Geoscience Reference
In-Depth Information
Figure 13.9
Annual mean surface air temperatures for the decade 1998-2007, expressed as anomalies
with respect to the base period 1951-1980. Areas in grey have insufficient data to compute anomalies.
Source:NASA, Goddard Institute for Space Sciences (http://data.giss.nasa.gov/gistemp/).
in autumn and winter and over the Arctic Ocean.
It is linked to both changes in atmospheric
circulation and declining sea ice extent. Regarding
the latter, anomalous areas of open water in
autumn and winter allow for large heat fluxes
from the ocean surface to the lower atmosphere.
Note also the strong warming over the Antarctic
Peninsula.
One of the manifestations of recent warming
is a longer growing season. For example, in central
England, the growing season (defined as daily
mean temperature >5
to shrub vegetation. A further tendency of
the past 50 years or so is a decrease in the
diurnal temperature range; night-time mini-
mum temperatures increased by 0.8
C during
1951-1990 over at least half of the northern
land areas compared with only 0.3
°
C for daytime
maximum temperatures. This appears to be
mainly a result of increased cloudiness, which, in
turn,
may
be a response to increased greenhouse
gases and tropospheric aerosols. However, the
linkages are not yet adequately determined.
Precipitation changes are much more difficult
to characterize. The period since 1900 has seen an
overall increase in precipitation north of about
30
o
N. By contrast, since the 1970s, there have been
decreases over much of the tropics and subtropics.
However, these general features mask strong
seasonal, regional and temporal variations. As an
°
C for five days in succes-
sion) lengthened by 28 days over the twentieth
century and was about 270 days in the 1990s
compared with around 230-250 days in the
eighteenth to nineteenth centuries. In the Arctic,
there is strong evidence of links between recent
warming and regional transitions from tundra
°
