Precipitation Extremes Vary in Space and Time: Implications for Designing Experiments

Abstract:

There is much evidence suggesting that worldwide, precipitation regimes (amount, variability, and extremes) have been and are being altered, with an increase in extremes expected to have the greatest impacts on ecosystem function. Determining how and why both aquatic and terrestrial ecosystems differ in their sensitivity to precipitation extremes (wet and dry) is important to forecasting future ecosystem structure and function at the global scale. Coordinated multi-site experiments can be invaluable for assessing differential sensitivity of ecosystems to precipitation extremes. However, determining treatment levels in these experiments presents unique problems because extremes in precipitation are defined statistically, based on historical context, and thus can differ dramatically among sites. In an analysis of 144 long-term (100 yr) daily precipitation records from 12 major ecoregions, we show that statistically extreme wet and dry years differ from average years not only in precipitation amount, but also in the number of extreme precipitation events and statistically extreme periods of consecutive dry days. Further, the decadal frequency of extreme wet years has been increasing over the last 100-yrs. This has not been observed for extreme dry years, however. We conclude that while multi-site experiments with fixed treatment levels may be appropriate for assessing ecosystem sensitivity to CO₂ or warming, they may provide less mechanistic insight for studying extremes. We propose that for multi-site experiments focused on extremes, the amount of precipitation removed or added or the length of dry periods imposed should be site-specific (not fixed across sites) and matched to the historical climate record.