[1] Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO 2 concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO 2 in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO 2 with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions.

The response of tropical forests to droughts is highly uncertain 1 . During the dry season, canopy photosynthesis of some tropical forests can decline, whereas in others it can be maintained at the same or a higher level than during the wet season 2 . However, it remains uncertain to what extent water availability is responsible for productivity declines of tropical forests during the dry season 2,3 . Here we use global satellite observations of two independent measures of vegetation photosynthetic properties (enhanced vegetation index from 2002 to 2012 and solar-induced chlorophyll fluorescence from 2007 to 2012) to investigate links between hydroclimate and tropical forest productivity. We find that above an annual rainfall threshold of approximately 2,000 mm yr −1 , the evergreen state is sustained during the dry season in tropical rainforests worldwide, whereas below that threshold, this is not the case. Through a water-budget analysis of precipitation, potential evapotranspiration and satellite measurements of water storage change, we demonstrate that this threshold determines whether the supply of seasonally redistributed subsurface water storage from the wet season can satisfy plant water demands in the subsequent dry season. We conclude that water availability exerts a first-order control on vegetation seasonality in tropical forests globally. Our framework can also help identify where tropical forests may be vulnerable or resilient to future hydroclimatic changes.Photosynthetic metabolism in tropical forests controls ecosystem carbon uptake from the atmosphere, and it also influences critical ecosystem services, including carbon storage 4 , freshwater delivery 5 , maintenance of biodiversity 5 , and regulation of regional and global climate 6 . The photosynthetic metabolism of many tropical forests exhibits a recurring seasonality 2,7 . Understanding how climate influences these seasonal dynamics is an essential prerequisite for realistically predicting tropical forest responses to inter-annual and longer-term climate variation and change 3 . In particular, with a wide spectrum of varying total annual precipitation and dry-season length in the tropics ( Supplementary Fig. 7), the extent to which seasonality of vegetation productivity in tropical forests responds to water limitation remains unclear 2,3 . Although tropical forest seasonal dynamics have been studied at site and regional scales using eddy flux-tower networks and/or satellite remote sensing in Amazonia 2,8,9 , Insular Southeast (SE) Asia 7 and Africa 10 , a globally consistent functional inter-comparison of tropical forests is lacking. Thus, in this paper we address the following questions: What is the extent to which the seasonality of vegetation photosynthesis is limited by water availability in global tropical forests? Are there critical environmental thresholds that explain these seasonal variations? If so, what are the underlying physical mechanisms? What are the implications of such mechanisms on the future of tropical forests under cli...

Using •13C measurements in atmospheric CO 2 from a cooperative global air sampling network, we determined the partitioning of the net uptake of CO2 between ocean and land as a function of latitude and time. The majority of •13C measurements were made at the Institute of Arctic and Alpine Research (INSTAAR) of the University of Colorado. The network included 40 sites in 1992 and constitutes the most extensive data set available. We perform an inverse deconvolution of both CO2 and •13C observations, using a two-dimensional model of atmospheric transport. New features of the method include a detailed calculation of the isotopic disequilibrium of the terrestrial biosphere from global runs of the CENTURY soil model. Also, the discrimination against •3C by plant photosynthesis, as a function of latitude and time,is calculated from global runs of the SiB biosphere model. Uncertainty due to the longitudinal structure of the data, which is not represented by the model, is studied through a bootstrap analysis by adding and omitting measurement sites. The resulting error estimates for our inferred sources and sinks are of the order of 1 GTC (1 GTC = 10 •5 gC). Such error bars do not reflect potential systematic errors arising from our estimates of the isotopic disequilibria between the atmosphere and the oceans and biosphere, which are estimated in a separate sensitivity analysis. With respect to global totals for 1992 we found that 3.1 GTC of carbon dissolved into the ocean and that 1.5 GTC were sequestered by land ecosystems. Northern hemisphere ocean gyres north of 15øN absorbed 2.7 GTC. The equatorial oceans between 10øS and 10øN were a net source to the atmosphere of 0.9 GTC. We obtained a sink of 1.6 GTC in southern ocean gyres south of 20øS, although the deconvolution is poorly constrained by sparse data coverage at high southern latitudes. The seasonal uptake of CO2 in northern gyres appears to be correlated with a bloom of phytoplankton in surface waters. On land, northern temperate and boreal ecosystems between 35øN and 65øN were found to be a major sink of CO2 in 1992, as large as 3.5 GTC. Northern tropical ecosystems (equator-30øN) appear to be a net source to the atmosphere of 2 GTC which could reflect biomass burning. A small sink, 0.3 GTC, was inferred for southern tropical ecosystems (30øS-equator). 1.Paper number 94JD02847. 0148-0227/95/94JD-02847505.00 burning have increased the concentration of this gas in the atmosphere by approximately 80 ppm. However, this increased CO2 burden represents only about 50% of the cumulative loading due to fossil fuels alone. This demonstrates that strong natural sinks are currently absorbing atmospheric CO2 at the surface of the Earth. Both the oceans and the terrestrial ecosystems can absorb CO2 and store large quantities of carbon on this timescale. The ocean dissolves CO2 through air-sea exchange processes and stores it in deep waters. Terrestrial ecosystems can store carbon if plant photosynthesis exceeds the release to the atmosphere by respiration. The long-term stor...

Abstract. The goal of the Boreal Ecosystem-Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified. IntroductionPersuasive arguments indicate that there will be global warming resulting from the continuing increase in atmospheric CO 2 concentration [Houghton et al., 1995; Hasselmann, 1997].However, there are uncertainties about the magnitude and regional patterns of projected global change because of shortcomings in the atmospheric general circulation models (AGCMs) used for climate simulation. There is a real need to improve (1) our understanding of basic climatic physical and dynamic processes so that we can enhance the realism and accuracy of AGCMs and (2) our ability to quantify global-scale climate variables and parameters to better initialize and vali- The exchanges of energy, water, and carbon between the atmosphere and the continents represent the lower boundary condition for the atmospheric physical climate system and the climatic forcing to terrestrial biota and biogeochemical cycles.• •øCanada Center for Remote Sensing, Ottawa.• •Atmospheric Sciences Resource Center, Albany, New York.•2University of Edinburgh, Edinburgh, Scotland.•3University of Wisconsin, Madison.•4Forestry Canada, Edmonton, Alberta, Canada.•5Atmospheric Environment Service, Downsview, Ontario, Canada.•'NASA Headquarters, Washington, D.C.

Biogeochemical models must include a broad variety of biological and physical processes to test our understanding of the terrestrial carbon cycle and to predict ecosystem biomass and carbon fluxes. We combine the photosynthesis and biophysical calculations in the Simple Biosphere model, Version 2.5 (SiB2.5) with the biogeochemistry from the Carnegie‐Ames‐Stanford Approach (CASA) model to create SiBCASA, a hybrid capable of estimating terrestrial carbon fluxes and biomass from diurnal to decadal timescales. We add dynamic allocation of Gross Primary Productivity to the growth and maintenance of leaves, roots, and wood and explicit calculation of autotrophic respiration. We prescribe leaf biomass using Leaf Area Index (LAI) derived from remotely sensed Normalized Difference Vegetation Index. Simulated carbon fluxes and biomass are consistent with observations at selected eddy covariance flux towers in the AmeriFlux network. Major sources of error include the steady state assumption for initial pool sizes, the input weather data, and biases in the LAI.