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Marcos Heil Costa and Jonathan A. Foley

Abstract

It is generally expected that the Amazon basin will experience at least two major environmental changes during the next few decades and centuries: 1) increasing areas of forest will be converted to pasture and cropland, and 2) concentrations of atmospheric CO2 will continue to rise. In this study, the authors use the National Center for Atmospheric Research GENESIS atmospheric general circulation model, coupled to the Integrated Biosphere Simulator, to determine the combined effects of large-scale deforestation and increased CO2 concentrations (including both physiological and radiative effects) on Amazonian climate.

In these simulations, deforestation decreases basin-average precipitation by 0.73 mm day−1 over the basin, as a consequence of the general reduction in vertical motion above the deforested area (although there are some small regions with increased vertical motion). The overall effect of doubled CO2 concentrations in Amazonia is an increase in basin-average precipitation of 0.28 mm day−1. The combined effect of deforestation and doubled CO2, including the interactions among the processes, is a decrease in the basin-average precipitation of 0.42 mm day−1. While the effects of deforestation and increasing CO2 concentrations on precipitation tend to counteract one another, both processes work to warm the Amazon basin. The effect of deforestation and increasing CO2 concentrations both tend to increase surface temperature, mainly because of decreases in evapotranspiration and the radiative effect of CO2. The combined effect of deforestation and doubled CO2, including the interactions among the processes, increases the basin-average temperature by roughly 3.5°C.

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Christine Delire, Jonathan A. Foley, and Starley Thompson

Abstract

A fully coupled atmosphere–biosphere model, version 3 of the NCAR Community Climate Model (CCM3) and the Integrated Biosphere Simulator (IBIS), is used to illustrate how vegetation dynamics may be capable of producing long-term variability in the climate system, particularly through the hydrologic cycle and precipitation. Two simulations of the global climate are conducted with fixed climatological sea surface temperatures: one including vegetation as a dynamic boundary condition, and the other keeping vegetation cover fixed. A comparison of the precipitation power spectra over land from these two simulations shows that dynamic interactions between the atmosphere and vegetation enhance precipitation variability at time scales from a decade to a century, while damping variability at shorter time scales.

In these simulations, the two-way coupling between the atmosphere and the dynamic vegetation cover introduces persistent precipitation anomalies in several ecological transition zones: between forest and grasslands in the North American midwest, in southern Africa, and at the southern limit of the tropical forest in the Amazon basin, and between savanna and desert in the Sahel, Australia, and portions of the Arabian Peninsula. These regions contribute most to the long-term variability of the atmosphere–vegetation system.

Slow changes in the vegetation cover, resulting from a “red noise” integration of high-frequency atmospheric variability, are responsible for generating this long-term variability. Lead and lag correlation between precipitation and vegetation leaf area index (LAI) shows that LAI influences precipitation in the following years, and vice versa. A mechanism involving changes in LAI resulting in albedo, roughness, and evapotranspiration changes is proposed.

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Samuel Levis, Jonathan A. Foley, and David Pollard

Abstract

Changes in vegetation cover are known to influence the climate system by modifying the radiative, momentum, and hydrologic balance of the land surface. To explore the interactions between terrestrial vegetation and the atmosphere for doubled atmospheric CO2 concentrations, the newly developed fully coupled GENESIS–IBIS climate–vegetation model is used. The simulated climatic response to the radiative and physiological effects of elevated CO2 concentrations, as well as to ensuing simulated shifts in global vegetation patterns is investigated.

The radiative effects of elevated CO2 concentrations raise temperatures and intensify the hydrologic cycle on the global scale. In response, soil moisture increases in the mid- and high latitudes by 4% and 5%, respectively. Tropical soil moisture, however, decreases by 5% due to a decrease in precipitation minus evapotranspiration.

The direct, physiological response of plants to elevated CO2 generally acts to weaken the earth’s hydrologic cycle by lowering transpiration rates across the globe. Lowering transpiration alone would tend to enhance soil moisture. However, reduced recirculation of water in the atmosphere, which lowers precipitation, leads to more arid conditions overall (simulated global soil moisture decreases by 1%), particularly in the Tropics and midlatitudes.

Allowing structural changes in the vegetation cover (in response to changes in climate and CO2 concentrations) overrides the direct physiological effects of CO2 on vegetation in many regions. For example, increased simulated forest cover in the Tropics enhances canopy evapotranspiration overall, offsetting the decreased transpiration due to lower leaf conductance. As a result of increased circulation of moisture through the hydrologic cycle, precipitation increases and soil moisture returns to the value simulated with just the radiative effects of elevated CO2. However, in the highly continental midlatitudes, changes in vegetation cover cause soil moisture to decline by an additional 2%. Here, precipitation does not respond sufficiently to increased plant-water uptake, due to a limited source of external moisture into the region.

These results illustrate that vegetation feedbacks may operate differently according to regional characteristics of the climate and vegetation cover. In particular, it is found that CO2 fertilization can cause either an increase or a decrease in available soil moisture, depending on the associated changes in vegetation cover and the ability of the regional climate to recirculate water vapor. This is in direct contrast to the view that CO2 fertilization will enhance soil moisture and runoff across the globe: a view that neglects changes in vegetation structure and local climatic feedbacks.

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Tracy E. Twine, Christopher J. Kucharik, and Jonathan A. Foley

Abstract

Climatic and hydrologic observations and results from a terrestrial ecosystem model coupled to a regional-scale river-routing algorithm are used to document the associations between the El Niño–Southern Oscillation (ENSO) phenomenon and anomalies in climate, surface water balance, and river hydrology within the Mississippi River basin. While no ENSO signal is found in streamflow at the outlet of the basin in Vicksburg, Mississippi, significant anomalies in all water balance components are found in certain regions within the basin. ENSO is mainly associated with positive winter temperature anomalies, but hydrologic patterns vary with season, location, and ENSO phase. El Niño precipitation anomalies tend to affect evapotranspiration (ET) in the western half of the basin and runoff in the eastern half. La Niña events are associated with ET anomalies in the central portion of the basin and runoff anomalies in the southern and eastern portions of the basin. Both ENSO phases are associated with decreased snow depth. Anomalous soil moisture patterns occur at seasonal time scales and filter noisier spatial patterns of precipitation anomalies into coherent patterns with larger field significance; however, for all water budget components, there is a large amount of variability in response within a particular ENSO phase. With anomalies that are up to 4 times those of a typical event, it is clear that improved predictability of the onset and strength of an upcoming ENSO event is important for both water resource management and disaster mitigation.

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Tracy E. Twine, Christopher J. Kucharik, and Jonathan A. Foley

Abstract

The effects of land cover change on the energy and water balance of the Mississippi River basin are analyzed using the Integrated Biosphere Simulator (IBIS) model. Results of a simulated conversion from complete forest cover to crop cover over a single model grid cell show that annual average net radiation and evapotranspiration decrease, while total runoff increases. The opposite effects are found when complete grass cover is replaced with crop cover. Basinwide energy and water balance changes are then analyzed after simulated land cover change from potential vegetation to the current cover (natural vegetation and crops). In general, net radiation decreases over crops converted from forest and increases over crops converted from grasslands. Evapotranspiration rates decrease over summer crops (corn and soybean) converted from forest and increase over summer crops converted from grassland. The largest decreases (∼0.75 mm day−1; 20%) are found in summer over former forests, and the largest increases (∼0.4 mm day−1; 45%) are found in spring over former northern grasslands. Drainage rates increase over summer crops converted from savanna and forest and decrease over summer crops converted from grasslands. The largest increases (∼0.6 mm day−1; 45%) are found in winter over summer crops in former southern forests, and the largest decreases (∼0.4 mm day−1; 25%) are found in summer over summer crops grown in former northern grasslands. The simulated energy and water balance changes resulting from land cover change depend on season, crop type (winter, spring, or summer plantings) and management, and the type of natural vegetation that is removed.

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Justin E. Bagley, Ankur R. Desai, Paul C. West, and Jonathan A. Foley

Abstract

The impacts of changing land cover on the soil–vegetation–atmosphere system are numerous. With the fraction of land used for farming and grazing expected to increase, extensive alterations to land cover such as replacing forests with cropland will continue. Therefore, quantifying the impact of global land-cover scenarios on the biosphere is critical. The Predicting Ecosystem Goods and Services Using Scenarios boundary layer (PegBL) model is a new global soil–vegetation–boundary layer model designed to quantify these impacts and act as a complementary tool to computationally expensive general circulation models and large-eddy simulations. PegBL provides high spatial resolution and inexpensive first-order estimates of land-cover change on the surface energy balance and atmospheric boundary layer with limited input requirements. The model uses a climatological-data-driven land surface model that contains only the physics necessary to accurately reproduce observed seasonal cycles of fluxes and state variables for natural and agricultural ecosystems. A bulk boundary layer model was coupled to the land model to estimate the impacts of changing land cover on the lower atmosphere. The model most realistically simulated surface–atmosphere dynamics and impacts of land-cover change at tropical rain forest and northern boreal forest sites. Further, simple indices to measure the potential impact of land-cover change on boundary layer climate were defined and shown to be dependent on boundary layer dynamics and geographically similar to results from previous studies, which highlighted the impacts of land-cover change on the atmosphere in the tropics and boreal forest.

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Justin E. Bagley, Ankur R. Desai, Keith J. Harding, Peter K. Snyder, and Jonathan A. Foley

Abstract

Expansion of agricultural lands and inherent variability of climate can influence the water cycle in the Amazon basin, impacting numerous ecosystem services. However, these two influences do not work independently of each other. With two once-in-a-century-level droughts occurring in the Amazon in the past decade, it is vital to understand the feedbacks that contribute to altering the water cycle. The biogeophysical impacts of land cover change within the Amazon basin were examined under drought and pluvial conditions to investigate how land cover and drought jointly may have enhanced or diminished recent precipitation extremes by altering patterns and intensity. Using the Weather Research and Forecasting (WRF) Model coupled to the Noah land surface model, a series of April–September simulations representing drought, normal, and pluvial years were completed to assess how land cover change impacts precipitation and how these impacts change under varied rainfall regimes. Evaporative sources of water vapor that precipitate across the region were developed with a quasi-isentropic back-trajectory algorithm to delineate the extent and variability that terrestrial evaporation contributes to regional precipitation. A decrease in dry season latent heat flux and other impacts of deforestation on surface conditions were increased by drought conditions. Coupled with increases in dry season moisture recycling over the Amazon basin by ~7% during drought years, land cover change is capable of reducing precipitation and increasing the amplitude of droughts in the region.

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