How the climate change ?
If nothing is done to reduce emissions, current climate models predict a global warming of about 1.4
– 5.8°C between 1990 and 2100. These projections are based on a wide range of assumptions
about the main forces driving future emissions (such as population growth and technological change) but do
not assume any climate change policies for reducing emissions. Even a 1.4oC rise would be larger
than any century-time-scale trend for the past 10,000 years. These projections takes into account the effects
of aerosols and the delaying effect of the oceans. Oceanic inertia means that the earth's surface and
lower atmosphere would continue to warm for hundreds of years even if greenhouse gas concentrations stopped
rising in 2100.
The average sea level is predicted to rise by 9 to 88 cm by 2100. This would be caused mainly by the
thermal expansion of the upper layers of the ocean as they warm, with some contribution from melting
glaciers. The uncertainty range is large, and changing ocean currents, local land movement and other factors
could cause local and regional sea levels to rise much more or much less than the global average. Slightly
faster melting of the Greenland and Antarctica ice sheets is likely to be balanced counteracted by increased
snowfall in both regions. As the warming penetrates deeper into the oceans and ice continues to melt, the sea
level will continue rising well long after surface temperatures have leveled off.
Regional and seasonal warming predictions are much more uncertain. Although most areas are expected to
warm, some will warm much more than others. The largest warming is predicted for cold northern regions in
winter. The reason is that snow and ice reflect sunlight, so less snow means more heat is absorbed from the
sun, which enhances any warming: a strong positive feedback effect. By the year 2100, winter temperatures in
northern Canada, Greenland and northern Asia are predicted to rise by 40% more than the global average.
Inland regions are projected to warm faster than oceans and coastal zones. The reason is simply the
ocean delay, which prevents the sea surface from warming as fast as the land. The size of this delay depends
on how deep any warming penetrates into the oceans. Over most of the oceans, the uppermost few hundred metres
do not mix with the water beneath them. These upper layers will warm within just a few years, while the deep
ocean stays cold. Water mixes down into the ocean depths in only a few very cold regions, such as the
Atlantic south of Greenland and the Southern Ocean near Antarctica. In these regions, warming will be delayed
because much more water needs to be warmed up to get the same temperature change at the surface.
Global precipitation is predicted to increase, but at the local level trends are much less certain. By
the second half of the 21st century, it is likely that wintertime precipitation in the northern
mid- to high latitudes and in Antarctica will rise. For the tropics, models suggest that some land areas will
see more precipitation, and others less. Australia, Central American and southern Africa show consistent
decreases in winter rainfall.
More rain and snow will mean wetter soil conditions in high-latitude winters, but higher temperatures may
mean drier soils in summer. Local changes in soil moisture are clearly important for agriculture, but
models still find it difficult to simulate them. Even the sign of the global change in summertime soil
moisture - whether there will be an increase or a decrease - is uncertain.
The frequency and intensity of extreme weather events such as storms and hurricanes may are likely to
change. With increasing global temperatures the world is likely to experience more hot days and heat
waves and fewer frost days and cold spells. Climate models also consistently show extreme precipitation
events becoming more frequent over many areas and the risk of drought becoming greater over continental areas
in summer. There is also some evidence to show that hurricanes could be more intense (with stronger winds and
more rainfall) in some areas. There is little agreement amongst models concerning changes in mid-latitude
storms. There are also other phenomena, such as thunderstorms and tornadoes, where knowledge is currently
inadequate for making projections. However, models still cannot predict how. The models used to simulate
climate change cannot themselves simulate these extreme weather events, so the evidence is indirect. There is
some concern that patterns of extreme weather may change because the models predict changes in ocean surface
temperatures and other factors that are known to affect storm and hurricane development. However, it will be
many years before scientists can predict whether individual regions will become more or less stormy.
Rapid and unexpected climate transitions cannot be ruled out. The most dramatic such change, the
collapse of the West Antarctic ice sheet, which would lead to a catastrophic rise in sea level, is now
considered unlikely during the 21st century. There is evidence that changes in ocean circulation
having a significant impact on regional climate (such as a weakening of the Gulf Stream that warms Europe)
can take place in only a few decades, but it is unknown whether or not greenhouse warming could trigger any
such change. Climate models that do show a weakening in the Gulf Stream still project warming over