132
Summary
The earth’s climate is controlled by the net incident solar
radiation and the strength of the greenhouse effect. The lat-
ter is a measure of the ability of the greenhouse gases and
the clouds to absorb and emit infrared radiation. The green-
house effect provides an insulating layer without which life
on earth in its present form would not be possible.
To define climate is not an easy task. A basic reason for
this is that climate is better characterised as something that
varies instead of as something stationary. One of the prima-
ry objectives in climate research is to understand the
mechanisms behind climate variations and changes.
The climate during the Holocene, i.e. the period after the
end of the last ice age, was quite stable in comparison with
the variations during the ice age. However, there were some
variations in the Holocene. It is likely that about 5,000
years ago, the temperature in central Greenland was above
two degrees warmer than it is today. Whether the global cli-
mate was also warmer than it is today is, however, ques-
tionable. The climate was also mild about 1,000 years ago,
in particular in Greenland. There is some ongoing scientific
debate on whether or not it was actually warmer than today.
In any case, the global warming of 0.7-0.8˚C during the last
100 years or so has been quite marked.
Observations have shown that the temperature in the
stratosphere (20-50 km above the earth’s surface) has
decreased in recent decades. This is the opposite of what
has happened near the surface. There have also been varia-
tions in other climate indices such as the North Atlantic
Oscillation Index, the NAO. Variations in the NAO are
important for the winter climate in many regions around the
North Atlantic, including the North Sea and the Wadden
Sea. One important example is that the overall level of win-
ter storm activity - both strength and number of storms -
seems to co-vary with the NAO. There have been increases
in both storm activity and the NAO index from about 1970
to the 1990s.
Obviously the climate can vary if it is heated or cooled
by external processes such as increased solar radiation, lar-
ge volcanic eruptions or anthropogenic emissions of green-
house gases and aerosol particles. But the climate also
varies without such external forcing. During the last ice
age, these so-called internal climate variations were very
large and were related to global couplings between the
atmosphere-ocean-ice system.
The greenhouse effect has always been essential to the
earth’s climate. Natural variations in the carbon dioxide
(CO
2
) and methane concentrations have co-varied with the
temperature for millions (or even billions) of years. One
should, however, be careful not to claim that greenhouse
gases have been the ultimate reason for all climatic varia-
tions. This is because the atmospheric concentrations of
greenhouse gases are strongly dependent on the temperatu-
re. So if an external forcing, e.g. from increased solar radi-
ation, heats the climate, the greenhouse gases will follow
with higher concentrations. In this way greenhouse gases
act as a fundamental amplifier of external forcings because
the increased greenhouse effect heats the climate conside-
rably more than the direct effect of the initial solar forcing.
Once initially heated by external forcings, including the
effects of increased greenhouse gas concentrations, the cli-
mate will react with higher temperatures. These higher tem-
peratures will - in turn - initiate a number of additional cli-
matic feedbacks leading to even higher temperatures, and
so on. The most important feedbacks are related to snow
and ice and to water vapour in the atmosphere.
Although there are some uncertainties, it seems almost
certain that the present concentrations of CO
2
are the
highest during many hundreds of thousands of years, and
probably the highest in more than 20 million years. Metha-
ne concentrations are now also very high. These high con-
centrations can only be due to human emissions, and it
seems inevitable than some global warming will take place
as a result of this. Many types of climate observations show
that this warming has already begun.
Climate models are used to estimate the magnitude and
nature of future climate changes and to understand past
changes. These are computer models describing the funda-
mental physics of the climate system. They include several
components: the atmosphere, the land surface, the sea ice
1...,122,123,124,125,126,127,128,129,130,131 133,134,135,136,137,138,139,140,141,142,...176