GFDL - Geophysical Fluid Dynamics Laboratory

Highlighted Papers

• Shevliakova, E., S. W. Pacala, S. Malyshev, G. C. Hurtt, P.C.D. Milly, J. P. Caspersen, L. T. Sentman, J. P. Fisk, C. Wirth, and C. Crevoisier (2009), Carbon Cycling under 300 Years of Land-use Change: the Importance of the Secondary Vegetation Sink, Global Biogeochem. Cycles, doi:10.1029/2007GB003176, in press. [ Abstract PDF]

  We have developed a dynamic land model (LM3V) able to simulate ecosystem dynamics and exchanges of water, energy and CO2 between land and atmosphere. LM3V is specifically designed to address the consequences of land-use/land-management changes including cropland and pasture dynamics, shifting cultivation, logging, fire, and resulting patterns of secondary regrowth. Here we analyze the behavior of LM3V, forced with the output from the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model AM2, observed precipitation data and four historic scenarios of land-use change for 1700-2000. Our analysis suggests a net terrestrial carbon source due to land-use activities from 1.1 to 1.3 GtC/yr during the 1990s, where the range is due to the difference in the historic cropland distribution. This magnitude is substantially smaller than previous estimates from other models, largely due to our estimates of a secondary vegetation sink of 0.35 to 0.6 GtC/yr in the 1990s and decelerating agricultural land clearing since the1960s. For the 1990s, our estimates for the pastures carbon flux vary from a source of 0.37 to a sink of 0.15 GtC/yr, and for the croplands our model shows a carbon source of 0.6 to 0.9 GtC/yr. Our process-based model suggests a smaller net deforestation source than earlier bookkeeping models because it accounts for decelerated net conversion of primary forest to agriculture and for stronger secondary vegetation regrowth in tropical regions. The overall uncertainty is likely to be higher than the range reported here because of uncertainty in the biomass recovery under changing ambient conditions, including atmospheric CO2 concentration, nutrients availability and climate.

• Findell, K. L., E. Shevliakova, P. C. D. Milly, and R. J. Stouffer, 2007: Modeled impact of anthropogenic land cover change on climate. Journal of Climate, 20(4), 3621-3634. [ Abstract PDF]

  Equilibrium experiments with the Geophysical Fluid Dynamics Laboratory’s climate model are used to investigate the impact of anthropogenic land cover change on climate. Regions of altered land cover include large portions of Europe, India, eastern China, and the eastern United States. Smaller areas of change are present in various tropical regions. This study focuses on the impacts of biophysical changes associated with the land cover change (albedo, root and stomatal properties, roughness length), which is almost exclusively a conversion from forest to grassland in the model; the effects of irrigation or other water management practices and the effects of atmospheric carbon dioxide changes associated with land cover conversion are not included in these experiments. The model suggests that observed land cover changes have little or no impact on globally averaged climatic variables (e.g., 2-m air temperature is 0.008 K warmer in a simulation with 1990 land cover compared to a simulation with potential natural vegetation cover). Differences in the annual mean climatic fields analyzed did not exhibit global field significance. Within some of the regions of land cover change, however, there are relatively large changes of many surface climatic variables. These changes are highly significant locally in the annual mean and in most months of the year in eastern Europe and northern India. They can be explained mainly as direct and indirect consequences of model-prescribed increases in surface albedo, decreases in rooting depth, and changes of stomatal control that accompany deforestation.

• Dunne, J. P., J. L. Sarmiento, and A. Gnanadesikan, 2007: A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor. Global Biogeochemical Cycles, 21, GB4006, doi:10.1029/2006GB002907. [ Abstract ]

  We present a new synthesis of the oceanic cycles of organic carbon, silicon, and calcium carbonate. Our calculations are based on a series of algorithms starting with satellite-based primary production and continuing with conversion of primary production to sinking particle flux, penetration of particle flux to the deep sea, and accumulation in sediments. Regional and global budgets from this synthesis highlight the potential importance of shelves and near-shelf regions for carbon burial. While a high degree of uncertainty remains, this analysis suggests that shelves, less than 50 m water depths accounting for 2% of the total ocean area, may account for 48% of the global flux of organic carbon to the seafloor. Our estimates of organic carbon and nitrogen flux are in generally good agreement with previous work while our estimates for CaCO₃ and SiO₂ fluxes are lower than recent work. Interannual variability in particle export fluxes is found to be relatively small compared to intra-annual variability over large domains with the single exception of the dominating role of El Niño-Southern Oscillation variability in the central tropical Pacific. Comparison with available sediment-based syntheses of benthic remineralization and burial support the recent theory of mineral protection of organic carbon flux through the deep ocean, pointing to lithogenic material as an important carrier phase of organic carbon to the deep seafloor. This work suggests that models which exclude the role of lithogenic material would underestimate the penetration of POC to the deep seafloor by approximately 16–51% globally, and by a much larger fraction in areas with low productivity. Interestingly, atmospheric dust can only account for 31% of the total lithogenic flux and 42% of the lithogenically associated POC flux, implying that a majority of this material is riverine or directly erosional in origin.

• Gnanadesikan, A., and R. J. Stouffer, 2006: Diagnosing atmosphere-ocean general circulation model errors relevant to the terrestrial biosphere using the Köppen climate classification. Geophysical Research Letters, 33, L22701, doi:10.1029/2006GL028098. [ Abstract PDF]

  Coupled atmosphere-ocean-land-sea ice climate models (AOGCMs) are often tuned using physical variables like temperature and precipitation with the goal of minimizing properties such as the root-mean-square error. As the community moves towards modeling the earth system, it is important to note that not all biases have equivalent impacts on biology. Bioclimatic classification systems provide means of filtering model errors so as to bring out those impacts that may be particularly important for the terrestrial biosphere. We examine one such diagnostic, the classic system of Köppen, and show that it can provide an “early warning” of which model biases are likely to produce serious biases in the land biosphere. Moreover, it provides a rough evaluation criterion for the performance of dynamic vegetation models. State-of-the art AOGCMs fail to capture the correct Köppen zone in about 20–30% of the land area excluding Antarctica, and misassign a similar fraction to the wrong subzone.

• Westley, M. B., H. Yamagishi, B. N. Popp, and N. Yoshida, 2006: Nitrous oxide cycling in the Black Sea inferred from stable isoptope and isotopomer distributions. Deep-Sea Research II, 53(17-19), doi:10.1016/j.dsr2.2006.03.012. [ Abstract ]

  The low-oxygen regions of the world's oceans have been shown to be major sources of nitrous oxide, a trace gas in the atmosphere that contributes to both greenhouse warming and the destruction of stratospheric ozone. Nitrous oxide can be produced as a by-product of nitrification or an intermediate of denitrification; low oxygen conditions enhance the yield of nitrous oxide from both pathways. We measured the concentration and isotopic composition of dissolved nitrous oxide at several stations in the Black Sea, an anoxic basin with a well-defined suboxic layer that separates the ventilated surface waters from the sulfidic deep waters. Our data show that in contrast to other low-oxygen marine regions, nitrous oxide does not accumulate in the Black Sea at significant levels. Moreover, whereas the reduction of nitrous oxide by denitrification usually yields residual gas that is enriched in both stable isotopes, in the Black Sea declining nitrous oxide concentrations are accompanied by enrichment in 18O-N2O but depletion in 15N-N2O. We measured a minimum δ15N-N2O value of −10.8±0.8‰ vs. air N2, by far the lowest measured to date for seawater. Measurements of the distribution of 15N within the linear nitrous oxide molecule reveal that this unusual isotopic signal is most pronounced in the end-position nitrogen, and that site preference, or the tendency for 15N to be found in the center-position nitrogen, co-varies positively with 18O-N2O. We surmise that the highly unusual isotopic composition of Black Sea nitrous oxide is the result of two processes: production of 15N-depleted nitrous oxide by ammonium oxidation followed by its reduction by denitrification, which causes enrichment in 18O and enhancement of 15N-site preference. Bottle incubation experiments with 15N-ammonium and 15N-nitrite reveal that both oxidation and reduction pathways to nitrous oxide are active in the Black Sea suboxic zone.