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Simon F. B. Tett
,
Jonathan M. Gregory
,
Nicolas Freychet
,
Coralia Cartis
,
Michael J. Mineter
, and
Lindon Roberts

Abstract

Uncertainty in climate projections is large as shown by the likely uncertainty ranges in equilibrium climate sensitivity (ECS) of 2.5–4 K and in the transient climate response (TCR) of 1.4–2.2 K. Uncertainty in model projections could arise from the way in which unresolved processes are represented, the parameter values used, or the targets for model calibration. We show that, in two climate model ensembles that were objectively calibrated to minimize differences from observed large-scale atmospheric climatology, uncertainties in ECS and TCR are about 2–6 times smaller than in the CMIP5 or CMIP6 multimodel ensemble. We also find that projected uncertainties in surface temperature, precipitation, and annual extremes are relatively small. Residual uncertainty largely arises from unconstrained sea ice feedbacks. The more than 20-year-old HadAM3 standard model configuration simulates observed hemispheric-scale observations and preindustrial surface temperatures about as well as the median CMIP5 and CMIP6 ensembles while the optimized configurations simulate these better than almost all the CMIP5 and CMIP6 models. Hemispheric-scale observations and preindustrial temperatures are not systematically better simulated in CMIP6 than in CMIP5 although the CMIP6 ensemble seems to better simulate patterns of large-scale observations than the CMIP5 ensemble and the optimized HadAM3 configurations. Our results suggest that most CMIP models could be improved in their simulation of large-scale observations by systematic calibration. However, the uncertainty in climate projections (for a given scenario) likely largely arises from the choice of parameterization schemes for unresolved processes (“structural uncertainty”), with different tuning targets another possible contributor.

Significance Statement

Climate models represent unresolved phenomena controlled by uncertain parameters. Changes in these parameters impact how well a climate model simulates current climate and its climate projections. Multiple calibrations of a single climate model, using an objective method, to large-scale atmospheric observations are performed. These models produce very similar climate projections at both global and regional scales. An analysis that combines uncertainties in observations with simulated sensitivity to observations and climate response also has small uncertainty showing that, for this model, current observations constrain climate projections. Recently developed climate models have a broad range of abilities to simulate large-scale climate with only some improvement in their ability to simulate this despite a decade of model development.

Full access
Zhiyuan Hu
,
Haiyan Li
,
Jiawei Liu
,
Shaobo Qiao
,
Dongqian Wang
,
Nicolas Freychet
,
Simon F. B. Tett
,
Buwen Dong
,
Fraser C. Lott
,
Qingxiang Li
, and
Wenjie Dong
Open access
Chunhui Lu
,
Jie Jiang
,
Ruidan Chen
,
Safi Ullah
,
Rong Yu
,
Fraser C. Lott
,
Simon F. B. Tett
, and
Buwen Dong
Open access
Rouke Li
,
Delei Li
,
Nergui Nanding
,
Xuan Wang
,
Xuewei Fan
,
Yang Chen
,
Fangxing Tian
,
Simon F. B. Tett
,
Buwen Dong
, and
Fraser C. Lott
Open access
Sabine Undorf
,
Simon F. B. Tett
,
Joseph Hagg
,
Marc J. Metzger
,
Chris Wilson
,
Graham Edmond
,
Miranda Jacques-Turner
,
Stuart Forrest
, and
Marion Shoote
Full access
Haosu Tang
,
Jun Wang
,
Yang Chen
,
Simon F. B. Tett
,
Ying Sun
,
Lijing Cheng
,
Sarah Sparrow
, and
Buwen Dong

Current human-induced warming has led to approximately a 30-fold increase in the occurrence probability of 2021 northwestern Pacific concurrent marine and terrestrial summer heat.

Open access
Sabine Undorf
,
Simon F. B. Tett
,
Joseph Hagg
,
Marc J. Metzger
,
Chris Wilson
,
Graham Edmond
,
Miranda Jacques-Turner
,
Stuart Forrest
, and
Marion Shoote

Abstract

Anthropogenic climate change calls for rapid and enormous cuts in emissions of CO2 and other greenhouse gases to mitigate future impacts. Even with these, however, many changes will continue to occur over the next 20–30 years adding to those already observed. Adaptation is crucial and urgent, but identifying strategies is complex and requires dialogue and cooperation among stakeholders, especially for infrastructure that exhibits interdependent risks in that failure in one type may impact others. A serious game was codeveloped with infrastructure operators to communicate climate projections and climate hazards to them; identify potential interdependencies, cascading impacts, cumulative effects, and vulnerability hot spots; and engage them to improve cooperation and enable a shared understanding of cross-cutting climate risks and interdependencies. In the game, players provide present-day infrastructure services in the Inverclyde district, Scotland, as they experience a plausible decade of 2050s weather characterized by a sequence of hazard events. This sequence was extracted from climate model projections to ensure scientific plausibility. The infrastructure operators were responsible for drinking water and gas supplies, road and rail transport, wastewater treatment, and civil infrastructure. When playing the game the participating U.K. infrastructure providers felt that although there were challenges, they could cope with 2050s climate change. None of the projected hazard events were anticipated to cause catastrophic impact cascades on infrastructure. The game was positively received, and the study suggests it is a useful tool to both communicate climate hazards and explore potential interdependent risks by bringing together stakeholders’ individual expertise in an engaging way.

Free access
Yang Chen
,
Wei Chen
,
Qin Su
,
Feifei Luo
,
Sarah Sparrow
,
David Wallom
,
Fangxing Tian
,
Buwen Dong
,
Simon F. B. Tett
, and
Fraser C. Lott
Full access
Wenxia Zhang
,
Wei Li
,
Lianhua Zhu
,
Yuanyuan Ma
,
Linyun Yang
,
Fraser C Lott
,
Chunxiang Li
,
Siyan Dong
,
Simon F B Tett
,
Buwen Dong
, and
Ying Sun
Free access
Liwen Ren
,
Dongqian Wang
,
Ning An
,
Shuoyi Ding
,
Kai Yang
,
Nicolas Freychet
,
Simon F. B. Tett
,
Buwen Dong
, and
Fraser C. Lott
Free access