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The general circulation model (GCM) that we used is based on the third Hadley Centre coupled ocean-atmosphere model, HadCM3 (7), which we have coupled to an ocean carbon-cycle model (HadOCC) [...], The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate (1). About half of the current emissions are being absorbed by the ocean and by land ecosystems (2), but this absorption is sensitive to climate (3,4) as well as to atmospheric carbon dioxide concentrations (5), creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and C[O.sub.2] concentrations from simple carbon-cycle models that do not include climate change (6). Here we present results from a fully coupled, three-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt [Cyr.sup.1] is balanced by the terrestrial carbon source, and atmospheric C[O.sub.2] concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon [models.sup.2], resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.

The article discusses the need for carbon dioxide removal (CDR) to achieve net-zero emissions and the challenges of burden sharing for CDR. It highlights the inequitable distribution of past emissions and explores different definitions of equity for allocating the responsibility of CDR. The study finds that the feasibility of delivering CDR at the national level varies depending on land availability and geological storage. It concludes that international trading and a better understanding of CDR expectations and limits are necessary. The article emphasizes the urgency of addressing climate change and the growing gap between required and feasible CDR. [Extracted from the article]

Abstract The 50-year global CO2 record led the way in establishing a scientific fact: modern civilization is changing important properties of the global atmosphere, oceans and biosphere. The evidence on which this scientific fact is based will be refined further, but the next challenge for scientists is broader. In addition to its traditional role in providing discovery, diagnosis, and prediction of the changes that are taking place on our planet, science has now also a role in helping society mitigate emissions by objectively quantifying them, and in helping adaptation by providing environmental forecasts on regional scales. Science is also expected to provide new options for society to tackle the transition to a new energy system, and to provide thorough environmental evaluation of all such options. This is what the meeting recognized as planetary responsibilities for scientists in the next 50 years.

Future climate projections from Earth system models (ESMs) typically focus on the timescale of this century. We use a set of four ESMs and one Earth system model of intermediate complexity (EMIC) to explore the dynamics of the Earth's climate and carbon cycles under contrasting emissions trajectories beyond this century, to the year 2300. The trajectories include a very high emissions, unmitigated fossil-fuel driven scenario, as well as a second mitigation scenario that diverges from the first scenario after 2040 and features an "overshoot", followed by stabilization of atmospheric CO2 concentrations by means of large net-negative CO2 emissions. In both scenarios, and for all models considered here, the terrestrial system switches from being a net sink to either a neutral state or a net source of carbon, though for different reasons and centered in different geographic regions, depending on both the model and the scenario. The ocean carbon system remains a sink, albeit weakened by climate-carbon feedbacks, in all models under the high emissions scenario, and switches from sink to source in the overshoot scenario. The global mean temperature anomaly generally follows the trajectories of cumulative carbon emissions, except that 23rd-century warming continues after the cessation of carbon emissions in several models, both in the high emissions scenario and in one model in the overshoot scenario. While ocean carbon cycle responses qualitatively agree both in globally integrated and zonal-mean dynamics in both scenarios, the land models qualitatively disagree in zonal-mean dynamics, in the relative roles of vegetation and soil in driving C fluxes, in the response of the sink to CO2, and in the timing of the sink-source transition, particularly in the high emissions scenario. The lack of agreement among land models on the mechanisms and geographic patterns of carbon cycle feedbacks, alongside the potential for lagged physical climate dynamics to cause warming long after CO2 concentrations have stabilized, point to the possibility of surprises in the climate system beyond the 21st century time horizon, even under relatively mitigated global warming scenarios, which should be taken into consideration when setting global climate policy. [ABSTRACT FROM AUTHOR]

We present the compatible CO2 emissions from fossil fuel (FF) burning and industry, calculated from the historical and Shared Socioeconomic Pathway (SSP) experiments of nine Earth system models (ESMs) participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel mean FF emissions match the historical record well and are close to the data-based estimate of cumulative emissions (394 ± 59 GtC vs 400 ± 20 GtC, respectively). Only two models fall inside the observed uncertainty range; while two exceed the upper bound, five fall slightly below the lower bound, due primarily to the plateau in CO2 concentration in the 1940s. The ESMs' diagnosed FF emission rates are consistent with those generated by the integrated assessment models (IAMs) from which the SSPs' CO2 concentration pathways were constructed; the simpler IAMs' emissions lie within the ESMs' spread for seven of the eight SSP experiments, the other being only marginally lower, providing confidence in the relationship between the IAMs' FF emission rates and concentration pathways. The ESMs require fossil fuel emissions to reduce to zero and subsequently become negative in SSP1-1.9, SSP1-2.6, SSP4-3.4, and SSP5-3.4over. We also present the ocean and land carbon cycle responses of the ESMs in the historical and SSP scenarios. The models' ocean carbon cycle responses are in close agreement, but there is considerable spread in their land carbon cycle responses. Land-use and land-cover change emissions have a strong influence over the magnitude of diagnosed fossil fuel emissions, with the suggestion of an inverse relationship between the two. [ABSTRACT FROM AUTHOR]

Lockdowns to avoid the spread of COVID-19 have created an unprecedented reduction in human emissions. While the country-level scale of emissions changes can be estimated in near-real-time, the more detailed, gridded emissions estimates that are required to run General Circulation Models (GCM) of the climate will take longer to collect. In this paper we use recorded and projected country-and-sector activity levels to modify gridded predictions from the MESSAGE-GLOBIOM SSP2-4.5 scenario. We provide updated projections for concentrations of greenhouse gases, emissions fields for aerosols and precursors, and the ozone and optical properties that result from this. The codebase to perform similar modifications to other scenarios is also provided. We outline the means by which these results may be used in a model intercomparison project (CovidMIP) to investigate the impact of national lockdown measures on climate. This includes three strands: an assessment of short-term effects (5-year period), of longer-term effects (30 years) and an investigation into the separate effects of changes in emissions of greenhouse gases and aerosols. This last strand supports possible attribution of observed changes in the climate system, hence these simulations will also form part of the Detection and Attribution Model Intercomparison Project (DAMIP). [ABSTRACT FROM AUTHOR]

Results from the fully and biogeochemically coupled simulations in which CO2 increases at a rate of 1 % yr -1 (1pctCO2) from its preindustrial value are analyzed to quantify the magnitude of carbon–concentration and carbon–climate feedback parameters which measure the response of ocean and terrestrial carbon pools to changes in atmospheric CO2 concentration and the resulting change in global climate, respectively. The results are based on 11 comprehensive Earth system models from the most recent (sixth) Coupled Model Intercomparison Project (CMIP6) and compared with eight models from the fifth CMIP (CMIP5). The strength of the carbon–concentration feedback is of comparable magnitudes over land (mean ± standard deviation = 0.97 ± 0.40 PgC ppm -1) and ocean (0.79 ± 0.07 PgC ppm -1), while the carbon–climate feedback over land (-45.1 ± 50.6 PgC ∘ C -1) is about 3 times larger than over ocean (-17.2 ± 5.0 PgC ∘ C -1). The strength of both feedbacks is an order of magnitude more uncertain over land than over ocean as has been seen in existing studies. These values and their spread from 11 CMIP6 models have not changed significantly compared to CMIP5 models. The absolute values of feedback parameters are lower for land with models that include a representation of nitrogen cycle. The transient climate response to cumulative emissions (TCRE) from the 11 CMIP6 models considered here is 1.77 ± 0.37 ∘ C EgC -1 and is similar to that found in CMIP5 models (1.63 ± 0.48 ∘ C EgC -1) but with somewhat reduced model spread. The expressions for feedback parameters based on the fully and biogeochemically coupled configurations of the 1pctCO2 simulation are simplified when the small temperature change in the biogeochemically coupled simulation is ignored. Decomposition of the terms of these simplified expressions for the feedback parameters is used to gain insight into the reasons for differing responses among ocean and land carbon cycle models. [ABSTRACT FROM AUTHOR]

Understanding future changes in the terrestrial carbon cycle is important for reliable projections of climate change and impacts on ecosystems. It is known that nitrogen could limit plants' response to increased atmospheric carbon dioxide and is therefore important to include in Earth System Models. Here we present the implementation of the terrestrial nitrogen cycle in the JULES land surface model (JULES-CN). Two versions are discussed - the one implemented within the UK Earth System Model (UKESM1) which has a bulk soil biogeochemical model and a development version which resolves the soil biogeochemistry with depth. The nitrogen cycle is based on the existing carbon cycle in the model. It represents all the key terrestrial nitrogen processes in an efficient way. Biological fixation and nitrogen deposition are external inputs, and loss occurs via leaching and a bulk gas loss parameterisation. Nutrient limitation reduces carbon-use efficiency (CUE - ratio of net to gross primary productivity) and can slow soil decomposition. We show that ecosystem level limitation of net primary productivity by nitrogen is consistent with observational estimates and that simulated carbon and nitrogen pools and fluxes are comparable to the limited available observations. The impact of N limitation is most pronounced in northern mid-latitudes. The introduction of a nitrogen cycle improves the representation of interannual variability of global net ecosystem exchange which was much too pronounced in the carbon cycle only versions of JULES (JULES-C). It also reduces the CUE and alters its response over the twentieth century and limits the CO2-fertilisation effect, such that the simulated current day land carbon sink is reduced by about 0.5 Pg C yr-1. The inclusion of a prognostic land nitrogen scheme marks a step forward in functionality and realism for the JULES and UKESM models. [ABSTRACT FROM AUTHOR]