Climate Change: The Redistribution of Rainfall Patterns
Global warming/cooling, increased hurricane intensity & frequency, and changing weather patterns are all part of the larger issue of climate change. But one such topic rarely discussed by most people is the redistribution of precipitation around the world. The occurrence of changing rainfall patterns is another key part of climate change that scientists are studying right now. Humans have only been measuring and recording rainfall statistics for a very short period of time, but still slight trends can be seen in the data. So what does the current scientific data on this subject predict for our future?
Changing Rainfall Patterns over the Sahara Desert Region
When scientists looked at satellite and aerial imagery of the Sahara Desert Region for the period of 1982 to 2002, they found evidence that the Sahara Desert was getting smaller. In a 2,400 mile long zone bordering the southern boundary of the Sahara, extensive re-greening and increased plant growth was observed. Increased vegetation is clearly indicative of a trend of increasing rainfall in this region of the world. If this trend continues, the much needed rainfall could help bring this part of the globe back to life, turning this arid region into the lush paradise that it was over 12,000 years ago. More evidence that pointed to an increasing rate of precipitation was later discovered in a 2007 IPCC study on precipitation trends.
Changing Rainfall Patterns in America
As many of you know, the west and southwestern United States are experiencing a drought. According to the National Academy of Sciences as well as the United States Geological Survey, this region of the Country is likely the driest is has been in over 500 years. Additionally, predictions about future water levels indicate that there is a 50% chance that Lake Mead behind Hoover Dam in Arizona will run dry by 2021. Increasing demand for water combined with decreasing rainfall amounts could spell disaster for the American southwest in the future.
Moreover, the desert states aren't the only ones suffering from decreased precipitation. Currently, northern Florida is experiencing record low flows and reservoir levels. The extended drought continues to plummet the region's springs, rivers, lakes and groundwater levels to historic lows. If things don't change for the area, water use restrictions could be in Florida's future. You can read more about Florida's water shortage in this article posted in the The Gainsville Sun.
Its not all bad news for America though. There are many parts of the country, especially in the northern regions, that are experiencing increased rainfall. Parts of New York and many areas near the Atlantic coast have experienced more rainfall recently than they have ever seen before.
Global Precipitation Trends
In general, from 1900 to 2005 the amount of precipitation falling to earth's surface has increased in regions north of 30th latitude. The precipitation trends calculated with climate models also predict that this increase will continue well into the future. Moreover, rainfall has mostly declined over tropical regions since the 1970s leading to increased periods of drought and even water shortages.
According to a 2001 study by the Royal Meteorological Society, total global precipitation has increased over land surface by about 0.09mm per year during the twentieth century. With the exception of northern Africa and a few other regions, most land areas of the globe experienced an overall increase in total precipitation. However, it must be noted that according to the study, there was no significant change in the overall total amount of precipitation that is occurring on Earth.
In a more recent IPCC study (2007), scientists looked at all rainfall records from the twentieth century. By analyzing rainfall data from 1901 to 2005, the original conclusions in the Royal Meteorological Society's study were essentially confirmed. When the period of record was shortened to just the more recent period of 1979-2005, the precipitation patterns and trends nearly reversed their direction for many parts of the globe. By isolating data for the more recent years, scientists were able to show several key items that were not apparently obvious when the complete dataset was analyzed as a whole. For example, in North America, the western and southwestern regions showed a downward trend in total rainfall. However, northern Africa experienced a marked increased in the overall total precipitation that it received. This suggests that rainfall patterns have made a quick and dramatic change in recent years.
Please refer to figure 3.13 of the IPCC report for a great graphic showing these trends.
A Simple Analysis Using Oregon State University's PRISM Climate Database
Oregon State University's PRISM Climate Group has developed a great online spatial database of climate statistics. For some datasets, you can even download information from as far back as 1895. Data from PRISM is recognized world wide as the highest quality spatial data that is currently available for this particular type of information. For this article, I will download a complete set of annual precipitation data for four locations in America and analyze it for trends. Focusing on locales that have experienced changing rainfall patterns, I chose the following four areas of the country for analysis:
- El Paso International Airport (ELP), Texas
- Gainesville Regional Airport (GNV), Florida
- Pierre Regional Airport (PIR), South Dakota
- Syracuse Hancock International Airport (SYR), New York
This first graph shows the total annual precipitation for each location for the period of 1895 to 2011.
Overall, you can see that the annual rainfall amounts for each location vary widely from year to year. It is also somewhat difficult to seen any significant trends in rainfall patterns at this scale.
However, when we add a linear trend to each of the data sets, you can more easily see that the northern states (South Dakota and New York) have a slight upward trend in precipitation while the southern states have been nearly flat or slightly downward over the last 117 years.
Next, I will look at the same areas of the country but for just the period of record from 1979 to 2011. By shrinking the period of analysis just to the last 3 decades, the upward and downward trends are more clearly visible, especially for the southern states. It is also more relevant and representative to present day. The following graph shows the linear trends of these four areas.
And finally, the table below shows the decadal rates of rainfall change as well as the average net change for these four regions for both of the time periods analyzed using the PRISM rainfall data.
117 Year Precipitation Trends
33 Year Precipitation Trends
The PRISM data clearly shows that rainfall patterns are changing. However, this analysis only considered the total annual precipitation and did not look into other rainfall characteristics such as intensity or temporal distribution.
While it is apparent that rainfall patterns are indeed changing, the cause of this is up for debate. Many theories such as global warming/cooling, changing energy output from the sun, etc, have been developed to try to explain this as well as the many other aspects of climate change. Perhaps this change is really just part of mother nature's cyclical character similar to the shorter cycles of El Nino & La Nina or even the Pacific Decadal Oscillation. However, no theory regarding long term variability in precipitation patterns has been definitively proven.
References and Resources
Environmental Protection Agency. "Precipitation and Storm Changes" Climate Change-Science. April 14, 2011. <http://epa.gov/climatechange/science/recentpsc.html>
Hulme, Mike. Todd, Martin. New, Mark. Jones, Phil. "Precipitation Measurements and Trends in the Twentieth Century" International Journal of Climatology. April 7, 2001. <http://mikehulme.org/wp-content/uploads/2007/04/2001-new_etal_ijc.pdf>
Oregon State University PRISM Climate Group. <http://www.prism.oregonstate.edu/>
Owen, James. "Sahara Desert Greening Due to Climate Change?" National Geographic News. July 31, 2009. <http://news.nationalgeographic.com/news/2009/07/090731-green-sahara.html>
Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.) "Fourth Assessment Report: Climate Change 2007: Section 18.104.22.168 Spatial Patterns of Precipitation Trends" Intergovernmental Pattern on Climate Change (IPCC). 2007. <http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-3-2-2.html>
Walton, Brett. Forecasting Western U.S. Water Supply in 2012: La Nina Again Delivers a Wet North and a Dry South. Circle of Blue. May 1, 2012. <http://www.circleofblue.org/waternews/2012/world/north-america/western-us-water-supply-la-nina-again-delivers-a-wet-north-and-a-dry-south/>
Yahya, Ayisha. "Are the Deserts Getting Greener?" BBC World News. July 16, 2009. <http://news.bbc.co.uk/2/hi/8150415.stm>