Attributing Causes to "Old" Extremes

Many efforts are underway worldwide to attribute causes to recent events. Analysis of extreme events in the long historical record is an important sanity test of our understanding of the contributors and causes to extreme events. Two example cases will be presented:

a) During the Dustbowl period in the Central US, record breaking heat waves were recorded which are far outside anything that occurred over the 20th century. An analysis reveals that spring dry conditions made these events more likely, and that SSTs, particularly in the Atlantic may have contributed to the record heat waves by contributing to drought. However, changes in land cover appear a key driver in climate model simulations which may explain the exceptional nature also in the context of CMIP5 simulations. In addition, even then increased greenhouse gas levels slightly contributed to extreme temperatures. This case study emphasizes the potential for strong feedback processes between human activity and climate variability and its impact on extremes.

b) the 'year without a summer' caused severe impacts across Europe, where instrumental data and analyses show a wet and cold summer in 1816. The contribution of the volcanic eruption of Mount Tambora to changing risk for these conditions is analyzed, using a combination of climate model simulations, data analysis and analogue methods. Results show that the volcanic eruption was the key driver for the cold conditions, and that some model simulations suggest that rainfall anomalies may be linked to forced circulation anomalies, yet the latter result is model-dependent.

A case will be made to reanalyze old extremes in order to better understand and evaluate the mechanisms and feedbacks in very rare events.