Expert Report

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As climate change warms the atmosphere, Earth's hydrology is shifting—with the potential to make floods and droughts more extreme. There is now a pressing need for decision-makers to better understand the ongoing changes in hydrologic extremes in order to make preparations for changing conditions. This report assesses changes in the frequency and severity of floods and droughts, abilities of communities to understand and forecast these changes, and strategies for better communicating the science to water resources practitioners.

Key Messages

  • Global climate models and the basic laws of physics predict that the hydrologic cycle will accelerate as climate warms. Changing patterns of precipitation could potentially lead to more extreme floods and droughts.
  • Recent analyses of a broad spectrum of water cycle variables, including precipitation, snow cover, and droughts, show that climate change is already affecting hydrology. However, some changes in hydrology have been unexpected.
  • There is a disconnect between climate model predictions and observed patterns of floods and droughts, in part to the complexity of interactions between the atmosphere and land-surface systems. Often, the global climate models that predict increased precipitation are too large in scale and coarse in spatial resolution to tell scientists exactly how hydrology will change at local and regional scales. Smaller-scale regional climate models are not yet sophisticated enough to add significant information.
  • However, these models don’t comprehensively address non-climate issues such as the construction of dams and changes in land cover that can also affect water cycles. More information on all these factors, and how they interact, is needed to gain a better understanding of how climate change will translate to floods and droughts on a regional and local scale.
  • To prepare for shifting conditions, water resource managers will need new flood-frequency guidelines that draw on advances in hydrologic and climate science over the past 25 years.
  • Many different groups, from climate scientists to atmospheric modelers to water resource managers, can help build a better understanding of changing hydrology; but currently, communication between disciplines is limited. A common vocabulary would facilitate collaboration between the scientists and practitioner communities.
  • In the past, researchers placed considerable emphasis on the probability component of risk, but much less emphasis on vulnerability to hazards, which in this context means susceptibility to and ability to cope with losses caused by extreme floods or droughts. Without research to better understand all the dimensions of risk, the design of effective climate change adaptation strategies will remain unrealized.
  • Uncertain flood or drought frequencies cause major problems with water engineering projects that have long life-spans, such as dams, levees, and sewers. One solution to this problem is to construct infrastructure in smaller units that have shorter expected longevities or design re-visit times (on the order of 10-20 years). In addition, engineers could use reconstructions of conditions during past periods of climate change to design projects that could adapt to predicted future conditions.
  • Firm commitments by federal, state, and municipal agencies to retain observational networks are needed to advance abilities to prepare for, adapt to, and mitigate the impacts of hydrologic extremes as climate conditions change.