At what horizontal scale can significant surface warming be detected?

The assessment of the possible causes of observed climate change in the IPCC Third Assessment Report concluded that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations” (Mitchell et al. 2001). This conclusion was based on many studies of global and very large scale climate variations. That assessment also concluded that “surface temperature changes are detectable only on scales greater than 5,000 km”. Since then, it has been shown that an anthropogenic climate change signal is detectable in continental-scale regions using surface temperature changes over the 20th century. For example, Stott (2003) used simulations with the HadCM3 model to show that most of the observed warming over the last 50 years in six separate regions of the globe, including North America, Eurasia and Australia, was likely to be due to the increase in greenhouse gases in the atmosphere.

Almost no studies have considered the detection of surface temperature trends at a regional scale such as at the scale of an individual model grid box. This is because detection of anthropogenic climate change is a signal-to-noise problem and the noise associated with natural variations of surface temperatures at a regional scale is greater than at larger continental or global scales.

Here, we report the results of exactly such a study. We show that observed trends in surface air temperature at individual grid boxes of size 500km are significant compared with estimates of internal variability at the majority of grid boxes over the globe and over a range of trend periods from 100 years down to the most recent 30 years. These trends are consistent with the response to increasing greenhouse gases in the atmosphere, but we cannot eliminate the possibility that other forcing factors, such as land surface change may have contributed to the observed trends in some regions. The IPCC TAR conclusion on the scale of detectable surface temperature changes is no longer correct.