Global changes in observed heat waves and warm spells

During the last decade, a suite of severe heat waves has occurred over various global regions. A handful of modeling studies have projected global increases in heat wave and warm spell intensity, frequency and duration over the next century and large Northern Hemisphere and Southern Hemisphere regions. However, there has been little research conducted on changes in observed events, which, at least in part is due to the subjective nature of heat waves and the corresponding metrics employed to describe these events.

Perkins and Alexander (2013) present a framework in which to measure heat waves, taking into account various definitions and characteristics, such that the subjective nature of heat waves are preserved, as well as reducing the amount of applicable metrics. Moreover, the framework is designed such that anomalously warm events that occur outside of summer (i.e. warm spells) may be measured. The HadGHCND gridded observed dataset is employed over the period 1950-2011, making it the first analysis of changes in observed heat waves and warm spells conducted and the global scale. Results demonstrate that the definition of a heat wave may result in slightly different trend magnitudes, although the definitions agree on the trend sign and time series of events globally and for most regions. There is evidence that warm spells, which include wintertime and summertime events, are increasing faster than heat waves only occurring during summer. Moreover, for the time period analysed statistically significant changes in the number of heat wave days are evident for much of Europe and Asia, as well as parts of Australia and the United States of America, although in some regions heat waves do not exhibit a significant trend if a longer period is considered .

References Perkins, S. E. and L. V. Alexander, 2013: On the Measurement of Heat Waves. J. Climate, 26, 4500–4517. doi: http://dx.doi.org/10.1175/JCLI-D-12-00383.1