Thornton Peter E.

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Peter E.

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  • Article
    Preindustrial-control and twentieth-century carbon cycle experiments with the Earth System Model CESM1(BGC)
    (American Meteorological Society, 2014-12-15) Lindsay, Keith ; Bonan, Gordon B. ; Doney, Scott C. ; Hoffman, Forrest M. ; Lawrence, David M. ; Long, Matthew C. ; Mahowald, Natalie M. ; Moore, J. Keith ; Randerson, James T. ; Thornton, Peter E.
    Version 1 of the Community Earth System Model, in the configuration where its full carbon cycle is enabled, is introduced and documented. In this configuration, the terrestrial biogeochemical model, which includes carbon–nitrogen dynamics and is present in earlier model versions, is coupled to an ocean biogeochemical model and atmospheric CO2 tracers. The authors provide a description of the model, detail how preindustrial-control and twentieth-century experiments were initialized and forced, and examine the behavior of the carbon cycle in those experiments. They examine how sea- and land-to-air CO2 fluxes contribute to the increase of atmospheric CO2 in the twentieth century, analyze how atmospheric CO2 and its surface fluxes vary on interannual time scales, including how they respond to ENSO, and describe the seasonal cycle of atmospheric CO2 and its surface fluxes. While the model broadly reproduces observed aspects of the carbon cycle, there are several notable biases, including having too large of an increase in atmospheric CO2 over the twentieth century and too small of a seasonal cycle of atmospheric CO2 in the Northern Hemisphere. The biases are related to a weak response of the carbon cycle to climatic variations on interannual and seasonal time scales and to twentieth-century anthropogenic forcings, including rising CO2, land-use change, and atmospheric deposition of nitrogen.
  • Article
    Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks : results from an atmosphere-ocean general circulation model
    (Copernicus Publications on behalf of the European Geosciences Union, 2009-10-08) Thornton, Peter E. ; Doney, Scott C. ; Lindsay, Keith ; Moore, J. Keith ; Mahowald, Natalie M. ; Randerson, James T. ; Fung, Inez Y. ; Lamarque, J.-F. ; Feddema, J. J. ; Lee, Y.-H.
    Inclusion of fundamental ecological interactions between carbon and nitrogen cycles in the land component of an atmosphere-ocean general circulation model (AOGCM) leads to decreased carbon uptake associated with CO2 fertilization, and increased carbon uptake associated with warming of the climate system. The balance of these two opposing effects is to reduce the fraction of anthropogenic CO2 predicted to be sequestered in land ecosystems. The primary mechanism responsible for increased land carbon storage under radiatively forced climate change is shown to be fertilization of plant growth by increased mineralization of nitrogen directly associated with increased decomposition of soil organic matter under a warming climate, which in this particular model results in a negative gain for the climate-carbon feedback. Estimates for the land and ocean sink fractions of recent anthropogenic emissions are individually within the range of observational estimates, but the combined land plus ocean sink fractions produce an airborne fraction which is too high compared to observations. This bias is likely due in part to an underestimation of the ocean sink fraction. Our results show a significant growth in the airborne fraction of anthropogenic CO2 emissions over the coming century, attributable in part to a steady decline in the ocean sink fraction. Comparison to experimental studies on the fate of radio-labeled nitrogen tracers in temperate forests indicates that the model representation of competition between plants and microbes for new mineral nitrogen resources is reasonable. Our results suggest a weaker dependence of net land carbon flux on soil moisture changes in tropical regions, and a stronger positive growth response to warming in those regions, than predicted by a similar AOGCM implemented without land carbon-nitrogen interactions. We expect that the between-model uncertainty in predictions of future atmospheric CO2 concentration and associated anthropogenic climate change will be reduced as additional climate models introduce carbon-nitrogen cycle interactions in their land components.
  • Article
    Representing the function and sensitivity of coastal interfaces in earth system models
    (Nature Research, 2020-05-18) Ward, Nicholas D. ; Megonigal, J. Patrick ; Bond-Lamberty, Benjamin ; Bailey, Vanessa L. ; Butman, David ; Canuel, Elizabeth A. ; Diefenderfer, Heida ; Ganju, Neil K. ; Goni, Miguel ; Graham, Emily B. ; Hopkinson, Charles S. ; Khangaonkar, Tarang ; Langley, J. Adam ; McDowell, Nate G. ; Myers-Pigg, Allison N. ; Neumann, Rebecca B. ; Osburn, Christopher L. ; Price, René M. ; Rowland, Joel ; Sengupta, Aditi ; Simard, Marc ; Thornton, Peter E. ; Tzortziou, Maria ; Vargas, Rodrigo ; Weisenhorn, Pamela B. ; Windham-Myers, Lisamarie
    Between the land and ocean, diverse coastal ecosystems transform, store, and transport material. Across these interfaces, the dynamic exchange of energy and matter is driven by hydrological and hydrodynamic processes such as river and groundwater discharge, tides, waves, and storms. These dynamics regulate ecosystem functions and Earth’s climate, yet global models lack representation of coastal processes and related feedbacks, impeding their predictions of coastal and global responses to change. Here, we assess existing coastal monitoring networks and regional models, existing challenges in these efforts, and recommend a path towards development of global models that more robustly reflect the coastal interface.
  • Article
    Desert dust and anthropogenic aerosol interactions in the Community Climate System Model coupled-carbon-climate model
    (Copernicus Publications on behalf of the European Geosciences Union, 2011-02-15) Mahowald, Natalie M. ; Lindsay, Keith ; Rothenberg, D. ; Doney, Scott C. ; Moore, J. Keith ; Thornton, Peter E. ; Randerson, James T. ; Jones, C. D.
    Coupled-carbon-climate simulations are an essential tool for predicting the impact of human activity onto the climate and biogeochemistry. Here we incorporate prognostic desert dust and anthropogenic aerosols into the CCSM3.1 coupled carbon-climate model and explore the resulting interactions with climate and biogeochemical dynamics through a series of transient anthropogenic simulations (20th and 21st centuries) and sensitivity studies. The inclusion of prognostic aerosols into this model has a small net global cooling effect on climate but does not significantly impact the globally averaged carbon cycle; we argue that this is likely to be because the CCSM3.1 model has a small climate feedback onto the carbon cycle. We propose a mechanism for including desert dust and anthropogenic aerosols into a simple carbon-climate feedback analysis to explain the results of our and previous studies. Inclusion of aerosols has statistically significant impacts on regional climate and biogeochemistry, in particular through the effects on the ocean nitrogen cycle and primary productivity of altered iron inputs from desert dust deposition.
  • Article
    Atmospheric carbon dioxide variability in the Community Earth System Model : evaluation and transient dynamics during the twentieth and twenty-first centuries
    (American Meteorological Society, 2013-07-01) Keppel-Aleks, Gretchen ; Randerson, James T. ; Lindsay, Keith ; Stephens, Britton B. ; Moore, J. Keith ; Doney, Scott C. ; Thornton, Peter E. ; Mahowald, Natalie M. ; Hoffman, Forrest M. ; Sweeney, Colm ; Tans, Pieter P. ; Wennberg, Paul O. ; Wofsy, Steven C.
    Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.
  • Article
    Observed 20th century desert dust variability : impact on climate and biogeochemistry
    (Copernicus Publications on behalf of the European Geosciences Union, 2010-11-19) Mahowald, Natalie M. ; Kloster, S. ; Engelstaedter, S. ; Moore, J. Keith ; Mukhopadhyay, S. ; McConnell, Joseph R. ; Albani, S. ; Doney, Scott C. ; Bhattacharya, A. ; Curran, M. A. J. ; Flanner, M. G. ; Hoffman, Forrest M. ; Lawrence, David M. ; Lindsay, Keith ; Mayewski, P. A. ; Neff, Jason C. ; Rothenberg, D. ; Thomas, E. ; Thornton, Peter E. ; Zender, Charles S.
    Desert dust perturbs climate by directly and indirectly interacting with incoming solar and outgoing long wave radiation, thereby changing precipitation and temperature, in addition to modifying ocean and land biogeochemistry. While we know that desert dust is sensitive to perturbations in climate and human land use, previous studies have been unable to determine whether humans were increasing or decreasing desert dust in the global average. Here we present observational estimates of desert dust based on paleodata proxies showing a doubling of desert dust during the 20th century over much, but not all the globe. Large uncertainties remain in estimates of desert dust variability over 20th century due to limited data. Using these observational estimates of desert dust change in combination with ocean, atmosphere and land models, we calculate the net radiative effect of these observed changes (top of atmosphere) over the 20th century to be −0.14 ± 0.11 W/m2 (1990–1999 vs. 1905–1914). The estimated radiative change due to dust is especially strong between the heavily loaded 1980–1989 and the less heavily loaded 1955–1964 time periods (−0.57 ± 0.46 W/m2), which model simulations suggest may have reduced the rate of temperature increase between these time periods by 0.11 °C. Model simulations also indicate strong regional shifts in precipitation and temperature from desert dust changes, causing 6 ppm (12 PgC) reduction in model carbon uptake by the terrestrial biosphere over the 20th century. Desert dust carries iron, an important micronutrient for ocean biogeochemistry that can modulate ocean carbon storage; here we show that dust deposition trends increase ocean productivity by an estimated 6% over the 20th century, drawing down an additional 4 ppm (8 PgC) of carbon dioxide into the oceans. Thus, perturbations to desert dust over the 20th century inferred from observations are potentially important for climate and biogeochemistry, and our understanding of these changes and their impacts should continue to be refined.