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This paper shows that the ratio of global warming to cumulative CO₂ emissions is constant due to complex interactions of physical and biogeochemical processes, and not because heat and carbon are mixed into the ocean by similar processes.

Estimates of carbon budgets compatible with limiting warming to below specific temperature limits are reviewed, and reasons underlying their differences discussed along with their respective strengths and limitations.

A re-assessment of the global carbon budget shows the natural land sink is substantially smaller than previously estimated, indicating emerging impacts of climate change on the evolution of the carbon sinks.

There is no single correct procedure for the attribution of responsibility for growth in atmospheric CO₂ concentrations because results are closely dependant on how carbon sinks are accounted for and linked to emissions. Now research that uses two different approaches—one assuming geographically constrained sinks and the other unconstrained—unambiguously attributes the largest share of the historical increase in CO₂ to developed countries.

Strong mitigation of anthropogenic emissions is necessary, but it is not clear how fast these efforts would lead to temperature changes. Here, the authors find that there is a substantial delay between reductions of emissions and a detectable change in surface temperature for a number of climate forcers.

The authors combine climate simulations with observations to estimate carbon budgets which are better constrained and find they are more than 10% larger than the mean value from CMIP6 models.

A linear relationship between the sensitivity of tropical land carbon storage to warming and the sensitivity of the annual growth rate of atmospheric CO₂ to tropical temperature anomalies provides a tight constraint on the sensitivity of tropical land carbon to climate change.

Northern Hemisphere photosynthesis is thought to respond positively to temperature variations, yet the strength of this relationship may change over time. Here, using a combination of satellite data and models, the authors assess the temporal change of this relationship over the past three decades.

The global net land sink is relatively well constrained. However, the responsible drivers and above/below-ground partitioning are highly uncertain. Model issues regarding turnover of individual plant and soil components are responsible.

Efforts to control climate change require the stabilization of atmospheric carbon dioxide concentrations. An assessment of the trends in sources and sinks of atmospheric carbon dioxide suggests that the sinks are not keeping up with the increase in carbon dioxide emissions, but uncertainties are still large.

Many models assume a universal carbon use efficiency across forest biomes, in contrast to assumptions of other process-based models. Here the authors analyse forest production efficiency across a wide range of climates to show a positive relationship with annual temperature and precipitation, indicating that ecosystem models are overestimating forest carbon losses under warming.

Growth in CO₂ emissions has slowed since the Paris Agreement 5 years ago. The COVID-19 pandemic has caused a drop in emissions of about 7% in 2020 relative to 2019, but strong policy is needed to address underlying drivers and to sustain a decline in global emissions beyond the current crisis.

COVID-19 pandemic lockdowns have altered global energy demands. Using government confinement policies and activity data, daily CO₂ emissions have decreased by ~17% to early April 2020 against 2019 levels; annual emissions could be down by 7% (4%) if normality returns by year end (mid-June).

Decisions about how soon, how quickly and by how much carbon dioxide emissions are reduced will determine whether the climate target of limiting warming to 2 °C can be met. Research reveals that it will probably only be possible if ambitious reductions are implemented within the next two decades and emissions eventually fall to zero.

Permafrost loss can be projected by considering its distribution against warming air temperatures. Using observations to constrain loss estimates, this study investigates loss under different levels of warming.

The model–inventory discrepancy in net land-use carbon emissions mainly results from conceptual differences in estimating anthropogenic forest sinks. A revised disaggregation of global land model results allows greater comparability with inventories.

Global net ecosystem production (NEP) from a number of atmospheric inversions and dynamic global vegetation models is analysed to attribute trends to potential drivers. CO₂ is found to have a positive effect on NEP that is constrained by climate warming.

Observed daily changes in CO₂ emissions from across the globe reveal the sectors and countries where pandemic-related emissions declines were most pronounced in 2020.

Analysis of peatland carbon accumulation over the last millennium and its association with global-scale climate space indicates an ongoing carbon sink into the future, but with decreasing strength as conditions warm.

Atmospheric CO₂ concentration measured across a network of towers in North America shows that continent- and biome-scale measurements of the temperature sensitivity of ecosystem respiration are lower than have previously been estimated from plot-level studies.

Nature-based climate solutions are widely incorporated into climate change mitigation plans and need firm scientific foundations. Through literature review and expert elicitation, this analysis shows that for some major pathways there is strong support, while for others their efficacy remains uncertain.

A failure to recognize the factors behind continued emissions growth could limit the world’s ability to shift to a pathway consistent with 1.5 °C or 2 °C of global warming. Continued support for low-carbon technologies needs to be combined with policies directed at phasing out the use of fossil fuels.

Climate sensitivity models may inaccurately characterize the full Earth system response, as they ignore changes in the carbon cycle, aerosols, land use and land cover. A combination of a simplified climate model, a range of simulations from a recent model intercomparison and historical constraints now show that, independent of the timing of emissions or the atmospheric concentration of CO₂, emitting a trillion tonnes of carbon will cause global warming of 1.0 to 2.1 degrees Celsius.

Microbial carbon use efficiency (CUE) is crucial for carbon storage, but its variability is difficult to capture due to inconsistent measurements and complex interactions. This perspective proposes integrating diverse data and models to improve CUE in carbon cycle models

In order to limit climate warming, CO₂ emissions must remain below fixed quota. An evaluation of past emissions suggests that at 2014 emissions rates, the total quota will probably be exhausted within the next 30 years.

Including passive CO₂ uptake as an anthropogenic removal in greenhouse gas accounting systems could undermine the Paris Agreement; measures to address this include acknowledging the need for Geological Net Zero and disaggregated accounting for carbon sinks.

Considering the combined effects of CO₂ fertilization and climate change drivers on plant physiology leads to a modest increase in simulated European forest transpiration in spite of the effects of CO₂-induced stomatal closure.

The Paris Agreement has increased the incentive to verify reported anthropogenic carbon dioxide emissions with independent Earth system observations. Reliable verification requires a step change in our understanding of carbon cycle variability.

Uncertainties and subjective choices affecting remaining carbon budgets should be fully considered when applying them to international and national climate policies.

Future cumulative CO₂ emissions consistent with a given warming limit are a finite common global resource that countries need to share — a carbon quota. Strategies to share a quota consistent with a 2 °C warming limit range from keeping the present distribution to reaching an equal per-capita distribution of cumulative emissions. This Perspective shows that a blend of these endpoints is the most viable solution.

Scenario analyses suggest that negative emissions technologies (NETs) are necessary to limit dangerous warming. Here the authors assess the biophysical limits to, and economic costs of, the widespread application of NETs.

A satellite-based estimate of forest regrowth carbon flux across the Northern Hemisphere suggests forest disturbance and regrowth are transient but important aspects of the carbon sink that may explain underestimates from dynamic global vegetation models

The Brazilian Amazon was a net carbon source during recent climate extremes and the south-eastern Amazon was a net land carbon source between 2010 and 2020 due to increasing human-induced disturbance and drought, suggest bottom-up and top-down estimates of land carbon fluxes.

A long-term goal for climate policy can only be agreed through political processes, but science can inform these through mapping policy choices and the risks they create. Recommendations for the practical use of the IPCC's Fifth Assessment Report are provided.

The net carbon flux on land comprises emissions from land-use change and uptake by vegetation. An evaluation of vegetation models suggests that emissions from land-use change, and thus the terrestrial sink, may be substantially underestimated.