Are humans causing climate change?

Is part of the erratic weather being experienced globally since the 1970s due to a human impact on climate? It is an unquestioned fact that humans have raised the level of carbon dioxide, a strong greenhouse gas, in the Earth's atmosphere by about 30% since the Industrial Revolution. Simulations of climate of the 20th and 21st century by global climate models give strong evidence that natural variability of the Earth's climate in the absence of anthropogenically enhanced greenhouse gas concentrations cannot account for increases in global temperatures over the last 30 years.

Scientifically based attribution of some fraction of unusual weather to human influences, if it could be done, would have profound implications for national and international policy. Armed with such evidence, litigious societies such as the US would find their courts filled with lawsuits blaming greenhouse gas producers (i.e., anyone who uses energy derived from fossil fuels) for everything from extreme hurricanes to crop losses to loss of permafrost that causes an Alaskan village to slide into the sea.

Previous work on attributing some fraction of climate change to humans has been reported by Karoly et al. (2003), Stott (2003), and Zwiers and Zhang (2003). More recently Stott et al. (2004) provide a conceptual framework to estimate how much human activities have contributed to the increased risk of a heatwave such as the one that affected Europe in 2003. They conclude that human influence has doubled the risk of such a heat wave exceeding a temperature threshold such as was exceeded in 2003. And their results are statistically significant at the 90% confidence level. Schar and Jendritzky (2004) document the excess mortality due to 2003 European heatwave, and Allen and Lord (2004) discuss the potential political and legal consequences of the ability to allocated some part of climate change to humans.

References

Allen, M.R., and R. Lord, 2004: The blame game. Nature, 432, 551-552.

Schar, C., and G. Jendritzky, 2004: Hot news from summer 2003. Nature, 432, 559-560.

Stott, P.A., 2003: Attribution of regional-scale temperature changes to anthropogenic and natural causes. Geophy. Res. Lett., 30, doi: 10.1029/2000JD000028.

Zwiers, F.W., and X. Zhang, 2003: Towards regional scale climate change detection. J. Clim., 16, 793-797.