Geoscience Reference
In-Depth Information
Precipitation
There are two global precipitation data sets: Hulme and the Global Historical Climate Net-
work (GHCN). Unfortunately, unlike for temperature, rainfall and snow data are not as well
documented and the records have not been carried out for as long. It is also known that pre-
cipitation overland tends to be underestimated by up to 10-15 per cent owing to the effects
of airflow around the collecting dish. Without correction of this effect, there could be a
spurious upward trend in global precipitation. Despite these problems there seems to be a
significant increase of precipitation over the last 25 years (see
Figure 8
) particularly in the
Northern Hemisphere middle latitudes. This is supported by evidence that since the 1980s
atmospheric water content has increased over the land and ocean as well as in the upper
troposphere. This is consistent with the extra water vapour that the warmer atmosphere can
hold.
There is evidence for a global increase in precipitation but the evidence for this change is
much stronger when considering individual regions. The latest IPCC report suggest that
significant increases in precipitation have occurred in the eastern parts of North and South
America, northern Europe, and northern and central Asia. It seems that seasonality of pre-
cipitation is also changing, for example in the high latitudes in the Northern Hemisphere,
with increased rainfall in the winter and a decrease in the summer. Long-term drying trends
have been observed onthe Sahel, in the Mediterranean, southern Africa, and parts of south-
ern Asia. It has also been observed that the amount of rain falling during heavy 'extreme'
rain events has increased. These observations are supported by detailed compilation of all
precipitation records for the Northern Hemisphere published in
Nature
in 2011 by Dr
Seung-Ki Min and his colleagues, which showed a significant increase in the intensity of
rainfall over the last 60 years.
