It is now unequivocal that the Earth’s climate system is warming. 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. 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. This project assesses the potential for triggering several types of abrupt climate change (ACC) during the 21st century. The types of ACC considered mean that the climate system is forced across a threshold into a new state; the onset rate is set by climate dynamics and is faster than the rate of climate forcing increase; the change persists for years or longer and affects regions sub-continental or larger in size; and the magnitude of the change exceeds the magnitudes of comparable natural modes of variability. The focus here is on the risk of ACC on decadal rather than centennial time scales. The project investigates some of the most significant mechanisms proposed for ACC through a series of linked projects that examine:
- Dynamics of ice shelf: Ocean interaction and evaluation of marine ice sheet instability
- Boreal/Arctic-climate positive feedbacks and ACC
- Rapid destabilization of methane hydrates in Arctic Ocean sediments
- Mega droughts in North America, including the role of biosphere-atmosphere feedbacks
Since this project is focused on the future risk of abrupt phenomena, the onset of these phenomena is predicted using a detailed representation in the Community Earth System Model (CESM). The team includes many of the primary scientific and software developers of the CESM and its component models. In order to quantify the risk of ACC, the team is adding new capabilities and functionality to CESM and its accompanying diagnostic packages. The team is enhancing CESM 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 CESM that can generate equilibrated solutions for the coupled ocean-atmosphere system much more rapidly than is feasible using standard forward solution methods. The team tests that the new physics, chemistry, and biogeochemistry is consistent with the comparative stability of the recent climate record. The enhancements to CESM and its diagnostics together with the resulting model simulations will be shared with the wider CESM community.