Estimation of natural and anthropogenic contributions to twentieth century temperature change

Abstract

Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well‐mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well‐mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well‐mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an “optimal detection” methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well‐mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near‐surface temperature from well‐mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near‐surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.

Climate change in Central America and Mexico: regional climate model validation and climate change projections

“…In fact, mountain regions throughout the world have experienced large upward shifts in freezing levels in recent decades (Diaz et al 2003;Vuille and Bradley 2000). Climate models consistently show tropospheric amplification of surface warming in response to well-mixed greenhouse gases (Santer et al 1996;Hansen et al 2002;Tett et al 2002). Maximum warming in these models occurs in the middle and upper troposphere.…”

The effects of changing solar activity on climate: contributions from palaeoclimatological studies

“…Model simulations that include most known forcing factors (TSI, greenhouse gasses, volcanic activity, aerosols and ozone) show that for the first half of the 20th century, most of the global warming can be attributed to natural forcing (Stott et al 2000;Tett et al 2002;Meehl et al 2004;Gray et al 2010). However, temperature changes during the second half of the 20th century are considered to be most likely the result of the increased concentration of greenhouse gasses.…”

Solar Influence on Global and Regional Climates

“…A comprehensive review of detection-attribution methods and results is given as part of the IPCC 4th Fourth Assessment Report (Hegerl et al 2007). Some key conclusions in this context are that (a) the combined effects of solar and volcanic forcings has resulted in a net cooling over recent decades; (b) models simulate much less warming over the 20th century in response to solar forcing alone than for greenhouse gas forcing alone (Cubasch et al 1997;Broccoli et al 2003;Meehl et al 2003Meehl et al , 2004Tett et al 1999Tett et al , 2002; (c) in combination, the greenhouse effect on surface temperatures is dominant over the solar effect (Stone et al 2007a, b), although there is some debate about the precise contributions ; and (d) the changes in the altitude profiles also reveal a dominant greenhouse gas effect .…”