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Christopher E. Doughty, Scott R. Loarie, and Christopher B. Field

Abstract

South America has undergone a large increase in albedo over the past decade as forests have been converted to crops and wetlands have been drained. Recent modeling literature and paleoclimate precipitation proxies have highlighted how changes in surface energy balance could affect the position of the intertropical convergence zone (ITCZ) in South America. Here, the authors investigate whether large continental increases in albedo in South America can likewise affect the southward migration of the ITCZ into South America using the NCAR Community Atmosphere Model, version 3.0 (CAM3.0) coupled with the Community Land Model, version 3.5 (CLM3.5) and a slab ocean model. Moderate Resolution Imaging Spectroradiometer (MODIS) albedo data show that between 2001 and 2008 average albedo increased by 0.0025 albedo units across all South America and by 0.0032 albedo units between 0° and 24° latitude in South America and, because of this effect, the authors’ simulations estimate an average ~23 mm yr−1 decrease in rainfall in the southern migration of the ITCZ (SMI) and an average ~9 mm yr−1 decrease in the entire Amazon basin. Large increases in albedo in South America decrease the northward atmospheric energy transport at the equator during the months the region of increased albedo is south of the ITCZ (May–July), leading to an apparent delay in its arrival to the SMI region and reduced rainfall in this region. However, because changing albedo is often associated with changing surface roughness, the authors model this separately and find that decreased surface roughness will have an opposite, increasing effect on precipitation. Therefore, they expect increasing albedo in South America associated with the drainage of wetlands to decrease precipitation, especially in the SMI region; however, in the case of deforestation, some of the decrease in precipitation from increased albedo may be offset by a corresponding decrease in surface roughness.

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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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Robert E. Dickinson, Joseph A. Berry, Gordon B. Bonan, G. James Collatz, Christopher B. Field, Inez Y. Fung, Michael Goulden, William A. Hoffmann, Robert B. Jackson, Ranga Myneni, Piers J. Sellers, and Muhammad Shaikh

Abstract

Most evapotranspiration over land occurs through vegetation. The fraction of net radiation balanced by evapotranspiration depends on stomatal controls. Stomates transpire water for the leaf to assimilate carbon, depending on the canopy carbon demand, and on root uptake, if it is limiting. Canopy carbon demand in turn depends on the balancing between visible photon-driven and enzyme-driven steps in the leaf carbon physiology. The enzyme-driven component is here represented by a Rubisco-related nitrogen reservoir that interacts with plant–soil nitrogen cycling and other components of a climate model. Previous canopy carbon models included in GCMs have assumed either fixed leaf nitrogen, that is, prescribed photosynthetic capacities, or an optimization between leaf nitrogen and light levels so that in either case stomatal conductance varied only with light levels and temperature.

A nitrogen model is coupled to a previously derived but here modified carbon model and includes, besides the enzyme reservoir, additional plant stores for leaf structure and roots. It also includes organic and mineral reservoirs in the soil; the latter are generated, exchanged, and lost by biological fixation, deposition and fertilization, mineralization, nitrification, root uptake, denitrification, and leaching. The root nutrient uptake model is a novel and simple, but rigorous, treatment of soil transport and root physiological uptake. The other soil components are largely derived from previously published parameterizations and global budget constraints.

The feasibility of applying the derived biogeochemical cycling model to climate model calculations of evapotranspiration is demonstrated through its incorporation in the Biosphere–Atmosphere Transfer Scheme land model and a 17-yr Atmospheric Model Inter comparison Project II integration with the NCAR CCM3 GCM. The derived global budgets show land net primary production (NPP), fine root carbon, and various aspects of the nitrogen cycling are reasonably consistent with past studies. Time series for monthly statistics averaged over model grid points for the Amazon evergreen forest and lower Colorado basin demonstrate the coupled interannual variability of modeled precipitation, evapotranspiration, NPP, and canopy Rubisco enzymes.

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