An algorithm based on the physics of radiative transfer in vegetation canopies for the retrieval of vegetation green leaf area index (LAI) and fraction of absorbed photosynthetically active radiation (FPAR) from surface reflectances was developed and implemented for operational processing prior to the launch of the moderate resolution imaging spectroradiometer (MODIS) aboard the TERRA platform in December of 1999. The performance of the algorithm has been extensively tested in prototyping activities prior to operational production. Considerable attention was paid to characterizing the quality of the product and this information is available to the users as quality assessment (QA) accompanying the product. The MODIS LAI/FPAR product has been operationally produced from day one of science data processing from MODIS and is available free of charge to the users from the Earth Resources Observation System (EROS) Data Center Distributed Active Archive Center. Current and planned validation activities are aimed at evaluating the product at several field sites representative of the six structural biomes. Example results illustrating the physics and performance of the algorithm are presented together with initial QA and validation results. Potential users of the product are advised of the provisional nature of the product in view of changes to calibration, geolocation, cloud screening, atmospheric correction and ongoing validation activities. D

China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978. In this article, we present evidence for a significant urbanization effect on climate based on analysis of impacts of land-use changes on surface temperature in southeast China, where rapid urbanization has occurred. Our estimated warming of mean surface temperature of 0.05°C per decade attributable to urbanization is much larger than previous estimates for other periods and locations. The spatial pattern and magnitude of our estimate are consistent with those of urbanization characterized by changes in the percentage of urban population and in satellite-measured greenness. Land-use changes from urbanization, creating an urban heat island (UHI), have been suspected as partially being responsible for the observed warming over land during the last few decades because of (i) the observed decrease in the diurnal temperature range (DTR) resulting from a larger increase or a smaller decrease in minimum temperature relative to maximum temperature and (ii) a lower rate of warming observed over the past 20 years in the lower troposphere compared with the surface (1). The area-weighted average warming effect of UHI over land during the 20th century has been estimated to be Ͻ0.06°C per century (1-4) globally and approximately 0.06ϳ0.15°C per century (5, 6) in the U.S. based on differences in temperature trends between rural and urban stations. A much larger estimate of 0.27°C per century in the U.S. has been reported recently (7) by comparing trends in observed and reanalysis surface temperatures over the period from 1950 to 1999.China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978. From 1978 to 2000, China's gross domestic product grew at an average annual rate of 9.5%, compared with 2.5% for developed countries and 5% for developing countries; the number of small towns soared from 2,176 to 20,312, nearly double that of the world average during this period; the number of cities increased from 190 to 663; and the proportion of urban population rose from 18% to 39% (see the Peopledaily article at http:͞͞english. peopledaily.com.cn͞200111͞27͞eng2001112785410.shtml and the State Family Planning Commission of China web site at www.sfpc.gov.cn͞EN͞enews20030320-1.htm). In this article, we present evidence for a significant urbanization effect on climate based on analysis of impacts of land-use changes on surface temperature in southeast China, where most of China's urbanization has occurred. Data and MethodsThe UHI effect has been estimated by comparing observed temperatures in urban stations with those in their surrounding rural stations, but such results largely depend on how rural versus urban stations are classified and whether the data are homogeneous (7-9). Population data often are used to identify a station as urban and rural, but such information generally is out-of-date, and thus satellite measurements of night lights have been substituted re...

Tropical forests are global epicentres of biodiversity and important modulators of climate change, and are mainly constrained by rainfall patterns. The severe short-term droughts that occurred recently in Amazonia have drawn attention to the vulnerability of tropical forests to climatic disturbances. The central African rainforests, the second-largest on Earth, have experienced a long-term drying trend whose impacts on vegetation dynamics remain mostly unknown because in situ observations are very limited. The Congolese forest, with its drier conditions and higher percentage of semi-evergreen trees, may be more tolerant to short-term rainfall reduction than are wetter tropical forests, but for a long-term drought there may be critical thresholds of water availability below which higher-biomass, closed-canopy forests transition to more open, lower-biomass forests. Here we present observational evidence for a widespread decline in forest greenness over the past decade based on analyses of satellite data (optical, thermal, microwave and gravity) from several independent sensors over the Congo basin. This decline in vegetation greenness, particularly in the northern Congolese forest, is generally consistent with decreases in rainfall, terrestrial water storage, water content in aboveground woody and leaf biomass, and the canopy backscatter anomaly caused by changes in structure and moisture in upper forest layers. It is also consistent with increases in photosynthetically active radiation and land surface temperature. These multiple lines of evidence indicate that this large-scale vegetation browning, or loss of photosynthetic capacity, may be partially attributable to the long-term drying trend. Our results suggest that a continued gradual decline of photosynthetic capacity and moisture content driven by the persistent drying trend could alter the composition and structure of the Congolese forest to favour the spread of drought-tolerant species.

[1] This paper analyzes the relation between satellite-based measures of vegetation greenness and climate by land cover type at a regional scale (2°Â 2°grid boxes) between 1982 and 1999. We use the normalized difference vegetation index (NDVI) from the Global Inventory Monitoring and Modeling Studies (GIMMS) data set to quantify climate-induced changes in terrestrial vegetation. Climatic conditions are represented with monthly data for land surface air temperature and precipitation. The relation between NDVI and the climate variables is represented using a quadratic specification, which is consistent with the notion of a physiological optimum. The effects of spatial heterogeneity and unobserved variables are estimated with specifications and statistical techniques that allow coefficients to vary among grid boxes. Using this methodology, we are able to estimate statistically meaningful relations between NDVI and climate during spring, summer, and autumn for forests between 40°N and 70°N in North America and Eurasia. Of the variables examined, changes in temperature account for the largest fraction of the change in NDVI between the early 1980s and the late 1990s. Changes in stratospheric aerosol optical depth and precipitation have a smaller effect, while artifacts associated with variations in solar zenith angle are negligible. These results indicate that temperature changes between the early 1980s and the late 1990s are responsible for much of the observed increase in satellite measures of northern forest greenness.

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.[1] The aerosol direct solar effect under clear sky is assessed by (1) combining multiple aerosol characterizations and (2) using the satellite-retrieved land surface albedo. The aerosol characterization is made through an integration of the MODerate resolution Imaging Spectroradiometer (MODIS) retrievals and the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model simulations. The spectral and bidirectional albedo of land surface is derived from MODIS. On a global average, the solar forcing at the top of atmosphere (TOA) DF TOA is À4.5 Wm À2 , of which about 1/3 is contributed by a sum of natural and anthropogenic sulfate and carbonaceous aerosols. Though the optical depth is about 50% higher over land than over ocean, no significant land-ocean contrast in this TOA forcing is observed. It is reduced by larger aerosol absorption and higher surface albedo over land. As a result of absorption by soot and dust, a much larger surface cooling and substantial atmospheric absorption coexist over land and adjacent oceans. Globally, the surface cooling DF SFC is about À9.9 Wm À2 , and the atmospheric absorption DF AIR is about 5.4 Wm À2 , suggesting that more than half of the surface cooling results from the atmospheric absorption. Sensitivity tests show that an inclusion of MODIS-derived anisotropy of land surface reflection reduces the diurnal variation of TOA solar forcing, because of aerosol-induced changes in the fraction of direct beam and hence in the effective reflection from the surface. Constraining the GOCART dust absorption with recent measurements reduces DF AIR and DF SFC by 1.3 Wm À2 and 0.9 Wm À2 , respectively, and increases the TOA cooling by 0.4 Wm À2 .

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Increased clouds and precipitation normally decrease the diurnal temperature range (DTR) and thus have commonly been offered as explanation for the trend of reduced DTR observed for many land areas over the last several decades. Observations show, however, that the DTR was reduced most in dry regions and especially in the West African Sahel during a period of unprecedented drought. Furthermore, the negative trend of DTR in the Sahel appears to have stopped and may have reversed after the rainfall began to recover. This study develops a hypothesis with climate model sensitivity studies showing that either a reduction in vegetation cover or a reduction in soil emissivity would reduce the DTR by increasing nighttime temperature through increased soil heating and reduced outgoing longwave radiation. Consistent with empirical analyses of observational data, our results suggest that vegetation removal and soil aridation would act to reduce the DTR during periods of drought and human mismanagement over semiarid regions such as the Sahel and to increase the DTR with more rainfall and better human management. Other mechanisms with similar effects on surface energy balance, such as increased nighttime downward longwave radiation due to increased greenhouse gases, aerosols, and clouds, would also be expected to have a larger impact on DTR over drier regions.drought ͉ surface emissivity ͉ longwave radiation ͉ sensitivity test ͉ surface energy balance T he global mean land surface air temperature has been steadily rising since the 1950s, primarily attributed to increased greenhouse gases, and this rising results largely from nighttime warming over some land areas as minimum temperature (T min ) increased much faster than maximum temperature (T max ) (1). Associated with such asymmetric warming is the reduction of the diurnal temperature range (DTR) (DTR ϭ T max Ϫ T min ) (1). The decrease in DTR has been observed over most locations over land since 1950 (2), but in many parts of the world this decline abated in the early 1980s (3), perhaps coincident with the reversal of global dimming (4). The DTR appears to continue to decrease in some rapidly industrializing locations such as southern China (5).Changes in DTR can result from a number of mechanisms, all connected to the surface energy balance. The most obvious and widely recognized mechanism from a meteorological viewpoint is the reduction of DTR as a consequence of the increase of clouds, precipitation, and soil moisture (6, 7). Clouds, especially low clouds, decrease the daytime surface solar heating and increase nighttime downward longwave radiation. Evapotranspiration associated with soil moisture and precipitation can balance solar heating and further reduce T max . These day-to-day changes in DTR aggregate to seasonal to multidecadal statistical connections between DTR and clouds/precipitation/soil moisture (7).Other mechanisms such as changes in atmospheric circulation, greenhouse gases, aerosols, and land cover/use may also contribute to a decrease of the DTR (6,8,...