Climate Change 2007 Update
3. How is the climate going to change in the future?
- 3.1 What are the projected changes in temperature for the 21st century?
- 3.2 What are the other projected changes for the 21st century?
- 3.3 What changes are projected on the longer term?
3.1 What are the projected changes in temperature for the 21st century?
According to a series of emission
temperature is projected to increase by about 0.2°C per decade
for the next two decades. Previous projections had suggested a
warming of 0.15 to 0.3°C per decade for 1990 to 2005.
Even if the concentrations of all
greenhouse gases and
aerosols had been kept
constant at year 2000 levels, a further warming of about 0.1°C
per decade would be expected, mainly due to the long time it
takes for oceans to release the heat they accumulated. Continued
greenhouse gas emissions at
or above current rates would cause further warming and induce
many changes in the global climate system during the
21st century that would very likely be larger than
those observed during the 20th century. In addition,
warming tends to reduce land and ocean uptake of atmospheric
increasing the amount of human-induced emissions that remains in
Best estimates for globally average surface air warming
between the 1980s and the 2090s in the selected
scenarios range from 1.8°C
(likely range is 1.1°C to 2.9°C) to 4.0°C (likely range is 2.4°C
to 6.4°C). Uncertainties are related to model differences and to
differences in energy use scenarios.
3.2 What are the other projected changes for the 21st century?
Global average sea level is projected to rise by 18 to 59 cm
by the end of the 21st century (2090-2099), depending
on the scenario
However, models used to date do not include uncertainties about
certain climate mechanisms because of lack of knowledge. For
instance, projections of sea level rise do not take into account
the fact that the flow of ice from the
ice sheets in Greenland and
Antarctica could be faster in the future than they were in
recent years. These changes could increase the projections by 10
to 20 cm, maybe even more, but understanding is still too
limited to include them in the models with any level of
Geographical patterns in climate changes are expected to
remain similar to those observed over the past several decades.
Warming is expected to be greatest over land and at most high
northern latitudes, and smallest over the Southern Ocean and
parts of the North Atlantic Ocean.
Other projected changes include:
- increased acidification of the oceans caused by
concentrations in the
- shrinking snow cover and
sea ice, and decreased
- increasingly frequent hot extremes, heat waves, and
heavy precipitation events
- more intense tropical cyclones (typhoons and
- a moving of extra-tropical storm tracks towards the
poles, with consequent changes in wind, precipitation, and
- greater amounts of precipitation in high-latitudes and
less rain in most subtropical land regions; and
- a slowing of the Atlantic Ocean
3.3 What changes are projected on the longer term?
From 1980 to the end of the 21st
temperatures are projected to increase by 1.8°C to 4.0°C.
Warming and sea level rise caused by human activities will
continue for centuries, even if
concentrations were to be stabilized, because of the long
timescales associated with climate processes and
Warming is expected to affect the carbon cycle, resulting in
even higher carbon dioxide
concentration in the
atmosphere, but the
magnitude of this is uncertain.
If greenhouse gas
concentrations in the
atmosphere were to
stabilize in 2100 at levels projected in the B1 and A1B emission
scenarios, a further
increase in global average temperature of about 0.5°C would
still be expected around 2200. Under this A1B scenario, the
thermal expansion of the oceans alone would lead to an increase
of 30 to 80 cm of global sea level by 2300, and this rise would
continue over many centuries after that.
The melting of the
Greenland ice sheet is
projected to continue and to contribute to sea level rise after
2100. If it were to keep melting for millennia until Greenland
ice disappeared completely, global sea level would rise by about
The vulnerability of the
ice sheets to warming could
be increased by dynamical processes related to ice flow (not
included in current models but suggested by recent observations)
thereby increasing future sea level rise. Current global model
studies project that the Antarctic ice sheet will remain too
cold for widespread surface melting and is expected to gain in
mass due to increased snowfall.
Both past and future human emissions of
carbon dioxide will
continue to contribute to warming and sea level rise for more
than a millennium, due to the long time it takes for this gas to
disappear from the
The Emission Scenarios of the IPCC Special Report on
Emission Scenarios (SRES)