Latin America may have less water, as a consequence population. Current models project that such changes
both of reduced precipitation and retreating glaciers, would occur over very long time scales (millennia) if
significant species loss, and by mid-century it may global temperature were to be sustained at 1.9–4.6°C
expect the gradual replacement of tropical forest by over pre-industrial levels. Complete melting of the
savanna in eastern Amazonia. Yields of food crops may Greenland ice sheet would raise sea level by 7m and
diminish, exposing more people to the risk of hunger. could be irreversible.
North America faces water scarcity, more heat waves,
coastal threats and problems for some crops.
Meridional overturning circulation
The possibility which continues to fascinate the media,
The risk of abrupt or irreversible changes
where potentially abrupt change could take place, is in
Climate warming could lead to some impacts that are
the meridional overturning circulation of ocean water.
abrupt or irreversible, depending upon the rate and Ocean currents have specific patterns that are deter-
magnitude of the change. Abrupt climate change on mined by different water densities. Based on current
time scales of a decade or so is normally thought of
model simulations, it is very likely that the meridional
as involving ocean circulation changes (like the me-
overturning circulation (MOC) of the Atlantic Ocean (the
ridional overturning circulation – see box). On longer
mixing of cold and warm water around the meridian) will
time scales, ice sheet and ecosystem changes may
slow down during this century (the Gulf Stream bring-
also play a role.
ing warm water to Northern latitudes of Europe is part of
this circulation system); nevertheless temperatures in the
region are projected to increase. It is very unlikely that
If a large-scale abrupt climate change effect were to
the MOC will undergo a large abrupt transition during
occur, its impact could be quite high. Partial loss of
the 21st century, and longer-term changes in the MOC
ice sheets on polar land and/or the thermal expan-
cannot be confidently assessed. Impacts of large-scale
sion of seawater over very long time scales could im- and persistent changes in the MOC are likely to include
ply metres of sea level rise, with the greatest impacts changes in marine ecosystem productivity, fisheries,
on coasts, river deltas and islands, implying major
ocean CO
2
uptake, oceanic oxygen concentrations and
changes in coastlines and inundation of low-lying ar-
terrestrial vegetation.
6
eas. The number of people in the world who live within
100 km of the coast and no more than 100 m above
6. About one-third of anthropogenic CO
2
emissions are thought
sea level has been calculated at 600 millions to 1.2
to be entering the oceans, which form the largest active carbon
“sink” – absorber – on Earth.
billion – between 10 and 23 per cent of the world’s
Projected climate change and its impacts 39
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