Assessing Likelihood of Some Extreme Events during 2013: Australian Annual Mean Temperatures and U.S. Regional Annual Mean Precipitation Anomalies

The degree to which particular extreme weather and climate events are assessed as being attributable to anthropogenic climate change (e.g., that anthropogenic forcing influenced their probability of occurrence or other characteristics) can vary dramatically from case to case. We present some examples of such assessments for two extreme events occurring during 2013.

One example is the record or near-record annual mean temperature over a large region of Australia in 2013. According to this analysis of the CMIP5 models, the event was largely attributable to anthropogenic forcing of the climate system. Another 2013 case was the extreme positive annual mean precipitation anomalies in 5x5 degree gridded (GHCN) precipitation data that were observed along the U.S./Canadian border region. This is a region with a detectable long-term increase of precipitation since 1900. Nonetheless, the 2013 event is assessed as primarily attributable to internal (unforced) climate variability and only partly attributable to external forcing (natural and anthropogenic forcing combined). There are many outstanding challenges to these studies. Among these are the limitations to historical data length, data quality, model ensemble size, and model control run length, and the important underlying question of how well current models simulate internal (intrinsic) climate variability. Furthermore, there is room for improvement in addressing model biases, station/gridcell scale mismatches, and modeling the extreme ends of the distributions (e.g. with Generalized Extreme Value methods).