FIGURE 4.1 Simplified diagram of Earth’s radiation budget. Energy is exchanged between three main sources: space (gray), the atmosphere (blue), and the surface (brown), expressed in watts per square meter (W/m2) and derived from Kiehl and Trenberth (1997). Short-wavelength solar radiation (yellow arrow) enters the atmosphere and reaches land. A fraction is reflected back into space by the atmosphere or the surface (brown arrow). Another fraction of the energy is absorbed by the atmosphere and reemitted as long-wavelength radiation (white arrow) into space or back to the surface. Adding greenhouse gases increases the fraction of the energy absorbed by the atmosphere and reemitted back to the surface, which increases the surface temperature to balance the radiation budget between the atmosphere and surface. Under stable conditions, the total amount of energy entering the system from solar radiation will exactly balance the amount being radiated into space, thus allowing Earth to maintain a constant average temperature over time. Recent measurements indicate that the Earth is presently absorbing 0.85 ± 0.15 W/m2 more than it emits into space (Hansen et al. 2005). SOURCE: Data from Kiehl and Trenberth (1997) and Hansen et al. (2005). Drawing by R. A. Rohde, University of California, Berkeley. Robert A. Rohde/Global Warming Art.

of the greenhouse effect. With measurements of the outgoing longwave radiation and observations of the surface temperature and emissivity, the greenhouse effect of the atmosphere at any location can be computed. The average strength of Earth’s greenhouse effect is about 155 W/m−2, but it varies from about 270 W/m−2 in moist, cloudy regions of the tropics to about 100 W/m−2 at high latitudes. The role of water vapor in the greenhouse effect has also been measured in this way (Raval and Ramanathan 1989, Rind et al. 1999, Inamdar and Ramanathan 1998). Global satellite measurements of water vapor using infrared sounding and microwave imaging data allowed isolation of the water vapor contributions to the greenhouse effect and essential validation of the water vapor greenhouse effect in climate models.

Earth radiation budget measurements are being used to study climate feedback mechanisms and to observe interannual variations and trends in the albedo and thermal emissions of Earth (Wong et al. 2006). Earth radiation budget measurements are now sufficiently well calibrated that long-term changes in the Earth’s energy balance can be estimated from space-based measurements (Wielicki et al. 2002, 2005, Loeb et al. 2007). Long-term monitoring of Earth’s energy balance allows greater understanding of the climate system’s response to natural events such as El Niño and volcanic eruptions (see Box 4.3) and also may reveal aspects of the onset of human-induced global warming.

GLOBAL DISTRIBUTION OF CLOUD PROPERTIES

Knowledge of global distribution of cloud properties is required to understand the role of clouds in Earth’s climate. Prior to the satellite era, observations of clouds were based on estimates made by human observers on the surface, providing only limited data coverage, particularly over the oceans. Beginning in the 1980s, an international climate research project under the World Climate Research Programme used satellite measurements taken for purposes of weather observation to create a data set of global cloud observations, giving the first estimates of the global distribution of cloud amount, optical depth, and cloud top temperature based on instrumental data (Schiffer and Rossow 1985, Rossow and Schiffer 1999). These results originate from the International Satellite Cloud Climatology Project, which continues today using a constellation of six operational geosynchronous (GEO) and low earth orbit (LEO) satellites. It is the longest continuous project using international satellites for climate monitoring.

Combining radiation budget measurements with cloud amount and type measurements from space has shown how different types of clouds contribute to the radiation budget, indicating that deep convective tropical clouds have a relatively small effect on the radiation balance of Earth but that marine stratocumulus clouds have a strongly negative impact on the radiation balance (Figure 4.5; Hartmann et al. 1992, Chen et al. 2000). The response of clouds to climate change remains one of the outstanding uncertainties in making projections into the future.

Estimates of global cloud properties from existing meteorological instruments are limited by the precision and spectral coverage of the instruments on the meteorological satellite platforms. New instruments with better calibration and more information about clouds are providing new opportunities to understand clouds and their role in climate. Moderate Resolution Imaging Spectroradiometer (MODIS) data provide much better calibration and spectral resolution than current or former meteorological satellites (King et al. 2003). Multiangle Imaging Spectroradimeter (MISR) data provide multiangle, multiwavelength visible views of clouds that can provide important information on cloud geometry and reflective properties (Diner et al. 2005). Measurements of clouds with cloud radar and light detection and ranging (lidar)



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