because the dynamical forcing is weak and small changes in this forcing and in the boundary layer properties can have a large impact on the mean relative humidity and hence on the cloud cover. Note that the importance of marine boundary layer clouds to the climate sensitivity of GCMs does not necessarily imply that such clouds are equally important to the climate sensitivity of the Earth, nor is it necessarily the case that changes in CRF will be greatest in regions where climatologic CRF is currently largest. Nevertheless, marine boundary layer clouds can potentially exert greater radiative leverage on the climate system than any other cloud type.

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