Land Carbon Storage Research Articles

Sources of Uncertainty in Modeled Land Carbon Storage within and across Three MIPs: Diagnosis with Three New Techniques

Terrestrial carbon cycle models have incorporated increasingly more processes as a means to achieve more-realistic representations of ecosystem carbon cycling. Despite this, there are large across-model variations in the simulation and projection of carbon cycling. Several model intercomparison projects (MIPs), for example, the fifth phase of the Coupled Model Intercomparison Project (CMIP5) (historical simulations), Trends in Net Land–Atmosphere Carbon Exchange (TRENDY), and Multiscale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP), have sought to understand intermodel differences. In this study, the authors developed a suite of new techniques to conduct post-MIP analysis to gain insights into uncertainty sources across 25 models in the three MIPs. First, terrestrial carbon storage dynamics were characterized by a three-dimensional (3D) model output space with coordinates of carbon residence time, net primary productivity (NPP), and carbon storage potential. The latter represents the potential of an ecosystem to lose or gain carbon. This space can be used to measure how and why model output differs. Models with a nitrogen cycle generally exhibit lower annual NPP in comparison with other models, and mostly negative carbon storage potential. Second, a transient traceability framework was used to decompose any given carbon cycle model into traceable components and identify the sources of model differences. The carbon residence time (or NPP) was traced to baseline carbon residence time (or baseline NPP related to the maximum carbon input), environmental scalars, and climate forcing. Third, by applying a variance decomposition method, the authors show that the intermodel differences in carbon storage can be mainly attributed to the baseline carbon residence time and baseline NPP (>90% in the three MIPs). The three techniques developed in this study offer a novel approach to gain more insight from existing MIPs and can point out directions for future MIPs. Since this study is conducted at the global scale for an overview on intermodel differences, future studies should focus more on regional analysis to identify the sources of uncertainties and improve models at the specified mechanism level.

1980-2010年北京市农用地碳储量对土地利用变化的响应

1980-2010年北京市农用地碳储量对土地利用变化的响应

Challenges and prospects in connectivity analysis in agricultural systems: Actions to implement policies on land management and carbon storage at EU level

There is an increasing interest in climate change mitigation agricultural activities and the effects on carbon (C) sinks. It is essential to study, propose and implement agricultural practices to protect the environment and increase crop yields, whilst at the same time aiming to reduce agricultural emissions and increase C sequestration. Further study of these aspects is important in order to identify the best land management practices and policies to protect the environment. So with this objective in mind, the study of connectivity is an opportunity to gain insight into the sustainability of agroecosystems, due to the close relationship among connectivity, biodiversity and C storage. In fact, high connectivity values can be associated with healthy ecosystems and high levels of biodiversity, which can assure better soil protection and can favor sustainability, implying a greater soil C sequestration capacity.The measure of connectivity’s temporal variation using analysis of current connectivity (inter- and intra-compartments) and the development of risk and vulnerability maps are key facts to, on one side, identify areas most prone to habitat degradation and fragmentation, and then, on another side to establish priorities. This proposed analysis must be carried out by sectors and in a stratified way. For it, “Sector agricultural connectivity units” are proposed as a dimensionless measuring concept to implement this assessment.To establish agricultural land management schemes and programs for policy intervention at European level in order to guide the strategy to address environmental issues in the agricultural sector, we propose a set of lines of action focused mainly on three aspects: the development of National Action Plans, the establishment of Monitoring Programs, and the integration of their outcomes with the information from available databases through Big Data treatments and other analysis tools. Furthermore, to achieve this goal, the role of farmers can not be underestimated and the communication problems between farmers and scientists should be solved.

Transient Traceability Analysis of Land Carbon Storage Dynamics: Procedures and Its Application to Two Forest Ecosystems

AbstractUptake of anthropogenically emitted carbon (C) dioxide by terrestrial ecosystem is critical for determining future climate. However, Earth system models project large uncertainties in future C storage. To help identify sources of uncertainties in model predictions, this study develops a transient traceability framework to trace components of C storage dynamics. Transient C storage (X) can be decomposed into two components, C storage capacity (Xc) and C storage potential (Xp). Xc is the maximum C amount that an ecosystem can potentially store and Xp represents the internal capacity of an ecosystem to equilibrate C input and output for a network of pools. Xc is codetermined by net primary production (NPP) and residence time (τN), with the latter being determined by allocation coefficients, transfer coefficients, environmental scalar, and exit rate. Xp is the product of redistribution matrix (τch) and net ecosystem exchange. We applied this framework to two contrasting ecosystems, Duke Forest and Harvard Forest with an ecosystem model. This framework helps identify the mechanisms underlying the responses of carbon cycling in the two forests to climate change. The temporal trajectories of X are similar between the two ecosystems. Using this framework, we found that different mechanisms lead to a similar trajectory between the two ecosystems. This framework has potential to reveal mechanisms behind transient C storage in response to various global change factors. It can also identify sources of uncertainties in predicted transient C storage across models and can therefore be useful for model intercomparison.

The potential of Eucalyptus plantations to restore degraded soils in semi-arid Morocco (NW Africa)

Short-rotation forestry using eucalyptus in degraded oak forests in the semi-arid area of NW Morocco can be a useful strategy to avoid further degradation and carbon loss from this ecosystem, but it might be constrained by nutrient and water supply in the long term. Land degradation and deforestation of natural forests are serious issues worldwide, potentially leading to altered land use and carbon storage capacity. Our objectives were to investigate if short-rotation plantations can restore carbon pools of degraded soils, without altering soil fertility. Carbon and nutrient pools in above- and below-ground biomass and soils were assessed using stand inventories, harvested biomass values, allometric relationships and selective sampling for chemical analyses. Carbon pools in the total ecosystem were low in the degraded land and in croplands (6–13 Mg ha−1) and high in forests (66–94 in eucalyptus plantations; 86–126 in native forests). The soil nutrient status of eucalyptus stands was intermediate between degraded land and native forests and increased over time after eucalyptus introduction. All harvest scenarios for eucalyptus are likely to impoverish the soil but, for the moment, the soil nutrient status has not been affected. Afforestation of degraded land with eucalyptus can be a useful restoration tool relative to carbon storage and soil fertility, provided that non-intensive forestry is applied.

Biophysical consequences of photosynthetic temperature acclimation for climate

AbstractPhotosynthetic temperature acclimation is a commonly observed process that is increasingly being incorporated into Earth System Models (ESMs). While short‐term acclimation has been shown to increase carbon storage in the future, it is uncertain whether acclimation will directly influence simulated future climate through biophysical mechanisms. Here, we used coupled atmosphere‐biosphere simulations using the Community Earth System Model (CESM) to assess how acclimation‐induced changes in photosynthesis influence global climate under present‐day and future (RCP 8.5) conditions. We ran four 30 year simulations that differed only in sea surface temperatures and atmospheric CO2 (present or future) and whether a mechanism for photosynthetic temperature acclimation was included (yes or no). Acclimation increased future photosynthesis and, consequently, the proportion of energy returned to the atmosphere as latent heat, resulting in reduced surface air temperatures in areas and seasons where acclimation caused the biggest increase in photosynthesis. However, this was partially offset by temperature increases elsewhere, resulting in a small, but significant, global cooling of 0.05°C in the future, similar to that expected from acclimation‐induced increases in future land carbon storage found in previous studies. In the present‐day simulations, the photosynthetic response was not as strong and cooling in highly vegetated regions was less than warming elsewhere, leading to a net global increase in temperatures of 0.04°C. Precipitation responses were variable and rates did not change globally in either time period. These results, combined with carbon‐cycle effects, suggest that models without acclimation may be overestimating positive feedbacks between climate and the land surface in the future.

Carbon cycle and climate effects of forcing from fire-emitted aerosols

Aerosols emitted by landscape fires affect many climatic processes. Here, we combined an aerosol–climate model and a coupled climate–carbon model to study the carbon cycle and climate effects caused by fire-emitted aerosols (FEA) forcing at the top of the atmosphere and at the surface. This forcing (‘best guess’ present-day values of −0.10 and −1.3 W m−2 at the top of the atmosphere and surface, respectively) had a predominant cooling influence that altered regional land carbon stocks on decadal timescales by modifying vegetation productivity and soil–litter decomposition. Changes in regional land and ocean carbon stocks became much stronger for FEA forcing acting on multi-century timescales; this occurred because carbon stocks responded to the forcing itself on such timescales and also due to gradual effects on the climate (e.g. through increased sea ice cover) that further affected the carbon cycle. Carbon increases and decreases in different regions partly offset each other, so that absolute changes in global land, atmosphere, and ocean stocks were all <2 Pg C after 30 years of FEA forcing and <6 Pg C after more than 1000 years of FEA forcing. FEA-caused changes in land carbon storage did not substantially modify the magnitude of FEA emissions, suggesting there is no consequential regional-scale positive feedback loop between these two elements. However, we found indications that the FEA-caused cooling from frequently-burning regions in Africa and Australia increased land carbon stocks in eastern South America and equatorial Asia, respectively. This suggests the potential for remote carbon cycle effects from regions emitting large amounts of fire aerosols.

Mapping peat in Scotland with remote sensing and site characteristics

Mapping the spatial distribution of organic soil is important for decision making in land management. Organic soil types have a strong effect on carbon storage, water availability and quality, biodiversity and many other ecosystem services. A large proportion of Scotland's soil is organic, and is classed as peats or organomineral. Existing soil maps for Scotland include a 1:25 000 map of eastern and southern parts of the country that identifies the major soil subgroup at each location, or a 1:250 000 map covering the whole country that gives proportions of the major soil group within mapping units. A detailed map of the organic soils of Scotland that disaggregates these mapping units is therefore required. Existing data were used from several Scottish soil surveys and remote sensing data (Landsat 8) to develop a model for predicting whether a site has mineral, organomineral (mineral with peaty topsoil) or organic (peat) soil. The model was applied across Scotland at a grid resolution of 100 m. Predictive accuracies of 68.1% (mineral), 61.3% (organomineral) and 81.8% (organic) were obtained, with an overall prediction accuracy of 69.8%. Training the model with two classes only, non‐organic (including mineral and organomineral) and organic, gave accuracies of 83.9 and 93.6%, respectively, and an overall accuracy of 86.4%. A map of prediction confidence was also produced. The results show how the approach can be used to disaggregate the existing map units and predict mineral and organic soils at locations within a map unit. The map is being used to prioritize monitoring efforts in catchments across Scotland.Highlights Use of remote sensing and spatial covariates to map peat in Scotland. Resulting peat maps will be useful for decision making on land management and carbon storage in soil. Peat is mapped accurately in most areas, although uncertainty is greater in complex landscapes. Combination of remote sensing and spatial covariates improves accuracy of mapping.

Current Browning of Surface Waters Will Be Further Promoted by Wetter Climate

Browning of surface waters because of increasing terrestrial dissolved organic carbon (OC) concentrations is a concern for drinking water providers and can impact land carbon storage. We show that positive trends in OC in 474 streams, lakes, and rivers in boreal and subarctic ecosystems in Norway, Sweden, and Finland between 1990 and 2013 are surprisingly constant across climatic gradients and catchment sizes (median, +1.4% year–1; interquartile range, +0.8–2.0% year–1), implying that water bodies across the entire landscape are browning. The largest trends (median, +1.7% year–1) were found in regions impacted by strong reductions in sulfur deposition, while subarctic regions showed the least browning (median, +0.8% year–1). In dry regions, precipitation was a strong and positive driver of OC concentrations, declining in strength moving toward high rainfall sites. We estimate that a 10% increase in precipitation will increase mobilization of OC from soils to freshwaters by at least 30%, demonstrating the ...

Effect of biogenic carbon inventory on the life cycle assessment of bioenergy: challenges to the neutrality assumption

Biogenic carbon is defined as carbon contained in biomass that is accumulated during plant growth. In spite of the considerable progress towards the inventory of biogenic carbon in the life cycle assessment (LCA) of bioenergy in policy guidelines, many scientific articles tend to give no consideration to biogenic carbon, due to the neutrality assumption, rather than to employ a complete inventory according to the LCA principles. Meanwhile, the assumption of biogenic carbon neutrality has been previously challenged on the basis of changes in soil carbon stock due to land use change and carbon storage capacities of long-rotation trees or wood products. Supporting this argument, we investigate three other inventory aspects which strongly affect final results, namely, differences in framing system boundaries (cradle to grave vs. cradle to gate), forms of carbon emissions (carbon dioxide vs. methane), and valuation of biogenic carbon (relative economic values of biomass products vs. biomass residues). Referring to a generic bioenergy system, our analysis is focused on eight scenarios of various carbon flows encompassing biomass decomposition in fields and its alternative utilization as bioenergy feedstocks. These scenarios are applicable to both biomass products and biomass residues, for which the impacts proportionally depend on the chosen allocation criteria between the two. Further, a framework to quantify the performances of the various possible carbon flows on global warming impacts is formulated. The operation of the framework demonstrates that the assumption of biogenic carbon neutrality introduces a bias to the ‘true’ values based on a complete inventory. This can make the values of global warming impacts substantially higher or lower than the real scores when different system boundaries, forms of carbon emissions, and biomass valuation are taken into account. The results of this study could contribute to the harmonization of future bioenergy LCA by directing further research to adopt more the concept of utilizing a complete inventory rather than the neutrality assumption.

Spatiotemporal Changes of Built-Up Land Expansion and Carbon Emissions Caused by the Chinese Construction Industry.

China is undergoing rapid urbanization, enlarging the construction industry, greatly expanding built-up land, and generating substantial carbon emissions. We calculated both the direct and indirect carbon emissions from energy consumption (anthropogenic emissions) in the construction sector and analyzed built-up land expansion and carbon storage losses from the terrestrial ecosystem. According to our study, the total anthropogenic carbon emissions from the construction sector increased from 3,905×10(4) to 103,721.17×10(4) t from 1995 to 2010, representing 27.87%-34.31% of the total carbon emissions from energy consumption in China. Indirect carbon emissions from other industrial sectors induced by the construction sector represented approximately 97% of the total anthropogenic carbon emissions of the sector. These emissions were mainly concentrated in seven upstream industry sectors. Based on our assumptions, built-up land expansion caused 3704.84×10(4) t of carbon storage loss from vegetation between 1995 and 2010. Cropland was the main built-up land expansion type across all regions. The study shows great regional differences. Coastal regions showed dramatic built-up land expansion, greater carbon storage losses from vegetation, and greater anthropogenic carbon emissions. These regional differences were the most obvious in East China followed by Midsouth China. These regions are under pressure for strong carbon emissions reduction.

Human land-use-driven reduction of forest volatiles cools global climate

The global radiative effects of historical cropland expansion are typically estimated as the trade-off between reduced land carbon storage (causing warming) and increased surface albedo (causing cooling). Now research shows that the net atmospheric chemistry effect (−0.11 ± 0.17 W m−2) is of comparable magnitude and should also be taken into account.

Emergent constraints on climate‐carbon cycle feedbacks in the CMIP5 Earth system models

AbstractAn emergent linear relationship between the long‐term sensitivity of tropical land carbon storage to climate warming (γLT) and the short‐term sensitivity of atmospheric carbon dioxide (CO2) to interannual temperature variability (γIAV) has previously been identified by Cox et al. (2013) across an ensemble of Earth system models (ESMs) participating in the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Here we examine whether such a constraint also holds for a new set of eight ESMs participating in Phase 5 of the Coupled Model Intercomparison Project. A wide spread in tropical land carbon storage is found for the quadrupling of atmospheric CO2, which is of the order of 252 ± 112 GtC when carbon‐climate feedbacks are enabled. Correspondingly, the spread in γLT is wide (−49 ± 40 GtC/K) and thus remains one of the key uncertainties in climate projections. A tight correlation is found between the long‐term sensitivity of tropical land carbon and the short‐term sensitivity of atmospheric CO2 (γLT versus γIAV), which enables the projections to be constrained with observations. The observed short‐term sensitivity of CO2 (−4.4 ± 0.9 GtC/yr/K) sharpens the range of γLT to −44 ± 14 GtC/K, which overlaps with the probability density function derived from the C4MIP models (−53 ± 17 GtC/K) by Cox et al. (2013), even though the lines relating γLT and γIAV differ in the two cases. Emergent constraints of this type provide a means to focus ESM evaluation against observations on the metrics most relevant to projections of future climate change.

Sensitivity of global and regional terrestrial carbon storage to the direct CO2 effect and climate change based on the CMIP5 model intercomparison.

Global and regional land carbon storage has been significantly affected by increasing atmospheric CO2 concentration and climate change. Based on fully coupled climate-carbon-cycle simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we investigate sensitivities of land carbon storage to rising atmospheric CO2 concentration and climate change over the world and 21 regions during the 130 years. Overall, the simulations suggest that consistently spatial positive effects of the increasing CO2 concentrations on land carbon storage are expressed with a multi-model averaged value of 1.04PgC per ppm. The stronger positive values are mainly located in the broad areas of temperate and tropical forest, especially in Amazon basin and western Africa. However, large heterogeneity distributed for sensitivities of land carbon storage to climate change. Climate change causes decrease in land carbon storage in most tropics and the Southern Hemisphere. In these regions, decrease in soil moisture (MRSO) and enhanced drought somewhat contribute to such a decrease accompanied with rising temperature. Conversely, an increase in land carbon storage has been observed in high latitude and altitude regions (e.g., northern Asia and Tibet). The model simulations also suggest that global negative impacts of climate change on land carbon storage are predominantly attributed to decrease in land carbon storage in tropics. Although current warming can lead to an increase in land storage of high latitudes of Northern Hemisphere due to elevated vegetation growth, a risk of exacerbated future climate change may be induced due to release of carbon from tropics.

Quantifying spatial–temporal change in land-cover and carbon storage among exurban residential parcels

The area of land occupied by exurban residential development is significant and has been increasing over the past several decades in the United States. Considerable attention has been drawn to the measurement of regional-scale patterns of land-cover change and assessment of its environmental and socioeconomic consequences. Yet little is known about the quantity of land-cover change within individual exurban residential parcels, which reflect homeowner preferences, land-management strategies, and the ecosystem services they generate. Similarly, little is known about the spatial autocorrelation of land cover among parcels and how it may change over time. Using a dataset delineating land-cover change within exurban residential parcels in southeastern Michigan from 1960 to 2000, the quantity and composition of land cover and carbon storage are examined. The spatial similarity of land-cover quantity among neighboring parcels is evaluated using local indicators of spatial association. Results show, among other findings, that (1) the number of exurban residential parcels, the quantity of tree cover, and amount of carbon storage increased steadily from 1960 to 2000; (2) the distribution of parcel sizes remained relatively constant and dominated by small parcels; (3) an increasing proportion of parcels were significantly similar to their neighbors; and (4) using a benefits transfer approach, new exurban parcels are estimated to store ~15,000–29,000 kg C over the study period. The measured changes in land cover and carbon storage improve our understanding of how ecosystem services may change in human-dominated landscapes and provide evidence that policy opportunities are available to increase carbon management.

Climate and carbon cycle response to the 1815 Tambora volcanic eruption

The sulphur released by the 1815 Tambora volcanic eruption resulted in a net cooling after the eruption. The cold climate was responsible for crop failures, leading to serious famine and high food prices in Europe and North America. The year 1816 became known as the “year without summer”. We performed a series of climate simulations with the UK Met Office model HadGEM2‐ES to assess the climate and carbon cycle consequences of the eruption. The model shows a temperature decrease of 1 ± 0.1°C and global precipitation decrease of 3.7% in 1816. The following net primary productivity (NPP) increase is caused by strongly reduced plant respiration and supports the overall increase in land carbon storage after the eruption. Most of the carbon is taken up by the soil reservoir, mainly due to increased litter influx. Overall, the change of combined land and ocean carbon implies an atmospheric CO2 decrease of over 6 ppmv. C3 and C4 grasses, used here as an analogy for crops, revealed globally increasing productivity for C3 grasses/crops (e.g., wheat) by ≥8%, while C4 grasses/crops (e.g., maize) decreased by over 12%. Regional positive C3 and negative C4 NPP are mainly found in the tropics and midlatitudes, whereas positive C4 NPP areas are distributed in marginal areas. Negative C3 grasses anomalies are found in high‐elevation and high‐latitude regions. These findings highlight the importance of including process‐based vegetation or crop model components to represent the potentially nonlinear dependencies on climatic changes.

Effect of Anthropogenic Land-Use and Land-Cover Changes on Climate and Land Carbon Storage in CMIP5 Projections for the Twenty-First Century

Abstract The 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 Global Environmental Model, version 2 (Earth System) (HadGEM2-ES); and Model for Interdisciplinary Research on Climate, Earth System Model (MIROC-ESM)—reveal statistically significant changes in mean annual surface air temperature. In addition, changes in land surface albedo, available energy, and latent heat fluxes are small but significant for most ESMs in regions affected by land-use changes. These climatic effects are relatively small, as land-use changes in the RCP2.6 and RCP8.5 scenarios are small in magnitude and mainly limited to tropical and subtropical regions. The relative importance of the climatic effects of land-use changes is higher for the RCP2.6 scenario, which considers an expansion of biofuel croplands as a climate mitigation option. The underlying similarity among all models is the loss in global land carbon storage due to land-use changes.

Global patterns of nitrogen limitation: confronting two global biogeochemical models with observations

Projections of future changes in land carbon (C) storage using biogeochemical models depend on accurately modeling the interactions between the C and nitrogen (N) cycles. Here, we present a framework for analyzing N limitation in global biogeochemical models to explore how C-N interactions of current models compare to field observations, identify the processes causing model divergence, and identify future observation and experiment needs. We used a set of N-fertilization simulations from two global biogeochemical models (CLM-CN and O-CN) that use different approaches to modeling C-N interactions. On the global scale, net primary productivity (NPP) in the CLM-CN model was substantially more responsive to N fertilization than in the O-CN model. The most striking difference between the two models occurred for humid tropical forests, where the CLM-CN simulated a 62% increase in NPP at high N addition levels (30 g N m(-2) yr(-1)), while the O-CN predicted a 2% decrease in NPP due to N fertilization increasing plant respiration more than photosynthesis. Across 35 temperate and boreal forest sites with field N-fertilization experiments, we show that the CLM-CN simulated a 46% increase in aboveground NPP in response to N, which exceeded the observed increase of 25%. In contrast, the O-CN only simulated a 6% increase in aboveground NPP at the N-fertilization sites. Despite the small response of NPP to N fertilization, the O-CN model accurately simulated ecosystem retention of N and the fate of added N to vegetation when compared to empirical (15) N tracer application studies. In contrast, the CLM-CN predicted lower total ecosystem N retention and partitioned more losses to volatilization than estimated from observed N budgets of small catchments. These results point to the need for model improvements in both models in order to enhance the accuracy with which global C-N cycle feedbacks are simulated.

Twenty-First-Century Compatible CO2 Emissions and Airborne Fraction Simulated by CMIP5 Earth System Models under Four Representative Concentration Pathways

Abstract The carbon cycle is a crucial Earth system component affecting climate and atmospheric composition. The response of natural carbon uptake to CO2 and climate change will determine anthropogenic emissions compatible with a target CO2 pathway. For phase 5 of the Coupled Model Intercomparison Project (CMIP5), four future representative concentration pathways (RCPs) have been generated by integrated assessment models (IAMs) and used as scenarios by state-of-the-art climate models, enabling quantification of compatible carbon emissions for the four scenarios by complex, process-based models. Here, the authors present results from 15 such Earth system GCMs for future changes in land and ocean carbon storage and the implications for anthropogenic emissions. The results are consistent with the underlying scenarios but show substantial model spread. Uncertainty in land carbon uptake due to differences among models is comparable with the spread across scenarios. Model estimates of historical fossil-fuel emissions agree well with reconstructions, and future projections for representative concentration pathway 2.6 (RCP2.6) and RCP4.5 are consistent with the IAMs. For high-end scenarios (RCP6.0 and RCP8.5), GCMs simulate smaller compatible emissions than the IAMs, indicating a larger climate–carbon cycle feedback in the GCMs in these scenarios. For the RCP2.6 mitigation scenario, an average reduction of 50% in emissions by 2050 from 1990 levels is required but with very large model spread (14%–96%). The models also disagree on both the requirement for sustained negative emissions to achieve the RCP2.6 CO2 concentration and the success of this scenario to restrict global warming below 2°C. All models agree that the future airborne fraction depends strongly on the emissions profile with higher airborne fraction for higher emissions scenarios.

Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability

The release of carbon from tropical forests may exacerbate future climate change, but the magnitude of the effect in climate models remains uncertain. Coupled climate-carbon-cycle models generally agree that carbon storage on land will increase as a result of the simultaneous enhancement of plant photosynthesis and water use efficiency under higher atmospheric CO(2) concentrations, but will decrease owing to higher soil and plant respiration rates associated with warming temperatures. At present, the balance between these effects varies markedly among coupled climate-carbon-cycle models, leading to a range of 330 gigatonnes in the projected change in the amount of carbon stored on tropical land by 2100. Explanations for this large uncertainty include differences in the predicted change in rainfall in Amazonia and variations in the responses of alternative vegetation models to warming. Here we identify an emergent linear relationship, across an ensemble of models, between the sensitivity of tropical land carbon storage to warming and the sensitivity of the annual growth rate of atmospheric CO(2) to tropical temperature anomalies. Combined with contemporary observations of atmospheric CO(2) concentration and tropical temperature, this relationship provides a tight constraint on the sensitivity of tropical land carbon to climate change. We estimate that over tropical land from latitude 30° north to 30° south, warming alone will release 53 ± 17 gigatonnes of carbon per kelvin. Compared with the unconstrained ensemble of climate-carbon-cycle projections, this indicates a much lower risk of Amazon forest dieback under CO(2)-induced climate change if CO(2) fertilization effects are as large as suggested by current models. Our study, however, also implies greater certainty that carbon will be lost from tropical land if warming arises from reductions in aerosols or increases in other greenhouse gases.