By Paul Homewood
A paper recently published finds that there has been no significant global precipitation change from since 1850.
Precipitation measurements made at nearly 1000 stations located in 114 countries were studied. Each station had at least 100 years of observations resulting in a dataset comprising over 1½ million monthly precipitation amounts. Data for some stations extend back to the 1700s although most of the data exist for the period after 1850. The total annual precipitation was found if all monthly data in a given year were present. 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: −1.2 ± 1.7, 2.6 ± 2.5 and −5.4 ± 8.1% per century for the periods 1850–2000, 1900–2000 and 1950–2000, respectively. A change of 1% per century corresponds to a precipitation change of 0.09 mm/year.
The stations selected offer good global coverage:
The study is particularly critical of other studies which claim to have detected significant changes over he last few decades:
Three large studies have examined global precipitation records for decades in the last part of the 20th century (Li et al., 2014). The Climate Prediction Center produced 17 years of monthly analysis (Climate Merged Analysis of Precipitation or CMAP) based on precipitation observations using rain gauges, satellite estimates and numerical model outputs (Xie and Arkin, 1997). A second dataset obtained using similar methods was found by the Global Precipitation Climatology Project (GPCP) for the period 1979–2005 (Adler et al., 2003; Huffman et al., 2009). A third data reanalysis has been developed by the National Center for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) (Kistler et al., 2001). The three datasets generate time series having significant differences (Li et al., 2014; Gu et al., 2007). For the period 1979–2008, the CMAP model shows a decreasing trend of 1 mm/year. In contrast, the GPCP trend shows a nearly flat trend of 0.1 mm/year while the NCEP/NCAR model shows an increasing trend of 3.5 mm/year.
These differences are not entirely surprising given that precipitation varies considerably over time scales of decades (van Wijngaarden, 2013). Hence, the resulting trends frequently are not statistically significant. This study examined monthly precipitation measurements taken at over 1000 stations, each having a record of at least 100 years of observations to detect long term changes in precipitation.
Amongst the paper’s conclusions :
1) 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.
2) 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.
3) Stations experiencing low, moderate and heavy annual precipitation did not show very different precipitation trends. This indicates deserts/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 full paper is here: