Heat Waves

Heat waves are often associated with another type of weather extreme involving very high surface ozone concentrations. In some regions, the impacts from high surface ozone levels during heat waves are more severe than impacts from the heat itself. How may these extremes evolve in the future? A global Earth system model is used to study the relationship between heat waves and surface ozone levels over land areas around the world that could experience changes in future ozone precursor emissions. The model is driven by emissions of greenhouse gases and ozone precursors from a medium-high emission scenario (RCP6.0), and is compared to an experiment with anthropogenic ozone precursor emissions fixed at 2005 levels. With ongoing increases in greenhouse gases and corresponding increases in average temperature in both experiments, future heat waves become more intense (greater maximum temperatures during heat waves). However, surface ozone concentrations during heat waves decrease proportionately more than during non-heat wave days in areas where ozone precursors are prescribed to decrease in RCP6.0 (e.g. most of North America and Europe). Meanwhile, surface ozone concentrations during heat waves increase in areas where ozone precursors either increase or have little change in RCP6.0 (e.g. central Asia, the Mideast, northern Africa). In the stabilized ozone precursor experiment, surface ozone concentrations increase during future heat waves in most regions compared to non-heat wave days except where ozone suppression occurs. This produces decreases in ozone in future heat waves in those areas likely associated with changes in isoprene emissions from forests (e.g. west coast and southeastern North America, eastern Europe).

Meehl, G.A., C. Tebaldi, S. Tilmes, J.-F. Lamarque, S. Bates, A. Pendergrass, and D. Lombardozzi, 2018: Future heat waves and surface ozone. Env. Res. Lett., http://iopscience.iop.org/article/10.1088/1748-9326/aabcdc

A warmer atmosphere has more water vapor. Scientists have been trying to predict what this means for precipitation, but this is more complex and harder to model than temperature. One explanation has been that the intensity of extreme precipitation events will increase at a rate proportional to the increase in atmospheric moisture. But recent findings show that this explanation is too simplistic. There are many ways to define extreme precipitation, and the choice of definition affects how it responds to warming. Researchers must choose their definition of extreme precipitation with care and articulate it clearly, and users should consider how extreme precipitation is defined when interpreting analyses of its change with warming. This is because climate change projections show extreme precipitation increasing more than the mean, which complicates the relationship between mean and extreme precipitation.

Pendergrass, A.G., 2018: What precipitation is extreme? Science, 360(6393), 1072-1073. Doi: 10.1126/science.aat1871

Understanding changes in precipitation variability is essential for a complete explanation of the hydrologic cycle’s response to warming and its impacts. While changes in mean and extreme precipitation have been studied intensively, precipitation variability has received less attention, despite its theoretical and practical importance. Single- and multi-model ensembles of climate simulations as well as station records of daily precipitation observations going back to 1950 are analyzed to show that precipitation variability in most climate models increases over a majority of global land area in response to warming. Comparing simulations of recent decades to simulations forced with higher greenhouse gases, in the global, multi-model mean, precipitation variability increases 3–4% K−1 globally, and is remarkably robust on a range of timescales from daily to decadal. Precipitation variability increases by at least as much as mean precipitation and less than moisture and extreme precipitation for most models, regions, and timescales. This is related to an increase in moisture which is partially mitigated by weakening circulation. Changes in observed daily variability in station data are consistent with increased variability.

Pendergrass, A. G., R. Knutti, F. Lehner, C. Deser, and B. M. Sanderson, 2017: Precipitation variability increases in a warmer climate. Sci. Rep., 7, 17966, Doi: 10.1038/s41598-017-17966-y