Papers by Jorge Gaspar Sarmiento
Proceedings of the National Academy of Sciences of the United States of America, Jan 22, 2015
The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the... more The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the 21st century as climate-driven losses exceed CO2-driven C gains, thereby accelerating global warming. Although it has long been recognized that tropical climate plays a critical role in regulating interannual climate variability, the causal link between changes in temperature and precipitation and terrestrial processes remains uncertain. Here, we combine atmospheric mass balance, remote sensing-modeled datasets of vegetation C uptake, and climate datasets to characterize the temporal variability of the terrestrial C sink and determine the dominant climate drivers of this variability. We show that the interannual variability of global land C sink has grown by 50-100% over the past 50 y. We further find that interannual land C sink variability is most strongly linked to tropical nighttime warming, likely through respiration. This apparent sensitivity of respiration to nighttime temperatur...
This paper explores the relationship between large-scale vertical exchange and the cycling of bio... more This paper explores the relationship between large-scale vertical exchange and the cycling of biologically active nutrients within the ocean. It considers how the parameterization of vertical and lateral mixing effects estimates of new production (defined as the net uptake of phosphate). A baseline case is run with low vertical mixing in the pycnocline and a relatively low lateral diffusion coefficient. The magnitude of the diapycnal diffusion coefficient is then increased within the pycnocline, within the pycnocline of the Southern Ocean, and in the top 50 m; while the lateral diffusion coefficient is increased throughout the ocean. It is shown that it is possible to change lateral and vertical diffusion coefficients so as to preserve the structure of the pycnocline while changing the pathways of vertical exchange and hence the cycling of nutrients. Comparisons between the different models reveal that new production is very sensitive to the level of vertical mixing within the pycnocline, but only weakly sensitive to the level of lateral and upper ocean diffusion. The results are compared with two estimates of new production based on ocean color and the annual cycle of nutrients. On a global scale, the observational estimates are most consistent with the circulation produced with a low diffusion coefficient within the pycnocline, resulting in a new production of around 10 GtC yr À1 : On a regional level, however, large differences appear between observational and model based estimates. In the tropics, the models yield systematically higher levels of new production than the observational estimates. Evidence from the Eastern Equatorial Pacific suggests that this is due to both biases in the data used to generate the observational estimates and problems with the models. In the North Atlantic, the observational estimates vary more than the models, due in part to the methodology by which the nutrient-based climatology is constructed. In the North Pacific, the modelled values of new production are all much lower than the observational estimates, probably as a result of the failure to form intermediate
Climate stabilization during the next 100 to 200 y will require significant reductions in atmosph... more Climate stabilization during the next 100 to 200 y will require significant reductions in atmospheric carbon dioxide emissions to avoid large increases in global temperature. While there is only mild disagreement concerning carbon management options such as energy efficiency, alternative energy sources, and even geologic C storage, ocean storage remains controversial, due to its potential impacts for deep-sea ecosystems. A cautionary approach to carbon management might avoid any ocean C storage. However, this approach does not consider the balance between ocean and terrestrial ecosystems, or societal concerns. Using a broader perspective, we might ask whether atmospheric CO2 storage (i.e. the status quo), or deep ocean sequestration is better for Earth's ecosystems and societies? We explored the potential storage capacity of the deep ocean for carbon dioxide, under scenarios producing a 0.2 pH unit reduction, a level similar to observed scale of pH variability in deep ocean basins, which may also represent coarse thresholds for deep-sea ecosystem impacts. Roughly 500 PgC could be stored in the deep ocean to lower pH by 0.2 units, yielding a long term (~250 y) ocean sequestration program of 2 PgCy-1. The mitigation value of such ocean C sequestration for upper ocean and terrestrial systems depends strongly on future emission scenarios. Under a low emission scenario (e.g. SRES scenario A1T, B1; atm CO2 ~575 ppm, global temperature change of ~+2 oC), a 2 PgCy-1 ocean CO2 injection program could mitigate global temperature by ~-0.4 oC (20%) by 2100. This could reduce significantly the number of people at risk of water shortage and tropical diseases, with lesser improvement expected for hunger or coastal flooding. Mitigation for terrestrial and shallow ocean ecosystems is difficult to predict. A 0.4 oC reduction in warming this century is expected to delay the progression of coral reef devastation by roughly 20 y. The mitigation potential of ocean storage under very high emission scenarios appears weaker, due partially to uncertainty in the trajectories of ecosystem change and societal issues. For high emission scenarios (e.g. SRES A1F1, A2; 900 ppm CO2, 4.2 oC global temperature increase by 2100), the mitigation effect of ocean sequestration is still ~ -0.4 oC, with a reduction of atmospheric CO2 near 50 ppm. However, under such high emissions, the effects of large global temperature on societal issues such as disease, hunger, and water are expected to be severe, with an unknown incremental benefit from ocean sequestration. These results indicate the importance of a careful consideration of the benefits and liabilities of ocean sequestration, in terms of ecosystem health for global ecosystems and terrestrial concerns. A cautionary approach to ocean carbon sequestration in consideration of the global consequences of anthropogenic climate change may differ from an approach considering deep-ocean ecosystems alone.
Global Change Biology, 2010
Abstract Previous projection of climate change impacts on global food supply focuses solely on pr... more Abstract Previous projection of climate change impacts on global food supply focuses solely on production from terrestrial biomes, ignoring the large contribution of animal protein from marine capture fisheries. Here, we project changes in global catch potential for 1066 species of exploited marine fish and invertebrates from 2005 to 2055 under climate change scenarios. We show that climate change may lead to large-scale redistribution of global catch potential, with an average of 30–70% increase in high-latitude regions and a drop of ...
Agu Fall Meeting Abstracts, Dec 1, 2008
Between 1960 and 2006, the ocean took up ~33% of the cumulative fossil fuel emissions of 251 Pg C... more Between 1960 and 2006, the ocean took up ~33% of the cumulative fossil fuel emissions of 251 Pg C and increases in atmospheric CO2 concentration accounted for ~56%. The remaining ~11% of fossil fuel emissions were taken up by the terrestrial biosphere despite emissions from deforestation occurring mostly in the tropics. Around 1990/91, the global carbon cycle appears to have undergone a major shift. The atmospheric growth rate decelerated from an average of 58.4 ' 1.8% of fossil fuel emissions prior to 1990, to 52.1% thereafter. Furthermore, ocean carbon model simulations suggest that the oceanic uptake leveled off after ~1990 instead of increasing as expected. Taken together, this implies an increase in the net uptake by the terrestrial biosphere of ~0.9 Pg C yr-1. While the impact of the 1991 Mt. Pinatubo eruption can account for about one-third of this increase, we cannot explain the remainder. Estimates of the regional distribution of sources and sinks over the oceans have improved greatly in the last decade, but remain a vexing challenge for the land biosphere. The difficulty in accounting for land sources and sinks and the associated uncertainty with regard to mechanisms leads to major uncertainties in the future behavior of the global carbon sinks, with major implications for climate policy.
We provide a modeling framework that fully couples a one-dimensional physical mixed layer model, ... more We provide a modeling framework that fully couples a one-dimensional physical mixed layer model, a biogeochemical model, and an upper trophic level fisheries model. For validation purposes, the model has been parameterized for the pelagic Eastern Pacific Subarctic Gyre ecosystem. This paper presents a thorough description of the model itself, as well as an ensemble-based parameterization process that allows the model to incorporate the high level of uncertainty associated with many upper trophic level predator-prey processes. Through a series of model architecture experiments, we demonstrate that the use of a consistent functional response for all predator-prey interactions, as well as the use of densitydependent mortality rates for planktonic functional groups, are important factors in reproducing annual and seasonal observations. We present the results of a 50-year climatological simulation, which demonstrates that under contemporary physical forcing, the model is capable of reproducing long-term seasonal dynamics in primary production and biogeochemical cycling, while maintaining steady-state coexistence of upper trophic level functional groups at levels consistent with observations.
Global Biogeochem Cycle, 1999
Iop Conference Series Earth and Environmental Science, 2009
This article was submitted without an abstract, please refer to the full-text PDF file.
The oceanic transport of carbon by the large-scale ocean circulation and the associated air-sea e... more The oceanic transport of carbon by the large-scale ocean circulation and the associated air-sea exchange of CO2 are a significant factor controlling the distribution of atmospheric CO2, from which major inferences are made with regard to global-scale sources and sinks of CO2. We present here new estimates of pre-industrial and present CO2 air-sea exchange, their implied ocean carbon transport, and their impact on atmospheric CO2 on the basis of an ocean inverse modeling method that uses ocean interior observations of dissolved inorganic carbon (DIC) and associated anthropogenic CO2 estimates. The inversely estimated pre-industrial air-sea fluxes of CO2 reveal the expected pattern of CO2 uptake by the oceans in the mid to high latitudes and release back into the atmosphere in the low latitudes. By contrast, the air-sea flux of anthropogenic CO2 is found to be into the ocean everywhere, totaling about 1.8 Pg C yr-1. The pre-industrial CO2 flux results imply a net oceanic transport of carbon in pre-industrial times from the high latitudes to the low latitudes and a corresponding transport in the opposite direction in the atmosphere. We find, however, a strong asymmetry between the two hemispheres. This asymmetry is largely caused by the Atlantic Ocean, where CO2 is taken up at high latitudes and transported across the equator into the southern hemisphere with magnitudes in agreement with estimates derived from direct hydrographic inversions. This Atlantic transport gives rise to a modest cross-equatorial transport of about 0.4 PgC yr-1. Our inversely estimated pre-industrial air-sea CO2 fluxes nevertheless produce a south pole to north pole gradient of atmospheric CO2 in pre-industrial times of nearly 1 ppm. Although this south-north interhemispheric gradient is significant, it is too small to negate the need for a large northern hemisphere land carbon sink in order to explain the currently observed atmospheric CO2 gradients.
Journal of Climate, Oct 31, 2014
Dominance of the Southern Ocean in anthropogenic carbon and heat uptake in CMIP5 models. J. Climate.
The diVerence between Mauna Loa and South Pole atmospheric CO 2 concentrations from 1959 to the p... more The diVerence between Mauna Loa and South Pole atmospheric CO 2 concentrations from 1959 to the present scales linearly with CO 2 emissions from fossil fuel burning and cement production (together called fossil CO 2 ). An extrapolation to zero fossil CO 2 emission has been used to suggest that the atmospheric CO 2 concentration at Mauna Loa was 0.8
Direct estimates of the net exchange rates of CO2 across the air-sea interface are associated wit... more Direct estimates of the net exchange rates of CO2 across the air-sea interface are associated with large uncertainties caused by the high spatial and temporal variability of the air-sea partial pressure difference and uncertainties in the parameterization of air-sea gas exchange. We have developed an inverse modeling technique that avoids these problems by estimating the net pre-industrial air-sea flux based
Progress in Oceanography, 2014
Biogeosciences Discussions, 2010
In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes... more In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which 5 biological productivity at low latitudes would be catastrophically reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In ad-10 dition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global primary productivity between 30 • S and 30 • N. The high 15 end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, the high preformed nutrients subducted in the SAMW layer are converted rapidly (in less than 40 years) to remineralized nutrients, explain-20 ing previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.
Under increasing atmospheric CO2 concentrations the changing Earth's radiative balance will i... more Under increasing atmospheric CO2 concentrations the changing Earth's radiative balance will influence the atmospheric and oceanic circulation. One of the consequences will be a modification of the ability of the ocean to absorb and store CO2. To a first degree of approximation the ocean's carbon uptake can be separated between solubility-driven and biologically-driven pumps. The separate impact of the changing ocean circulation on these two pumps is still unclear though a partial compensation between opposite responses is expected. We design a suite of model experiments to quantify these responses analyzing a preindustrial steady state and an evolving state with atmospheric carbon concentrations rising according to historical (1880-2009) and projected IPCC scenarios. All experiments are carried out in CM2Mc, a coarse version of one of the climate models (the GFDL CM2) used in the IPCC Fourth Assessment report. The ocean biogeochemical component is solved by the Biology-Light...
Generalized models of thorium and particle cycling, data from Station P, and an inversion techniq... more Generalized models of thorium and particle cycling, data from Station P, and an inversion technique are used to obtain rate estimates of important biological and chemical transformations occurring in the water column. We first verify the inversion technique using an idealized data set generated by a finite difference model, and then apply the inversion technique to data from Station P.
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Papers by Jorge Gaspar Sarmiento