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The climate system responds to changes in the amount of atmospheric greenhouse gases or aerosols through rapid processes, triggered within hours and days, and through slower processes, where the full response may only be seen after centuries. In this paper, we aim to elucidate the mechanisms operating on time scales of hours to years to better understand the response of key climate quantities such as energy fluxes, temperature, and precipitation after a sudden increase in either carbon dioxide (CO2), black carbon (BC), or sulfate (SO4) aerosols. The results are based on idealized simulations from six global climate models. We find that the effect of changing ocean temperatures kicks in after a couple of months. Rapid precipitation reductions start occurring instantly and are established after just a few days. For BC, they constitute most of the equilibrium response. For CO2 and SO4, the magnitude of the precipitation response gradually increases as surface warming/cooling evolves, and for CO2, the sign of the response changes from negative to positive after 2 years. Rapid cloud adjustments are typically established within the first 24 h, and while the magnitude of cloud feedbacks for CO2 and SO4 increases over time, the geographical pattern of the equilibrium cloud change is present already after the first year. While there are model differences, our work underscores the overall similarity of the major time-varying processes and responses simulated by current global models and hence the robustness of key features of simulated responses to historical and future anthropogenic forcing. Significance Statement How does the climate system respond to a change in the amount of atmospheric greenhouse gases or aerosols? Some processes are rapid, responding within hours and days. Others are slow, and the full response to a forcing of the climate may only be seen after centuries. In this paper, we use six global climate models to investigate the time scales of climate responses to carbon dioxide, black carbon, and sulfate, focusing on key climate quantities, such as temperature, precipitation, and clouds. While there are ample model differences, our work underscores the overall similarity of the major time-varying processes and responses simulated by current global models and hence the robustness of key features of simulated responses to historical and future anthropogenic forcing.

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Estimating the equilibrium climate sensitivity (ECS; the equilibrium warming in response to a doubling of CO2) from observations is one of the big problems in climate science. Using observations of interannual climate variations covering the period 2000 to 2017 and a model-derived relationship between interannual variations and forced climate change, we estimate ECS is likely 2.4-4.6 K (17-83% confidence interval), with a mode and median value of 2.9 and 3.3 K, respectively. This analysis provides no support for low values of ECS (below 2 K) suggested by other analyses. The main uncertainty in our estimate is not observational uncertainty, but rather uncertainty in converting observations of short-term, mainly unforced climate variability to an estimate of the response of the climate system to long-term forced warming.

One of the major uncertainties in the ability to predict future climate change, and hence its impacts, is the lack of knowledge of the earth's climate sensitivity. Here, data are combined from the 1985-96 Earth Radiation Budget Experiment (ERBE) with surface temperature change information and estimates of radiative forcing to diagnose the climate sensitivity. Importantly, the estimate is completely independent of climate model results. A climate feedback parameter of 2.3 +/- 1.4 W m(-2) K-1 is found. This corresponds to a 1.0-4.1-K range for the equilibrium warming due to a doubling of carbon dioxide (assuming Gaussian errors in observable parameters, which is approximately equivalent to a uniform "prior" in feedback parameter). The uncertainty range is due to a combination of the short time period for the analysis as well as uncertainties in the surface temperature time series and radiative forcing time series, mostly the former. Radiative forcings may not all be fully accounted for; however, all argument is presented that the estimate of climate sensitivity is still likely to be representative of longer-term climate change. The methodology can be used to 1) retrieve shortwave and longwave components of climate feedback and 2) suggest clear-sky and cloud feedback terms. There is preliminary evidence of a neutral or even negative longwave feedback in the observations, suggesting that current climate models may not be representing some processes correctly if they give a net positive longwave feedback.

On 30 September 2021, the EU CONSTRAIN project hosted an online workshop ���The Role of Climate Model Emulators in the IPCC 6th Assessment Report���. The workshop appraised recent developments in the use of climate model emulators in the 6th Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) which was published on 9 August 2021. The workshop highlighted the many ways in which climate model emulators contributed to AR6. Some of the notable benefits are: Synthesis of multiple lines of evidence and production of 100s of projections central to the assessments of WG1 and WG3. Supporting the IPCC���s ambition to produce open science using transparent, accessible models and data. Facilitating communication, collaborative working, and consistency between the working groups and between the chapter teams in WG1. The discussion at the workshop made it clear that emulators offer considerable opportunities to enhance both future climate research and the communication of climate science, in terms of: Advancing emulations from global means to regional climate changes, and from changes in climate means to changes in climate extremes. Facilitating capacity building, and promoting communication and dialogue about climate science with a wider community of scientists and interested parties such as policymakers and decision makers. This workshop was dedicated to the early career scientists who made a substantial contribution to the work on emulators in the IPCC AR6.

The remaining carbon budget represents the total amount of CO2 that can still be emitted in the future while limiting global warming to a given temperature target. Remaining carbon budget estimates range widely, however, and this uncertainty can be used to either trivialize the most ambitious mitigation targets by characterizing them as impossible, or to argue that there is ample time to allow for a gradual transition to a low-carbon economy. Neither of these extremes is consistent with our best understanding of the policy implications of remaining carbon budgets. Understanding the scientific and socio-economic uncertainties affecting the size of the remaining carbon budgets, as well as the methodological choices and assumptions that underlie their calculation, is essential before applying them as a policy tool. Here we provide recommendations on how to calculate remaining carbon budgets in a traceable and transparent way, and discuss their uncertainties and implications for both international and national climate policies.

The annual ZERO IN reports by the CONSTRAIN project provide information on scientific topics that are fundamental to the Paris Agreement, as well as background and context on new developments at the science-policy interface. This includes new insights into the complex processes represented in climate models and what they mean for temperature change and other climate impacts over the coming decades. This third report provides additional context and background on the latest IPCC report on the physical science basis of climate change (IPCC AR6 WGI)1 , and addresses important questions around how likely we are to reach 1.5��C of global temperature increase.

The annual ZERO IN reports by the CONSTRAIN project provide information on scientific topics that are fundamental to the Paris Agreement, as well as background and context on new developments at the science-policy interface. This includes new insights into the complex processes represented in climate models and what they mean for temperature change and other climate impacts over the coming decades. This third report provides additional context and background on the latest IPCC report on the physical science basis of climate change (IPCC AR6 WGI)1 , and addresses important questions around how likely we are to reach 1.5°C of global temperature increase.

On 30 September 2021, the EU CONSTRAIN project hosted an online workshop “The Role of Climate Model Emulators in the IPCC 6th Assessment Report”. The workshop appraised recent developments in the use of climate model emulators in the 6th Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) which was published on 9 August 2021. The workshop highlighted the many ways in which climate model emulators contributed to AR6. Some of the notable benefits are: Synthesis of multiple lines of evidence and production of 100s of projections central to the assessments of WG1 and WG3. Supporting the IPCC’s ambition to produce open science using transparent, accessible models and data. Facilitating communication, collaborative working, and consistency between the working groups and between the chapter teams in WG1. The discussion at the workshop made it clear that emulators offer considerable opportunities to enhance both future climate research and the communication of climate science, in terms of: Advancing emulations from global means to regional climate changes, and from changes in climate means to changes in climate extremes. Facilitating capacity building, and promoting communication and dialogue about climate science with a wider community of scientists and interested parties such as policymakers and decision makers. This workshop was dedicated to the early career scientists who made a substantial contribution to the work on emulators in the IPCC AR6.

Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability.

A collection of ScienceBrief Reviews synthesising the latest research on climate change impacts, understanding and future developments. This collection brings ScienceBrief Reviews together in one volume covering the impacts of climate change on wildfires, cyclones, extreme rainfall, marine heatwaves, Arctic amplification, carbon sinks and the role of independent expert advisory bodies. The evidence discussed in these reviews was collected at ScienceBrief.org and can be explored via links within each review.