P. Cadule

Climate-carbon cycle feedback analysis: Results from the C4MIP model intercomparison

Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5°C. All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.

Permafrost carbon-climate feedbacks accelerate global warming

Permafrost soils contain enormous amounts of organic carbon, which could act as a positive feedback to global climate change due to enhanced respiration rates with warming. We have used a terrestrial ecosystem model that includes permafrost carbon dynamics, inhibition of respiration in frozen soil layers, vertical mixing of soil carbon from surface to permafrost layers, and CH4 emissions from flooded areas, and which better matches new circumpolar inventories of soil carbon stocks, to explore the potential for carbon-climate feedbacks at high latitudes. Contrary to model results for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), when permafrost processes are included, terrestrial ecosystems north of 60°N could shift from being a sink to a source of CO2 by the end of the 21st century when forced by a Special Report on Emissions Scenarios (SRES) A2 climate change scenario. Between 1860 and 2100, the model response to combined CO2 fertilization and climate change changes from a sink of 68 Pg to a 27 + -7 Pg sink to 4 + -18 Pg source, depending on the processes and parameter values used. The integrated change in carbon due to climate change shifts from near zero, which is within the range of previous model estimates, to a climate-induced loss of carbon by ecosystems in the range of 25 + -3 to 85 + -16 Pg C, depending on processes included in the model, with a best estimate of a 62 + -7 Pg C loss. Methane emissions from high-latitude regions are calculated to increase from 34 Tg CH4/y to 41–70 Tg CH4/y, with increases due to CO2 fertilization, permafrost thaw, and warming-induced increased CH4 flux densities partially offset by a reduction in wetland extent.

Carbon-concentration and carbon-climate feedbacks in CMIP5 Earth System Models

AbstractThe magnitude and evolution of parameters that characterize feedbacks in the coupled carbon–climate system are compared across nine Earth system models (ESMs). The analysis is based on results from biogeochemically, radiatively, and fully coupled simulations in which CO2 increases at a rate of 1% yr−1. These simulations are part of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CO2 fluxes between the atmosphere and underlying land and ocean respond to changes in atmospheric CO2 concentration and to changes in temperature and other climate variables. The carbon–concentration and carbon–climate feedback parameters characterize the response of the CO2 flux between the atmosphere and the underlying surface to these changes. Feedback parameters are calculated using two different approaches. The two approaches are equivalent and either may be used to calculate the contribution of the feedback terms to diagnosed cumulative emissions. The contribution of carbon–concentration feedback to...

Carbon residence time dominates uncertainty in terrestrial vegetation responses to future climate and atmospheric CO2

Future climate change and increasing atmospheric CO2 are expected to cause major changes in vegetation structure and function over large fractions of the global land surface. Seven global vegetation models are used to analyze possible responses to future climate simulated by a range of general circulation models run under all four representative concentration pathway scenarios of changing concentrations of greenhouse gases. All 110 simulations predict an increase in global vegetation carbon to 2100, but with substantial variation between vegetation models. For example, at 4 °C of global land surface warming (510–758 ppm of CO2), vegetation carbon increases by 52–477 Pg C (224 Pg C mean), mainly due to CO2 fertilization of photosynthesis. Simulations agree on large regional increases across much of the boreal forest, western Amazonia, central Africa, western China, and southeast Asia, with reductions across southwestern North America, central South America, southern Mediterranean areas, southwestern Africa, and southwestern Australia. Four vegetation models display discontinuities across 4 °C of warming, indicating global thresholds in the balance of positive and negative influences on productivity and biomass. In contrast to previous global vegetation model studies, we emphasize the importance of uncertainties in projected changes in carbon residence times. We find, when all seven models are considered for one representative concentration pathway × general circulation model combination, such uncertainties explain 30% more variation in modeled vegetation carbon change than responses of net primary productivity alone, increasing to 151% for non-HYBRID4 models. A change in research priorities away from production and toward structural dynamics and demographic processes is recommended.

Effect of anthropogenic land-use and land cover changes on climate and land carbon storage in CMIP5 projections for the 21st century

AbstractThe effects of land-use changes on climate are assessed using specified-concentration simulations complementary to the representative concentration pathway 2.6 (RCP2.6) and RCP8.5 scenarios performed for phase 5 of the Coupled Model Intercomparison Project (CMIP5). This analysis focuses on differences in climate and land–atmosphere fluxes between the ensemble averages of simulations with and without land-use changes by the end of the twenty-first century. Even though common land-use scenarios are used, the areas of crops and pastures are specific for each Earth system model (ESM). This is due to different interpretations of land-use classes. The analysis reveals that fossil fuel forcing dominates land-use forcing. In addition, the effects of land-use changes are globally not significant, whereas they are significant for regions with land-use changes exceeding 10%. For these regions, three out of six participating models—the Second Generation Canadian Earth System Model (CanESM2); Hadley Centre Glo...

Quantifying carbon cycle feedbacks.

Abstract Perturbations to the carbon cycle could constitute large feedbacks on future changes in atmospheric CO2 concentration and climate. This paper demonstrates how carbon cycle feedback can be expressed in formally similar ways to climate feedback, and thus compares their magnitudes. The carbon cycle gives rise to two climate feedback terms: the concentration–carbon feedback, resulting from the uptake of carbon by land and ocean as a biogeochemical response to the atmospheric CO2 concentration, and the climate–carbon feedback, resulting from the effect of climate change on carbon fluxes. In the earth system models of the Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP), climate–carbon feedback on warming is positive and of a similar size to the cloud feedback. The concentration–carbon feedback is negative; it has generally received less attention in the literature, but in magnitude it is 4 times larger than the climate–carbon feedback and more uncertain. The concentration–carbon feed...

Spatiotemporal patterns of terrestrial carbon cycle during the 20th century

[1] We evaluated how climate change, rising atmospheric CO 2 concentration, and land use change influenced the terrestrial carbon (C) cycle for the last century using a process-based ecosystem model. Over the last century, the modeled land use change emitted about 129 Pg of C to the atmosphere. About 76% (or 98 Pg C) of this emission, however, was offset by net C uptake on land driven by climate changes and rising atmospheric CO 2 concentration. Thus, the modeled net release of C from the terrestrial ecosystems to the atmosphere from 1901 to 2002 is about 31 Pg C. Global net primary productivity (NPP) has significantly increased by 14% during the last century, especially since the 1970s. From 1980 to 2002, global NPP increased with an average increase rate of 0.4% yr ―1 . At global scale, such an increase seems to be primarily attributed to the increase in atmospheric CO 2 concentration, and then to precipitation change. Over the last 2 decades, climate change and rising CO 2 forced the land carbon sink (1.6 Pg C yr ―1 for 1980s and 2.2 Pg C yr ―1 for 1990s) to be larger than land use change driven carbon emissions (1.0 Pg C yr ―1 for 1980s and 1.2 Pg C yr ―1 for 1990s), resulting a net land sink of 0.5 Pg C yr ―1 in the 1980s and of 1.0 Pg C yr ―1 in the 1990s. The largest C emission from land use change appeared in tropical regions with an average emission of 0.6 Pg C yr ―1 in 1980s and 0.7 Pg C yr ―1 in 1990s, which is slightly larger than net carbon uptake due to CO 2 fertilization and climate change. Thus, net carbon balance of tropical lands is close to neutral over the past 2 decades (about 0.13 Pg C yr ―1 in 1980s and 0.03 Pg C yr ―1 in 1990s). We also found that current global warming has already started accelerating C loss from terrestrial ecosystems, by enhanced decomposition of soil organic carbon. In response to warming trends only, the global net carbon uptake significantly decreased, offsetting about 70% of the increase in global net carbon uptake owing to CO 2 fertilization during 1980―2002. The global terrestrial C cycle also shows large year-to-year variations, and different regions have quite distinct dominant drivers. Generally, interannual changes of carbon fluxes in tropical and temperate ecosystems are mainly explained by precipitation variability, while temperature variability plays a major role in boreal ecosystems.

Stratospheric ozone depletion reduces ocean carbon uptake and enhances ocean acidification

Observational and atmospheric inversion studies find that the strength of the Southern Ocean carbon dioxide (CO2) sink is not increasing, despite rising atmospheric CO2. However, this is yet to be captured by contemporary coupled-climate-carbon-models used to predict future climate. We show that by accounting for stratospheric ozone depletion in a coupled-climate-carbon-model, the ventilation of carbon rich deep water is enhanced through stronger winds, increasing surface water CO2 at a rate in good agreement with observed trends. We find that Southern Ocean uptake is reduced by 2.47 PgC (1987-2004) and is consistent with atmospheric inversion studies. The enhanced ventilation also accelerates ocean acidification, despite lesser Southern Ocean CO2 uptake. Our results link two important anthropogenic changes: stratospheric ozone depletion and greenhouse gas increases; and suggest that studies of future climate that neglect stratospheric ozone depletion likely overestimate regional and global oceanic CO2 uptake and underestimate the impact of ocean acidification.

Ten years of global burned area products from spaceborne remote sensing—A review: Analysis of user needs and recommendations for future developments

Abstract Early global estimates of carbon emissions from biomass burning were based on empirical assumptions of fire return interval in different biomes in the 1980s. Since then, significant improvements of spaceborne remote sensing sensors have resulted in an increasing number of derived products characterizing the detection of active fire or the subsequent burned area (GFED, MODIS MCD45A1, L3JRC, Globcarbon, GBS, GLOBSCAR, GBA2000). When coupled with global land cover and vegetation models allowing for spatially explicit fuel biomass estimates, the use of these products helps to yield important information about the spatial and the temporal variability of emission estimates. The availability of multi-year products (>10 years) leads to a better understanding of uncertainties in addition to increasing accuracy. We surveyed a wide range of users of global fire data products whilst also undertaking a review of the latest scientific literature. Two user groups were identified, the first being global climate and vegetation modellers and the second being regional land managers. Based on this review, we present here the current needs covering the range of end-users. We identified the increasing use of BA products since the year 2000 with an increasing use of MODIS as a reference dataset. Scientific topics using these BA products have increased in diversity and area of application, from global fire emissions (for which BA products were initially developed) to regional studies with increasing use for ecosystem management planning. There is a significant need from the atmospheric science community for low spatial resolution (gridded, 1/2 degree cell) and long time series data characterized with supplementary information concerning the accuracy in timing of the fire and reductions of omission/commission errors. There is also a strong need for precisely characterizing the perimeter and contour of the fire scar for better assimilation with land cover maps and fire intensity. Computer and earth observation facilities remain a significant gap between ideal accuracies and the realistic ones, which must be fully quantified and comprehensive for an actual use in global fire emissions or regional land management studies.

Regional Impacts of Climate Change and Atmospheric CO2 on Future Ocean Carbon Uptake: A Multimodel Linear Feedback Analysis

The increase in atmospheric CO2 over this century depends on the evolution of the oceanic air–sea CO2 uptake, which will be driven by the combined response to rising atmospheric CO2 itself and climate change. Here, the future oceanic CO2 uptake is simulated using an ensemble of coupled climate–carbon cycle models. The models are driven by CO2 emissions from historical data and the Special Report on Emissions Scenarios (SRES) A2 high-emission scenario. A linear feedback analysis successfully separates the regional future (2010–2100) oceanic CO2 uptake into a CO2-induced component, due to rising atmospheric CO2 concentrations, and a climate-induced component, due to global warming. The models capture the observationbased magnitude and distribution of anthropogenic CO2 uptake. The distributions of the climate-induced component are broadly consistent between the models, with reduced CO2 uptake in the subpolar Southern Ocean and the equatorial regions, owing to decreased CO2 solubility; and reduced CO2 uptake in the midlatitudes, owing to decreased CO2 solubility and increased vertical stratification. The magnitude of the climate-induced component is sensitive to local warming in the southern extratropics, to large freshwater fluxes in the extratropical North Atlantic Ocean, and to small changes in the CO2 solubility in the equatorial regions. In key anthropogenic CO2 uptake regions, the climate-induced component offsets the CO2induced component at a constant proportion up until the end of this century. This amounts to approximately 50% in the northern extratropics and 25% in the southern extratropics and equatorial regions. Consequently, the detection of climate change impacts on anthropogenic CO2 uptake may be difficult without monitoring additional tracers, such as oxygen.