Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate

Abstract

Ensemble simulations are run with a global coupled climate model employing five forcing agents that influence the time evolution of globally averaged surface air temperature during the twentieth century. Two are natural (volcanoes and solar) and the others are anthropogenic [e.g., greenhouse gases (GHGs), ozone (stratospheric and tropospheric), and direct effect of sulfate aerosols]. In addition to the five individual forcing experiments, an additional eight sets are performed with the forcings in various combinations. The late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing (mainly GHGs), while the early twentieth-century warming is mainly caused by natural forcing in the model (mainly solar). However, the signature of globally averaged temperature at any time in the twentieth century is a direct consequence of the sum of the forcings. The similarity of the response to the forcings on decadal and interannual time scales is tested by performing a principal component analysis of the 13 ensemble mean globally averaged temperature time series. A significant portion of the variance of the reconstructed time series can be retained in residual calculations compared to the original single and combined forcing runs. This demonstrates that the statistics of the variances for decadal and interannual time-scale variability in the forced simulations are similar to the response from a residual calculation. That is, the variance statistics of the response of globally averaged temperatures in the forced runs are additive since they can be reproduced in the responses calculated as a residual from other combined forcing runs.

Recent intense hurricane response to global climate change

“…From this perspective the global warming signal appeared around 1960 and has increased to a current level of *0.8°C [as has previously been implied by Meehl et al (2004Meehl et al ( , 2007Meehl et al ( , 2012 and IPCC (2007)]. …”

“…However, comparison of global climate model simulations with and without anthropogenic forcing ( Fig. 1) indicates that the net anthropogenic climate forcing-including cooling as well as warming agents-was indistinguishable from natural variability until the midtwentieth century (Meehl et al 2004(Meehl et al , 2007(Meehl et al , 2012IPCC 2007). The ensembles of model simulations with and without anthropogenic emissions closely track each other and the observed global surface temperature until around 1970.…”

“…After then, the anthropogenic simulations continue to track the observed global warming, but those with only natural forcing indicate a cooling trend back to nineteenth century temperature levels. This change has been attributed to reductions in emissions of sulfate aerosols, which counteracted the warming component due to greenhouse gases before 1960 (Meehl et al 2004(Meehl et al , 2012Mann and Emanuel 2006;Thompson et al 2010).…”

“…We thank James Done, Asuka Suzuki-Parker and Tom Galarneau for helpful discussions, Ming Ge for extracting the land proximity data, Roger Pielke Jr for providing the Weinkle et al (2012) landfall data, Jerry Meehl and Julie Arblaster for providing the climate simulation data from Meehl et al (2004Meehl et al ( , 2007Meehl et al ( , 2012, and Jim Kossin and Chris Velden for providing the homogenized satellite data.…”

Detection and attribution of anthropogenic forcing to diurnal temperature range changes from 1950 to 1999: comparing multi-model simulations with observations

“…Global coupled atmosphere-ocean general circulation models (AOGCMs) are generally able to reproduce the observed warming in the 20th century when natural (i.e., volcanic aerosols and solar variability) and anthropogenic (i.e., GHGs and sulfate aerosols) forcings are included (IPCC 2007). The global mean temperature changes in the first half century were partly a response to natural forcings but the warming in the later half century has mostly resulted from anthropogenic forcing (e.g., Meehl et al 2004). The models also show a decrease in DTR but of magnitude much smaller than that observed (Stenchikov and Robock 1995;Weaver 2002, 2003;Karoly et al 2003;Braganza et al 2004;Zhou et al 2009).…”