Raimund Muscheler of Lund University explained that there are a number of natural factors which, at different time scales and with varying intensities, affect the earth's climate. Some of these factors are changes in the earth's orbit around the sun and the inclination of the earth's axis, solar activity and volcanic eruptions.
Sunspots
Sunspots are a sign of high radiation activity, and they have been documented since the 17th century. During the 17th century and parts of the 18th century, solar activity was especially low and this coincided with the "little ice age" on earth. For the past 30 years, solar activity has been monitored from satellites, and it has varied up and down by 0.1 per cent in cycles of 7-8 years. It is estimated that a change by 0.1 per cent may alter the earth's mean temperature by 0.1 – 0.2 degrees Celsius. The present solar activity is considered to be the lowest for about 100 years. It is therefore not likely that a change in solar activity is an important cause of the temperature rise observed on earth for the past 100 years, said Muscheler.
The atmosphere's content of isotopes such as C-14 and Be-10 increases when solar activity is low. Isotopes in natural archives such as tree rings, cores from ice sheets, or stalactites, can therefore indirectly show what historic changes in solar activity there have been.
Cosmic radiation
According to Henrik Svensmark from Danish National Space Institute, cosmic radiation can affect cloud formation, which, in turn, affects the earth's energy balance. Variations in cosmic radiation can be large and rapid. He has analysed in detail some events with especially rapid and large reductions in cosmic radiation, and saw that cloud formation in these events considerably decreases, but with a delay of 5-6 days.
Cosmic radiation at present is unusually high, and will probably soon diminish. If the theory is right, this should be followed by reduced cloud formation globally. The global temperature should also decrease, since clouds are considered to have a greater retarding effect on the earth's long wave outward radiation than they have on the incoming short wave radiation.
Greenhouse gases
Gunilla Svensson of Stockholm University often referred to IPCC and thus had its analysis as the basis of her presentation. She emphasised that at present there is a relative lack of actual measured data relating to the poles, but that temperature changes shown by the land and sea based measurements are in good agreement with the results of satellite measurements in the atmosphere.
Theoretically, the causes of the rises in global mean temperature may be both internal factors in the climate system and external influencing factors – for example solar radiation, volcanic eruptions or greenhouse gases.
Since there is only one earth, hypotheses cannot be tested by actual experiments, and this must be done through computer models. This IPCC has done and drawn the conclusion that greenhouse gases are with the greatest probability the principal cause of the observed temperature rise.
Carbon dioxide and methane
Th earth is habitable because of a natural greenhouse effect due to the water vapour and carbon dioxide in the atmosphere. The atmospheric content of the greenhouse gases carbon dioxide and methane has not, however, been as high as now for at least the past 600,000 years.
Jan Egil Kristjansson, Oslo University, summed up by saying that the climate is now dominated by human activities and that, with great probability, carbon dioxide is the single most important anthropogenic factor.
Climate change – man made?
More than 100 people had come to the Salén Building in Stockholm to listen to research findings from an American, two Swedes and three professors from the other Nordic countries, and to a concluding panel discussion. The aim of the seminar Climate change – Man made? was to present the latest research news concerning possible contributory causes of the greenhouse effect other than the greenhouse gases created by human activities. Such factors are primarily radiation from the sun and the cosmos, as well as particles and clouds. The programme had been drawn up by a committee under the direction of Erland Källén of Stockholm University. The meeting was chaired by Arne Johansson, Secretary General of the Research Council's NT Committee.
Author
:
Hans-Örjan Nohrstedt
is Director of International Affairs, Formas