Moisture and Precipitation

Read...

Another outcome of a warming atmosphere and ocean is increased moisture. Earlier, we discussed how rising temperatures lead to higher evaporation rates. Eventually, condensation rates catch up, reaching a new balance, but in this warmer state, the number of water vapor molecules in the air is greater. In the atmosphere, this has a major impact: if warmer air holds more water vapor, then when it rises and cools to the point where net condensation occurs, there’s more water available for cloud formation and precipitation.

To gauge the moisture available for precipitation, climate scientists use a metric called "precipitable water." This represents the amount of rain that would fall if all the water vapor in a column of air—from the Earth's surface up to the top of the troposphere—condensed and fell as rain. The image below shows the simulated percentage change in precipitable water between the 1984-2013 average and the 1871-1900 average. The widespread blue shading indicates that in most regions, precipitable water has increased by several percent, and in some areas, by as much as 15 percent as the world has warmed.

The difference between the 1984-2013 average of atmospheric precipitable water and the 1871-1900 average shows that most areas of the globe have had an increase in precipitable water ranging from a few percent to 15 percent or more.

Credit: NOAA

This trend has critical implications for precipitation intensity and frequency, as more moisture in the atmosphere can lead to more intense downpours, amplifying the risk of floods and extreme rainfall events in many areas. Therefore, it's not surprising to see an increase in heavy rain events. For example, the percentage of the contiguous United States receiving an unusually large portion of total annual rainfall from extreme one-day rainfall events has increased (here's the corresponding graph from NOAA; the orange curve represents a running nine-year average). But, the increase in heavy rain events hasn't occurred equally everywhere.

The frequency and intensity of heavy precipitation events have increased across much of the United States, particularly the eastern part of the continental US, with implications for flood risk and infrastructure planning. Maps show observed changes in three measures of extreme precipitation: (a) total precipitation falling on the heaviest 1% of days, (b) daily maximum precipitation in a 5-year period, and (c) the annual heaviest daily precipitation amount over 1958–2021. Numbers in black circles depict percent changes at the regional level. Data were not available for the US-Affiliated Pacific Islands and the US Virgin Islands.

Figure credits: (a) adapted from Easterling et al. 2017;282 (b, c) NOAA NCEI and CISESS NC. Source: https://nca2023.globalchange.gov/chapter/2/

The figure above, from the U.S. National Climate Assessment, highlights trends in “heavy precipitation”—defined here as the top one percent of all rainfall events in each region—from 1958 through 2021. In the leftmost panel (the one labeled "a"), we can see that the Northeast has experienced the largest increase in rainfall from these intense events, while the Pacific Northwest has seen a smaller increase, and Hawaii actually shows a slight decrease in rainfall from its heaviest events during this period.

The other two panels display different measures of extreme precipitation, such as the maximum daily rainfall over one- and five-year windows. The overall story remains consistent: significant increases in the eastern United States, with smaller yet still notable, increases in the West. These trends aren't limited to the United States; as we’ll see shortly, similar patterns are evident globally, with variations across regions.