Geoscience Reference
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(black line) and an average of model predictions for the
same period using natural and human-induced forcings.
For comparison, the orange band shows the range of
predictions using only natural forcings due to solar
activity changes and volcanoes.
The Fourth Assessment report of IPCC was published
in 2007 and gave firm indication that the recent trend of
climate warming is due to an increase in the concentration
of greenhouse gases in the atmosphere. The main source
of carbon dioxide is from fossil fuel consumption, with
changes in land use forming a significant but small
contribution. Methane has also increased in concentra-
tion, though the rate of increase has declined since the
1990s. However, there is concern that methane release
may increase if areas of permafrost and peat become
sufficiently warm for biogenic activity to liberate more
methane; an example of positive feedback. The third main
greenhouse gas, nitrous oxide, has increased steadily since
the 1980s. About one-third of emissions are due to human
influences, mainly in agriculture.
There are few long-term measurements of climate
that have experienced uniform site conditions through-
out their period of record. Many sites have become
surrounded by urbanization, others have changed location
and many have closed, to be replaced by new ones. It is
not easy to be absolutely certain that an observed change
of climate in an acceptable statistical sense is the result of
climate change, changed site factors or a combination of
the two. There are relatively few observing stations over
ocean, mountain and polar areas, so our knowledge of
change in these environments is more limited. Careful
analysis of the data record has indicated that many loca-
tions are experiencing change. Table 28.1 shows recent
trends in some climate parameters, the likelihood of these
changes being due to human influence, and the likelihood
of future trends.
Our expectations of future climate are likely to depend
upon how much and how rapidly the human-induced
changes to the atmosphere and the planet's surface
proceed. These have to be taken into account in order to
make predictions. Starting with the Special Report on
Emission Scenarios (SRES) published in 2001, thirty-five
scenarios were developed based on a mix of environ-
mental constraints, economic assumptions and global
equalization or fragmentation. Of these, six most likely
scenarios are used for calculations. Present-day demog-
raphy, technological development, as far as it can be
forecast, socio-economic factors and geopolitical consid-
erations would suggest that future levels of emission are
likely to be in the middle to higher levels of expectation.
The lack of progress in reducing emissions following the
opening of the Kyoto Protocol in 1998 indicates the
difficulties involved.
For each emission scenario, the emission figures have
to be converted to atmospheric concentrations over time.
The impact of the changed pollutant levels can then be
Table 28.1 Recent trends, assessment of human influence on the trend and projections for extreme weather events
for which there is an observed late twentieth-century trend
Likelihood that trend
Likelihood of a human
Likelihood of future trends
Phenomenon and
occurred in late
contribution to
based on projections
direction of trend
twentieth century
observed trend
for twenty-first century
Warmer and fewer cold days
Very likely
Likely
Virtually certain
and nights over most land areas
Warmer and more frequent hot days
Very likely
Likely (nights)
Virtually certain
and nights over most land areas
Warm spells/heat waves.
Likely
More likely than not
Very likely
Frequency increases over
most land areas
Heavy precipitation events.
Likely
More likely than not
Very likely
Frequency (or proportion of total
rainfall from heavy falls) increases
over most areas
Area affected by drought
Likely in many regions
More likely than not
Likely
increases
since 1970s
Intense tropical cyclone activity
Likely in some regions
More likely than not
Likely
increases
since 1970
Increased incidence of extreme
Likely
More likely than not
Likely
high sea level (excluding tsunamis)
Source: after IPCC 2007.
 
 
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