About the global rainfall trend maps
Analyses available
Global and hemispheric land surface rainfall trend maps are available for annual and seasonal rainfall, covering periods starting from the beginning of each decade from 1900 to 1970 until present. Seasonal means are December-February (DJF), March-May (MAM), June-August (JJA) and September-November (SON). Since rainfall is such a highly variable parameter, these different starting times demonstrate the importance of the analysis period on rainfall trend values. For example, the post-1900 maps look quite different to the post-1970 maps for different regions. Analysis periods starting after 1970 are considered too short to calculate meaningful trend values. Large gaps are evident in the analyses reflecting the poor observation of rainfall in some regions, particularly over the oceans.
Interpreting the analyses
The trend maps are a useful way to compare how the rainfall has changed in different regions of the globe over time. However, they need to be interpreted with caution. Trend values have been determined from a linear (straight line) fit to the data, but the change may not have been gradual. For example, a calculated trend could be due to a relatively rapid "step" change, with the remainder of the series being fairly flat (see some of the timeseries graphs). Also, because rainfall is such a variable element, trend values are highly dependent on the start and end dates of the analysis. Consequently trend maps starting in different decades can look remarkably different. Users are advised to keep in mind the period over which trend values have been calculated and interpret them alongside the timeseries of spatially averaged values.
In addition, the effect of missing data on grid point trends should also be considered as a caveat to this analysis. Unlike global temperature records, which generally improve in spatial coverage and consistency with time, several problems exist with the recording and digitising of rainfall records that has resulted in variable coverage for specific regions throughout time. In order for a trend value to be shown, gridpoints were required to have no more than one third of the relevent time period as missing data. For the Australian region, much better confidence can be placed in the Bureau's high quality rainfall data. (See Australian rainfall trend maps.)
Finally, the trend values calculated here using past observations should not be used to imply future rates of change. Due to the complex interactions between the natural and human drivers of climate change and variability, the climate of any location is always changing. Future rates of change will depend on how these drivers interact in future, which will not necessarily be the same as in the past.
Data used
The rainfall trend maps are calculated from the US National Climatic Data Center (NCDC) Global Historical Climatology Network (GHCN) observed gridded precipitation anomalies. These data are publically available from the NCDC. A full overview and description of this data, including references can be found at http://www.ncdc.noaa.gov/oa/climate/research/ghcn/ghcngrid-prcp.html.
Please note that any use of these analyses should be acknowledged to the Bureau of Meteorology. Apart from the purposes of study, research, criticism and review, no part of these data may be reproduced, or redistributed for any commercial purposes, or distributed to a third party for such purpose, without written permission from the Director of Meteorology.
Further information
Peterson, Thomas C. and Russell S. Vose, 1997. An overview of the Global Historical Climatology Network temperature data base, Bulletin of the American Meteorological Society, 78, 2837-2849.
Easterling, David R., Thomas C. Peterson, and Thomas R. Karl, 1996. On the development and use of homogenized climate data sets. Journal of Climate, 9, 1429-1434.
Easterling, David R. and Thomas C. Peterson, 1995. A new method for detecting and adjusting for undocumented discontinuities in climatological timeseries. International Journal of Climatology, 15, 369-377.
Peterson, Thomas C. and David R. Easterling, 1994. Creation of homogeneous composite climatological reference series. International Journal of Climatology, 14, 671-679.
