122 research outputs found
The climate time scale in the approach to radiative-convective equilibrium
In this paper, we discuss the importance of the surface boundary condition (fixed versus interactive surface temperature) for the long time scale of approach to Radiative-Convective Equilibrium (RCE). Using a simple linearized two-variable model for surface-atmosphere interaction, we derive an analytic expression for τ[subscript C], a long climate relaxation time scale that remains well defined and much longer than either mixing time scale of Tompkins and Craig (1998b), even in the limit that the heat capacity of the surface vanishes. We show that the size of τ[subscript C] is an intrinsic property of the coupling between the atmosphere and surface, and not a result of the thermal inertia of the surface alone. When the surface heat capacity is low, τ[subscript C] can be several times longer than expected, due to the effects of moisture on the effective heat capacity of the atmosphere. We also show that the theoretical expression for τ[subscript C] is a good predictor of best fit exponential relaxation time scales in a single-column model with full physics, across a range of surface temperatures and surface heat capacities.National Science Foundation (U.S.) (Grant 1136480
Aerosol invigoration of atmospheric convection through increases in humidity
Cloud-aerosol interactions remain a major obstacle to understanding climate
and severe weather. Observations suggest that aerosols enhance tropical
thunderstorm activity; past research, motivated by the importance of
understanding aerosol impacts on clouds, has proposed several mechanisms that
could explain that observed link. Here, we show that high-resolution
atmospheric simulations can reproduce the observed link between aerosols and
convection. However, we also show that previously proposed mechanisms are
unable to explain the invigoration. Examining underlying processes reveals
that, in our simulations, high aerosol concentrations increase environmental
humidity by producing clouds that mix more condensed water into the surrounding
air. In turn, higher humidity favors large-scale ascent and stronger
convection. Our results provide a physical reason to expect invigorated
thunderstorms in high-aerosol regions of the tropics.Comment: 10 pages, 4 figures, under review at Scienc
Convective dynamics and the response of precipitation extremes to warming in radiative-convective equilibrium
Tropical precipitation extremes are expected to strengthen with warming, but
quantitative estimates remain uncertain because of a poor understanding of
changes in convective dynamics. This uncertainty is addressed here by analyzing
idealized convection-permitting simulations of radiative-convective equilibrium
in long-channel geometry. Across a wide range of climates, the thermodynamic
contribution to changes in instantaneous precipitation extremes follows
near-surface moisture, and the dynamic contribution is positive and small, but
sensitive to domain size. The shapes of mass flux profiles associated with
precipitation extremes are determined by conditional sampling that favors
strong vertical motion at levels where the vertical saturation specific
humidity gradient is large, and mass flux profiles collapse to a common shape
across climates when plotted in a moisture-based vertical coordinate. The
collapse, robust to changes in microphysics and turbulence schemes, implies a
thermodynamic contribution that scales with near-surface moisture despite
substantial convergence aloft and allows the dynamic contribution to be defined
by the pressure velocity at a single level. Linking the simplified dynamic mode
to vertical velocities from entraining plume models reveals that the small
dynamic mode in channel simulations (<~2 %/K) is caused by opposing
height-dependences of vertical velocity and density, together with the
buffering influence of cloud-base buoyancies that vary little with surface
temperature. These results reinforce an emerging picture of the response of
extreme tropical precipitation rates to warming: a thermodynamic mode of about
7 %/K dominates, with a minor contribution from changes in dynamics.Comment: 28 pages, 15 figures, 1 table. This work has been accepted to Journal
of the Atmospheric Sciences. The AMS does not guarantee that the copy
provided here is an accurate copy of the final published wor
Green House Gas Mitigation Policy, Bio-fuels and Land-use Change- a Dynamic Analysis
Research and Development/Tech Change/Emerging Technologies, Resource /Energy Economics and Policy,
Correction to “Importance of carbon-nitrogen interactions and ozone on ecosystem hydrology during the 21st century”
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 114 (2009): G03009, doi:10.1029/2009JG001083
Unintended Environmental Consequences of a Global Biofuels Program
Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).Biofuels are being promoted as an important part of the global energy mix to meet the climate change challenge. The environmental costs of biofuels produced with current technologies at small scales have been studied, but little research has been done on the consequences of an aggressive global biofuels program with advanced technologies using cellulosic feedstocks. Here, with simulation modeling, we explore two scenarios for cellulosic biofuels production and find that both could contribute substantially to future global-scale energy needs, but with significant unintended environmental consequences. As the land supply is squeezed to make way for vast areas of biofuels crops, the global landscape is defined by either the clearing of large swathes of natural forest, or the intensification of agricultural operations worldwide. The greenhouse gas implications of land-use conversion differ substantially between the two scenarios, but in both, numerous biodiversity hotspots suffer from serious habitat loss. Cellulosic biofuels may yet serve as a crucial wedge in the solution to the climate change problem, but must be deployed with caution so as not to jeopardize biodiversity, compromise ecosystems services, or undermine climate policy.This study received funding from the MIT Joint Program on the Science and Policy of Global Change, which is supported by a onsortium of government, industry and foundation sponsors
Impacts of ozone on trees and crops
Author Posting. © The Author(s), 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Comptes Rendus Geosciences 339 (2007): 784-798, doi:10.1016/j.crte.2007.08.008.In this review article, we explore how surface-level ozone affects trees and crops with special emphasis on consequences for productivity and carbon sequestration. Vegetation exposure to ozone reduces photosynthesis, growth, and other plant functions. Ozone formation in the atmosphere is a product of NOx that are also a source of nitrogen deposition. Reduced carbon sequestration of temperate forests resulting from ozone is likely offset by increased carbon sequestration from nitrogen fertilization. However, since fertilized croplands are generally not nitrogen-limited, capping ozone-polluting substances in the U.S., Europe, and China can reduce future crop yield loss substantially.This study was funded by the Biocomplexity Program of the U.S. National Science Foundation (ATM-0120468), the Methods and Models for Integrated Assessment Program of the U.S. National Science Foundation (DEB-9711626) and the Earth Observing System Program of the U.S. National Aeronautics and Space Administration (NAG5-10135)
Nitrogen effect on carbon-water coupling in forests, grasslands, and shrublands in the arid western United States
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): G03023, doi:10.1029/2010JG001621.As greenhouse gases, including CO2, accumulate in the atmosphere, the western United States is predicted to undergo large-scale climate warming and reduced summer precipitation in the coming decades. In this study we explore the role of these climate changes with elevated CO2 to determine the plant physiological response on primary productivity and associated feedbacks on evapotranspiration (ET) and runoff using a biogeochemistry model, TEM-Hydro, with downscaled climate data for the western United States from the NCAR CCSM3 A2 scenario. Net primary productivity increases by 32% in forests due to feedbacks between warmer temperatures and enhanced nitrogen mineralization but decreases in shrublands by 24% due to excessive drying and reduced nitrogen mineralization. Warming directly increases nitrogen mineralization rates but indirectly decreases them by reducing soil moisture, so the net effect is highly dependent on climatic conditions within each biome. Increased soil moisture resulting from larger water use efficiency from the elevated CO2 leads to more net nitrogen mineralization in forests, which reduces N-limiting conditions. The effect of CO2 on stomatal conductance is therefore enhanced because of its effect on reducing nitrogen limiting conditions. Runoff decreases over the 21st century by 22% in forests, 58% in grasslands, and 67% in shrublands due to the reduced precipitation in each region but is modulated by the plant-induced changes in ET. The role of moisture limitation is therefore a crucial regulator of nitrogen limitation, which determines the future productivity and water availability in the West.This study was funded by the Department
of Energy, Basic Research and Modeling to Support Integrated Assessment,
DE‐FG02‐08ERG64648
Historical carbon emissions and uptake from the agricultural frontier of the Brazilian Amazon
Author Posting. © Ecological Society of America, 2011. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 21 (2011): 750–763, doi:10.1890/09-1957.1.Tropical ecosystems play a large and complex role in the global carbon cycle. Clearing of natural ecosystems for agriculture leads to large pulses of CO2 to the atmosphere from terrestrial biomass. Concurrently, the remaining intact ecosystems, especially tropical forests, may be sequestering a large amount of carbon from the atmosphere in response to global environmental changes including climate changes and an increase in atmospheric CO2. Here we use an approach that integrates census-based historical land use reconstructions, remote-sensing-based contemporary land use change analyses, and simulation modeling of terrestrial biogeochemistry to estimate the net carbon balance over the period 1901–2006 for the state of Mato Grosso, Brazil, which is one of the most rapidly changing agricultural frontiers in the world. By the end of this period, we estimate that of the state's 925 225 km2, 221 092 km2 have been converted to pastures and 89 533 km2 have been converted to croplands, with forest-to-pasture conversions being the dominant land use trajectory but with recent transitions to croplands increasing rapidly in the last decade. These conversions have led to a cumulative release of 4.8 Pg C to the atmosphere, with 80% from forest clearing and 20% from the clearing of cerrado. Over the same period, we estimate that the residual undisturbed ecosystems accumulated 0.3 Pg C in response to CO2 fertilization. Therefore, the net emissions of carbon from Mato Grosso over this period were 4.5 Pg C. Net carbon emissions from Mato Grosso since 2000 averaged 146 Tg C/yr, on the order of Brazil's fossil fuel emissions during this period. These emissions were associated with the expansion of croplands to grow soybeans. While alternative management regimes in croplands, including tillage, fertilization, and cropping patterns promote carbon storage in ecosystems, they remain a small portion of the net carbon balance for the region. This detailed accounting of a region's carbon balance is the type of foundation analysis needed by the new United Nations Collaborative Programmme for Reducing Emissions from Deforestation and Forest Degradation (REDD).This work was supported by NASA’s Earth and Space
Science Fellowship (G. L. Galford) and NASA’s Large-Scale
Biosphere–Atmosphere Experiment in Amazonia (grant number NNG06GE20A)
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