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)

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.