GFDL - Geophysical Fluid Dynamics Laboratory

Highlighted Papers

• Shevliakova, Elena, S. W. Pacala, Sergey Malyshev, G. C. Hurtt, P. C. D. Milly, J. P. Caspersen, Lori 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 Biogeochemical Cycles, 23, GB2022, doi:10.1029/2007GB003176. [ 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 and 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/a 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/a in the 1990s and decelerating agricultural land clearing since the 1960s. For the 1990s, our estimates for the pastures' carbon flux vary from a source of 0.37 to a sink of 0.15 GtC/a, and for the croplands our model shows a carbon source of 0.6 to 0.9 GtC/a. 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.

• Henson, S A., D Raitsos, John P Dunne, and A McQuatters-Gollop, November 2009: Decadal variability in biogeochemical models: Comparison with a 50-year ocean colour dataset. Geophysical Research Letters, 36(L21601), doi:10.1029/2009GL040874. [ Abstract]

  Assessing the skill of biogeochemical models to hindcast past variability is challenging, yet vital in order to assess their ability to predict biogeochemical change. However, the validation of decadal variability is limited by the sparsity of consistent, long-term biological datasets. The Phytoplankton Colour Index (PCI) product from the Continuous Plankton Recorder survey, which has been sampling the North Atlantic since 1948, is an example of such a dataset. Converting the PCI to chlorophyll values using SeaWiFS data allows a direct comparison with model output. Here we validate decadal variability in chlorophyll from the GFDL TOPAZ model. The model demonstrates skill at reproducing interannual variability, but cannot simulate the regime shifts evident in the PCI data. Comparison of the model output, data and climate indices highlights under-represented processes that it may be necessary to include in future biogeochemical models in order to accurately simulate decadal variability in ocean ecosystems.

• Henson, S A., John P Dunne, and Jorge L Sarmiento, April 2009: Decadal variability in North Atlantic phytoplankton blooms. Journal of Geophysical Research, 114, C04013, doi:10.1029/2008JC005139. [ Abstract]

  The interannual to decadal variability in the timing and magnitude of the North Atlantic phytoplankton bloom is examined using a combination of satellite data and output from an ocean biogeochemistry general circulation model. The timing of the bloom as estimated from satellite chlorophyll data is used as a novel metric for validating the model's skill. Maps of bloom timing reveal that the subtropical bloom begins in winter and progresses northward starting in May in subpolar regions. A transition zone, which experiences substantial interannual variability in bloom timing, separates the two regions. Time series of the modeled decadal (1959–2004) variability in bloom timing show no long-term trend toward earlier or delayed blooms in any of the three regions considered here. However, the timing of the subpolar bloom does show distinct decadal-scale periodicity, which is found to be correlated with the North Atlantic Oscillation (NAO) index. The mechanism underpinning the relationship is identified as anomalous wind-driven mixing conditions associated with the NAO. In positive NAO phases, stronger westerly winds result in deeper mixed layers, delaying the start of the subpolar spring bloom by 2–3 weeks. The subpolar region also expands during positive phases, pushing the transition zone further south in the central North Atlantic. The magnitude of the bloom is found to be only weakly dependent on bloom timing, but is more strongly correlated with mixed layer depth. The extensive interannual variability in the timing of the bloom, particularly in the transition region, is expected to strongly impact the availability of food to higher trophic levels.

• Christensen, V., C. J. Walters, R. Ahrens, J. Alder, J. Buszowski, L. B. Christensen, W. W. L. Cheung, John P Dunne, R. Froese, V. Karpouzi, K. Kaschner, K. Kearney, S. Lai, V. Lam, M. L. D. Palomares, A. Peters-Mason, C. Piroddi, Jorge L Sarmiento, J. Steenbeek, R. Sumaila, R. Watson, D. Zeller, and D. Pauly, 2009: Database-driven models of the world's large marine ecosystems. Ecological Modelling, 220, 1984-1996. [ Abstract]

  We present a new methodology for database-driven ecosystem model generation and apply the methodology to the world's 66 currently defined Large Marine Ecosystems. The method relies on a large number of spatial and temporal databases, including FishBase, SeaLifeBase, as well as several other databases developed notably as part of the /Sea Around Us/ project. The models are formulated using the freely available Ecopath with Ecosim (EwE) modeling approach and software. We tune the models by fitting to available time series data, but recognize that the models represent only a first-generation of database-driven ecosystem models. We use the models to obtain a first estimate of fish biomass in the world's LMEs. The biggest hurdles at present to further model development and validation are insufficient time series trend information, and data on spatial fishing effort.

• E. D. Galbraith, Anand Gnanadesikan, John P Dunne, and M. R. Hiscock (accepted): Regional impacts of iron-light colimitation in a global biogeochemical model. Biogeosciences Discussions, 6, 7517-7564. [ Abstract]

  Laboratory and field studies have revealed that iron has multiple roles in phytoplankton physiology, with particular importance for light-harvesting cellular machinery. However, although iron-limitation is explicitly included in numerous biogeochemical/ecosystem models, its implementation varies, and its effect on the efficiency of light harvesting is often ignored. Given the complexity of the ocean environment, it is difficult to predict the consequences of applying different iron limitation schemes. Here we explore the interaction of iron and nutrient cycles using a new, streamlined model of ocean biogeochemistry. Building on previously published parameterizations of photoadaptation and export production, the Biogeochemistry with Light Iron Nutrients and Gasses (BLING) model is constructed with only three explicit tracers but including macronutrient and micronutrient limitation, light limitation, and an implicit treatment of community structure. The structural simplicity of this computationally inexpensive model allows us to clearly isolate the global effects of iron availability on maximum light-saturated photosynthesis rates from those of photosynthetic efficiency. We find that the effect on light-saturated photosynthesis rates is dominant, negating the importance of photosynthetic efficiency in most regions, especially the cold waters of the Southern Ocean. The primary exceptions to this occur in iron-rich regions of the Northern Hemisphere, where high light-saturated photosynthesis rates cause photosynthetic efficiency to play a more important role. Additionally, we speculate that the small phytoplankton dominating iron-limited regions tend to have relatively high photosynthetic efficiency, such that iron-limitation has less of a deleterious effect on growth rates than would be expected from short-term iron addition experiments.

• Friedrichs, M. A. M., M. -E. Carr, Richard T. Barber, M. Scardi, D. Antione, R. A. Armstrong, I. Asanuma, M. J. Behrenfeld, E. T. Buitenhuis, F. Chai, J. R. Christian, A. M. Ciotti, S. C. Doney, M. Dowell, John P Dunne, B. Gentili, W. Gregg, N. Hoepffner, J. Ishizaka, T. Kameda, I. Lima, J. Marra, F. Mèlin, J. K. Moore, A. Morel, R. T. O'Malley, J. O'Reilly, V. S. Saba, M. Schmeltz, T. J. Smyth, J. Tjiputra, K. Waters, T. K. Westberry, and A. Winguth, 2009: Assessing the uncertainties of model estimates of primary productivity in the tropical Pacific Ocean. Journal of Marine Systems, 76(1-2), doi:10.1016/j.jmarsys.2008.05.010. [ Abstract]

  Depth-integrated primary productivity (PP) estimates obtained from satellite ocean color-based models (SatPPMs) and those generated from biogeochemical ocean general circulation models (BOGCMs) represent a key resource for biogeochemical and ecological studies at global as well as regional scales. Calibration and validation of these PP models are not straightforward, however, and comparative studies show large differences between model estimates. The goal of this paper is to compare PP estimates obtained from 30 different models (21 SatPPMs and 9 BOGCMs) to a tropical Pacific PP database consisting of not, vert, similar 1000 ^14 C measurements spanning more than a decade (1983–1996). Primary findings include: skill varied significantly between models, but performance was not a function of model complexity or type (i.e. SatPPM vs. BOGCM); nearly all models underestimated the observed variance of PP, specifically yielding too few low PP (< 0.2 g C m^- 2 d^- 1 ) values; more than half of the total root-mean-squared model–data differences associated with the satellite-based PP models might be accounted for by uncertainties in the input variables and/or the PP data; and the tropical Pacific database captures a broad scale shift from low biomass-normalized productivity in the 1980s to higher biomass-normalized productivity in the 1990s, which was not successfully captured by any of the models. This latter result suggests that interdecadal and global changes will be a significant challenge for both SatPPMs and BOGCMs. Finally, average root-mean-squared differences between in situ PP data on the equator at 140°W and PP estimates from the satellite-based productivity models were 58% lower than analogous values computed in a previous PP model comparison 6 years ago. The success of these types of comparison exercises is illustrated by the continual modification and improvement of the participating models and the resulting increase in model skill.

• Saba, V. S., M. A. M. Friedrichs, M. -E. Carr, D. Antoine, R. A. Armstrong, I. Asanuma, O. Aumont, N. R. Bates, M. J. Behrenfeld, V. Bennington, L. Bopp, J. Bruggeman, E. T. Buitenhuis, M. J. Church, A. M. Ciotti, S. C. Doney, M. Dowell, John P Dunne, S. Dutkiewicz, W. Gregg, N. Hoepffner, K. J. W. Hyde, J. Ishizaka, T. Kameda, D. M. Karl, I. Lima, M. W. Lomas, J. Marra, G. A. McKinley, F. Mèlin, J. K. Moore, A. Morel, J. O'Reilly, B. Salihoqlu, M. Scardi, T. J. Smyth, S. Tang, J. Tjiputra, J. Uitz, M. Vichi, K. Waters, and T. K. Westberry, A. Yool (submitted): The challenges of modeling depth-integrated marine primary productivity over multiple decades: A case study at BATS and HOT. Global Biogeochemical Cycles.
• Rodgers, K. B., R. M. Key, Anand Gnanadesikan, Jorge L Sarmiento, O Aumont, L. Bopp, S. C. Doney, John P Dunne, D. M. Glover, A. Ishida, M. Ishii, A. R. Jacobson, C. L. Monaco, E. Maier-Reimer, H. Mercier, N. Metzl, F. F. Pérez, A. F. Rios, R. Wanninkhof, P. Wetzel, C. D. Winn, and Y. Yamanaka, 2009: Using altimetry to help explain patchy changes in hydrographic carbon measurements. Journal of Geophysical Research, 114, C09013, doi:10.1029/2008JC005183. [ Abstract]

  Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O_2 ) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O_2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O_2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O_2 column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O_2 column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictability's CO_2 /Repeat Hydrography program).

• Findell, Kirsten L., Elena Shevliakova, P. C. D. Milly, and Ronald 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, John P., Jorge L. Sarmiento, and Anand 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 CaCO_3 and SiO_2 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, Anand, and Ronald 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.

• G.C. Hurtt, S. Frolking, M.G. Fearon, B. Moore III, Elena Shevliakova, Sergey Malyshev, S.W. Pacala, and R.A. Houghton, 2006: The underpinnings of land-use history: Three centuries of global gridded land-use transitions, wood harvest activity, and resulting secondary landscapes. Global Change Biology, 12(7), doi:10.1111/j.1365-2486.2006.01150.x.
• Westley, Marian 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]

  To accurately assess the impacts of human land use on the Earth system, information is needed on the current and historical patterns of land-use activities. Previous global studies have focused on developing reconstructions of the spatial patterns of agriculture. Here, we provide the first global gridded estimates of the underlying land conversions (land-use transitions), wood harvesting, and resulting secondary lands annually, for the period 1700–2000. Using data-based historical cases, our results suggest that 42–68% of the land surface was impacted by land-use activities (crop, pasture, wood harvest) during this period, some multiple times. Secondary land area increased 10–44 × 106 km2 ; about half of this was forested. Wood harvest and shifting cultivation generated 70–90% of the secondary land by 2000; permanent abandonment and relocation of agricultural land accounted for the rest. This study provides important new estimates of globally gridded land-use activities for studies attempting to assess the consequences of anthropogenic changes to the Earth's surface over time.