"The warming of the earth is clear"

Interview on climate change and the fourth report of the UN climate council

Global anomalies of air temperature at the surface of the earth. © IPCC, 2007
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Climate change is in full swing and has been largely human-influenced - the Intergovernmental Panel on Climate Change (IPCC) came a few months ago. In an interview, Peter Lemke, climatologist and head of the Climate Sciences Department at the Alfred Wegener Institute, reports on the current results and their significance for the future of the earth.


GeoUnion: Climate change is on everyone's lips - but is there now clarity about whether it was actually caused by humans?

Lemke: Model calculations and the comparison with observations show that the warming of the last 50 years was very probably caused mainly by anthropogenic greenhouse gases, mainly carbon dioxide. Changes in solar radiation, on the other hand, have only a minor influence.

Since 1750, the carbon dioxide content of the air has increased by 35 percent from 280 ppm to 379 ppm in 2005. The growth rate of the last ten years is the largest in 50 years and today's value is the highest in the last 650, 000 years. About 78 percent of the increase is attributable to the use of fossil fuels and 22 percent to land use changes such as clearing land. But other important greenhouse gases such as methane and nitrous oxide, whose concentrations have increased by 148 percent and 18 percent since 1750, contribute to the change in the radiation balance. These long-lived greenhouse gases together make up about half as much as the CO2 increase. display

GeoUnion: What is the most important indicator of climate change?

Lemke: This is primarily the temperature. Because the warming of the climate system is available without any doubt and clearly measurable. So the global surface temperature has risen by 0.74 degrees Celsius from 1906 to 2005 and eleven of the last twelve years were the warmest since records began. The temperature increase of the past 50 years is twice as high as the last 100 years, and the Arctic has warmed twice as much as the Earth on a global average. Reconstructed data from observations and other sources, such as tree ring data, even suggest that temperatures over the past 50 years were likely to be higher than ever in the past 500 years, and probably higher than in the past 1, 300 years.

GeoUnion: What effect does this have on the chill regions of the earth?

Lemke: The snow-covered area has decreased by about five percent since 1980. Glaciers worldwide are shrinking and currently contribute 0.8 mm per year to sea-level rise. Also, the sea ice in the Arctic recorded since 1978, a decline in the annual average by eight percent and in the summer even by 22 percent. In Antarctica, on the other hand, no significant decline can be seen so far.

Likewise, the ice sheets on Greenland and the Antarctic currently lose mass due to melting and glacial erosion. They thus contribute 0.4 mm per year to sea-level rise. The temperature increase in the upper layers of the permafrost soils has also been worrying by up to three degrees Celsius since 1980. Accordingly, the maximum extent of the seasonally frozen soil has fallen by seven percent since 1900, and by as much as 15 percent in the spring.

GeoUnion: What are the consequences for the oceans?

Lemke: The sea level has risen on average by about three millimeters per year since 1993, in the 20th century by a total of 17 centimeters. Of this, just over half is caused by the thermal expansion of the warmer ocean, about 25 percent by melting of the mountain glaciers, and about 15 percent by the melting of the ice sheets. However, changes in the meridional orbital movement in the Atlantic, often simply referred to as the Gulf Stream, can not be inferred from the available data. However, it is very unlikely that an abrupt collapse will occur in the 21st century.

GeoUnion: What is the view into the future?

Lemke: Climate projections for the next 100 years can be convincingly simulated by climate models driven by energy use scenarios. Such models predicted Temperatur depending on energy use a further temperature increase and a sea-level rise until the end of the 21st century. For the last decade of the 21st century, the most probable value of global warming for the lowest scenario is 1.8 degrees Celsius (1.1 2.9 degrees Celsius), and for the h The next scenario is 4.0 degrees Celsius (2.4 6.4 degrees Celsius). The largest warming takes place in high northern latitudes. For the next two to three decades, projected warming will have little bearing on assumptions about future emissions. Even with an immediate end to all emissions, the inertia of the climate system would cause a further increase in temperature up to about 0.6 degrees Celsius.

When the second child is born, the first seems suddenly much larger than before. SXC

GeoUnion: That probably means a further rise in sea level?

Lemke: Yes. There are projections for the sea level rise from 2090 to 2100: 18 38 centimeters for the lowest and 26 59 centimeters for the highest scenario. Even after the emissions have completely ceased, sea levels will continue to rise over many centuries due to further warming of the deep ocean. However, there is considerable uncertainty regarding the further development of the Greenlandic and Antarctic ice sheet, a higher contribution to the future increase can not be ruled out. Model results suggest that a permanent warming well over three degrees Celsius over millennia would lead to a complete melting of the Greenland ice sheet, corresponding to a sea level rise of seven meters.

GeoUnion: What does this mean for the weather in Central Europe?

Lemke: It is very likely that the Meridional revival in the Atlantic will decrease by an average of 25 percent in the 21st century. This would theoretically also reduce the potential heat input. But temperatures in the Atlantic region will continue to increase as global warming dominates. Precipitation is likely to increase in higher latitudes, while precipitation is likely to decrease in the tropics and subtropics, including the Mediterranean.

GeoUnion: Thank you very much for the interview


Further information on the IPCC can be found on the Internet at www.ipcc.ch and on WCRP at wcrp.wmo.int.

(Alfred Wegener Institute for Polar and Marine Research, 21.09.2007 - AHE)