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Jeffrey A. Hicke, David B. Lobell, and Gregory P. Asner

Abstract

Croplands cover large areas of the globe and contribute significantly to the global carbon cycle. However, like other ecosystems, limited information exists on spatially explicit, ground-based estimates of carbon fluxes. In this study, county-level cropland area and harvest information reported in the U.S. Department of Agriculture (USDA) National Agricultural Statistics Service (NASS) from 1972 to 2001 was utilized to calculate the temporal behavior of net primary production (NPP) for croplands across the United States. Production data for individual crops were converted to estimates of NPP using crop-specific factors. Because NASS does not include all crops of interest during all years, only a crop type in a county estimate was included if the entire time series was complete. Incomplete reporting occurred primarily with hay. Trends in crop area, NPP, and total production (area times NPP) exhibited significant spatial variation. The largest increases in production occurred in the Midwest, Great Plains, and Mississippi River Valley regions. Cropland area exhibited a range of trends from large percent increases in counties across the Great Plains and the West to decreases across the South. Generally, NPP increased in counties throughout the United States and for the country as a whole. It was estimated that total coterminous cropland production increased during 1972–2001 from 0.37 to 0.53 Pg C yr−1, a 40% increase over 1972 values. Since total cropland area changed little during the 30-yr period, production increases were driven primarily by gains in NPP.

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David B. Lobell, Céline Bonfils, and Jean-Marc Faurès

Abstract

Expansion of irrigated land can cause local cooling of daytime temperatures by up to several degrees Celsius. Here the authors compare the expected cooling associated with rates of irrigation expansion in developing countries for historical (1961–2000) and future (2000–30) periods with climate model predictions of temperature changes from other forcings, most notably increased atmospheric greenhouse gas levels, over the same periods. Indirect effects of irrigation on climate, via methane production in paddy rice systems, were not considered. In regions of rapid irrigation growth over the past 40 yr, such as northwestern India and northeastern China, irrigation’s expected cooling effects have been similar in magnitude to climate model predictions of warming from greenhouse gases. A masking effect of irrigation can therefore explain the lack of significant increases in observed growing season maximum temperatures in these regions and the apparent discrepancy between observations and climate model simulations. Projections of irrigation for 2000–30 indicate a slowing of expansion rates, and therefore cooling from irrigation expansion over this time period will very likely be smaller than in recent decades. At the same time, warming from greenhouse gases will likely accelerate, and irrigation will play a relatively smaller role in agricultural climate trends. In many irrigated regions, therefore, temperature projections from climate models, which generally ignore irrigation, may be more accurate in predicting future temperature trends than their performance in reproducing past observed trends in irrigated regions would suggest.

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Scott R. Loarie, David B. Lobell, Gregory P. Asner, and Christopher B. Field

Abstract

Albedo is an important factor affecting global climate, but uncertainty in the sources and magnitudes of albedo change has led to simplistic treatments of albedo in climate models. Here, the authors examine nine years (2000–08) of historical 1-km Moderate Resolution Imaging Spectroradiometer (MODIS) albedo estimates across South America to advance understanding of the magnitude and sources of large-scale albedo changes. The authors use the magnitude of albedo change from the arc of deforestation along the southeastern edge of the Brazilian Amazon (+2.8%) as a benchmark for comparison. Large albedo increases (>+2.8%) were 2.2 times more prevalent than similar decreases throughout South America. Changes in surface water drove most large albedo changes that were not caused by vegetative cover change. Decreased surface water in the Santa Fe and Buenos Aires regions of Argentina was responsible for albedo increases exceeding that of the arc of deforestation in magnitude and extent. Although variations in the natural flooding regimes were likely the dominant mechanism driving changes in surface water, it is possible that human manipulations through dams and other agriculture infrastructure contributed. This study demonstrates the substantial role that land-cover and surface water change can play in continental-scale albedo trends and suggests ways to better incorporate these processes into global climate models.

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