ETH Zurich's weekly web journal - auf deutsch
ETH Life - wissen was laeuft ETH Life - wissen was laeuft

ETH Life - wissen was laeuft ETH Life - wissen was laeuft

ETH - Eidgenoessische Technische Hochschule Zuerich - Swiss Federal Institute of Technology Zurich
Section: Science Life
deutsche Version english Version
Print-Version Drucken

Published: 08.05.2003, 06:00
Modified: 07.05.2003, 20:18
Climate change
"Climate is a very complicated, variable system"

"Climate time bomb" or "intellectual hot air"? The discussion on changes to our climate is being heatedly conducted by the media. And not always with scientifically accurate data. One thing seems certain: our planet's climate has grown warmer and temperatures will continue to rise. ETH researchers are calling for a sober discussion.

By Michael Breu

Our climate is a complex system, influenced by many factors. This makes predictions difficult. The United Nations set up an Intergovernmental Panel on Climate Change (IPCC) to look into the matter. Nearly a thousand scientists from many disciplines have worked on this issue over the past 15 years (1). The third "Assessment Report on Climate Change" was published two years ago. The fourth, which takes new research results into account, is already in preparation and due to appear by 2007.

The crux of the current report is clear: during the 20th century average global temperature rose by 0.6 degrees centigrade. Scientists predict that the average surface temperature will rise by a further 1.4–5.8 degrees by 2100. The synthesis report concludes that climate change "poses grave risk to human health."

Greenpeace talks of a "time bomb" and warns of collapse. Prominent critics counter, "Constant repetition doesn't make something more true," and add, "the debate on climate is surrounded by myths and misunderstandings. Things are not as bad as they're often made out to be."(2)

Professor Christoph Schär, you are head of the ETH Institute for Atmospheric and Climate Science, and co-author of the third IPCC report. Which of the two scenarios is true, time bomb or myth?

Christoph Schär: It is becoming ever more clear that our climate is changing and that this will have a considerable effect on the climate system of our planet. We are talking about a degree of change that has not happened since the end of the last ice age, ten thousand years ago. We already have clear evidence that today's temperatures are higher that they have been in the last thousand years. Clearly, climate warming would cause a shift in the climate zones. Such a shift will effect the entire water cycle and the geographical distribution of natural catastrophes. It is still too early, however, to give detailed regional predictions of the future.

"Average temperatures have clearly risen": Earth's atmosphere, seen from the space shuttle Columbia. large

The latest IPCC report summarises current research and one finds, amongst other things, statements about a rise in the Earth's temperature. Sceptics say that the current warming does not always proceed continuously, but varies from region to region. One often cited example is the world's oldest meteorological records from Berlin-Dahlem. Can climate warming really be deduced from such a slight increase of 0.6 degrees?

Schär: Detection is extremely difficult. In 1860 the meteorological stations did not have the equipment to measure such slight signales. But existing test series has been very carefully examined and combined with other factors, such as the spread of urbanisation, and older measuring methods that have been simulated and re-calibrated. Nowadays we can identify system inherent measuring errors that were made at the time. The IPCC report shows that climate warming has a probability of 90 per cent certain.

Modern methods of measuring temperature have also come under attack. Measuring methods have changed (new thermometers) but on the other hand, many formerly rural regions are far more densely populated today. In South Africa, for example, it appears that over the past thirty years, the temperature was recorded as higher than it really was. Are the data being called into question?

Schär: There are indeed regions where the temperature is very uncertain, in others the periods of recorded data is too short. In addition, warming is not evenly spread. This is not surprising. Climate is a very complicated, variable system. When such a system heats up, regional patterns occur. This means that the climate of regions in different parts of the Earth react differently. We have very reliable data for many regions. In Europe's alpine region, for instance, a very finely woven network of meteorological stations has existed for over a hundred years. These data show quite clearly that average winter temperatures have risen in these areas by 1 degree. When we speak of global warming of 0.6 degrees since 1900 this figure includes a safety margin of 0.2 degrees. Indirect indicators, such as the global observation of the widespread retreat of non-polar glaciers, support the conclusion that our planet's atmosphere has warmed up.

Floods: Scenarios from the machine

"Climate change" usually crops up in the media when cities or landscapes are flooded or when a hurricane sweeps over the land. When this happens, the most frequently used headline is: "Are natural catastrophes increasing?" The final report of the National Research Programme "Climate Change and Natural Catastrophes" says: "Extreme flooding, such as takes place in the alpine regions towards the end of the summer and autumn, shows long-term fluctuations. The observed increase of such catastrophes since the 1970s lies within the range of natural fluctuation." Christoph Frei from the ETH Institute for Atmospheric and Climate Science, explains some of the difficulties involved in judging and weighting such catastrophes. Because they are (relatively) rare, a constant change over a long period of time cannot be proved for purely statistical reasons–even if a change really is taking place. But to draw the conclusion that changes in the global climate have no influence on catastrophes would be erroneous, says Frei. Observations show that precipitation in the alpine regions during the winter months has increased by 20 to 30 per cent during the past century. Even more striking: the frequency of intensive precipitation in winter has increased from 20 to 80 per cent. "The observed changes can be explained by natural climate fluctuations, by anthropogenic activity or by a combination of both," Frei explains and goes on, "our understanding of the process and observations of precipitation increase in midlatitudes support the view that the climate change is being influenced." Data on precipitation that Frei has compiled are used to check complex regional climate models with which futurist scenarios can be developed. A concrete scenario with a regional climate model will be produced in the coming months. "At the moment, it's still in the machine," says Frei. The simulations will then be evaluated and the scenarios published.


Earth's climate is being scientifically examined. Collected data flow into the report of the Intergovernmental Panel on Climate Change (Pictures: NASA) large

Sceptics argue that the length of records is not long enough to provide firm evidence and that warmer phases have always occurred. How do you answer this criticism?

Schär: It's true that there is nothing new about warm climate phases. However, two things are important here. First of all, the global warming of recent decades goes beyond the natural variability of the last millennium. And secondly, there is no plausible explanation for recent warming, in contrast to bygone fluctuations, which were caused by solar variability, for example, or by changes in the elements of the Earth's orbit. Because of this, the IPCC report concludes that, to a great extent, current warming is the result of human-induced, or so-called anthropogenic, activities.

Some delve even deeper into climate history and examine the annual growth rings of trees or stalagmitic deposits. How much can methods such as these tell us?

Schär: Stalagmites mirror geological periods and because they are extremely old, they make a good instrument to investigate long-term climate changes. As for annual rings, they can be helpful to show rapid changes but whether or not they are good indicators for long periods of time is controversial. Ice drilling is an excellent method for examining climate fluctuations on scales ranging from a few years to hundreds of thousands of years. Ice cores from Greenland, for example, can be dated to within a very few years. A very high degree of accuracy can be obtained by counting the layers of frozen snow. It is, however, difficult to interpret these climate archives. For example, there is no direct connection between the locally measured proportion of the oxygen isotope O18 and the global temperature development. For researchers this means that they must observe, compare and combine the data from a number of climate archives to gain insight into global temperature. This is an area of research where intensive and very successful work is being done.

Let's stay with measurements, for a moment. The concentration of carbon dioxide, has risen considerably–from 270 ppm in 1870 to 370 ppm today. A wide range of emission scenarios predicts that the rise in CO2 concentration will lead to higher average surface temperatures of the Earth. Sceptics argue that a higher concentration CO2 will not necessarily mean higher temperatures because the saturation potential of CO2 sets a limit. What is your view?

Schär: The saturation effect in certain spectral bands has been well known for a long time. This effect is taken into account in climate models. Nevertheless, all climate models are extremely sensitive to CO2 concentration.

Solar activity plays its part. The consequences have been underestimated in the past. Changes in the sun's magnetic field also influence the Earth's climate. Has the sun, as a factor, been underestimated?

Schär: This criticism comes from an outdated study, which took only data until the 1970s into account and that also contained a crass methodological error. The revised version of the work, which was published three years ago, shows that global fluctuation since 1950 cannot be explained by variations in solar activity. Indeed, solar output has decreased over the past 20 years.

Computer models have also come in for criticism. James Hansen, climatologist at NASA, was the first person to conjure up on-screen scenarios in 1988. In the meantime he has distanced himself from the doom-mongers and has become far more optimistic in his predictions on climate development in the coming 50 years. The IPCC is based on computer models. How valid are the scenarios contained in the report?

Schär: Hansen’s evidence is often misunderstood and the central point not correctly reproduced. The IPCC report states unequivocally that CO2 will become the dominant climate force, if emissions continue to increase and aerosol effects level off. Hansen also says it is necessary to reduce CO2 emissions in order to stabilise the climate. As far as this important principle is concerned, he has by no means distanced himself from his early predictions. In his report Hansen proposes alternative stabilisation scenarios that no longer weight CO2 greenhouse gases or aerosols. It is far-fetched to interpret this as criticism of the IPCC based.

What aspects support the view of global warming in your opinion?

Schär: First of all, observations that show that warming has taken place over the past century as confirmed by such independent indicators as the shrinking of the glaciers. Secondly, the understanding of the process, which has confirmed the greenhouse effect as one of the most important causes motors of our climate system. And third, the climate model that simulates the fluctuation of relevant factors based on laws of physics. All these factors point in the same direction: the observed warming cannot be put down to natural factors. In order to achieve the stabilisation of the climate system, human-induced greenhouse gas emission must be reduced.

The fourth IPCC report is being prepared. Which areas call for more research?

Schär: We need valid data on natural climate fluctuations over past centuries and millennia. We need to come to a better understanding of critical climate relevant processes, for example, the role that clouds or aerosols play in a climate system. Research on these questions also calls for new field experiments. We need better climate models that take relevant physical processes into account and which produce higher spatial resolution. And finally, it is important that probabilistic climate predictions be developed (instead of today's customary scenarios) that contain testimony on regional consequences (e.g. natural catastrophes). Methods must be further developed to meet these requirements.

What projects are being carried out at the ETH?

Numerous groups at the ETH are carrying out research on the issue of climate change. Taken together they cover a wide range of questions, from the reconstruction of Earth's climate in the past to the role the sun plays in our climate. At our institute, researchers are working on climate processes and climate modelling. The latter is part of a number of EU projects that deal with global and regional climate calculations. ETH is also a leading participant in the National Research Programme's project on climate. One major goal of this project is the better understanding of the connection and interdependence of climate variability, climate change and natural catastrophes.

(1) Intergovernmental Panel on Climate Change:
(2) Institute for Atmospheric and Climate Science, ETH Zürich:

You can write a feedback to this article or read the existing comments.

!!! Dieses Dokument stammt aus dem ETH Web-Archiv und wird nicht mehr gepflegt !!!
!!! This document is stored in the ETH Web archive and is no longer maintained !!!