Carbon-climate Feedbacks Research Articles

Feedbacks in Emission-Driven and Concentration-Driven Global Carbon Budgets

Abstract Emissions of CO2 into the atmosphere affect the carbon budgets of the land and ocean as biogeochemical processes react to increased CO2 concentrations. Biogeochemical processes also react to changes in temperature and other climate parameters. This behavior is characterized in terms of carbon–concentration and carbon–climate feedback parameters. The results of this study include 1) the extension of the direct carbon feedback formalism of Boer and Arora to include results from radiatively coupled simulations, as well as those from the biogeochemically coupled and fully coupled simulations used in earlier analyses; 2) a brief analysis of the relationship between this formalism and the integrated feedback formalism of Friedlingstein et al.; 3) the feedback analysis of simulations based on each of the representative concentration pathways (RCPs) RCP2.6, RCP4.5, and RCP8.5; 4) a comparison of the effects of specifying atmospheric CO2 concentrations or CO2 emissions; and 5) the quantification of the relative importance of the two feedback mechanisms in terms of their cumulative contribution to the change in atmospheric CO2. Feedback results are broadly in agreement with earlier studies in that carbon–concentration feedback is negative for the atmosphere and carbon–climate feedback is positive. However, the magnitude and evolution of feedback behavior depends on the formalism employed, the scenario considered, and the specification of CO2 from emissions or as atmospheric concentrations. Both feedback parameters can differ by factors of two or more, depending on the scenario and on the specification of CO2 emissions or concentrations. While feedback results are qualitatively useful and illustrative of carbon budget behavior, they apply quantitatively to particular scenarios and cases.

MESMO 2: a mechanistic marine silica cycle and coupling to a simple terrestrial scheme

Abstract. Here we describe the second version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 2), an earth system model of intermediate complexity, which consists of a dynamical ocean, dynamic-thermodynamic sea ice, and energy moisture balanced atmosphere. The new version has more realistic land ice masks and is driven by seasonal winds. A major aim in version 2 is representing the marine silica cycle mechanistically in order to investigate climate-carbon feedbacks involving diatoms, a critically important class of phytoplankton in terms of carbon export production. This is achieved in part by including iron, on which phytoplankton uptake of silicic acid depends. Also, MESMO 2 is coupled to an existing terrestrial model, which allows for the exchange of carbon, water and energy between land and the atmosphere. The coupled model, called MESMO 2E, is appropriate for more complete earth system simulations. The new version was calibrated, with the goal of preserving reasonable interior ocean ventilation and various biological production rates in the ocean and land, while simulating key features of the marine silica cycle.

Causes of variation in soil carbon simulations from CMIP5 Earth system models and comparison with observations

Abstract. Stocks of soil organic carbon represent a large component of the carbon cycle that may participate in climate change feedbacks, particularly on decadal and centennial timescales. For Earth system models (ESMs), the ability to accurately represent the global distribution of existing soil carbon stocks is a prerequisite for accurately predicting future carbon–climate feedbacks. We compared soil carbon simulations from 11 model centers to empirical data from the Harmonized World Soil Database (HWSD) and the Northern Circumpolar Soil Carbon Database (NCSCD). Model estimates of global soil carbon stocks ranged from 510 to 3040 Pg C, compared to an estimate of 1260 Pg C (with a 95% confidence interval of 890–1660 Pg C) from the HWSD. Model simulations for the high northern latitudes fell between 60 and 820 Pg C, compared to 500 Pg C (with a 95% confidence interval of 380–620 Pg C) for the NCSCD and 290 Pg C for the HWSD. Global soil carbon varied 5.9 fold across models in response to a 2.6-fold variation in global net primary productivity (NPP) and a 3.6-fold variation in global soil carbon turnover times. Model–data agreement was moderate at the biome level (R2 values ranged from 0.38 to 0.97 with a mean of 0.75); however, the spatial distribution of soil carbon simulated by the ESMs at the 1° scale was not well correlated with the HWSD (Pearson correlation coefficients less than 0.4 and root mean square errors from 9.4 to 20.8 kg C m−2). In northern latitudes where the two data sets overlapped, agreement between the HWSD and the NCSCD was poor (Pearson correlation coefficient 0.33), indicating uncertainty in empirical estimates of soil carbon. We found that a reduced complexity model dependent on NPP and soil temperature explained much of the 1° spatial variation in soil carbon within most ESMs (R2 values between 0.62 and 0.93 for 9 of 11 model centers). However, the same reduced complexity model only explained 10% of the spatial variation in HWSD soil carbon when driven by observations of NPP and temperature, implying that other drivers or processes may be more important in explaining observed soil carbon distributions. The reduced complexity model also showed that differences in simulated soil carbon across ESMs were driven by differences in simulated NPP and the parameterization of soil heterotrophic respiration (inter-model R2 = 0.93), not by structural differences between the models. Overall, our results suggest that despite fair global-scale agreement with observational data and moderate agreement at the biome scale, most ESMs cannot reproduce grid-scale variation in soil carbon and may be missing key processes. Future work should focus on improving the simulation of driving variables for soil carbon stocks and modifying model structures to include additional processes.

The responses of soil and rhizosphere respiration to simulated climatic changes vary by season

Responses of soil respiration (Rs) to anthropogenic climate change will affect terrestrial carbon storage and, thus, feed back to warming. To provide insight into how warming and changes in precipitation regimes affect the rate and temperature sensitivity of Rs and rhizosphere respiration (Rr) across the year, we subjected a New England old-field ecosystem to four levels of warming and three levels of precipitation (ambient, drought, and wet treatments). We measured Rs and heterotrophic respiration (Rh) monthly (in areas of the plots with and without plants, respectively) and estimated Rr by calculating the difference in respiration between Rs and Rh. Even in this mesic ecosystem, Rs and Rr responded strongly to the precipitation treatments. Drought reduced Rs and Rr, both annually and during the growing season. Annual cumulative Rs responded nonlinearly to precipitation treatments; both drought and supplemental precipitation suppressed Rs compared to the ambient treatment. Warming increased Rs and Rr in spring and winter when soil moisture was optimal but decreased these rates in summer when moisture was limiting. Cumulative winter Rr increased by about 200% in the high warming (approximately 3.5 degrees C) treatment. The effect of climate treatments on the temperature sensitivity of Rs depended on the season. In the fall, the drought treatment decreased apparent Q10 relative to the other precipitation treatments. The responses of Rs to warming and altered precipitation were largely driven by changes in Rr. We emphasize the importance of incorporating realistic soil moisture responses into simulations of soil carbon fluxes; the long-term effects of warming on carbon--climate feedback will depend on future precipitation regimes. Our results highlight the nonlinear responses of soil respiration to soil moisture and, to our knowledge, quantify for the first time the loss of carbon through winter rhizosphere respiration due to warming. While this additional loss is small relative to the cumulative annual flux in this system, such increases in rhizosphere respiration during the non-growing season could have greater consequences in ecosystems where they offset or reduce subsequent warming-induced gains in plant growth.

A Sensitivity Analysis of Surface Biophysical, Carbon, and Climate Impacts of Tropical Deforestation Rates in CCSM4-CNDV*

Abstract The biophysical–climate and combined biophysical and carbon–climate feedbacks of tropical deforestation rates are explored through sensitivity analyses using the Community Climate System Model 4 with prognostic carbon–nitrogen and dynamic vegetation. Simulations test 5%, 2%, 1%, and 0.5% annual deforestation rates, each paired with preservation targets of 10% per tropical tree type. Perturbations are applied over pan-tropical land but analyses also investigate responses over the subcontinental areas of the Amazon basin, central Africa, and Southeast Asia. Sensitivities [expressed as the change in a variable per million square kilometers (Mkm2) of change in tree cover] and means of selected biophysical, carbon, and climate variables during and after deforestation are compared across rates. The most apparent effect of the rates is in hastening/postponing climate change, but otherwise results show no consistent differences across rates and vary more across subcontinents (with the Amazon basin reflecting highest sensitivities in albedo and ground temperatures, and Southeast Asia for total ecosystem carbon). Additionally, biophysical feedbacks alone were found to have significant impact on climate over subcontinental scales. In the Amazon, ground temperature increase due to biophysical feedbacks is as much as 55%, and precipitation decrease up to 61%, of combined biophysical and carbon impacts. Replication with other models is required. Although it is still unclear whether a slow but prolonged deforestation differs in impacts from one that is rapid but short, the rate can still be relevant to planning with regards to the timing of impacts.

Biome <i>Q<sub>10</sub></i> and Dryness

Temperature sensitivity of soil respiration (Q10) is a critical parameter in carbon cycle models with important implications for climate-carbon feedbacks in the 21st century. The common assumption of a constant Q10, usually with a value of 2.0, was shown to be invalid by a previous model-data fusion study that reported biome-specific values of this parameter. We extend the previous analysis by demonstrating that these biome-level values of Q10 also are a function of dryness (R 2 = 0.54). When tundra and cultivated lands are excluded, the correlation is much stronger (R 2 = 0.92). Therefore dryness is the primary driver for variability in respiration-temperature sensitivity in forest and grassland ecosystems. This finding has important implications for the response of the terrestrial carbon cycle to climate change, as it implies that the increasing dryness would potentially accelerate the respiration temperature sensitivity feedback.

Positive climate feedbacks of soil microbial communities in a semi‐arid grassland

Soil microbial communities may be able to rapidly respond to changing environments in ways that change community structure and functioning, which could affect climate-carbon feedbacks. However, detecting microbial feedbacks to elevated CO(2) (eCO(2) ) or warming is hampered by concurrent changes in substrate availability and plant responses. Whether microbial communities can persistently feed back to climate change is still unknown. We overcame this problem by collecting microbial inocula at subfreezing conditions under eCO(2) and warming treatments in a semi-arid grassland field experiment. The inoculant was incubated in a sterilised soil medium at constant conditions for 30days. Microbes from eCO(2) exhibited an increased ability to decompose soil organic matter (SOM) compared with those from ambient CO(2) plots, and microbes from warmed plots exhibited increased thermal sensitivity for respiration. Microbes from the combined eCO(2) and warming plots had consistently enhanced microbial decomposition activity and thermal sensitivity. These persistent positive feedbacks of soil microbial communities to eCO(2) and warming may therefore stimulate soil C loss.

The role of the terrestrial biosphere in the glacial CO2 problem and implilcations for future carbon-climate feedback

The role of the terrestrial biosphere in the glacial CO2 problem and implilcations for future carbon-climate feedback

Reading to the end of the last glacial epoch

The Search and Discovery piece “Carbon dioxide drove the ending of the last glacial epoch” (Physics Today, June 2012, page 16) could easily mislead readers who do not read through to the final paragraph. In reference to the temporal trends in temperature and atmospheric CO2 during the last glacial retreat, early on the writer asks, “Was the increasing CO2 an important driving mechanism of the glacial retreats, or was it mostly just a consequence of those retreats? … The Antarctic data, in isolation, seem to show CO2 increase generally lagging temperature rise by a few centuries, thus suggesting that CO2 may have been more a passenger than a driver.”Deniers of the consensus view on global warming have used the above argument to try to discredit the idea that anthropogenic CO2 increase is causing temperature rise. But that argument is fallacious. It is widely recognized today that temperature and CO2 are locked in a positive feedback relation to each other, and the Vostok ice-core data provide a means of quantifying the strength of that feedback over paleoclimatic time scales.11. M. Torn, J. Harte, Geophys. Res. Lett. 33, L10703, (2006). https://doi.org/10.1029/2005GL025540 In such a feedback system, the magnitude of the effective time constants governing how temperature influences CO2 and how CO2 influences temperature determine the degree of asynchrony of temperature and CO2.As correctly stated at the end of the article, the ultimate driver of glacial–interglacial transitions has been the weak influence of varying insolation. Lag or lead, CO2 enhances that influence and is responsible for much of the paleoclimatic variation during the past million years. Moreover, the positive feedback evident in the Vostok data suggests that carbon–climate feedback could greatly enhance future warming as well.REFERENCESSection:ChooseTop of pageREFERENCES <<1. M. Torn, J. Harte, Geophys. Res. Lett. 33, L10703, (2006). https://doi.org/10.1029/2005GL025540, Google ScholarCrossref, ISI© 2012 American Institute of Physics.

Response of salt-marsh carbon accumulation to climate change

About half of annual marine carbon burial takes place in shallow water ecosystems where geomorphic and ecological stability is driven by interactions between the flow of water, vegetation growth and sediment transport. Although the sensitivity of terrestrial and deep marine carbon pools to climate change has been studied for decades, there is little understanding of how coastal carbon accumulation rates will change and potentially feed back on climate. Here we develop a numerical model of salt marsh evolution, informed by recent measurements of productivity and decomposition, and demonstrate that competition between mineral sediment deposition and organic-matter accumulation determines the net impact of climate change on carbon accumulation in intertidal wetlands. We find that the direct impact of warming on soil carbon accumulation rates is more subtle than the impact of warming-driven sea level rise, although the impact of warming increases with increasing rates of sea level rise. Our simulations suggest that the net impact of climate change will be to increase carbon burial rates in the first half of the twenty-first century, but that carbon-climate feedbacks are likely to diminish over time.

Climate extremes and grassland potential productivity

The considerable interannual variability (IAV) (∼5 PgC yr−1) observed in atmospheric CO2 is dominated by variability in terrestrial productivity. Among terrestrial ecosystems, grassland productivity IAV is greatest. Relationships between grassland productivity IAV and climate drivers are poorly explained by traditional multiple-regression approaches. We propose a novel method, the perfect-deficit approach, to identify climate drivers of grassland IAV from observational data. The maximum daily value of each ecological or meteorological variable for each day of the year, over the period of record, defines the ‘perfect’ annual curve. Deficits of these variables can be identified by comparing daily observational data for a given year against the perfect curve. Links between large deficits of ecosystem activity and extreme climate events are readily identified. We applied this approach to five grassland sites with 26 site-years of observational data. Large deficits of canopy photosynthetic capacity and evapotranspiration derived from eddy-covariance measurements, and leaf area index derived from satellite data occur together and are driven by a local-dryness index during the growing season. This new method shows great promise in using observational evidence to demonstrate how extreme climate events alter yearly dynamics of ecosystem potential productivity and exchanges with atmosphere, and shine a new light on climate–carbon feedback mechanisms.

The balance of the carbon budget

Careful analysis reveals that the global uptake of anthropogenic carbon dioxide emissions by carbon sinks has doubled during the past 50 years — but the fractions of this absorbed by land and by sea remain unclear. See Letter p.70 The current state of land and ocean carbon sinks has been the subject of intense debate, because it has implications for how the carbon cycle might respond to climate change. About half of the current carbon dioxide emissions are taken up by land and ocean carbon sinks. Model studies predict a decline in future carbon sinks, resulting in a positive carbon-climate feedback, and several recent studies have suggested that land and ocean carbon sinks are beginning to wane. These authors use a global mass balance approach to audit the global carbon cycle, focusing on well-constrained observations of atmospheric carbon dioxide and estimates of anthropogenic emissions and a rigorous analysis of uncertainties. They find that carbon sinks have actually doubled during the past 50 years and continue to increase significantly. There were no signs, as of 2010, that carbon uptake has started to diminish on the global scale

Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years

One of the greatest sources of uncertainty for future climate predictions is the response of the global carbon cycle to climate change. Although approximately one-half of total CO(2) emissions is at present taken up by combined land and ocean carbon reservoirs, models predict a decline in future carbon uptake by these reservoirs, resulting in a positive carbon-climate feedback. Several recent studies suggest that rates of carbon uptake by the land and ocean have remained constant or declined in recent decades. Other work, however, has called into question the reported decline. Here we use global-scale atmospheric CO(2) measurements, CO(2) emission inventories and their full range of uncertainties to calculate changes in global CO(2) sources and sinks during the past 50 years. Our mass balance analysis shows that net global carbon uptake has increased significantly by about 0.05 billion tonnes of carbon per year and that global carbon uptake doubled, from 2.4 ± 0.8 to 5.0 ± 0.9 billion tonnes per year, between 1960 and 2010. Therefore, it is very unlikely that both land and ocean carbon sinks have decreased on a global scale. Since 1959, approximately 350 billion tonnes of carbon have been emitted by humans to the atmosphere, of which about 55 per cent has moved into the land and oceans. Thus, identifying the mechanisms and locations responsible for increasing global carbon uptake remains a critical challenge in constraining the modern global carbon budget and predicting future carbon-climate interactions.

Large methane emission upon spring thaw from natural wetlands in the northern permafrost region

The permafrost carbon–climate feedback is one of the major mechanisms in controlling the climate–ecosystem interactions in northern high latitudes. Of this feedback, methane (CH4) emission from natural wetlands is critically important due to its high warming potential. The freeze–thaw transition has been confirmed to play an important role in annual CH4 budget, yet the magnitude of this effect is uncertain. An intensive field campaign was carried out in the Sanjiang Plain, Northeast China to estimate the CH4 emission in the spring freeze–thaw transition period. The observation concluded that a large CH4 source was caused by spring thaw; the maximum hourly emission rate was 48.6 g C m−2 h−1, more than three orders of the regularly observed CH4 emission rate in the growing season. In some sporadically observed ‘hot spots’, the spring thawing effect contributed to a large CH4 source of 31.3± 10.1 g C m −2, which is approximately 80% of the previously calculated annual CH4 emission in the same study area. If our results are typical for natural wetlands in the Northern Hemisphere permafrost region, we estimate a global CH4 source strength of 0.5–1.0 Tg C (1 Tg =1012 g) caused by spring thaw in the Northern Hemisphere permafrost region in the year 2011. Combining with available satellite and flask data, a regional extrapolation reaches a temporal pattern of CH4 emission during 2003–2009 which is consistent with recently observed changes in atmospheric CH4 concentration in the high latitudes. This suggests that the CH4 emission upon spring thaw in the high latitudes might be enhanced by the projected climate warming. These findings indicate that the spring thawing effect is an important mechanism in the permafrost carbon–climate feedback and needs to be incorporated in Earth system models.

Quantification of terrestrial ecosystem carbon dynamics in the conterminous United States combining a process-based biogeochemical model and MODIS and AmeriFlux data

Abstract. Satellite remote sensing provides continuous temporal and spatial information of terrestrial ecosystems. Using these remote sensing data and eddy flux measurements and biogeochemical models, such as the Terrestrial Ecosystem Model (TEM), should provide a more adequate quantification of carbon dynamics of terrestrial ecosystems. Here we use Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI), Land Surface Water Index (LSWI) and carbon flux data of AmeriFlux to conduct such a study. We first modify the gross primary production (GPP) modeling in TEM by incorporating EVI and LSWI to account for the effects of the changes of canopy photosynthetic capacity, phenology and water stress. Second, we parameterize and verify the new version of TEM with eddy flux data. We then apply the model to the conterminous United States over the period 2000–2005 at a 0.05° × 0.05° spatial resolution. We find that the new version of TEM made improvement over the previous version and generally captured the expected temporal and spatial patterns of regional carbon dynamics. We estimate that regional GPP is between 7.02 and 7.78 Pg C yr−1 and net primary production (NPP) ranges from 3.81 to 4.38 Pg C yr−1 and net ecosystem production (NEP) varies within 0.08–0.73 Pg C yr−1 over the period 2000–2005 for the conterminous United States. The uncertainty due to parameterization is 0.34, 0.65 and 0.18 Pg C yr−1 for the regional estimates of GPP, NPP and NEP, respectively. The effects of extreme climate and disturbances such as severe drought in 2002 and destructive Hurricane Katrina in 2005 were captured by the model. Our study provides a new independent and more adequate measure of carbon fluxes for the conterminous United States, which will benefit studies of carbon-climate feedback and facilitate policy-making of carbon management and climate.

A framework for representing microbial decomposition in coupled climate models

Accurate prediction of future atmospheric CO2 concentrations is essential for evaluating climate change impacts on ecosystems and human societies. One major source of uncertainty in model predictions is the extent to which global warming will increase atmospheric CO2 concentrations through enhanced microbial decomposition of soil organic carbon. Recent advances in microbial ecology could help reduce this uncertainty, but current global models do not represent direct microbial control over decomposition. Instead, all of the coupled climate models reviewed in the most recent Intergovernmental Panel on Climate Change (IPCC) report assume that decomposition is a first-order decay process, proportional to the size of the soil carbon pool. Here we argue for the development of a new generation of models that link decomposition directly to the size and activity of microbial communities in coupled global models. This process begins with the formulation and validation of fine-scale models that capture fundamental microbial mechanisms without excessive mathematical complexity. These mechanistic models must then be scaled up through an aggregation process and validated at ecosystem to global scales prior to incorporation into global climate models (GCMs). Parameterizing microbial models at the global scale is challenging because some microbial properties such as in situ extracellular enzyme activities are very difficult to measure directly. New parameter fitting procedures may therefore be needed to infer the values of important microbial variables. Validating decomposition models at the global scale is also a challenge, and has not yet been accomplished with the land carbon models embedded in current GCMs. Fortunately new global datasets on soil carbon stocks and fluxes offer promising opportunities to validate both existing land carbon models and new microbial models. If challenges in scaling, parameterization, and validation can be overcome, a new generation of microbially-based decomposition models could substantially improve predictions of carbon–climate feedbacks in the Earth system. Development of new models structures would also reduce any bias due to the assumption of first-order decomposition across all of the models currently referenced in reports of the IPCC.

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 CH(4) 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 CO(2) 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 CO(2) 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 CH(4)/y to 41-70 Tg CH(4)/y, with increases due to CO(2) fertilization, permafrost thaw, and warming-induced increased CH(4) flux densities partially offset by a reduction in wetland extent.

Controls on winter ecosystem respiration in temperate and boreal ecosystems

Abstract. Winter CO2 fluxes represent an important component of the annual carbon budget in northern ecosystems. Understanding winter respiration processes and their responses to climate change is also central to our ability to assess terrestrial carbon cycle and climate feedbacks in the future. However, the factors influencing the spatial and temporal patterns of winter ecosystem respiration (Reco) of northern ecosystems are poorly understood. For this reason, we analyzed eddy covariance flux data from 57 ecosystem sites ranging from ~35° N to ~70° N. Deciduous forests were characterized by the highest winter Reco rates (0.90 &amp;amp;pm; 0.39 g C m−2 d−1), when winter is defined as the period during which daily air temperature remains below 0 °C. By contrast, arctic wetlands had the lowest winter Reco rates (0.02 &amp;amp;pm; 0.02 g C m−2 d−1). Mixed forests, evergreen needle-leaved forests, grasslands, croplands and boreal wetlands were characterized by intermediate winter Reco rates (g C m−2 d−1) of 0.70(±0.33), 0.60(±0.38), 0.62(±0.43), 0.49(±0.22) and 0.27(±0.08), respectively. Our cross site analysis showed that winter air (Tair) and soil (Tsoil) temperature played a dominating role in determining the spatial patterns of winter Reco in both forest and managed ecosystems (grasslands and croplands). Besides temperature, the seasonal amplitude of the leaf area index (LAI), inferred from satellite observation, or growing season gross primary productivity, which we use here as a proxy for the amount of recent carbon available for Reco in the subsequent winter, played a marginal role in winter CO2 emissions from forest ecosystems. We found that winter Reco sensitivity to temperature variation across space (QS) was higher than the one over time (interannual, QT). This can be expected because QS not only accounts for climate gradients across sites but also for (positively correlated) the spatial variability of substrate quantity. Thus, if the models estimate future warming impacts on Reco based on QS rather than QT, this could overestimate the impact of temperature changes.

Role of volcanic forcing on future global carbon cycle

Abstract. Using a fully coupled global climate-carbon cycle model, we assess the potential role of volcanic eruptions on future projection of climate change and its associated carbon cycle feedback. The volcanic-like forcings are applied together with a business-as-usual IPCC-A2 carbon emissions scenario. We show that very large volcanic eruptions similar to Tambora lead to short-term substantial global cooling. However, over a long period, smaller eruptions similar to Pinatubo in amplitude, but set to occur frequently, would have a stronger impact on future climate change. In a scenario where the volcanic external forcings are prescribed with a five-year frequency, the induced cooling immediately lower the global temperature by more than one degree before it returns to the warming trend. Therefore, the climate change is approximately delayed by several decades, and by the end of the 21st century, the warming is still below two degrees when compared to the present day period. Our climate-carbon feedback analysis shows that future volcanic eruptions induce positive feedbacks (i.e., more carbon sink) on both the terrestrial and oceanic carbon cycle. The feedback signal on the ocean is consistently smaller than the terrestrial counterpart and the feedback strength is proportionally related to the frequency of the volcanic eruption events. The cooler climate reduces the terrestrial heterotrophic respiration in the northern high latitude and increases net primary production in the tropics, which contributes to more than 45 % increase in accumulated carbon uptake over land. The increased solubility of CO2 gas in seawater associated with cooler SST is offset by a reduced CO2 partial pressure gradient between the ocean and the atmosphere, which results in small changes in net ocean carbon uptake. Similarly, there is nearly no change in the seawater buffer capacity simulated between the different volcanic scenarios. Our study shows that even in the relatively extreme scenario where large volcanic eruptions occur every five-years period, the induced cooling leads to a reduction of 46 ppmv atmospheric CO2 concentration as compared to the reference projection of 878 ppmv, at the end of the 21st century.

Monitoring and interpreting the ocean uptake of atmospheric CO 2

The oceans are an important sink for anthropogenically produced CO(2), and on time scales longer than a century they will be the main repository for the CO(2) that humans are emitting. Our knowledge of how ocean uptake varies (regionally and temporally) and the processes that control it is currently observation-limited. Traditionally, and based on sparse observations and models at coarse resolution, ocean uptake has been thought to be relatively invariant. However, in the few places where we have enough observations to define the uptake over periods of many years or decades, it has been found to change substantially at basin scales, responding to indices of climate variability. We illustrate this for three well-studied regions: the equatorial Pacific, the Indian Ocean sector of the Southern Ocean, and the North Atlantic. A lesson to take from this is that ocean uptake is sensitive to climate (regionally, but presumably also globally). This reinforces the expectation that, as global climate changes in the future owing to human influences, ocean uptake of CO(2) will respond. To evaluate and give early warning of such carbon-climate feedbacks, it is important to track trends in both ocean and land sinks for CO(2). Recent coordinated observational programmes have shown that, by organization of an observing network, the atmosphere-ocean flux of CO(2) can, in principle, be accurately tracked at seasonal or better resolution, over at least the Northern Hemisphere oceans. This would provide a valuable constraint on both the ocean and (by difference) land vegetation sinks for atmospheric CO(2).