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Dramatic changes in precipitation, such as heavier storms and less snow, are another sign that humans are causing global climate change.

As human-produced greenhouse gases heat the planet, increased humidity (water vapor in the atmosphere) results. Water vapor is itself a greenhouse gas. [112] In a process known as a positive feedback loop, more warming causes more humidity which causes even more warming. [113] Higher humidity levels also cause changes in precipitation. According to a report published in the Proceedings of the National Academy of Sciences, the recorded changes in precipitation over land and oceans “are unlikely to arise purely due to natural climate variability.” [48]

According to researchers at the Scripps Institution of Oceanography, up to 60% of the changes in river flow, winter air temperature, and snow pack in the western United States (1950-1999) were human-induced. [111] Since 1991, heavy precipitation events have been 30% above the 1901-1960 average in the Northeast, Midwest, and upper Great Plains regions. [16] A study found that global warming caused by human actions has increased extreme precipitation events by 18% across the globe, and that if temperatures continue to rise an increase of 40% can be expected. [174]

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