Geoscience Reference
In-Depth Information
methane and nitrous oxide relative to the preceding one million years. The increase
began around 1750 AD (and possibly earlier in the case of methane), as the Industrial
Revolution gained momentum, resulting in the increasing burning of coal and, later,
oil, as well as widespread forest clearance in Europe and, more recently, in the tropics.
The increase in atmospheric methane concentration may have begun about 5,000 years
ago, as inefficient rice cultivation began in Asia. Before the Industrial Revolution, the
maximum concentration of atmospheric carbon dioxide (pCO
2
) amounted to about
280-300 ppmv. In April 2013, it was 400 ppmv, and is increasing at an accelerating
rate. There have been fluctuations in pCO
2
throughout the Cenozoic, but what seems
to be new is the speed with which the concentration of the various greenhouse gases
is increasing, which appears to be without equivalent in geologically recent times.
An expected consequence of the increase in the concentration of greenhouse-
enhancing gases in the lower atmosphere has been a net warming of the surface of
both land and sea amounting to about 1
C on average, with greater warming evident
in higher latitudes. This warming is evident in rising sea levels and in the widespread
melting of ice caps and mountain glaciers. About half of the sea level rise is estimated
to be from melting ice and about half from thermal expansion of the warmer ocean
surface.
Efforts to predict how the earth's climate might respond in the near future draw on
a combination of global atmospheric circulation models, historical archives, meteor-
ological observations and insights from the study of past climates. There is enormous
uncertainty over possible future changes in precipitation around the globe. Present-
day trends suggest that winter rainfall may continue to decrease in those regions
in the Southern Hemisphere that experience a Mediterranean type of climate. Con-
versely, summer rainfall is likely to increase along the margins of the tropical deserts,
although precipitation incidence may become more erratic, with more frequent floods
and droughts. The adverse impacts of possible future changes in climate are likely to
be greatest among the poorest communities that are already subject to erratic rainfall
regimes and widespread land degradation. Many such communities now live along
the desert margins and in the dry subhumid tropics, and they have a long history of
adapting to climatic vicissitudes. They will need to display similar levels of resilience
and adaptable strategies of grazing and farming in the foreseeable future. Good gov-
ernance, food security, access to clean drinking water and health-care facilities are
necessary to ensure success. However, it is not only the poorer rural societies who
will be most vulnerable. The more advanced societies that are highly dependent on
technology and major infrastructure will also find it hard to adapt, especially large
cities, where good drinking water is already in scarce supply.
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