sity are other metrics of an extreme cold event. Metrics of duration can be the length of time (e.g., number of days) that a certain minimum threshold of temperature is exceeded or the time for which the multiday average temperature is below a prescribed threshold; intensity, on the other hand, is often measured by the lowest temperature attained. In some instances, the severity of a cold event has been quantified as the product of the event duration and intensity.

Prior Knowledge and Overview of Attribution Studies

Extreme cold events are driven by a combination of thermodynamics (cold air mass formation) and dynamics (the large-scale circulation, advection). Horton and colleagues (2015) have used self-organizing maps derived from atmospheric reanalyses to show that both factors have played roles in recent changes in extreme cold events. In particular, increasing trends in northerly flow have led to an increasing trend in winter cold extremes over central Asia.

The research to date indicates that extreme cold events are less frequent and less severe than in previous decades, although interannual variability is still large enough to allow extreme cold events such as occurred in North America in 2014 and Europe in 2012. Even over 60-year periods, trends in the coldest temperature of the year are not compellingly positive over Europe and the United States (van Oldenborgh et al., 2015, Figure 4b). The increases in cold extreme daily minimum temperatures (i.e., warming) are generally greater than are the increases in extreme daily maximum temperatures, and there is no indication of increased variability of daily or monthly winter temperatures over the United States (Kunkel et al., 2015; Screen et al., 2015). A similar warming of the coldest temperatures over other land areas of the world emerged from Sillmann and colleagues’analysis (2013a,b) of the ETCCDI indices for 1948-2005 in 4 different atmospheric reanalyses and 31 Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The tendency for cold extremes to warm by more than hot extremes also is apparent in Collins and colleagues’ (2013) Figures 12.13 and 12.14 as well as the U.S. National Climate Assessment’s Figure 2.20 (Melillo et al., 2014).

The general expectation is that cold events defined relative to fixed temperature thresholds should become less frequent and less severe as the climate warms on the global scale. But, it is nonetheless possible for them to increase in frequency or intensity regionally for periods of time (e.g., due to increases in the intensity of cold air advection from polar to lower-latitude regions).

Extreme cold events in eastern North America have characterized a few recent winters (2014, 2012), but such events are less frequent and their actual temperatures