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      Large influence of soil moisture on long-term terrestrial carbon uptake


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          The terrestrial biosphere absorbs about 25% of anthropogenic CO 2 emissions, yet the rate of land carbon uptake remains highly uncertain, leading to uncertainties in climate projections 1, 2 . Understanding the factors that are limiting or driving land carbon storage is therefore important for improved climate predictions. One potential limiting factor for land carbon uptake is soil moisture, which can reduce gross primary production due to ecosystem water stress 3, 4 , cause vegetation mortality 5 , and further exacerbate climate extremes due to land-atmosphere feedbacks 6 . Previous work has explored the impact of soil moisture availability on past carbon flux variability 3, 7, 8 . However, the magnitude of the effect of soil moisture variability and trends on the long-term carbon sink and the mechanisms responsible for associated carbon losses remain uncertain. Here we use four global land-atmosphere models 9 , and find that soil moisture variability and trends induce large CO 2 sources (~2–3 GtC/year) throughout the twenty-first century; on the order of the land carbon sink itself 1 . Subseasonal and interannual soil moisture variability generates a CO 2 source as a result of the nonlinear response of photosynthesis and net ecosystem exchange to soil water availability and the increased temperature and vapour pressure deficit caused by land-atmosphere interactions. Soil moisture variability reduces the present land carbon sink while soil moisture variability and its drying trend reduce it in the future. Our results emphasize that the capacity of continents to act as a future carbon sink critically depends on the nonlinear response of carbon fluxes to soil moisture and on land-atmosphere interactions. This suggests that with the drying trend and increase in soil moisture variability projected in several regions, the current carbon uptake rate may not be sustained past mid-century and could result in an accelerated atmospheric CO 2 growth rate.

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          Most cited references45

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          An Overview of CMIP5 and the Experiment Design

          The fifth phase of the Coupled Model Intercomparison Project (CMIP5) will produce a state-of-the- art multimodel dataset designed to advance our knowledge of climate variability and climate change. Researchers worldwide are analyzing the model output and will produce results likely to underlie the forthcoming Fifth Assessment Report by the Intergovernmental Panel on Climate Change. Unprecedented in scale and attracting interest from all major climate modeling groups, CMIP5 includes “long term” simulations of twentieth-century climate and projections for the twenty-first century and beyond. Conventional atmosphere–ocean global climate models and Earth system models of intermediate complexity are for the first time being joined by more recently developed Earth system models under an experiment design that allows both types of models to be compared to observations on an equal footing. Besides the longterm experiments, CMIP5 calls for an entirely new suite of “near term” simulations focusing on recent decades and the future to year 2035. These “decadal predictions” are initialized based on observations and will be used to explore the predictability of climate and to assess the forecast system's predictive skill. The CMIP5 experiment design also allows for participation of stand-alone atmospheric models and includes a variety of idealized experiments that will improve understanding of the range of model responses found in the more complex and realistic simulations. An exceptionally comprehensive set of model output is being collected and made freely available to researchers through an integrated but distributed data archive. For researchers unfamiliar with climate models, the limitations of the models and experiment design are described.
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            Land-atmosphere coupling and climate change in Europe.

            Increasing greenhouse gas concentrations are expected to enhance the interannual variability of summer climate in Europe and other mid-latitude regions, potentially causing more frequent heatwaves. Climate models consistently predict an increase in the variability of summer temperatures in these areas, but the underlying mechanisms responsible for this increase remain uncertain. Here we explore these mechanisms using regional simulations of recent and future climatic conditions with and without land-atmosphere interactions. Our results indicate that the increase in summer temperature variability predicted in central and eastern Europe is mainly due to feedbacks between the land surface and the atmosphere. Furthermore, they suggest that land-atmosphere interactions increase climate variability in this region because climatic regimes in Europe shift northwards in response to increasing greenhouse gas concentrations, creating a new transitional climate zone with strong land-atmosphere coupling in central and eastern Europe. These findings emphasize the importance of soil-moisture-temperature feedbacks (in addition to soil-moisture-precipitation feedbacks) in influencing summer climate variability and the potential migration of climate zones with strong land-atmosphere coupling as a consequence of global warming. This highlights the crucial role of land-atmosphere interactions in future climate change.
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              Atmospheric component of the MPI-M Earth System Model: ECHAM6


                Author and article information

                1 December 2018
                23 January 2019
                January 2019
                23 July 2019
                : 565
                : 7740
                : 476-479
                [1. ]Department of Earth and Environmental Engineering, Columbia University, New York, New York.
                [2. ]Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland.
                [3. ]Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey.
                [4. ]Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey.
                [5. ]Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Geesthacht, Germany.
                [6. ]Climate and Global Dynamics Laboratory, Terrestrial Sciences, National Center for Atmospheric Research, Boulder, Colorado.
                [7. ]The Earth Institute, Columbia University, New York, New York.
                Author notes

                Author Contributions

                JKG, and PG wrote the main manuscript text. JKG prepared the figures. JKG, PG and SIS designed the study. All authors reviewed and edited the manuscript.

                [* ] Corresponding author: Correspondence and requests for materials should be addressed to JKG ( jg3405@ 123456columbia.edu ).

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