Transient Climate Simulations with the HadGEM1 Climate Model: Causes of Past Warming and Future Climate Change

Abstract

The ability of climate models to simulate large-scale temperature changes during the twentieth century when they include both anthropogenic and natural forcings and their inability to account for warming over the last 50 yr when they exclude increasing greenhouse gas concentrations has been used as evidence for an anthropogenic influence on global warming. One criticism of the models used in many of these studies is that they exclude some forcings of potential importance, notably from fossil fuel black carbon, biomass smoke, and land use changes. Herein transient simulations with a new model, the Hadley Centre Global Environmental Model version 1 (HadGEM1), are described, which include these forcings in addition to other anthropogenic and natural forcings, and a fully interactive treatment of atmospheric sulfur and its effects on clouds. These new simulations support previous work by showing that there was a significant anthropogenic influence on near-surface temperature change over the last century. They demonstrate that black carbon and land use changes are relatively unimportant for explaining global mean near-surface temperature changes.The pattern of warming in the troposphere and cooling in the stratosphere that has been observed in radiosonde data since 1958 can only be reproduced when the model includes anthropogenic forcings. However, there are some discrepancies between the model simulations and radiosonde data, which are largest where observational uncertainty is greatest in the Tropics and high latitudes.Predictions of future warming have also been made using the new model. Twenty-first-century warming rates, following policy-relevant emissions scenarios, are slightly greater in HadGEM1 than in the Third Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) as a result of the extra forcing in HadGEM1. An experiment in which greenhouse gases and other anthropogenic forcings are stabilized at 2100 levels and held constant until 2200 predicts a committed twenty-second-century warming of less than 1 K, whose spatial distribution resembles that of warming during the twenty-first century, implying that the local feedbacks that determine the pattern of warming do not change significantly.

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

“…Volcanic eruptions in ACCESS-ESM1 are prescribed based on monthly global-mean stratospheric volcanic aerosol optical depth (Sato et al, 2002), which is then averaged over four equal-area latitude zones, similar to the way it is done in the Hadley Centre Global Environmental Model (HadGEM) (Stott et al, 2006;Jones et al, 2011). Globally significant volcanoes within the historical period are Krakatoa (1883), Santa Maria (1903), Agung (1963), El Chichón (1982 and Pinatubo (1991).…”

Quantifying and specifying the solar influence on terrestrial surface temperature

“…A significant cooling dominated by aerosols, is a robust feature of a wide range of detection analyses. A key factor in identifying the aerosol fingerprint, and therefore the amount of aerosol cooling is the change through time of the hemispheric temperature contrast, which is affected by the different evolution of aerosol forcing in the two hemispheres as well as the greater thermal inertia of the larger ocean area in the Southern Hemisphere (Santer et al, 1995(Santer et al, , 1996Hegerl et al, 2001;Stott et al, 2006). Regional and seasonal aspects of the temperature response may help to distinguish the response to greenhouse gas increases from the response to aerosols (e.g., Ramanathan et al, 2005;Nagashima et al, 2006).…”

Long-term natural variability and 20th century climate change

“…D elineating the relative role of anthropogenic forcing, natural forcing, and long-term natural variability in 20th century climate change presents a significant challenge to our understanding of the climate system (1)(2)(3)(4)(5)(6)(7). Observations suggest the warming of the 20th century global mean surface temperature has not been monotonic, even when smoothed by a 10-20 year low-pass filter.…”