Geoscience Reference
In-Depth Information
SUMMARY
mechanisms. Whereas longer term changes are probably
due to astronomical forcing mechanisms, short-term
changes (i.e. the past 100 years) appear to be more
obviously linked to anthropogenic factors. These are
mainly changes in atmospheric composition, including
aerosol loading, depletion of ozone and destruction of
world vegetation. Natural and anthropogenic aerosol
effects appear to be particularly important, but their net
effect remains uncertain.
Climate predictions are being made using a variety of
modelling strategies, of which coupled atmosphere-ocean
GCMs are the most sophisticated. Predictions covering
the next 100 years assuming a variety of emission scenarios
for greenhouse gases and aerosols indicate a mean global
temperature increase in the range 1.4 to 5.8°C by the
year 2100, together with sea-level rises of about 50 cm.
The magnitude of such predictions, based on computer
modelling, are still uncertain, however, and are subject to
large error bands due to our restricted knowledge of the
operations of the global atmosphere-earth-ocean-ice
system.
Alpine glaciers show a general twentieth-century
shrinkage, but even the sign of the mass balance of
Greenland and Antarctica is uncertain. Continental snow
cover has decreased over the past decade, especially in
spring, and Arctic sea ice was also less extensive in many
recent summers. Hydrological models suggest that spring
snow melt runoff would occur earlier and the variability
in rainfall amounts would intensify floods and droughts.
Vegetation cover and croplands will also be affected in the
long term, but human-induced changes will predominate.
Subtropical semi-arid areas are most likely to be affected
by climatic trends.
Critical research needs include better data on cloud
cover and radiation, ocean processes and their atmos-
pheric coupling, feedback processes in general, and the
relationships between large-scale and local/regional-scale
processes and phenomena.
Changes in climate involve factors both external to and
within the climate system. External ones include solar
variability, astronomical effects on the earth's orbit and
volcanic activity. Internal factors include natural variability
within the climate system, and feedbacks between the
atmosphere, ocean and land surface. During the twentieth
century, human-induced climatic change on local and
global scales has become a reality, primarily through
changes in atmospheric composition and surface prop-
erties.
Climatic changes on geological timescales involve
continental drift, volcanic activity and possible changes in
solar output. Over the past few million years, glacial-
interglacial cycles appear to have been strongly controlled
by astronomical variations in the earth's orbit, although
atmosphere-ocean-cryosphere feedbacks must also
be involved in amplifying the initial changes in solar
radiation.
During the twentieth century there has been a
significant average global temperature increase of 0.5°C,
greatest in the higher middle latitudes with warming,
particularly during the 1920 to 1940s and since the early
1970s; the 1990s included several of the warmest years on
record. Diurnal temperature ranges show a decrease over
the past few decades and some regions are experiencing
more frequent extreme conditions for temperature levels
and precipitation amounts. Precipitation trends are less
clear, particularly in the mid-latitudes, but the precipitation
of dry subtropical regions has tended to oscillate widely.
Climatic behaviour during the past twenty years supports
the view that the anthropogenically induced increase of
greenhouse gases and other pollutants is permanently
affecting global climate.
Possible causes of climatic change are examined from
the point of view of the global atmosphere-earth-
ocean-ice system and with respect to forcing and feedback
