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Earth radiation balance and climate: Why the moon is the wrong place to observe the Earth
Adv. Space Res. Vol. 14, No. 6, pp. (6)223-(6)232, 1994 Copyright © 1994 COSPAR Printed in Great Britain. All tights reserved. 0273-1177/94 $6.00 + 0.00
Pergamon
EARTH RADIATION BALANCE AND CLIMATE: WHY THE MOON IS THE WRONG PLACE TO OBSERVE THE EARTH Robert S. Kandel Laboratoire de Mdtdorologie Dynamique du CNRS, Ecole Polytechnique, F-91128 Palaiseau Cedex, France
ABSTRACT Increasing "greenhouse" gases in the Earth's atmosphere will perturb the Earth's radiation balance, forging climate change over coming decades. Climate sensitivity depends critically on cloud-radiation feedback: its evaluation requires continual observation of changing patterns of Earth radiation balance and cloud cover. The Moon is the wrong place for such observations, with many disadvantages compared to an observation system combining platforms in low polar, intermediate-inclination and geostationary orbits. From the Moon, active observations are infeasible; thermal infrared observations require very large instruments to reach spatial resolutions obtained at much lower cost from geostationary or lower orbits. The Earth's polar zones are never well observed from the Moon; other zones are invisible more than half the time. The monthly illumination cycle leads to further bias in radiation budget determinations. The Earth will be a pretty sight from the Earth-side of the Moon, but serious Earth observations will be made elsewhere. THE EARTH'S RADIATION BALANCE AND CLIMATE lmnortanee and Urgency of lmnroved Earth Observation Over the past two decades, growing awareness of the modification of the Earth's atmosphere by human activities has raised concern that climate may change significantly within the next century (cf. e.g./1-3/). Rapid climate changes will impose difficult adjustments on agriculture and on the remaining natural biosphere/4-5L Limiting the pace of expected global change will require costly changes in industrial and agricultural policy which cannot be instantaneously implemented/6-7/. Convincingly reliable model predictions are urgently needed, but at present predictions range from 1.5 to 5 K global warming for the reference case of carbon dioxide doubling. A critical factor of uncertainty is the strength and sign of climate-cloud- radiation feedback/8-9L Decisive progress requires more observational effort/10/. Space observation has played a fundamental role in providing uniform global coverage for monitoring global environmental change and for elucidating atmospheric and surface physical processes. Ambitious projects are under way to build a still more comprehensive Earth Observing System (EOS:/11-13/). Here we show that the Moon should not be considered as a platform for Earth observation, not only for reasons of urgency and cost, but also because the Moon is far inferior to satellites in lower orbits as a platform for observing our planet.
(6)223
(6)224
R . S . Kandel
Fundamental Physics of the Climate System Climate involves not only the atmosphere but also the oceans, other water bodies, ice masses, land surfaces and biosphere, in an ensemble that we call the climate system. Energy exchange between our planet and its cosmic environment is practically entirely radiative, and internal energy sources are negligible; the climate system functions essentially by absorbing part (70%) of the incident solar radiative flux, re-radiating this energy flux to space as thermal infrared radiation (Fig. 1). Albedo at the "top of the atmosphere" is significantly higher than global mean surface albedo because of the more or less high reflectivity (15-85%) of clouds, which vary strongly in space and time. Most of the solar (SW) radiant energy flux delivered to the climate system is absorbed at the Earth's surface, heating the atmosphere from the bottom. Evacuation of energy to space by longwave radiation (LW: 4-50 lam) is dominated by bands of polyatomic molecules (principally H20, CO2); clouds are also opaque in this domain. As a result, although the Earth's effective temperature (corresponding to the mean absorbed/reemitted radiation flux of 240 Wm-2) is 255 K, mean temperature at the surface is 288 K where the upward LW flux is 390 Wm"2. These differences characterize the greenhouse effecL
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Earth Radiation Balane and Climate
(6)225
l~aantring Atmospheric Comnosition and Global Warmin~ Atmospheric CO2 concentration rose from about 280 ppmv (parts per million by volume) in the mid18th century, to 315 ppmv during the International Geophysical Year (1957), and over 350 ppmv today (/1/, sect. 1), essentially as a result of human activities : deforestation and combustion of fossil fuels. Even more dramatic increases have occurred for methane and contribute significantly to enhancement of the greenhouse effect because this gas has unsaturated absorption bands in the atmospheric IR "window". The industrially produced chlorofluorocarbons (CFCs) also contribute, as can possible replacement products. Greenhouse gas abundances will continue to rise, depending on the one hand on emission rates which are more or less sensitive to industrial and agricultural policies, on the other hand on the global biogeochemical cycles whose sinks for these gases determine their atmospheric residence times. Increasing greenhouse gases increases tropospheric absorption of upgoing LW radiation from the surface and lower tropospheric levels, so that upgoing LW flux at the tropopause is reduced. Without changes in temperature and humidity structure, radiation balance at the tropopause would no longer be zero, the difference is the forcing term which tends to warm the troposphere and surface. Additional forcing due to rising abundances of other gases such as CH4 and the CFCs is often included in an "equivalent CO2" forcing, with the reference (equivalent CO 2 doubling) forcing of +4 Wm -2 expected between 2030 and 2100 (or later or never) according to various scenarios (/1/, sect. 2, and /14/). Response to this forcing is a steepening of the tropospheric temperature gradient with warming of the surface and lower troposphere restoring z e r o global radiation balance at the tropopause. It is generally (but not unanimously : cf./15/) expected that atmospheric humidity will increase with such wanning, providing positive feedback through the increase in IR absorption by added water vapor. Predictions of different models for climate sensitivity are in rough agreement regarding the contributions of water vapor and other feedbacks when clouds are not considered/gL yielding "clear-sky" sensitivity of order 0.5 K(Wm'2)"1, i.e. global mean warming of +2 K for a forcing of +4 Wm "2. The Critical Cloud-Radiati0n Feedback Clouds contribute significantly both to global mean albedo because of their high reflectance, and to the greenhouse effect because of their IR opacity. One can compute a "cloud radiative forcing" (CRF) corresponding to the changes in absorbed SW and emitted LW fluxes when clouds are made transparent in a model which has been previously run to equilibrium with radiatively interacting clouds /16/; different models yield quite different values of LW, SW and net CRF/8L Many but not all of these yield negative net CRF, implying that the cloud albedo effect dominates globally; analysis of data from the Earth Radiation Budget Experiment (ERBE : cf./17/) confirms this. Climate-cloud-radiation feedback depends on the change in net CRF when climate changes. Many simulations of global wanning lead to reduction in low-level cloudiness (which contributes principally to albedo while affecting outgoing LW very little) and increase in high-level clouds (which contribute a greenhouse effect because they reduce outgoing LW), yielding positive feedback and amplifying warming. For other models, feedback is negative, clouds becoming more reflective/18-19/as climate warms. One way to decide which models are incorrect and which hold promise of yielding reliable sensitivity estimates may be to observe how cloud radiative forcing changes with geographical, seasonal or interannual changes. This requires continuing and improved observations of clouds and the Earth's radiation budget from space. EXISTING AND PLANNED SPACE OBSERVATION SYSTEMS Snatial Coverage and Time Samolin~ Issues v
No single satellite can solve all Earth observation problems (cf. e.g./20/). A satellite in polar orbit can observe the entire globe at least twice in the course of 24 hours, but it cannot sample all local times. Physical conditions at the Earth's surface and in the atmosphere vary in response to the diurnal cycle of solar illumination, and this response cannot be calculated a priori. Radiative and non-radiative components of the surface-to-atmosphere energy budget over all land surfaces exhibit strong diurnal variation; so does cloudiness and the LW radiation flux at the top of the atmosphere/21-24/. Limited sampling of the diurnal cycle can bias daily or monthly means of surface or atmospheric parameters. Many Earth observation satellites have been placed in Sun-synchronous orbits for which local observation times remain nearly constant. This ensures comparability of observations from day to day,
(6)226
R.S. Kandel
but gives practically no information on the form of the diurnal cycle. The situation is improved with more than one satellite, such as the "morning" (07:30/19:30) and "afternoon" (02:30/14:30) pair of NOAA polar weather satellites. In circular orbit in the equatorial plane at an altitude of 35700 kin, with orbital period identical to the Earth's period of rotation, geostationary satellites make possible uninterrupted monitoring of rapidly evolving events and complete characterization of diurnal variations. This is at the price of geographical coverage : the poles are invisible, and five geostationary satellites are needed to cover the full 360 ° of longitude. Because of the distance, instruments must be much larger than in low polar orbits for a given desired resolution, especially at longer wavelengths; active measurements (radar, lidar) are out of the question. Intermediate-inclination orbits with observation times drifting fairly rapidly in local time provide a compromise : coverage of polar caps is sacrificed but all local times are observed in the course of a month, although gaps in time sampling occur on any particular day (Fig. 2). Meteorological Observations Temperature and humidity profdes, obtained by analysing narrow-spectral- band radiance data from the infrared and microwave "sounders" on board the NOAA operational weather satellites, constitute essential inputs to numerical weather prediction calculations. With two polar orbiters, such "soundings" are available four times in 24 hours for all of the globe. Comparable infrared sounders have been installed on NOAA geostationary weather satellites (GOES). From geostationaries, time sampling of the diurnal cycle is complete and longer signal integration is possible, but obtaining adequate spatial resolution requires a more cosily instrument than in low orbit. Microwave channels sensitive to atmospheric water vapor and liquid water have so far been implemented only on satellites in low orbits. Very high spatial resolution is possible from low orbit in the visible and near-IR : 0.5-1 km is currently obtained on the Russian Meteor and US NOAA weather satellites; 10-meter resolution is available for special studies from civilian land resources satellites such as SPOT. Thermal IR data with spatial resolution in the 1-12 km range is available not only from polar orbiters but also from the geostationary weather satellites. Such imagery, both in the visible and infrared, is routinely used for study of cloud fields : cloud displacements measured on images from geostationaries provide wind determinations over areas where conventional data are not available. For long-term studies of cloud cover, spatially and temporally sampled data from both NOAA polar orbiters and from the (more or less) complete set of geostationary weather satellites have been combined since 1983 in the International Satellite Cloud Climatology Project (ISCCP :/25-26/). Earth Radiation Budget Observations The goal of Earth radiation budget (ERB) observations is to obtain unbiased, absolutely calibrated, monthly (possibly daily) mean values of the reflected solar SW and emitted thermal LW radiative fluxes at the top of the atmosphere, for the entire globe, with adequate spatial resolution for comparison with climate models (100-500 kin). ERB data are only available from space, although estimates of the EarMs albedo were made in the 1930's by Danjon observing eal~hlight reflected by the Moon (cf./27/). NASA has conducted several ERB missions /28/; most recent was the Earth Radiation Budget Experiment (ERBE,/29-30/) with broad-band instruments on three satellites providing better sampling of the diurnal cycle/31, 22/. The ERBE scanner resolution makes it possible to identify cloud-free areas, yielding empirical estimates of cloud radiative forcing/17/. A scanner (or other imager), necessary for obtaining spatial resolution better than I000 kin, measures radiances. To convert these into fluxes one must take into account the anisotropy of reflected or emitted radiation/32/, which is known only in a statistical sense. If orbital viewing geometry depends systematically on geographical position (as from geostationary satellites) or on local observation time (as for solar zenith angle), there is a strong risk of bias in converting observed radiances into fluxes. One can measure flux at satellite altitude using a detector with a wide field of view integrating over the entire Earth disk; such sensors have exhibited great longevity on Nimbus-7 and the ERBE satellites. However, the flux thus measured involves integration over at least a few thousand kilometres, with contributions of radiation emerging at different angles from regions at different local times. Relating such measurements to quantifies relevant to the 100-500 km scale is extremely risky if not impossible.
Earth Radiation Balance and Climate
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Fig. 2. Day-hour matrix for ERBE observations in July 1985, for the region at the Equator and Greenwich meridian. Only NOAA-9 (0) and ERBS (black rectangle) were then in orbit. In some cases the region is seen on successive satellite orbits, and the local time of observation by NOAA-9 can shift slightly from day to day. Depending on the day of the month (left column), the ERBS/NOAA-9 combination has greater or smaller gaps in sampling the diurnal cycle. Individual daily mean values (obtained by averaging horizontally across the table) can be severely biased, as can the monthly mean diurnal cycle obtained by averaging columns vertically. Gaps can be filled by interpolation with diurnal models, or by using Meteosat ISCCP data (o) available every three hours.
(6)228
R.S. Kandel
Near-Term Prosnects Operational meteorological services are committed to maintaining and improving global monitoring of weather from space/13/. We expect further improvements in spatial resolution of imaging instruments and more and narrower spectral channels on infrared and microwave sounding instruments, with generalization of sounders on geostationary satellites. High-resolution infrared spectrometry will improve vertical resolution of temperature and humidity soundings. Also on line are improvements of radar determinations of liquid water and development of satellite-borne lidars to measure cirrus, aerosols, and humidity profiles, eventually also winds in cloud-free areas. Earth radiation budget monitoring will be reinforced beginning in 1993 by the French-Russian-German ScaRaB (Scanner for Radiation Budget) on board the Meteor-3 satellites/33/; NASA will fly the CERES (Clouds and the Earth's Radiant Energy System) scanners on TRMM and on the NASA EOS platforms beginning in 1997/32/. There should be fairly good coverage of diumal variations over the entire globe, with spatial resolution adequate for estimating cloud radiative forcing. Broad-band radiation budget data will be analysed together with simultaneous high-spatial-resolution data (better than 1 kin) in narrower spectral bands providing more information on cloud properties. Data from geostationary satellites will permit complete monitoring of short-period variations. Prospects are good for unbiased observations of changes in cloud radiative forcing over the next decade. DISADVANTAGES OF THE MOON AS AN OBSERVING PLATFORM The Moon Is Too Far The Moon is a natural satellite of the Earth at a mean distance of 3.8.105 kin, over ten times farther than artificial satellites in gcostationary orbits. For observations of the Earth's surface and atmosphere, high spatial and spectral resolution become significantly harder to obtain when going from low to geostationary orbits; difficulties are obviously compounded if one seeks to observe from the Moon. Power requirements and the inverse square law make active measurements (radar, lidar) difficult even from low orbit, but at least synthetic apem~re radar is facilitated by high orbital speed. Lidars have not yet been flown on satellites for Earth atmosphere and cloud observations; useful active (radar or lidar) measurements from geostationary satellites and from the Moon appear unlikely. Passive microwave measurements, which are today operational on polar orbiters and are envisaged for the next generation of geostationary meteorological satellites, would require extremely large structures or antenna arrays on the Moon to obtain mediocre performance, simply because of the fundamental limits on angular resolution. Angular resolution is not a problem for shortwave radiation. However in the longwave (e.g. at 10 Inn), 1-km spatial resolution which is possible with a 40-cm telescope on a geostationary satellite would require a 4-meter telescope on the Moon. For absolute radiometry at better than 1% accuracy, this would also require the construction of calibration sources filling the telescope input aper0are, i.e. a 4-meter black body operating at 250-330 K for the LW domain and a comparable appropriate SW source. Coarser resolution and a smaller telescope may be acceptable, but obtaining a good signal-to-noise ratio remains a problem when narrower spectral bands are used to characterize cloud fields or for atmospheric sounding. For broad-band measurements, the choice of detectors is limited. Certainly active cavity radiometers (ACRs) show considerable promise and could furnish accurate absolutely calibrated measurements of the reflected/emitted flux integrated over the whole Earth disk, but such measurements are of little use, and the advantage of ACRs baffled to very small angular field of view (10 -4 tad) is questionable. Inadeouate Global Coverage The Moon is a poor platform for observing polar zones, but it does permit observations of all longitudes (the Moon sees a rotating Earth), which is not the case for an individual geostationary platform. Orbital particularities lead to further limitations linked with angular and time sampling. Inclination of the lunar orbit with respect to the ecliptic is 5°9'; its inclination i with respect to the Earth's equatorial plane varies between 18018' and 28°36 ' with a period of 18.6 ye~ars/34/. In the course of a synodic month (29.53 days), the declination of the Moon thus varies between limits -+i : depending on the phase of the Moon
Earth Radiation Balance and Climate
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(6)230
R.S. Kandel
(or Earth), certain polar latitudes are unobservable. Depending on where we are in the 18.6-year nutation cycle and the annual cycle, these zones may be systematically unobservable during daytime or during nighttime (on the Earth). When lunar declination is at its maximum, the unobservable zone extends roughly from latitude 50 ° to the pole, excluding viewing zenith angles too close to 90 ° . Time SamDlin~ Biases Neglecting orbital eccentricity, the declination of the Moon as a function of time, 8(0, may be written roughly as: 5 (0
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where phase M(t) is zero at New Moon. Local solar time for the terrestrial meridian crossing the center of the Earth disk seen from the Moon is approximately 24.E(t)/2x . In practice, for lunar declination 8 (t) > 0, no useful observations can be made from the Moon of southern latitudes q0 < -(75 ° - ~ ) on Earth, whereas latitudes northward of 9 = 105° - 8 can be continually observed (the Moon remains 15° above the horizon through the 24 hr 50 min cycle). Depending on latitude, there is in the course of a month a systematic shift of times at which observations are possible. In Figure 3, we show the day-hour observation matrix computed for the special case of a month starting with New Moon in the ascending node of its orbit, considering points at three different latitudes on Earth. Daytime observations are possible only during part of the month, never at Full Moon except at the summer pole. Although for "full Earth", all useful times are observed (except over the invisible polar cap when solar and lunar declination are of opposite sign), viewing geometry varies systematically: for full Earth the lunar observer always observes backscattering, which yields extreme values of the bidirectional reflectance distribution function for most Earth scenes/35/. Other viewing geometries obtain at other Moon/Earth phases, but then only part of the day is observed for each point, and again viewing geometry varies systematically with solar time. Results obtained from reflected solar radiation measurements will therefore be subject to extreme bias. Biases also will affect results depending on longwave radiation emitted by the Earth-atmosphere system, because of systematic diumal variations of surface temperatures and cloud cover /21-22/. Monthly mean Earth radiation budget and other quantities, commonly used in comparing observed climatology with modeling results, will not be reliable. It may be possible to correct for some of these biases : results for the annual cycle with seasonal resolution may be relatively unaffected by lunar time sampling. However, many meteorological variations have typical time scales comparable to or less than the month, and there can be severe aliasing in results conceming these. In particular deep convection in the tropics nearly always exhibits some diurnal variation convoluted with intermediate-period variations such as the 4-5-day-period African Easterly waves/23/or monsoon breaks and Madden-Julian waves (40-50 days). CONCLUSIONS In the 35 years that have passed since the flight of Sputnik-l, more and more countries have launched an impressive array of satellites carrying instruments of many different types for the observation of our planet. Space observations have enormously enriched our knowledge concerning the global environment, thanks in particular to the wide and free distribution of data and to international cooperation. This same period has seen growing awareness of the potential impact of human activities on climate. More and better knowledge is urgent. The present informal satellite system is far from perfect, and the '~arth Observing System" being developed by the American, European and Japanese space agencies will not be
Earth Radiation Balance and Climate
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complete in the year 2000. We are far from having exhausted all the possibilities of existing satellite data; new ideas are needed and tools for data analysis (software as well as hardware) should be improved. Nevertheless, new imtruments providing improved accuracy or new kinds of information will always be welcome. Certainly we should not exclude a priori the possibility of putting instnnnents on satellite platforms in as yet unexploited orbits. However, as we have seen, the Moon's orbit has many drawbacks for observing the Earth. Space technology has demonsWated its capability to operate highperformance instruments on free-flying platforms in orbits providing complete global or diurnal coverage. Why go ten or five hundred times farther away simply to have solid Moon under the instrument's mounting ? The Moon is the wrong place to observe the Earth. REFERENCES 1. Houghton, J.T., et al. (eds.), 1990 : Climate Change - The IPCC Scientific Assessment, Cambridge Univ. Press, 365 pp. 2. Kandel, R., 1990 : Le Devenir des Climats, Hachette, Paris, 126 pp.; in English (transl. N. Hartmann) Our Changing Climate, McGraw-Hill, New York, 126 pp. 3. Schneider, S.H., 1989 : Global Warming, Sierra Club Books, San Francisco. 4. Tegart, W.J. McG., et al., (eds.) 1990 : Climate Change - The IPCC Impacts Assessment, Australian Govt. Publ. Serv., Canberra. 5. Leggett, J. (ed.), 1990 : Global Warming : The Greenpeace Report, Oxford Univ. Press, Oxford & New York, 554 pp. 6. Bemthal, F.M., et al., (eds.), 1990 : Climate Change - The IPCC Response Strategies, IPCCJWMO/ UNEP, Geneva, 270 pp. 7. Hammitt, J.K., et al., 1992 : A sequential-decision strategy for abating climate change, Nature,357, -318. 8. Cess, R.D., et al., 1989 : Interpretation of cloud-climate feedback as produced by 14 atmospheric general circulation models, Science, 245, 513-516. 9. Cess, R.D., et al., 1990 : Intercomparison of climate feedback processes in 19 atmospheric general circulation models, J.Geophys. Res.-Atmos., 95, 16601-16615. 10. Fouquart, Y., et al., 1990 : The influence of clouds an radiation : a climate-modeling perspective, Rev. Geophys., 28, 145-166. 11. ESA, 1991 : Report of the Earth observation user consultation meeting (ESTEC, Noordwilk), ESA SP-1143. 12. NASA, 1984 : Earth Observing System, Science and Mission Requirements, Working Group Rept., vol. 1 & app., NASA TM-86129, Washington, 58+59 pp. 13. NOAA/NASA, 1987 : Space-Based Remote Sensing of the Earth, Report to Congress, U.S. Govt. Printing Office, Washington, 123 pp. 14. Wigley, T.M.L., & S.C.B. Raper, 1992 : Implications for climate and sea level of revised IPCC emissions scenarios, Nature, 357, 293-300. 15. Lindzen, R.S., 1990 : Some coolness concerning global wanning, Bull. Amer. Meteorol. Soc., 71, 288-299. 16. Charlock, T.P., & V. Ramanathan, 1985 : The albedo field and cloud radiative forcing produced by a general circulation model with intemally generated cloud optics, J. Atmos. Sci., 42, 1408-1429. 17. Ramanathan, V., et al., 1989 : Cloud-radiative forcing and climate : results from the Earth Radiation Budget Experiment, Science, 243, 57-65. 18. Roeckner, E., et al., 1987 : Cloud optical depth feedbacks and climate modelling, Nature, 329, 138-140. 19. Mitchell, J.F.B., et al., 1989 : CO 2 and climate : a missing feedback ? Nature, 341, 132-134. 20. Kandel, R., 1990b : Satellite observation of the Earth radiation budget and clouds, Space Science Reviews, 52, 1-32. 21. Duvel, J.Ph., & R. Kandel, 1985 : Regional-scale diumal variations of outgoing infrared radiation observed by Meteosat, J. Clim. Appl. Meteorol., 24, 335-349. 22. Harrison, E.F., et al., 1988 : First estimates of the diurnal variation of longwave radiation from the multiple-satellite, Earth Radiation Budget Experiment (ERBE). Bull. Amer. Meteorol. Soc., 69, 1144-1151.
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23. Duvel, J.Ph., 1989 : Convection over tropical Africa and Atlantic Ocean during Northem Hemisphere summer. I : Interannual and diurnal variations, Mon, Wen. Rev., 117, 2782-2799. 24. Cheruy, F., et al., 1991 : Outgoing longwave radiation and its diurnal variations from combined Earth Radiation Budget Experiment and Meteosat observations - 2. Using Meteosat data to determine the longwave diurnal cycle, J. Geophys. Res.-Atmos., 96, 22623-22630. 25. Schiffer, R.A., & W.B. Rossow, 1983 : The International Satellite Cloud Climatology Project (ISCCP) : the first project of the World Climate Research Programme. Bull. Amer. Meteorol. Soc., 64, 779-784. 26. Schiffer, R.A., and W.B. Rossow, 1985 : ISCCP global radiance data set - a new resource for climate research. Bull. Amer. Meteorol. Soc., 66, 1498-1505. 27. Hunt, G.E., et al., 1986 : A history of presatellite investigations of the Earth's radiation budget, Rev. Geophys., 24, 351-356 28. House, F.B., et al., 1986 : History of satellite missions and measurements of the Earth radiation budget, Rev. Geophys., 24, 357-377. 29. Barkstrom, B.R., et al., 1986 : The Earth Radiation Budget Experiment (series of papers), Rev. Geophys., 24, 379-468. 30. Barkstrom, B.R., et al., 1989 : Earth Radiation Budget Experiment (ERBE) archival and April 1985 results, Bull. Amer. Meteorol. Soc., 70, 1254-1262. 31. Brooks, D.R., et al., 1986 : Development of algorithms for understanding the temporal and spatial variability of the Earth's radiation balance, Rev. Geophys., 24, 422-438. 32. Smith, G.L., et al., 1992 : Radiation budget measurements for the Eighties and Nineties, in COSPAR proceedings, A.2-S. 33. Kandel, R., et al., 1992 : The ScaRaB project - Earth radiation budget observations from the Meteor satellites, COSPAR proceedings, A.2-S. 34. Danjon, A., 1953 : Astronomie G~ndrale, J.& R. Sennac, Paris, 446 pp. 35. Suttles, J.T., et al., 1988 : Angular radiation models for Earth-atmosphere system - Vol.1 : Shortwave radiation, NASA RP 1184, Washington.
Pergamon
EARTH RADIATION BALANCE AND CLIMATE: WHY THE MOON IS THE WRONG PLACE TO OBSERVE THE EARTH Robert S. Kandel Laboratoire de Mdtdorologie Dynamique du CNRS, Ecole Polytechnique, F-91128 Palaiseau Cedex, France
ABSTRACT Increasing "greenhouse" gases in the Earth's atmosphere will perturb the Earth's radiation balance, forging climate change over coming decades. Climate sensitivity depends critically on cloud-radiation feedback: its evaluation requires continual observation of changing patterns of Earth radiation balance and cloud cover. The Moon is the wrong place for such observations, with many disadvantages compared to an observation system combining platforms in low polar, intermediate-inclination and geostationary orbits. From the Moon, active observations are infeasible; thermal infrared observations require very large instruments to reach spatial resolutions obtained at much lower cost from geostationary or lower orbits. The Earth's polar zones are never well observed from the Moon; other zones are invisible more than half the time. The monthly illumination cycle leads to further bias in radiation budget determinations. The Earth will be a pretty sight from the Earth-side of the Moon, but serious Earth observations will be made elsewhere. THE EARTH'S RADIATION BALANCE AND CLIMATE lmnortanee and Urgency of lmnroved Earth Observation Over the past two decades, growing awareness of the modification of the Earth's atmosphere by human activities has raised concern that climate may change significantly within the next century (cf. e.g./1-3/). Rapid climate changes will impose difficult adjustments on agriculture and on the remaining natural biosphere/4-5L Limiting the pace of expected global change will require costly changes in industrial and agricultural policy which cannot be instantaneously implemented/6-7/. Convincingly reliable model predictions are urgently needed, but at present predictions range from 1.5 to 5 K global warming for the reference case of carbon dioxide doubling. A critical factor of uncertainty is the strength and sign of climate-cloud- radiation feedback/8-9L Decisive progress requires more observational effort/10/. Space observation has played a fundamental role in providing uniform global coverage for monitoring global environmental change and for elucidating atmospheric and surface physical processes. Ambitious projects are under way to build a still more comprehensive Earth Observing System (EOS:/11-13/). Here we show that the Moon should not be considered as a platform for Earth observation, not only for reasons of urgency and cost, but also because the Moon is far inferior to satellites in lower orbits as a platform for observing our planet.
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Fundamental Physics of the Climate System Climate involves not only the atmosphere but also the oceans, other water bodies, ice masses, land surfaces and biosphere, in an ensemble that we call the climate system. Energy exchange between our planet and its cosmic environment is practically entirely radiative, and internal energy sources are negligible; the climate system functions essentially by absorbing part (70%) of the incident solar radiative flux, re-radiating this energy flux to space as thermal infrared radiation (Fig. 1). Albedo at the "top of the atmosphere" is significantly higher than global mean surface albedo because of the more or less high reflectivity (15-85%) of clouds, which vary strongly in space and time. Most of the solar (SW) radiant energy flux delivered to the climate system is absorbed at the Earth's surface, heating the atmosphere from the bottom. Evacuation of energy to space by longwave radiation (LW: 4-50 lam) is dominated by bands of polyatomic molecules (principally H20, CO2); clouds are also opaque in this domain. As a result, although the Earth's effective temperature (corresponding to the mean absorbed/reemitted radiation flux of 240 Wm-2) is 255 K, mean temperature at the surface is 288 K where the upward LW flux is 390 Wm"2. These differences characterize the greenhouse effecL
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Earth Radiation Balane and Climate
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l~aantring Atmospheric Comnosition and Global Warmin~ Atmospheric CO2 concentration rose from about 280 ppmv (parts per million by volume) in the mid18th century, to 315 ppmv during the International Geophysical Year (1957), and over 350 ppmv today (/1/, sect. 1), essentially as a result of human activities : deforestation and combustion of fossil fuels. Even more dramatic increases have occurred for methane and contribute significantly to enhancement of the greenhouse effect because this gas has unsaturated absorption bands in the atmospheric IR "window". The industrially produced chlorofluorocarbons (CFCs) also contribute, as can possible replacement products. Greenhouse gas abundances will continue to rise, depending on the one hand on emission rates which are more or less sensitive to industrial and agricultural policies, on the other hand on the global biogeochemical cycles whose sinks for these gases determine their atmospheric residence times. Increasing greenhouse gases increases tropospheric absorption of upgoing LW radiation from the surface and lower tropospheric levels, so that upgoing LW flux at the tropopause is reduced. Without changes in temperature and humidity structure, radiation balance at the tropopause would no longer be zero, the difference is the forcing term which tends to warm the troposphere and surface. Additional forcing due to rising abundances of other gases such as CH4 and the CFCs is often included in an "equivalent CO2" forcing, with the reference (equivalent CO 2 doubling) forcing of +4 Wm -2 expected between 2030 and 2100 (or later or never) according to various scenarios (/1/, sect. 2, and /14/). Response to this forcing is a steepening of the tropospheric temperature gradient with warming of the surface and lower troposphere restoring z e r o global radiation balance at the tropopause. It is generally (but not unanimously : cf./15/) expected that atmospheric humidity will increase with such wanning, providing positive feedback through the increase in IR absorption by added water vapor. Predictions of different models for climate sensitivity are in rough agreement regarding the contributions of water vapor and other feedbacks when clouds are not considered/gL yielding "clear-sky" sensitivity of order 0.5 K(Wm'2)"1, i.e. global mean warming of +2 K for a forcing of +4 Wm "2. The Critical Cloud-Radiati0n Feedback Clouds contribute significantly both to global mean albedo because of their high reflectance, and to the greenhouse effect because of their IR opacity. One can compute a "cloud radiative forcing" (CRF) corresponding to the changes in absorbed SW and emitted LW fluxes when clouds are made transparent in a model which has been previously run to equilibrium with radiatively interacting clouds /16/; different models yield quite different values of LW, SW and net CRF/8L Many but not all of these yield negative net CRF, implying that the cloud albedo effect dominates globally; analysis of data from the Earth Radiation Budget Experiment (ERBE : cf./17/) confirms this. Climate-cloud-radiation feedback depends on the change in net CRF when climate changes. Many simulations of global wanning lead to reduction in low-level cloudiness (which contributes principally to albedo while affecting outgoing LW very little) and increase in high-level clouds (which contribute a greenhouse effect because they reduce outgoing LW), yielding positive feedback and amplifying warming. For other models, feedback is negative, clouds becoming more reflective/18-19/as climate warms. One way to decide which models are incorrect and which hold promise of yielding reliable sensitivity estimates may be to observe how cloud radiative forcing changes with geographical, seasonal or interannual changes. This requires continuing and improved observations of clouds and the Earth's radiation budget from space. EXISTING AND PLANNED SPACE OBSERVATION SYSTEMS Snatial Coverage and Time Samolin~ Issues v
No single satellite can solve all Earth observation problems (cf. e.g./20/). A satellite in polar orbit can observe the entire globe at least twice in the course of 24 hours, but it cannot sample all local times. Physical conditions at the Earth's surface and in the atmosphere vary in response to the diurnal cycle of solar illumination, and this response cannot be calculated a priori. Radiative and non-radiative components of the surface-to-atmosphere energy budget over all land surfaces exhibit strong diurnal variation; so does cloudiness and the LW radiation flux at the top of the atmosphere/21-24/. Limited sampling of the diurnal cycle can bias daily or monthly means of surface or atmospheric parameters. Many Earth observation satellites have been placed in Sun-synchronous orbits for which local observation times remain nearly constant. This ensures comparability of observations from day to day,
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but gives practically no information on the form of the diurnal cycle. The situation is improved with more than one satellite, such as the "morning" (07:30/19:30) and "afternoon" (02:30/14:30) pair of NOAA polar weather satellites. In circular orbit in the equatorial plane at an altitude of 35700 kin, with orbital period identical to the Earth's period of rotation, geostationary satellites make possible uninterrupted monitoring of rapidly evolving events and complete characterization of diurnal variations. This is at the price of geographical coverage : the poles are invisible, and five geostationary satellites are needed to cover the full 360 ° of longitude. Because of the distance, instruments must be much larger than in low polar orbits for a given desired resolution, especially at longer wavelengths; active measurements (radar, lidar) are out of the question. Intermediate-inclination orbits with observation times drifting fairly rapidly in local time provide a compromise : coverage of polar caps is sacrificed but all local times are observed in the course of a month, although gaps in time sampling occur on any particular day (Fig. 2). Meteorological Observations Temperature and humidity profdes, obtained by analysing narrow-spectral- band radiance data from the infrared and microwave "sounders" on board the NOAA operational weather satellites, constitute essential inputs to numerical weather prediction calculations. With two polar orbiters, such "soundings" are available four times in 24 hours for all of the globe. Comparable infrared sounders have been installed on NOAA geostationary weather satellites (GOES). From geostationaries, time sampling of the diurnal cycle is complete and longer signal integration is possible, but obtaining adequate spatial resolution requires a more cosily instrument than in low orbit. Microwave channels sensitive to atmospheric water vapor and liquid water have so far been implemented only on satellites in low orbits. Very high spatial resolution is possible from low orbit in the visible and near-IR : 0.5-1 km is currently obtained on the Russian Meteor and US NOAA weather satellites; 10-meter resolution is available for special studies from civilian land resources satellites such as SPOT. Thermal IR data with spatial resolution in the 1-12 km range is available not only from polar orbiters but also from the geostationary weather satellites. Such imagery, both in the visible and infrared, is routinely used for study of cloud fields : cloud displacements measured on images from geostationaries provide wind determinations over areas where conventional data are not available. For long-term studies of cloud cover, spatially and temporally sampled data from both NOAA polar orbiters and from the (more or less) complete set of geostationary weather satellites have been combined since 1983 in the International Satellite Cloud Climatology Project (ISCCP :/25-26/). Earth Radiation Budget Observations The goal of Earth radiation budget (ERB) observations is to obtain unbiased, absolutely calibrated, monthly (possibly daily) mean values of the reflected solar SW and emitted thermal LW radiative fluxes at the top of the atmosphere, for the entire globe, with adequate spatial resolution for comparison with climate models (100-500 kin). ERB data are only available from space, although estimates of the EarMs albedo were made in the 1930's by Danjon observing eal~hlight reflected by the Moon (cf./27/). NASA has conducted several ERB missions /28/; most recent was the Earth Radiation Budget Experiment (ERBE,/29-30/) with broad-band instruments on three satellites providing better sampling of the diurnal cycle/31, 22/. The ERBE scanner resolution makes it possible to identify cloud-free areas, yielding empirical estimates of cloud radiative forcing/17/. A scanner (or other imager), necessary for obtaining spatial resolution better than I000 kin, measures radiances. To convert these into fluxes one must take into account the anisotropy of reflected or emitted radiation/32/, which is known only in a statistical sense. If orbital viewing geometry depends systematically on geographical position (as from geostationary satellites) or on local observation time (as for solar zenith angle), there is a strong risk of bias in converting observed radiances into fluxes. One can measure flux at satellite altitude using a detector with a wide field of view integrating over the entire Earth disk; such sensors have exhibited great longevity on Nimbus-7 and the ERBE satellites. However, the flux thus measured involves integration over at least a few thousand kilometres, with contributions of radiation emerging at different angles from regions at different local times. Relating such measurements to quantifies relevant to the 100-500 km scale is extremely risky if not impossible.
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Near-Term Prosnects Operational meteorological services are committed to maintaining and improving global monitoring of weather from space/13/. We expect further improvements in spatial resolution of imaging instruments and more and narrower spectral channels on infrared and microwave sounding instruments, with generalization of sounders on geostationary satellites. High-resolution infrared spectrometry will improve vertical resolution of temperature and humidity soundings. Also on line are improvements of radar determinations of liquid water and development of satellite-borne lidars to measure cirrus, aerosols, and humidity profiles, eventually also winds in cloud-free areas. Earth radiation budget monitoring will be reinforced beginning in 1993 by the French-Russian-German ScaRaB (Scanner for Radiation Budget) on board the Meteor-3 satellites/33/; NASA will fly the CERES (Clouds and the Earth's Radiant Energy System) scanners on TRMM and on the NASA EOS platforms beginning in 1997/32/. There should be fairly good coverage of diumal variations over the entire globe, with spatial resolution adequate for estimating cloud radiative forcing. Broad-band radiation budget data will be analysed together with simultaneous high-spatial-resolution data (better than 1 kin) in narrower spectral bands providing more information on cloud properties. Data from geostationary satellites will permit complete monitoring of short-period variations. Prospects are good for unbiased observations of changes in cloud radiative forcing over the next decade. DISADVANTAGES OF THE MOON AS AN OBSERVING PLATFORM The Moon Is Too Far The Moon is a natural satellite of the Earth at a mean distance of 3.8.105 kin, over ten times farther than artificial satellites in gcostationary orbits. For observations of the Earth's surface and atmosphere, high spatial and spectral resolution become significantly harder to obtain when going from low to geostationary orbits; difficulties are obviously compounded if one seeks to observe from the Moon. Power requirements and the inverse square law make active measurements (radar, lidar) difficult even from low orbit, but at least synthetic apem~re radar is facilitated by high orbital speed. Lidars have not yet been flown on satellites for Earth atmosphere and cloud observations; useful active (radar or lidar) measurements from geostationary satellites and from the Moon appear unlikely. Passive microwave measurements, which are today operational on polar orbiters and are envisaged for the next generation of geostationary meteorological satellites, would require extremely large structures or antenna arrays on the Moon to obtain mediocre performance, simply because of the fundamental limits on angular resolution. Angular resolution is not a problem for shortwave radiation. However in the longwave (e.g. at 10 Inn), 1-km spatial resolution which is possible with a 40-cm telescope on a geostationary satellite would require a 4-meter telescope on the Moon. For absolute radiometry at better than 1% accuracy, this would also require the construction of calibration sources filling the telescope input aper0are, i.e. a 4-meter black body operating at 250-330 K for the LW domain and a comparable appropriate SW source. Coarser resolution and a smaller telescope may be acceptable, but obtaining a good signal-to-noise ratio remains a problem when narrower spectral bands are used to characterize cloud fields or for atmospheric sounding. For broad-band measurements, the choice of detectors is limited. Certainly active cavity radiometers (ACRs) show considerable promise and could furnish accurate absolutely calibrated measurements of the reflected/emitted flux integrated over the whole Earth disk, but such measurements are of little use, and the advantage of ACRs baffled to very small angular field of view (10 -4 tad) is questionable. Inadeouate Global Coverage The Moon is a poor platform for observing polar zones, but it does permit observations of all longitudes (the Moon sees a rotating Earth), which is not the case for an individual geostationary platform. Orbital particularities lead to further limitations linked with angular and time sampling. Inclination of the lunar orbit with respect to the ecliptic is 5°9'; its inclination i with respect to the Earth's equatorial plane varies between 18018' and 28°36 ' with a period of 18.6 ye~ars/34/. In the course of a synodic month (29.53 days), the declination of the Moon thus varies between limits -+i : depending on the phase of the Moon
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27 28
31 3
6
-
9 LOCAL
F i g . 3.
Day-hour
-
12 Tn4E
15
-
-
O-
o+ 18
21
24
Ore)
matrix for observations, from the Moon,
of regions
on the Greenwich
m e r i d i a n , at t h e E q u a t o r a n d at l a t i t u d e s 75 ° N o r t h ( + ) a n d S o u t h (-). W e a s s u m e t h a t t h e l u n a r o r b i t ' s i n c l i n a t i o n r e l a t i v e t o t h e e c l i p t i c s t a y s f i x e d at + 1 ° 3 3 ' t h r o u g h o u t t h e m o n t h , w i t h t h e M o o n p a s s i n g t h r o u g h its a s c e n d i n g n o d e as a N e w M o o n o n t h e f i r s t d a y o f t h e m o n t h , a n d r e a c h i n g m a x i m u m d e c l i n a t i o n o f + 2 5 ° a p p r o x i m a t e l y o n e w e ~ k later. W e c o n s i d e r o n l y p o i n t s f o r w h i c h t h e z e n i t h a n g l e o f t h e e m e r g e n t r a d i a t i o n at t h e E a r t h is l e s s t h a n 7 5 °.
(6)230
R.S. Kandel
(or Earth), certain polar latitudes are unobservable. Depending on where we are in the 18.6-year nutation cycle and the annual cycle, these zones may be systematically unobservable during daytime or during nighttime (on the Earth). When lunar declination is at its maximum, the unobservable zone extends roughly from latitude 50 ° to the pole, excluding viewing zenith angles too close to 90 ° . Time SamDlin~ Biases Neglecting orbital eccentricity, the declination of the Moon as a function of time, 8(0, may be written roughly as: 5 (0
=
i(t) x
sin [21t (t-to)/T] ,
(1)
where T is the synodic lunar period and to is the time of the last passage of the Moon through the ascending node of its orbit. The phase of the Moon at ascending node M(to) and its orbital inclination i(t) vary slowly with the 18.6-year period. The phase E(t) of the Earth as seen from the Moon is E(O
=
M ( t O) + ~
=
M(O
+ 21t (t-to)/T
+ n
,
(2)
where phase M(t) is zero at New Moon. Local solar time for the terrestrial meridian crossing the center of the Earth disk seen from the Moon is approximately 24.E(t)/2x . In practice, for lunar declination 8 (t) > 0, no useful observations can be made from the Moon of southern latitudes q0 < -(75 ° - ~ ) on Earth, whereas latitudes northward of 9 = 105° - 8 can be continually observed (the Moon remains 15° above the horizon through the 24 hr 50 min cycle). Depending on latitude, there is in the course of a month a systematic shift of times at which observations are possible. In Figure 3, we show the day-hour observation matrix computed for the special case of a month starting with New Moon in the ascending node of its orbit, considering points at three different latitudes on Earth. Daytime observations are possible only during part of the month, never at Full Moon except at the summer pole. Although for "full Earth", all useful times are observed (except over the invisible polar cap when solar and lunar declination are of opposite sign), viewing geometry varies systematically: for full Earth the lunar observer always observes backscattering, which yields extreme values of the bidirectional reflectance distribution function for most Earth scenes/35/. Other viewing geometries obtain at other Moon/Earth phases, but then only part of the day is observed for each point, and again viewing geometry varies systematically with solar time. Results obtained from reflected solar radiation measurements will therefore be subject to extreme bias. Biases also will affect results depending on longwave radiation emitted by the Earth-atmosphere system, because of systematic diumal variations of surface temperatures and cloud cover /21-22/. Monthly mean Earth radiation budget and other quantities, commonly used in comparing observed climatology with modeling results, will not be reliable. It may be possible to correct for some of these biases : results for the annual cycle with seasonal resolution may be relatively unaffected by lunar time sampling. However, many meteorological variations have typical time scales comparable to or less than the month, and there can be severe aliasing in results conceming these. In particular deep convection in the tropics nearly always exhibits some diurnal variation convoluted with intermediate-period variations such as the 4-5-day-period African Easterly waves/23/or monsoon breaks and Madden-Julian waves (40-50 days). CONCLUSIONS In the 35 years that have passed since the flight of Sputnik-l, more and more countries have launched an impressive array of satellites carrying instruments of many different types for the observation of our planet. Space observations have enormously enriched our knowledge concerning the global environment, thanks in particular to the wide and free distribution of data and to international cooperation. This same period has seen growing awareness of the potential impact of human activities on climate. More and better knowledge is urgent. The present informal satellite system is far from perfect, and the '~arth Observing System" being developed by the American, European and Japanese space agencies will not be
Earth Radiation Balance and Climate
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complete in the year 2000. We are far from having exhausted all the possibilities of existing satellite data; new ideas are needed and tools for data analysis (software as well as hardware) should be improved. Nevertheless, new imtruments providing improved accuracy or new kinds of information will always be welcome. Certainly we should not exclude a priori the possibility of putting instnnnents on satellite platforms in as yet unexploited orbits. However, as we have seen, the Moon's orbit has many drawbacks for observing the Earth. Space technology has demonsWated its capability to operate highperformance instruments on free-flying platforms in orbits providing complete global or diurnal coverage. Why go ten or five hundred times farther away simply to have solid Moon under the instrument's mounting ? The Moon is the wrong place to observe the Earth. REFERENCES 1. Houghton, J.T., et al. (eds.), 1990 : Climate Change - The IPCC Scientific Assessment, Cambridge Univ. Press, 365 pp. 2. Kandel, R., 1990 : Le Devenir des Climats, Hachette, Paris, 126 pp.; in English (transl. N. Hartmann) Our Changing Climate, McGraw-Hill, New York, 126 pp. 3. Schneider, S.H., 1989 : Global Warming, Sierra Club Books, San Francisco. 4. Tegart, W.J. McG., et al., (eds.) 1990 : Climate Change - The IPCC Impacts Assessment, Australian Govt. Publ. Serv., Canberra. 5. Leggett, J. (ed.), 1990 : Global Warming : The Greenpeace Report, Oxford Univ. Press, Oxford & New York, 554 pp. 6. Bemthal, F.M., et al., (eds.), 1990 : Climate Change - The IPCC Response Strategies, IPCCJWMO/ UNEP, Geneva, 270 pp. 7. Hammitt, J.K., et al., 1992 : A sequential-decision strategy for abating climate change, Nature,357, -318. 8. Cess, R.D., et al., 1989 : Interpretation of cloud-climate feedback as produced by 14 atmospheric general circulation models, Science, 245, 513-516. 9. Cess, R.D., et al., 1990 : Intercomparison of climate feedback processes in 19 atmospheric general circulation models, J.Geophys. Res.-Atmos., 95, 16601-16615. 10. Fouquart, Y., et al., 1990 : The influence of clouds an radiation : a climate-modeling perspective, Rev. Geophys., 28, 145-166. 11. ESA, 1991 : Report of the Earth observation user consultation meeting (ESTEC, Noordwilk), ESA SP-1143. 12. NASA, 1984 : Earth Observing System, Science and Mission Requirements, Working Group Rept., vol. 1 & app., NASA TM-86129, Washington, 58+59 pp. 13. NOAA/NASA, 1987 : Space-Based Remote Sensing of the Earth, Report to Congress, U.S. Govt. Printing Office, Washington, 123 pp. 14. Wigley, T.M.L., & S.C.B. Raper, 1992 : Implications for climate and sea level of revised IPCC emissions scenarios, Nature, 357, 293-300. 15. Lindzen, R.S., 1990 : Some coolness concerning global wanning, Bull. Amer. Meteorol. Soc., 71, 288-299. 16. Charlock, T.P., & V. Ramanathan, 1985 : The albedo field and cloud radiative forcing produced by a general circulation model with intemally generated cloud optics, J. Atmos. Sci., 42, 1408-1429. 17. Ramanathan, V., et al., 1989 : Cloud-radiative forcing and climate : results from the Earth Radiation Budget Experiment, Science, 243, 57-65. 18. Roeckner, E., et al., 1987 : Cloud optical depth feedbacks and climate modelling, Nature, 329, 138-140. 19. Mitchell, J.F.B., et al., 1989 : CO 2 and climate : a missing feedback ? Nature, 341, 132-134. 20. Kandel, R., 1990b : Satellite observation of the Earth radiation budget and clouds, Space Science Reviews, 52, 1-32. 21. Duvel, J.Ph., & R. Kandel, 1985 : Regional-scale diumal variations of outgoing infrared radiation observed by Meteosat, J. Clim. Appl. Meteorol., 24, 335-349. 22. Harrison, E.F., et al., 1988 : First estimates of the diurnal variation of longwave radiation from the multiple-satellite, Earth Radiation Budget Experiment (ERBE). Bull. Amer. Meteorol. Soc., 69, 1144-1151.
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