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4 Earthâs Radiation Budget and the Role of Clouds and Aerosols in the Climate System Earth orbit is the ideal location from which to measure energy coming from the Sun (Box 4.1, Figure 4.2). Moni- the exchange of energy between Earth and space and its toring of variability and change has become an increasingly v Â ariability around the globe. The surface temperature of important goal because Earthâs climate is likely changing in Earth is in energy balance when the solar radiation absorbed response to human activities. by Earth is balanced by the emission of thermal energy Accurate observations of the radiation balance (Fig- from Earth to space (Figure 4.1). If Earth gains energy ureÂ 4.3) as a function of latitude allows direct measurement from space, the surface temperature will warm up until the of the annual mean poleward transport of energy. Earth gains energy exchange with space is again balanced. This chapter energy from space in the tropics and returns this energy to illustrates how satellite measurements of solar energy output, space at high latitudes. The poleward heat transport in the Earth radiation fluxes, clouds, water vapor, and aerosols have atmosphere and ocean warms the poles and cools the trop- improved our understanding of the climate system and its ics and also plays a key role in determining the response of sensitivity to changes in atmospheric composition. global climate to greenhouse gases. If atmospheric data are Solar energy emission is mostly in visible and near- used to compute the atmospheric heat transport, oceanic heat infrared wavelengths, while Earthâs emission is in thermal transports can be inferred by subtracting atmospheric transport infrared wavelengths. Basic starting points for understanding from the measured total transport. A measurement of the total the radiation balance are measurements of the energy coming required poleward energy flux from space provides indepen- from the Sun, the reflection and absorption of solar energy dent data that can be used to test estimates of atmospheric and by Earth, and the export of energy from Earth by the emis- ocean heat fluxes based on in situ measurements. Estimates of sion of thermal infrared radiation to space. The greenhouse oceanic heat fluxes from direct measurements of ocean cur- effect of the atmosphere is important in the energy balance rent and temperature are difficult. Measurements from space and is driven largely by water vapor, clouds, and carbon provided the first estimates of poleward heat flux in the ocean, dioxide. Noncloud aerosols are also very important in the which is nearly as large as the atmospheric flux but reaches climate system. 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). EARTHâS RADIATION BUDGET Measurements from the Earth Radiation Budget Experi- Measurement of Earthâs radiation budget was one of ment (ERBE; Barkstrom et al. 1984) had sufficient accuracy the earliest proposals for a scientific application of Earth- and spatiotemporal resolution to allow the inference of the orbiting satellites on Explorer 7 (see Chapter 2 and House clear-sky radiation balance and thus measure the effect of et al. 1986). Early measurements showed that Earth was clouds on Earthâs radiation balance (Figures 4.4 and 4.5). a warmer and darker planet compared to presatellite esti- This showed that clouds double Earthâs albedo from 0.15 mates indicating that a greater poleward energy transport by to 0.3 and reduce the emitted thermal radiation by 30 W/m2 the atmosphere and ocean was required (Vonder Haar and (Ramanathan et al. 1989, Harrison et al. 1990). These basic Suomi 1969, 1971). The quality of measurement has steadily measurements provide a standard against which to test cli- increased since those early days. Earth-orbiting satellites mate models. now allow precise global measurement of Earthâs thermal The amount by which the atmosphere reduces the loss of emission, the solar radiation reflected from Earth, and the thermal energy to space is one way to measure the strength 26
EARTHâS RADIATION BUDGET AND THE ROLE OF CLOUDS AND AEROSOLS IN THE CLIMATE SYSTEM 27 Thermal radiation study climate feedback mechanisms and to observe interan- into space: 195 Directly radiated nual variations and trends in the albedo and thermal emis- Solar radiation from surface: 40 sions of Earth (Wong et al. 2006). Earth radiation budget absorbed by Earth: 235 W/m2 measurements are now sufficiently well calibrated that long- term changes in the Earthâs energy balance can be estimated Greenhouse gas absorption: 350 from space-based measurements (Wielicki et al. 2002, 2005, 67 Heat and energy Loeb et al. 2007). Long-term monitoring of Earthâs energy 452 in the atmosphere balance allows greater understanding of the climate systemâs response to natural events such as El NiÃ±o and volcanic erup- tions (see Box 4.3) and also may reveal aspects of the onset The of human-induced global warming. 324 Greenhouse 168 Effect 492 GLOBAL DISTRIBUTION OF CLOUD PROPERTIES Earthâs land and ocean surface warmed to an average of 14Â°C Knowledge of global distribution of cloud properties is required to understand the role of clouds in Earthâs climate. Prior to the satellite era, observations of clouds were based on FIGURE 4.1â Simplified diagram of Earthâs radiation budget. Energy is exchanged between three main sources: space (gray), estimates made by human observers on the surface, provid- the atmosphere (blue), and the surface (brown), expressed in watts ing only limited data coverage, particularly over the oceans. per square meter (W/m2) and derived from Kiehl and Trenberth Beginning in the 1980s, an international climate research (1997). Short-wavelength solar radiation (yellow arrow) enters the project under the World Climate Research Programme used 4-1 atmosphere and reaches land. A fraction is reflected back into space satellite measurements taken for purposes of weather obser- by the atmosphere or the surfacewidtharrow). Another fraction column (brown vation to create a data set of global cloud observations, giving of the energy is absorbed by the atmosphere and reemitted as long- the first estimates of the global distribution of cloud amount, wavelength radiation (white arrow) into space or back to the sur- optical depth, and cloud top temperature based on instrumen- face. Adding greenhouse gases increases the fraction of the Âenergy tal data (Schiffer and Rossow 1985, Rossow and Schiffer a Â bsorbed by the atmosphere and reemitted back to the surface, 1999). These results originate from the International Satellite which increases the surface temperature to balance the radiation Cloud Climatology Project, which continues today using a budget between the atmosphere and surface. Under stable condi- tions, the total amount of energy entering the system from solar constellation of six operational geosynchronous (GEO) and radiation will exactly balance the amount being radiated into space, low earth orbit (LEO) satellites. It is the longest continuous thus Â allowing Earth to maintain a constant average temperature project using international satellites for climate monitoring. over time. Recent measurements indicate that the Earth is presently Combining radiation budget measurements with cloud absorbing 0.85Â Â±Â 0.15 W/m2 more than it emits into space (Hansen amount and type measurements from space has shown how et al. 2005). SOURCE: Data from Kiehl and Trenberth (1997) and different types of clouds contribute to the radiation budget, Hansen et al. (2005). Drawing by R. A. Rohde, University of Cali- indicating that deep convective tropical clouds have a rela- fornia, ÂBerkeley. Robert A. Rohde/Global Warming Art. tively 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 pro- of the greenhouse effect. With measurements of the outgoing jections into the future. longwave radiation and observations of the surface tempera- Estimates of global cloud properties from existing ture and emissivity, the greenhouse effect of the atmosphere meteorological instruments are limited by the precision and at any location can be computed. The average strength of spectral coverage of the instruments on the meteorological Earthâs greenhouse effect is about 155 W/mâ2, but it varies satellite platforms. New instruments with better calibra- from about 270 W/mâ2 in moist, cloudy regions of the tropics tion and more information about clouds are providing new to about 100 W/mâ2 at high latitudes. The role of water vapor opportunities to understand clouds and their role in climate. in the greenhouse effect has also been measured in this way Moderate Resolution Imaging Spectroradiometer (MODIS) (Raval and Ramanathan 1989, Rind et al. 1999, Inamdar and data provide much better calibration and spectral resolution Ramanathan 1998). Global satellite measurements of water than current or former meteorological satellites (King et al. vapor using infrared sounding and microwave imaging data 2003). Multiangle Imaging Spectroradimeter (MISR) data allowed isolation of the water vapor contributions to the provide multiangle, multiwavelength visible views of clouds greenhouse effect and essential validation of the water vapor that can provide important information on cloud geometry greenhouse effect in climate models. and reflective properties (Diner et al. 2005). Measurements of Earth radiation budget measurements are being used to clouds with cloud radar and light detection and ranging (lidar)
28 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS 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 atmosphereâs 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 Sunâs 11-year cycle is about 0.1 percent. Variations of 0.2 percent are associated with the Sunâs 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 Earthâs 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 4.2â 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. 4-2 SOURCE: Figure courtesy of Dr. Greg Kopp, University of Colorado, http://spot.colorado.edu/~koppg/TSI/.
EARTHâS RADIATION BUDGET AND THE ROLE OF CLOUDS AND AEROSOLS IN THE CLIMATE SYSTEM 29 Net Radiation 1985-1986 NO DATA -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 W/m2 FIGURE 4.3â The annual mean net radiation balance from the Earth Radiation Budget Experiment (ERBE), 1985-1986. Positive values indi- cate 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. 4-3 Longwave Cloud Forcing 1985-1986 NO DATA 0 10 20 30 40 50 60 70 80 90 100 W/m2 FIGURE 4.4â 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 4-4 equatorial continents. SOURCE: Graphic by D. Hartmann and M. Michelsen, University of Washington.
30 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS Annual ERBE Net Radiative Cloud Forcing NO DATA -90 -70 -50 -40 -30 -20 -10 0 10 20 30 40 W/m 2 Annual ISCCP C2 Inferred Stratus Cloud Amount NO DATA 0 10 20 30 40 50 60 70 80 Percent FIGURE 4.5â The top panel shows net cloud radiative forcing, annually averaged as observed by the ERBE. Negative values (red colors) 4-5 indicate that clouds reduced the energy balance of Earth by reflecting more solar radiation than the amount by which they reduced the escap- ing 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 are layered vertically, which was not possible with visible, radar in space can provide good vertical resolution of reason- infrared, or microwave passive instruments. ably thick clouds, including the tops and bottoms of layered The CloudSat radar and the Cloud Aerosol Lidar and clouds (Stephens et al. 2002). Lidar in space can provide very Infrared Pathfinder Satellite Observations (CALIPSO) lidar sensitive measurements of thin layers of clouds or aerosols provide a new dimension in observing the atmosphere. (Winker 1997). These data have provided an unprecedented Rather than providing horizontal distributions of cloud view of cloud structure, particularly in showing how clouds and aerosol features typical of more conventional satellite sensors, these new nadir-pointing active sensors measure
EARTHâS RADIATION BUDGET AND THE ROLE OF CLOUDS AND AEROSOLS IN THE CLIMATE SYSTEM 31 FIGURE 4.6â 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: ÂImage courtesy of G. Stephens. the vertical structure of clouds and aerosols. The vertical 4-6 particles, sea salt, organic particles, and smoke. Aerosols structure revealed by CloudSat, for instance, offers deeper play important roles in the energy budget of Earth, in the insights into the key processes that shape clouds and precipi- formation of clouds, and in the chemistry of the atmosphere. tation. For example, the image shown in Figure 4.6 is a cross Aerosol particles are produced naturally through biological section of the vertical distribution of radar reflectivity mea- emissions or elevation of particles by wind, but human activi- sured along a portion of one orbit. Also shown is the match- ties provide a substantial enhancement to the natural aerosol ing cloud mask information obtained from the combination loading of the atmosphere through agricultural and industrial of lidar and radar data. This example shows observations of activities. Aerosol particles can be produced either directly or clouds and precipitation associated with an active monsoon by the chemical conversion of precursor chemicals that exist over southern China. in solid or liquid form. Aerosols influence climate in several Observations such as these provide a way of observing ways. Because aerosol particles reflect and absorb radiation, the cloud structures with embedded precipitation and begin they can directly influence the energy balance of Earth. For to provide hints about the way precipitation is organized. many aerosols their primary effect is to reflect solar radiation When accumulated over the entire tropics, these observations and thereby cool the climate. Aerosols may also warm the are now beginning to reveal that not all precipitation falls atmosphere directly by absorption of radiation, however, and from deep convective clouds, as has generally been assumed, this is particularly important for highly absorbing aerosols but that significant accumulations of water come from pre- such as soot (Figure 4.7). cipitation that falls from shallower clouds, as highlighted in Space measurements have succeeded in depicting this one example. This result has further implications for the aerosols associated with human activity over the oceans nature of the vertical distribution of latent heating by precipi- by isolating fine-mode from coarse-mode aerosols such as tating cloud systems in the atmosphere, with ramifications on dust and sea salt that arise from natural processes. Plumes the way such clouds add (latent) heat to the atmosphere. The of fine aerosols are shown to result from biomass burning latter is essential for understanding the dynamic envelope of and from industrial activities (TanrÃ© et al. 2001). The ability monsoons as well as the topic of the prediction of medium- to distinguish fine from coarse aerosols has led to efforts to and longer-term variability of the tropical atmosphere. characterize the anthropogenic contribution to the aerosol direct forcing of climate (Bellouin et al. 2005, Kaufman et al. 2005). AEROSOLS FROM NATURAL PROCESSES AND HUMAN ACTIVITIES Indirect Effects of Aerosols An aerosol is a suspension of tiny liquid or solid par- ticles in the atmosphere. Aerosol particles are distinguished Another way that aerosols can influence climate is from clouds by requiring that aerosol particles be stable through their role as the small particles on which clouds in unsaturated air. Examples include dust, sulfuric acid form (cloud condensation nuclei). An important contribution
32 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS FIGURE 4.7â 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 NASAâs 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 âBrown Cloud.â 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 have been observed by satellite for the El Chichon aerosols in climate was the discovery of the ship track phe- and Pinatubo eruptions (McCormick 1992, Lambert et al. nomenon (Conover 1966, Coakley et al. 1987; Box 4.2, Fig- 1993, McCormick et al. 1993; Box 4.3, Figure 4.9). The ure 4.8). This heightened the awareness of the indirect effect initial development, transport, mixing, and gradual decline of human-produced aerosols have on the albedo of Earth. 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 Stratospheric Particles of large volcanic eruptions that have occurred in the more Stratospheric aerosols resulting from explosive volcanic distant past. Reflected solar measurements allow the evolv- eruptions and subsequent conversion of sulfur dioxide gas to
EARTHâS RADIATION BUDGET AND THE ROLE OF CLOUDS AND AEROSOLS IN THE CLIMATE SYSTEM 33 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 Earthâs climate. FIGURE 4.8â 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 4-8 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.
34 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS BOX 4.3 Response of Earthâs 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 indepen- dently 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 4.9â 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 60Â° N to 60Â° S due to the restriction of observed data to these latitudes. SOURCE: Soden et al. (2002). Reprinted with permission from AAAS, copyright 2002.
EARTHâS RADIATION BUDGET AND THE ROLE OF CLOUDS AND AEROSOLS IN THE CLIMATE SYSTEM 35 ing radiative impact of the aerosol cloud to be measured bution of Earth observations from space is the development (Stowe et al. 1992). of global climatologies of aerosols. These have been obtained Mineral aerosols are important for the trace metal bal- from visible measurements from weather satellites (Stowe et ances of the ocean, which in turn are important for ocean al. 1997, Nakajima and Higurashi 1998, Mishchenko et al. biology (see Chapters 8 and 9). Satellite images show the 1999), from ultraviolet measurements from the Total Ozone dramatic export of mineral aerosols from the Sahara Desert Mapping Spectrometer (Herman et al. 1997, Torres et al. to the Atlantic Ocean during dust storms (Figure 4.10). Min- 2002), and from the instruments on the Earth Observing eral dust can also have a significant impact on climate. 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 Global Climatologies of Aerosols polarization (TanrÃ© et al. 2001) and from lidar measurements Aerosol concentrations vary strongly over time and from space (Winker et al. 1996). Global aerosol measure- space, and quantifying the various effects of aerosols requires ments from space are greatly improved by their validation continuous global measurement, which can best be achieved with surface sun photometer measurements (Holben et al. from Earth-orbiting satellites (Figure 4.11). A major contri- 1998, Dubovik et al. 2000). FIGURE 4.10â 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 NASAâs 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.
36 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS FIGURE 4.11â 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 âmediumâ 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 esti- hemispheres, presumably due to human production of aero- mation of the global average optical depth of aerosols, which sols (Husar et al. 1997, Prospero et al. 2002). is presently unknown. Rapid global coverage also allows The ability to distinguish aerosols from clouds and fine sources of aerosols to be inferred from plumes of aerosols aerosols from coarse aerosols combined with the ability that can be observed over the oceans (Herman et al. 1997, to construct a long-term record of aerosols is a remark- Husar et al. 1997). Multiyear records of aerosol optical depth able accomplishment and demonstrates how sophisticated over water show reproducible seasonal patterns (Torres et satellite technology and analysis tools have become. This al. 2002). Measurements from space show a surprisingly newly gained observational capability greatly enhances our large contribution from Saharan dust and biomass burning understanding of climate forcing by aerosols from natural and distinct differences between the northern and southern and anthropogenic sources and leads to improvements in climate modeling.