Page 132 - Sjæklen2004

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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