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Investigation of the Magnitudes and Probabilities of Abrupt Climate TransitionS (IMPACTS) Project

It is now unequivocal that the Earth’s climate system is warming. The most recent IPCC assessment [2007a] concludes that the increased temperatures in the latter 20th century are very likely due to anthropogenic greenhouse gases, and continued greenhouse emissions will very likely result in even larger increases during the 21st century. However, the major risks to society and environment from climate change are posed primarily by abrupt and extreme climate phenomena rather than the continual warming trend [IPCC 2007b]. Potential forms of abrupt change include sudden deglaciation, reorganization of the thermohaline circulation system, and widespread melting of permafrost leading to large-scale shifts in the carbon cycle. Abrupt and extreme phenomena can exceed the thresholds for ecological and societal adaptation through either the rapid rate or magnitude of the associated climate change [IPCC 2007b]. In this proposal, we will assess the potential for triggering several types of abrupt climate change (ACC) during the 21st century.

We consider types of ACC that satisfy the conditions advanced by the NRC [2002]:

We will focus on the risk of ACC on decadal rather than centennial time scales. We will investigate some of the most significant mechanisms proposed for ACC through a series of linked projects that examine

Since this proposal is focused on the future risk of abrupt phenomena, we will predict the onset of these phenomena using a detailed representation of the Earth system called the Community Climate System Model [Collins 2007a]. This model has been developed through a partnership of federal agencies including DOE, and simulations from CCSM represent a significant contribution by DOE to the 4th IPCC Assessment [Meehl et al 2006; 2007]. The proposing team includes many of the primary scientific and software developers of the CCSM and its component models. In order to quantify the risk of ACC, the team will add new capabilities and functionality to CCSM and its accompanying diagnostic packages. The team will enhance CCSM with representations of ice shelves, terrestrial methanogenesis, gaseous oceanic plumes, and vegetative controls on soil moisture and evapotranspiration. The team will also create new versions of CCSM that can generate equilibrated solutions for the coupled ocean-atmosphere system much more rapidly than is feasible using standard forward solution methods. The team will test that the new physics, chemistry, and biogeochemistry is consistent with the comparative stability of the recent climate record. The enhancements to CCSM and its diagnostics together with the resulting model simulations will be shared with the wider CCSM community.

For each of these projects, we describe the scientific imperative and major issues to be addressed, the research methodology, and the potential impact of our findings for climate science.