How is the water cycle affected by climate change?
Water is constantly in motion, rising into the atmosphere as evaporation and falling as precipitation. How climate change impacts the water cycle is a topic of the Third National Climate Assessment.
In May 2014, the U.S. Global Change Research Program released The Third National Climate Assessment. The report details current and predicted impacts in the United States resulting from a changing climate. Over 300 experts contributed to the three-year effort to develop the report. Impacts are described as they affect geographic regions, and sectors such as health, water, forests and agriculture.
A major chapter covers how climate change impacts the water cycle, water resource use and management, and adaptation opportunities and challenges. This national perspective is helpful context for understanding impact we may experience in Michigan and the Great Lakes region.
Overall, precipitation has increased over the continental U.S. by approximately 5 percent since 1900. That pattern is not uniform everywhere – in more recent decades precipitation increased in the some parts of the country, including the Midwest (11 percent increase), while decreasing in part of the Southwest and Southeast. According to the report, these trends are expected to continue, with average annual precipitation increases in the northern U.S., and decreases in the South, especially the Southwest.
In addition to increased precipitation, the number of intense storm events – the heaviest one percent of all storms – is also on the rise. That trend is also projected to continue throughout the entire United States in coming decades. A greater frequency of heavy storms will likely alternate with increasing dry spells.
These changing precipitation patterns have significant impacts. The report outlines five additional key messages related to water cycle changes:
- Short and seasonal droughts are expected to increase in frequency throughout the U.S. The greatest drought impacts, though, are projected in the Southwest, Great Plains and Southeast, where trends point to longer-term, multi-seasonal dry periods. The widespread drought in the Southwest over the past decade resembles what could happen in the future.
- Many parts of the U.S. may experience increased flooding risk. There are four major flood types:
- flash floods caused by intense storms
- urban floods where heavy downpours overwhelm the capacity of stormwater drains (as occurred in the Detroit area during August 2014)
- riverine flooding where heavy precipitation is greater than river capacity in the watershed
- coastal flooding along ocean shorelines. All of these are impacted by increased overall annual precipitation in some parts of the continental U.S., and greater frequency of intense storms everywhere. Interestingly, flooding risk may increase even in areas experiencing decreased average yearly precipitation
- Availability of groundwater for irrigation, drinking water and industrial use may change in parts of the country. This impact is affected by changes in the amount of precipitation, evapotranspiration (from land and water surfaces, and plants) and withdrawals to compensate for hotter weather and decreased precipitation in some areas.
- The combination of rising sea levels and changing surface and groundwater patterns may seriously affect coastal wetlands and aquifers. Those environments are extremely important for fisheries and ecosystems.
- River and lake water quality may be affected through increases in nutrients, sediments and other pollutants. Harmful algae blooms, such as the one that affected Toledo’s water supply during the summer of 2014, are one potential impact. These changes result from higher air and water temperatures, more intense storms, and greater likelihood of droughts.
For additional information about climate change, visit the Michigan State University Extension website on climate change and variability. A related MSU Extension article summarizes the other portions of this National Climate Assessment water resources chapter, including water resources use and management, and adaptation opportunities and challenges.