PNNL

Abstract

Extremes in temperature and precipitation in an idealized climate engineering experiment are examined as part of the Geoengineering Model Intercomparison Project (GeoMIP). In GeoMIP experiment G1, an instantaneous quadrupling of CO2 from its preindustrial control value is offset by a reduction in solar irradiance. Significant changes in the probability density functions of anomalies of monthly surface temperature T and precipitation P were found between G1 and control simulations, featuring an extension of the high-T tail over land, of the low-T tail over ocean, and a shift of P to drier conditions. Using daily model output, we analyzed the frequency of extreme events, such as the coldest night (TNn), warmest day (TXx), and maximum 5-day precipitation amount, and also duration indicators such as cold and warm spells and consecutive dry days. The strong heating of the northern high latitudes simulated under CO2 quadrupling is much alleviated under G1, but a significant warming remains, particularly for TNn compared to TXx. Internal feedbacks tied to land-atmosphere surface energy balance and biogeochemical processes lead to regional increases in absorbed solar radiation at the surface, preferentially increasing monthly mean and maximum daily temperatures over Northern Hemisphere land in summer. Conversely, a cooling of the tropical oceans is reflected in both TNn and TXx, causing a marked increase in cold spell duration. Globally, G1 is more effective in reducing changes in temperature extremes compared to precipitation extremes and for reducing changes in precipitation extremes versus means, but less effective at reducing changes in temperature extremes compared to means.