Climate Change from 1850 to 2005 Simulated in CESM1(WACCM)

TLDR

The NCAR Community Earth System Model now includes WACCM, an atmospheric component extending into the lower thermosphere with fully interactive chemistry that self‑consistently simulates the stratospheric ozone hole and its tropospheric impacts. This study analyzes an ensemble of transient CESM1(WACCM) simulations spanning the preindustrial era to the present, conducted as part of CMIP5 phase 5. Stratospheric variability, including sudden warmings and ozone‑hole development, matches observations and propagates into the troposphere, altering winds, precipitation, and sea‑ice extent; while global‑mean temperature trends are unchanged compared to a CESM version without full stratosphere, significant extratropical differences—up to the size of the climate‑change signal—highlight stratosphere‑troposphere coupling as a major source of uncertainty.

Abstract

Abstract The NCAR Community Earth System Model (CESM) now includes an atmospheric component that extends in altitude to the lower thermosphere. This atmospheric model, known as the Whole Atmosphere Community Climate Model (WACCM), includes fully interactive chemistry, allowing, for example, a self-consistent representation of the development and recovery of the stratospheric ozone hole and its effect on the troposphere. This paper focuses on analysis of an ensemble of transient simulations using CESM1(WACCM), covering the period from the preindustrial era to present day, conducted as part of phase 5 of the Coupled Model Intercomparison Project. Variability in the stratosphere, such as that associated with stratospheric sudden warmings and the development of the ozone hole, is in good agreement with observations. The signals of these phenomena propagate into the troposphere, influencing near-surface winds, precipitation rates, and the extent of sea ice. In comparison of tropospheric climate change predictions with those from a version of CESM that does not fully resolve the stratosphere, the global-mean temperature trends are indistinguishable. However, systematic differences do exist in other climate variables, particularly in the extratropics. The magnitude of the difference can be as large as the climate change response itself. This indicates that the representation of stratosphere–troposphere coupling could be a major source of uncertainty in climate change projections in CESM.