Reto Knutti

Last Name

Knutti

First Name

Reto

ORCID

0000-0001-8303-6700

Organisational unit

03777 - Knutti, Reto / Knutti, Reto

Show all metadata

Search Results

Publications 1 - 10 of 39

A Distinct Role for Aerosol and GHG Forcing in Historical CMIP6 Evapotranspiration Trends
Egli, Marius; Humphrey, Vincent; Sippel, Sebastian; et al. (2024)
Earth's Future
Evapotranspiration (ET) is crucial for the global water balance, plant growth, and freshwater availability. It connects the surface water balance with surface energy fluxes, making its accurate representation vital for climate projections. However, global climate models (GCMs) struggle with ET representation due to resolution limitations and simplified depictions of soil, plant, and atmosphere interactions. Simulated future changes in ET are uncertain, and the role of driving processes remain unclear. Here, we explore the utility of a simple and interpretable method to disentangle these varying drivers. We investigate the sensitivity of JJA ET to different atmospheric variables through simple linear models predicting ET from atmospheric variables only. The model consistently yields good results across GCMs or forcing scenarios. We find that GCMs have shown strong decreases and subsequent increases in ET over the historical period, related to changes in net surface radiation. For future climate projections, decreases in water availability compete with higher available surface radiation, making future projections uncertain. Single forcing GCM realizations show that historical ET trends in densely populated regions have been more influenced by aerosol emissions than greenhouse gases. Finally, we investigate which atmospheric variables explain most short-term (year-to-year) and long-term (decadal) changes. While water availability may be the most important driver of short-term variability, for certain regions, radiation trends dominate long-term forcing. This paper leverages a simple approach to provide a comprehensive and understandable view into recent and future changes in ET, reconciling the evidence provided by more complex case studies.
The Detection and Attribution Model Intercomparison Project (DAMIP v2.0) contribution to CMIP7
Gillett, Nathan P.; Simpson, Isla R.; Hegerl, Gabi; et al. (2025)
Geoscientific Model Development
The first version of the Detection and Attribution Model Intercomparison Project (DAMIP v1.0) coordinated key simulations exploring the role of individual forcings in past, current and future climate as part of the Coupled Model Intercomparison Project, Phase 6 (CMIP6). The simulations have been used extensively in the literature for detection and attribution of long-term changes, constraining projections of climate change, attributing extreme events and understanding drivers of past and future simulated climate changes. Attribution studies using DAMIP v1.0 simulations underpinned prominent assessments of human-induced warming in the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report. Here, we describe the set of DAMIP v2.0 simulations, proposed for the next phase of CMIP, CMIP7. Detection and attribution studies rely on pre-industrial control simulations and historical simulations, which will be part of the Diagnostic, Evaluation and Characterization of Klima (DECK) set of simulations for CMIP7. In addition, we identify the three highest-priority single-forcing experiments for CMIP7 to be run as "Assessment Fast Track" simulations in support of the Seventh Assessment Report of the IPCC: simulations with natural forcings only, anthropogenic well-mixed greenhouse gases only and anthropogenic aerosols only. Beyond this, the DAMIP v2.0 experimental design includes full-column ozone-only simulations and land-use-only simulations, such that the set of individual forcing experiments, when these are considered together, represents the full set of historical forcings. While concentration-driven simulations are prioritised for attribution, emissions-driven versions of the DAMIP experiments are also proposed to support understanding of the influence of carbon-cycle feedbacks on the simulated responses to individual forcings.
Assessing observational constraints on future European climate in an out-of-sample framework
O'Reilly, Christopher H.; Brunner, Lukas; Qasmi, Said; et al. (2024)
npj Climate and Atmospheric Science
Observations are increasingly used to constrain multi-model projections for future climate assessments. This study assesses the performance of five constraining methods, which have previously been applied to attempt to improve regional climate projections from CMIP5-era models. We employ an out-of-sample testing approach to assess the efficacy of these constraining methods when applied to "pseudo-observational" datasets to constrain future changes in the European climate. These pseudo-observations are taken from CMIP6 simulations, for which future changes were withheld and used for verification. The constrained projections are more accurate and broadly more reliable for regional temperature projections compared to the unconstrained projections, especially in the summer season, which was not clear prior to this study. However, the constraining methods do not improve regional precipitation projections. We also analysed the performance of multi-method projections by combining the constrained projections, which are found to be competitive with the best-performing individual methods and demonstrate improvements in reliability for some temperature projections. The performance of the multi-method projection highlights the potential of combining constraints for the development of constraining methods.
Earth's Energy Imbalance More Than Doubled in Recent Decades
Mauritsen, Thorsten; Tsushima, Yoko; Meyssignac, Benoit; et al. (2025)
AGU Advances
Global warming results from anthropogenic greenhouse gas emissions which upset the delicate balance between the incoming sunlight, and the reflected and emitted radiation from Earth. The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather around the globe. Despite the fundamental role of the energy imbalance in regulating the climate system, as known to humanity for more than two centuries, our capacity to observe it is rapidly deteriorating as satellites are being decommissioned.
Bringing it all together: Science priorities for improved understanding of Earth system change and to support international climate policy
Jones, Colin G.; Adloff, Fanny; Booth, Ben B.B.; et al. (2024)
Earth System Dynamics
We review how the international modelling community, encompassing integrated assessment models, global and regional Earth system and climate models, and impact models, has worked together over the past few decades to advance understanding of Earth system change and its impacts on society and the environment and thereby support international climate policy. We go on to recommend a number of priority research areas for the coming decade, a timescale that encompasses a number of newly starting international modelling activities, as well as the IPCC Seventh Assessment Report (AR7) and the second UNFCCC Global Stocktake. Progress in these priority areas will significantly advance our understanding of Earth system change and its impacts, increasing the quality and utility of science support to climate policy. We emphasize the need for continued improvement in our understanding of, and ability to simulate, the coupled Earth system and the impacts of Earth system change. There is an urgent need to investigate plausible pathways and emission scenarios that realize the Paris climate targets - for example, pathways that overshoot 1.5 or 2 °C global warming, before returning to these levels at some later date. Earth system models need to be capable of thoroughly assessing such warming overshoots - in particular, the efficacy of mitigation measures, such as negative CO2 emissions, in reducing atmospheric CO2 and driving global cooling. An improved assessment of the long-term consequences of stabilizing climate at 1.5 or 2 °C above pre-industrial temperatures is also required. We recommend Earth system models run overshoot scenarios in CO2-emission mode to more fully represent coupled climate-carbon-cycle feedbacks and, wherever possible, interactively simulate other key Earth system phenomena at risk of rapid change during overshoot. Regional downscaling and impact models should use forcing data from these simulations, so impact and regional climate projections cover a more complete range of potential responses to a warming overshoot. An accurate simulation of the observed, historical record remains a fundamental requirement of models, as does accurate simulation of key metrics, such as the effective climate sensitivity and the transient climate response to cumulative carbon emissions. For adaptation, a key demand is improved guidance on potential changes in climate extremes and the modes of variability these extremes develop within. Such improvements will most likely be realized through a combination of increased model resolution, improvement of key model parameterizations, and enhanced representation of important Earth system processes, combined with targeted use of new artificial intelligence (AI) and machine learning (ML) techniques. We propose a deeper collaboration across such efforts over the coming decade. With respect to sampling future uncertainty, increased collaboration between approaches that emphasize large model ensembles and those focussed on statistical emulation is required. We recommend an increased focus on high-impact-low-likelihood (HILL) outcomes - in particular, the risk and consequences of exceeding critical tipping points during a warming overshoot and the potential impacts arising from this. For a comprehensive assessment of the impacts of Earth system change, including impacts arising directly as a result of climate mitigation actions, it is important that spatially detailed, disaggregated information used to generate future scenarios in integrated assessment models be available for use in impact models. Conversely, there is a need to develop methods that enable potential societal responses to projected Earth system change to be incorporated into scenario development. The new models, simulations, data, and scientific advances proposed in this article will not be possible without long-term development and maintenance of a robust, globally connected infrastructure ecosystem. This system must be easily accessible and useable by modelling communities across the world, allowing the global research community to be fully engaged in developing and delivering new scientific knowledge to support international climate policy.
Estimating climate sensitivity and future temperature in the presence of natural climate variability
Huber, Markus; Beyerle, Urs; Knutti, Reto (2014)
Geophysical Research Letters
We use an initial condition ensemble of an Earth System Model as multiple realizations of the climate system to evaluate estimates of climate sensitivity and future temperature change derived with a climate model of reduced complexity under “perfect” conditions. In our setup, the mean and most likely estimate of equilibrium climate sensitivity vary by about 0.4–0.8°C (±1σ) due to internal variability. Estimates of the transient climate response vary much less; however, the effect of the spread and bias in the transient response on future temperature projections increases with lead time. Future temperature projections are shown to be more robust for central ranges (i.e., likely range) than for single percentiles. The estimates presented here strongly depend on a delicate balance between a particular realization of the climate system, the emerging constraints on the estimates as well as on the signal, and the decreasing uncertainties in ocean heat uptake observations. © 2014 American Geophysical Union
Flash drought impacts on global ecosystems amplified by extreme heat
Gu, Lei; Schumacher, Dominik L.; Fischer, Erich M.; et al. (2025)
Nature Geoscience
Flash droughts-characterized by their rapid onset-can cause devastating socioeconomic and agricultural damage. During such events, soil moisture depletion is driven not only by precipitation shortages but also by the elevated atmospheric moisture demand arising due to extreme heat. However, the role of extreme heat in shaping the evolution of flash droughts and their ecological impacts remains uncertain. Here we investigate the processes involved by analysing global reanalysis data from 1950 to 2022. We find that, when flash droughts are accompanied by extreme heat, they exhibit 6.7-90.8% higher severity and 8.3-114.3% longer recovery time than flash droughts without extreme heat. The presence of extreme heat during flash droughts accelerates soil moisture drawdown over high latitudes, where wet soils and enhanced radiation foster evapotranspiration. By contrast, it slows the absolute onset speed in subtropical transitional climate zones owing to evapotranspiration throttling. Our machine learning approach further reveals that hot flash droughts lead to sharper declines in ecosystem productivity, particularly in croplands, thereby threatening global food security. These findings underscore the pressing need for enhanced infrastructure and ecosystem resilience to hot flash droughts in a warming future.
Increasing extreme precipitation variability plays a key role in future record-shattering event probability
de Vries, Iris; Sippel, Sebastian; Zeder, Joel; et al. (2024)
Communications Earth & Environment
Climate events that break records by large margins are a threat to society and ecosystems. Climate change is expected to increase the probability of such events, but quantifying these probabilities is challenging due to natural variability and limited data availability, especially for observations and very rare extremes. Here we estimate the probability of precipitation events that shatter records by a margin of at least one pre-industrial standard deviation. Using large ensemble climate simulations and extreme value theory, we determine empirical and analytical record shattering probabilities and find they are in high agreement. We show that, particularly in high emission scenarios, models project much higher record-shattering precipitation probabilities in a changing relative to a stationary climate by the end of the century for almost all the global land, with the strongest increases in vulnerable regions in the tropics. We demonstrate that increasing variability is an essential driver of near-term increases in record-shattering precipitation probability, and present a framework that quantifies the influence of combined trends in mean and variability on record-shattering behaviour in extreme precipitation. Probability estimates of record-shattering precipitation events in a warming world are crucial to inform risk assessment and adaptation policies.
How scientists’ collective climate advocacy affects public trust in scientists and voting behavior
Cologna, Viktoria; Freundt, Jana; Mede, Niels G.; et al. (2025)
Environmental Research Letters
Scientists increasingly engage in policy advocacy, especially on climate change. Does this advocacy undermine—or bolster—public trust in scientists and support for scientists' recommendations? We leveraged a unique opportunity to answer this question in a real-world setting: the 2023 referendum for the Swiss climate protection law (CPL), which was supported by a public statement of 252 renowned scientists across disciplines. We conducted a pre-registered, two-wave, quasi-field experiment (npre-vote = 1,622, npost-vote = 891) to test how scientists' collective support for the law affected public trust in scientists and voting behavior. We found that scientists' advocacy was associated with greater public trust, particularly among left-leaning individuals; only a minority viewed these scientists as not objective. However, perceptions of scientists' role in society and policymaking and voting behavior remained largely unaffected when participants were reminded of the CPL advocacy. Although we studied a restrained form of policy advocacy in a somewhat unique setting, our study challenges the widespread assumption that policy advocacy undermines public trust in scientists.
The Zero Emissions Commitment and climate stabilization
Palazzo Corner, Sofia; Siegert, Martin; Ceppi, Paulo; et al. (2023)
Frontiers in Science
How do we halt global warming? Reaching net zero carbon dioxide (CO2) emissions is understood to be a key milestone on the path to a safer planet. But how confident are we that when we stop carbon emissions, we also stop global warming? The Zero Emissions Commitment (ZEC) quantifies how much warming or cooling we can expect following a complete cessation of anthropogenic CO2 emissions. To date, the best estimate by the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report is zero change, though with substantial uncertainty. In this article, we present an overview of the changes expected in major Earth system processes after net zero and their potential impact on global surface temperature, providing an outlook toward building a more confident assessment of ZEC in the decades to come. We propose a structure to guide research into ZEC and associated changes in the climate, separating the impacts expected over decades, centuries, and millennia. As we look ahead at the century billed to mark the end of net anthropogenic CO2 emissions, we ask: what is the prospect of a stable climate in a post-net zero world?

Publications 1 - 10 of 39

Reto Knutti

Last Name

First Name

ORCID

Organisational unit

Filters

Search Results