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Sunday, July 13, 2008

The Correlation Between Temperature and Precipitation: Can’t Find One


In addition to Smith and Reynolds SST data, the NOAA NOMADS system also allows the retrieval of NCDC GHCN land surface temperature (LST) and precipitation data, based on global coordinates chosen by the user.

The LST is available from January 1880 to September 2005 and the precipitation data covers January 1900 to April 2004, allowing comparisons over the shared time period.

The media constantly bombards the public with reports of increases, decreases, maximums, minimums, droughts, floods, all associated with anthropogenic climate change. If there was a correlation in the data record between increases in anthropogenic greenhouse gases and temperature or between temperature and precipitation, those claims might have some basis in reality. We’ve all seen the numerous representations of greenhouse gases and temperature by both sides of the debate, each illustrating correlation or lack thereof. But I’ve never seen LST and precipitation compared side by side.

I, actually, was expecting more agreement between long-term trends, but I found few. Those were in Australian and South American data sets. There are agreements in short-term trends, but in some instances the change in precipitation precedes temperature, and in others, temperature comes first.


The following graphs illustrate LST and precipitation from 1900 to 1999, with the data smoothed with a 36-month filter. The scaling factor used for the precipitation data is illustrated in the Y-axis title. For the most part, the scaling factor used was 0.1, but there were those that varied.


Global precipitation has decreased since its peak around 1955.

Figure 1 – Global Temperature and Precipitation


The Northern Hemisphere has the same long-term trends as global; precipitation has decreased since its peak around 1955.

Figure 2 – Northern Hemisphere Temperature and Precipitation


Southern Hemisphere precipitation peaked around 1975 and shows a slight downward trend since then.

Figure 3 – Southern Hemisphere Temperature and Precipitation


Australia has the greatest swings in precipitation of the data sets illustrated in this post. There is no apparent temperature driver for those mighty swings. In another post, I’ll have to compare ENSO to those drastic changes in precipitation.

Figure 4 – Australia Temperature and Precipitation


The European temperature and precipitation curves both show steady rises. At times, the two curves appear to be in phase; in others, they’re nowhere close to being in agreement. In an effort to draw some information from the two noisy datasets, I added 6th-order polynomial trends, which really didn’t help identify anything.

Figure 5 – Europe Temperature and Precipitation


The two major swings in precipitation that appeared in the Australian illustration also show up in South America. Again, both curves rise over the century, but the oscillations in the two curves are well out of phase. I again added poly trends, but they offered no help.

Figure 6 – South America Temperature and Precipitation

The last data set comparison for this post is Greenland. Precipitation appears to have risen at a very slight trend, while temperature peaked in the 20s through 40s.

Figure 7 – Greenland Temperature and Precipitation

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