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Earth Observations from Space: The First 50 Years of Scientific Achievements
4
Earth’s Radiation Budget and the Role of Clouds and Aerosols in the Climate System
Earth orbit is the ideal location from which to measure the exchange of energy between Earth and space and its variability around the globe. The surface temperature of Earth is in energy balance when the solar radiation absorbed by Earth is balanced by the emission of thermal energy from Earth to space (Figure 4.1). If Earth gains energy from space, the surface temperature will warm up until the energy exchange with space is again balanced. This chapter illustrates how satellite measurements of solar energy output, Earth radiation fluxes, clouds, water vapor, and aerosols have improved our understanding of the climate system and its sensitivity to changes in atmospheric composition.
Solar energy emission is mostly in visible and near-infrared wavelengths, while Earth’s emission is in thermal infrared wavelengths. Basic starting points for understanding the radiation balance are measurements of the energy coming from the Sun, the reflection and absorption of solar energy by Earth, and the export of energy from Earth by the emission of thermal infrared radiation to space. The greenhouse effect of the atmosphere is important in the energy balance and is driven largely by water vapor, clouds, and carbon dioxide. Noncloud aerosols are also very important in the climate system.
EARTH’S RADIATION BUDGET
Measurement of Earth’s radiation budget was one of the earliest proposals for a scientific application of Earth-orbiting satellites on Explorer 7 (see Chapter 2 and House et al. 1986). Early measurements showed that Earth was a warmer and darker planet compared to presatellite estimates indicating that a greater poleward energy transport by the atmosphere and ocean was required (Vonder Haar and Suomi 1969, 1971). The quality of measurement has steadily increased since those early days. Earth-orbiting satellites now allow precise global measurement of Earth’s thermal emission, the solar radiation reflected from Earth, and the energy coming from the Sun (Box 4.1, Figure 4.2). Monitoring of variability and change has become an increasingly important goal because Earth’s climate is likely changing in response to human activities.
Accurate observations of the radiation balance (Figure 4.3) as a function of latitude allows direct measurement of the annual mean poleward transport of energy. Earth gains energy from space in the tropics and returns this energy to space at high latitudes. The poleward heat transport in the atmosphere and ocean warms the poles and cools the tropics and also plays a key role in determining the response of global climate to greenhouse gases. If atmospheric data are used to compute the atmospheric heat transport, oceanic heat transports can be inferred by subtracting atmospheric transport from the measured total transport. A measurement of the total required poleward energy flux from space provides independent data that can be used to test estimates of atmospheric and ocean heat fluxes based on in situ measurements. Estimates of oceanic heat fluxes from direct measurements of ocean current and temperature are difficult. Measurements from space provided the first estimates of poleward heat flux in the ocean, which is nearly as large as the atmospheric flux but reaches a maximum at a relatively low latitude of about 20 degrees, while the atmospheric flux peaks at about 50 degrees latitude (Vonder Haar and Oort 1973, Trenberth et al. 2001).
Measurements from the Earth Radiation Budget Experiment (ERBE; Barkstrom et al. 1984) had sufficient accuracy and spatiotemporal resolution to allow the inference of the clear-sky radiation balance and thus measure the effect of clouds on Earth’s radiation balance (Figures 4.4 and 4.5). This showed that clouds double Earth’s albedo from 0.15 to 0.3 and reduce the emitted thermal radiation by 30 W/m2 (Ramanathan et al. 1989, Harrison et al. 1990). These basic measurements provide a standard against which to test climate models.
The amount by which the atmosphere reduces the loss of thermal energy to space is one way to measure the strength
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Earth Observations from Space: The First 50 Years of Scientific Achievements
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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BOX 4.1
Total Solar Irradiance and Its Variability
The total solar irradiance, the total radiant energy coming from the Sun at the mean position of Earth, has been measured precisely from Earth-orbiting satellites for nearly 30 years, allowing the observation of nearly three solar cycles (Figure 4.2). To measure total solar irradiance precisely, it is important to remove the effect of the atmosphere’s absorption, which can be achieved by taking the measurements from Earth orbit. Also, satellite orbits can be chosen to be in constant sunlight, allowing continuous monitoring of solar irradiance changes. These measurements show that the variation in total solar irradiance associated with the Sun’s 11-year cycle is about 0.1 percent. Variations of 0.2 percent are associated with the Sun’s 27-day rotation at times of high solar activity (Hickey et al. 1980, Willson et al. 1981, Willson and Hudson 1988, Frohlich and Lean 2004). These changes are small compared to the effect of greenhouse gases on the energy balance of Earth. It is important to monitor the energy exchange between Earth and space so that observed changes in Earth’s climate can be attributed to and partitioned correctly among various causal mechanisms, including solar variability, atmospheric particles induced by volcanic eruptions, human-induced greenhouse gases, and aerosols.
FIGURE 4.2 Time history of total solar irradiance (TSI) observed from seven different orbiting TSI monitors, along with monthly sunspot number. The average change in TSI during the solar cycle is about 1.5 W/m2 or about 0.1 percent. SOURCE: Figure courtesy of Dr. Greg Kopp, University of Colorado, http://spot.colorado.edu/~koppg/TSI/.
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FIGURE 4.3 The annual mean net radiation balance from the Earth Radiation Budget Experiment (ERBE), 1985-1986. Positive values indicate net energy entering the Earth. In order to balance the energy budget, the atmosphere and ocean must transport heat from regions where the net input is positive to regions where it is negative. SOURCE: Graphic by D. Hartmann and M. Michelsen, University of Washington.
FIGURE 4.4 Longwave cloud forcing, the amount by which clouds reduce the escaping thermal emission from Earth, 1985-1986. Positive values indicate that clouds are reducing the thermal energy emission to space, a positive effect on the energy budget. Note the large positive forcing due to the deep convective clouds trapping longwave emission in the tropical West Pacific and Indian Ocean region and over the equatorial continents. SOURCE: Graphic by D. Hartmann and M. Michelsen, University of Washington.
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FIGURE 4.5 The top panel shows net cloud radiative forcing, annually averaged as observed by the ERBE. Negative values (red colors) indicate that clouds reduced the energy balance of Earth by reflecting more solar radiation than the amount by which they reduced the escaping infrared radiation. The bottom panel shows the fractional area coverage by low clouds as measured by the International Satellite Cloud Climatology Project (ISCCP). Note the close correspondence between low stratocumulus clouds over the ocean and strongly negative cloud radiative forcing. SOURCE: Graphic by D. Hartmann and M. Michelsen, University of Washington.
give unprecedented detail on vertical cloud structure. Cloud radar in space can provide good vertical resolution of reasonably thick clouds, including the tops and bottoms of layered clouds (Stephens et al. 2002). Lidar in space can provide very sensitive measurements of thin layers of clouds or aerosols (Winker 1997). These data have provided an unprecedented view of cloud structure, particularly in showing how clouds are layered vertically, which was not possible with visible, infrared, or microwave passive instruments.
The CloudSat radar and the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar provide a new dimension in observing the atmosphere. Rather than providing horizontal distributions of cloud and aerosol features typical of more conventional satellite sensors, these new nadir-pointing active sensors measure
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FIGURE 4.6 Portion of an orbit showing the cloud mask that combines CALIPSO lidar and CloudSat radar (upper panel) and the CloudSat radar reflectivity (lower panel). This example of monsoonal convection illustrates how precipitation (easily identified as regions of high reflectivity above the surface) falls from mixtures of deep and shallow convection. Shallow precipitating convection is often concealed from above by thick overlying cirrus clouds as apparent in the middle portion of this cross section. SOURCE: Image courtesy of G. Stephens.
the vertical structure of clouds and aerosols. The vertical structure revealed by CloudSat, for instance, offers deeper insights into the key processes that shape clouds and precipitation. For example, the image shown in Figure 4.6 is a cross section of the vertical distribution of radar reflectivity measured along a portion of one orbit. Also shown is the matching cloud mask information obtained from the combination of lidar and radar data. This example shows observations of clouds and precipitation associated with an active monsoon over southern China.
Observations such as these provide a way of observing the cloud structures with embedded precipitation and begin to provide hints about the way precipitation is organized. When accumulated over the entire tropics, these observations are now beginning to reveal that not all precipitation falls from deep convective clouds, as has generally been assumed, but that significant accumulations of water come from precipitation that falls from shallower clouds, as highlighted in this one example. This result has further implications for the nature of the vertical distribution of latent heating by precipitating cloud systems in the atmosphere, with ramifications on the way such clouds add (latent) heat to the atmosphere. The latter is essential for understanding the dynamic envelope of monsoons as well as the topic of the prediction of medium-and longer-term variability of the tropical atmosphere.
AEROSOLS FROM NATURAL PROCESSES AND HUMAN ACTIVITIES
An aerosol is a suspension of tiny liquid or solid particles in the atmosphere. Aerosol particles are distinguished from clouds by requiring that aerosol particles be stable in unsaturated air. Examples include dust, sulfuric acid particles, sea salt, organic particles, and smoke. Aerosols play important roles in the energy budget of Earth, in the formation of clouds, and in the chemistry of the atmosphere. Aerosol particles are produced naturally through biological emissions or elevation of particles by wind, but human activities provide a substantial enhancement to the natural aerosol loading of the atmosphere through agricultural and industrial activities. Aerosol particles can be produced either directly or by the chemical conversion of precursor chemicals that exist in solid or liquid form. Aerosols influence climate in several ways. Because aerosol particles reflect and absorb radiation, they can directly influence the energy balance of Earth. For many aerosols their primary effect is to reflect solar radiation and thereby cool the climate. Aerosols may also warm the atmosphere directly by absorption of radiation, however, and this is particularly important for highly absorbing aerosols such as soot (Figure 4.7).
Space measurements have succeeded in depicting aerosols associated with human activity over the oceans by isolating fine-mode from coarse-mode aerosols such as dust and sea salt that arise from natural processes. Plumes of fine aerosols are shown to result from biomass burning and from industrial activities (Tanré et al. 2001). The ability to distinguish fine from coarse aerosols has led to efforts to characterize the anthropogenic contribution to the aerosol direct forcing of climate (Bellouin et al. 2005, Kaufman et al. 2005).
INDIRECT EFFECTS OF AEROSOLS
Another way that aerosols can influence climate is through their role as the small particles on which clouds form (cloud condensation nuclei). An important contribution
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FIGURE 4.7 Against the arcing backdrop of the Himalayan Mountains (top of image), rivers of grayish haze follow the courses of the Ganges River and its tributaries (left) and the Brahmaputra River (right) on February 1, 2006. The plumes appear to combine like their watery counterparts and flow out together over the Bay of Bengal past the mouths of the Ganges, the multipronged delta of the river along the Bangladesh coast. This image was captured by MODIS on NASA’s Terra satellite. Scientists studying the cloud of haze that frequently lingers over parts of Asia from Pakistan to China and even the Indian and Pacific oceans have called the pollution the “Brown Cloud.” The mix of aerosols (tiny particles suspended in air) includes smoke from agricultural and home heating and cooking fires, vehicle exhaust, and industrial emissions. In addition to causing respiratory problems, the persistent haze appears to hinder crops by blocking sunlight and could be altering regional weather. SOURCE: NASA image created by Jesse Allen, Earth Observatory, using data obtained courtesy of the MODIS Rapid Response team, http://visibleearth.nasa.gov/view_rec.php?id=20461.
of satellite measurements to our understanding of the role of aerosols in climate was the discovery of the ship track phenomenon (Conover 1966, Coakley et al. 1987; Box 4.2, Figure 4.8). This heightened the awareness of the indirect effect human-produced aerosols have on the albedo of Earth.
STRATOSPHERIC PARTICLES
Stratospheric aerosols resulting from explosive volcanic eruptions and subsequent conversion of sulfur dioxide gas to aerosols have been observed by satellite for the El Chichon and Pinatubo eruptions (McCormick 1992, Lambert et al. 1993, McCormick et al. 1993; Box 4.3, Figure 4.9). The initial development, transport, mixing, and gradual decline of the aerosols associated with these eruptions provide a sound basis for understanding the effect of such volcanic eruptions on surface climate and better estimates of the likely effects of large volcanic eruptions that have occurred in the more distant past. Reflected solar measurements allow the evolv-
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BOX 4.2
Discovery of the Ship Track Phenomenon
Ship tracks can be observed in the atmosphere because very small airborne particles emitted in the exhaust of large ships attract water molecules, acting as cloud condensation nuclei, and leave bright streaks in the air after the ships have passed. Ship tracks are a visible example of how human-produced aerosols can indirectly change the energy balance of Earth by changing the properties of clouds by acting as cloud condensation nuclei (Figure 4.8). This indirect effect of clouds is currently one of the major uncertainties in computing the effect of human activities on Earth’s climate.
FIGURE 4.8 The top panel shows a true color image from the MODIS instrument taken over the Atlantic Ocean on January 27, 2003. Bright linear features are apparent in the low clouds in much of the scene. MODIS can independently measure the optical depth (lower left panel), which is enhanced in the bright regions, and the effective particle radius (lower right panel). The smaller particle radius in the ship tracks is what would be expected from the introduction of many more cloud condensation nuclei from the ship exhaust. Smaller particles are more effective in reflecting solar radiation. This strongly suggests that the cloud enhancements are caused by human-produced aerosols. SOURCE: Images courtesy of Jacques Descloitres, MODIS Land Rapid Response Team, and Mark Gray, MODIS Atmosphere Science Team, both at NASA Goddard Space Flight Center, http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=11271.
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BOX 4.3
Response of Earth’s Radiation Budget to a Volcanic Eruption
The response of the radiation balance to the eruption of Mount Pinatubo was directly measured with broadband radiation instruments on Earth-orbiting satellites. This allowed not only a direct confirmation of the effect of stratospheric volcanic aerosols in reducing the energy balance but also verification of the model-predicted surface cooling in response to the eruption, giving additional confidence in our ability to model climate variability and change (Hansen et al. 1992, Minnis et al. 1993, Soden et al. 2002; Figure 4.9). The stratospheric aerosols resulting from the eruption were independently measured from Advanced Very High Resolution Radiometer (AVHRR) data and Stratospheric Aerosol and Gas Experiment SAGE data (McCormick et al. 1995). This is a prime example of how the length and continuity of a given data record yields additional scientific benefits beyond the initial research results of the mission and beyond the monitoring implications for operaitonal agencies.
FIGURE 4.9 Comparison of the observed anomalies in absorbed shortwave (top panel) and emitted longwave (bottom panel) radiative fluxes at the top of the atmosphere from ERBE satellite observations (black) and three ensembles of Global Climate Model (GCM) simulations (red). The observed anomalies are expressed relative to a 1984-1990 base climatology, and the linear trend is removed. The GCM anomalies are computed as the difference between the control and Mount Pinatubo simulations for each ensemble member (the Mount Pinatubo eruption of May 1991 is marked on the bottom panel). The results are expressed relative to the preeruption (January to May 1991) value of the anomaly and smoothed with a 7-month running mean (thick line). Both the model and the observed global averages are from 60° N to 60° S due to the restriction of observed data to these latitudes. SOURCE: Soden et al. (2002). Reprinted with permission from AAAS, copyright 2002.
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ing radiative impact of the aerosol cloud to be measured (Stowe et al. 1992).
Mineral aerosols are important for the trace metal balances of the ocean, which in turn are important for ocean biology (see Chapters 8 and 9). Satellite images show the dramatic export of mineral aerosols from the Sahara Desert to the Atlantic Ocean during dust storms (Figure 4.10). Mineral dust can also have a significant impact on climate.
GLOBAL CLIMATOLOGIES OF AEROSOLS
Aerosol concentrations vary strongly over time and space, and quantifying the various effects of aerosols requires continuous global measurement, which can best be achieved from Earth-orbiting satellites (Figure 4.11). A major contribution of Earth observations from space is the development of global climatologies of aerosols. These have been obtained from visible measurements from weather satellites (Stowe et al. 1997, Nakajima and Higurashi 1998, Mishchenko et al. 1999), from ultraviolet measurements from the Total Ozone Mapping Spectrometer (Herman et al. 1997, Torres et al. 2002), and from the instruments on the Earth Observing System suite of instruments, especially MODIS (Chu et al. 2002, Remer et al. 2002, 2005) and MISR (Kahn et al. 2005; Figure 4.11). Aerosol properties can also be inferred from polarization (Tanré et al. 2001) and from lidar measurements from space (Winker et al. 1996). Global aerosol measurements from space are greatly improved by their validation with surface sun photometer measurements (Holben et al. 1998, Dubovik et al. 2000).
FIGURE 4.10 An intense African dust storm sent a massive dust plume northwestward over the Atlantic Ocean on March 2, 2003. In this true-color scene, acquired by MODIS aboard NASA’s Terra satellite, the thick dust plume (light brown) can be seen blowing westward and then routed northward by strong southerly winds. The plume extends more than 1,000 miles (1,600 km), covering a vast swath of ocean extending from the Cape Verde Islands (lower left), off the coast of Senegal, to the Canary Islands (top center), off the coast of Morocco. SOURCE: Image courtesy of Jacques Descloitres, MODIS Rapid Response Team, NASA GSFC, http://visibleearth.nasa.gov/view_rec.php?id=5619.
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FIGURE 4.11 These 19 global panels show the seasonal average distribution of atmospheric aerosol amounts across Africa and the Atlantic Ocean. The measurements capture airborne particles in the entire atmospheric column, for subvisible sizes ranging from tiny smoke particles to “medium” dust (about 0.5 to 2.5 microns). Such particles are produced by forest fires, deserts, volcanoes, breaking ocean waves, and urban and industrial pollution sources. SOURCE: NASA, GSFC, Langley Research Center (LaRC), Jet Propulsion Laboratory (JPL), Multiangle Imaging Spectroradimeter (MISR) Team.
Space observations have the potential to allow the estimation of the global average optical depth of aerosols, which is presently unknown. Rapid global coverage also allows sources of aerosols to be inferred from plumes of aerosols that can be observed over the oceans (Herman et al. 1997, Husar et al. 1997). Multiyear records of aerosol optical depth over water show reproducible seasonal patterns (Torres et al. 2002). Measurements from space show a surprisingly large contribution from Saharan dust and biomass burning and distinct differences between the northern and southern hemispheres, presumably due to human production of aerosols (Husar et al. 1997, Prospero et al. 2002).
The ability to distinguish aerosols from clouds and fine aerosols from coarse aerosols combined with the ability to construct a long-term record of aerosols is a remarkable accomplishment and demonstrates how sophisticated satellite technology and analysis tools have become. This newly gained observational capability greatly enhances our understanding of climate forcing by aerosols from natural and anthropogenic sources and leads to improvements in climate modeling.
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