Precipitation on Planet Earth

Precipitation, rainfall and snowfall, affect the amount of surface water and groundwater available for drinking, irrigation, and industry. The shortfall in the Colorado River is a reminder that precipitation is the source of river flows and can determine what types of animals and plants (including crops and people) can survive in a particular place. 

As average temperatures at the Earth’s surface rise, more evaporation occurs, which, in turn, should increase overall precipitation. Therefore, a warming climate is expected to increase precipitation in many areas. We are just beginning to see the precipitation impacts of changes in the climate; and as precipitation patterns vary across the planet the effects of climate change will too. By shifting the wind patterns and ocean currents that drive the world’s climate system, climate change will also cause some areas to experience decreased precipitation and some areas to experience increased precipitation.

Precipitation has been measured across the globe at various locations for centuries, beginning with simple and then more complex surface gauges. My weather station still uses a very simple rain gauge. As the record at individual locations and over regions lengthened, long-term means were calculated and seasonal and inter-annual variations at those locations were examined.

Over oceans, precipitation information was originally limited to islands and ships. Before the advent of satellites, global rainfall was tracked using the gauges over land and estimates of precipitation based on shipboard weather observations (e.g., Jaeger 1983). With satellite-based precipitation estimates available in the latter part of the twentieth century, efforts were made to provide analyses of monthly precipitation estimates combining the best observations over the entire globe with techniques to produce a consistent record during the period. 

Adler et al in their article published in 2017 found that “No overall significant trend is noted in the global precipitation mean value, unlike that for surface temperature and atmospheric water vapor. However, there is a pattern of positive and negative trends across the planet with increases over tropical oceans and decreases over some middle latitude regions.”

Adler, R.F., Gu, G., Sapiano, M. et al. Global Precipitation: Means, Variations and Trends During the Satellite Era (1979–2014). Surv Geophys 38, 679–699 (2017). https://doi.org/10.1007/s10712-017-9416-4

The U.S. EPA says: “On average, total annual precipitation has increased over land areas in the United States and worldwide. Since 1901, global precipitation has increased at an average rate of 0.04 inches per decade, while precipitation in the contiguous 48 states has increased at a rate of 0.20 inches per decade.”

from EPA

However, they note that “Data from the early 20th century are somewhat less precise because there were fewer stations collecting measurements at the time. To ensure that overall trends are reliable, the data have been adjusted where possible to account for any biases that might be introduced by factors such as station relocation or changes in measurement instruments. Hawaii and U.S. territories are not included, due to limitations in available data.”

The global number (an increase of 0.04 inches per decade) is a result of attempting to adjust the limited hand records of historic data. Until more satellite data is available I will stick with Adler et al and believe that until now there has been no statistically significant trend in the global precipitation. However, we are just beginning to see the impacts of climate change, and I did move to Virginia from California to make sure I have water.

from EPA