Data vs. Models #2: Droughts and Floods

This post compares observations with models’ projections regarding variable precipitation across the globe.

There have been many media reports that global warming produces more droughts and more flooding. That is, the models claim that dry places will get drier and wet places will get wetter because of warmer weather. And of course, the models predict future warming because CO2 continues to rise, and the model programmers believe only warming, never cooling, can be the result.

Now we have a recent data-rich study of global precipitation patterns and the facts on the ground lead the authors to a different conclusion.

Stations experiencing low, moderate and heavy annual precipitation did not show very different precipitation trends. This indicates deserts or jungles are neither expanding nor shrinking due to changes in precipitation patterns. It is therefore reasonable to conclude that some caution is warranted about claiming that large changes to global precipitation have occurred during the last 150 years.

The paper (here) is:

Changes in Annual Precipitation over the Earth’s Land Mass excluding Antarctica from the 18th century to 2013 W. A. van Wijngaarden, Journal of Hydrology (2015)

Study Scope

Fig. 1. Locations of stations examined in this study. Red dots show the 776 stations having 100–149 years of data, green dots the 184 stations having 150–199 years of data
and blue dots the 24 stations having more than 200 years of data.

This study examined the percentage change of nearly 1000 stations each having monthly totals of daily precipitation measurements for over a century. The data extended from 1700 to 2013, although most stations only had observations available beginning after 1850. The percentage change in precipitation relative to that occurring during 1961–90 was plotted for various countries as well as the continents excluding Antarctica. 

There are year to year as well as decadal fluctuations of precipitation that are undoubtedly influenced by effects such as the El Nino Southern Oscillation (ENSO) (Davey et al., 2014) and the North Atlantic Oscillation (NAO) (Lopez-Moreno et al., 2011). However, most trends over a prolonged period of a century or longer are consistent with little precipitation change.Similarly, data plotted for a number of countries and or regions thereof that each have a substantial number of stations, show few statistically significant trends.

Fig. 8. Effect of total precipitation on percentage precipitation change relative to 1961–90 for stations having total annual precipitation (a) less than 500 mm, (b) 500 to 1000 mm, (c) more than 1000 mm. The red curve is the moving 5 year average while the blue curve shows the number of stations. Considering only years having at least 10 stations reporting data, the trends in units of % per century are: (a) 1.4 ± 2.8 during 1854–2013, (b) 0.9 ± 1.1 during 1774–2013 and (c) 2.4 ± 1.2 during 1832–2013.

Fig. 8 compares the percentage precipitation change for dry stations (total precipitation <500 mm), stations experiencing moderate rainfall (between 500 and 1000 mm) and wet stations (total precipitation >1000 mm). There is no dramatic difference. Hence, one cannot conclude that dry areas are becoming drier nor wet areas wetter.

Summary

The percentage annual precipitation change relative to 1961–90 was plotted for 6 continents; as well as for stations at different latitudes and those experiencing low, moderate and high annual precipitation totals. The trends for precipitation change together with their 95% confidence intervals were found for various periods of time. Most trends exhibited no clear precipitation change. The global changes in precipitation over the Earth’s land mass excluding Antarctica relative to 1961–90 were estimated to be:

Periods	% per Century
1850–1900	1.2 ± 1.7
1900–2000	2.6 ± 2.5
1950–2000	5.4 ± 8.1

A change of 1% per century corresponds to a precipitation change of 0.09 mm/year or 9 mm/century.

As a background for how precipitation is distributed around the world, see the post: Here Comes the Rain Again. Along with temperatures, precipitation is the other main determinant of climates, properly understood as distinctive local and regional patterns of weather.  As the above study shows, climate change from precipitation change is vanishingly small.

Data vs. Models #1 was Arctic Warming.