Hspecially during the past century, land use changes and agricultural and industrial activities have been growing so rapidly that their effects on the environment, including the chemical composition of the global atmosphere have bccomc dearly noticcable on all scales. The first realization of the possibility of global effects was conncctcd with the growth of the "greenhouse" gas carbon dioxide measured by C. 1). Keeling and R. Revelle, on the basis of these measurements they stated that humanity had embarked on a global geophysical experiment potentially leading to climate warming. Other human-caused global disturbances in the atmosphere were discovered thereafter. In 1971 attention was called to the possible loss of stratospheric ozone, caused by NOx catalysts in the exhaust of supersonic aviation. The projected large fleets of aircraft were never built. However, in 1974 an already existing, but late recognized threat to the ozone layer by CiOx radicals produced in the stratosphere by the photochemical destruction of entirely man-made chlorofluorocarbon (CFC) gases was hypothesized and later confirmed by atmospheric observations. In fact, in 1985, scientists were caught totally by surprise when researchers of the British Antarctic survey reported much larger springtime ozone depletions, than originally estimated, on the order of 30%. It was found that the ozone loss was largest at altitudes between about 12 and 22 km, exactly the height region in which, under undisturbed conditions, maximum ozone concentrations had always been measured. At this location, it had always been thought that ozone was chemically inert. Since then, the "ozone hole" has grown in area and depth, so that by this year's spring total ozone had declined by more than 50% over a region three times the size of the United States. A couple of years of intensive research efforts showed that a chemical instability had developed, involving formation of CIOx catalysts on ice particles under sunlit conditions, followed by rapid ozone destruction. The combination of special natural factors in early spring, cold temperatures, and availability of sunlight, together with about six times larger than natural loadings of chlorine gases, had led to this chemical instability over the Antarctic. Since 1996 the production of CFC gases on the industrial world has been forbidden. I have dwelled in this issue in some detail for two reasons. First, international political action would not have been taken without convincing scientific evidence that the CFC emissions were the cause of the heavy ozone loss. Second, it will be particularly important to determine where the world's complex environmental system may be most vulnerable to human perturbation. For this purpose, modeling alone will be far from sufficient. Surprises arc not excluded, as the ozone hole story so drastically has demonstrated.

In the 1970s the substantial impact of the bioshpere on atmospheric chemistry was also realized. First, the main natural loss of stratospheric ozone occurs through reactions involving NOx radi cals that derived photochemically from the oxidation of N20, a by-product of the biological nitrogen cycle in soils and waters. Second, it was discovered that tropospheric ozone and its photochemical by-product, hydroxyl, are much influenced by chemical chain reactions involving CH4 and other hydrocarbons, carbon monoxide, and NOx. All these gases have both natural and anthropogenic sources. This is of the greatest importance, as the hydroxyl radicals, also called the "detergent of the atmosphere," to a large degree determine the chemical composition of the atmosphere by reacting with almost all gases that are emitted by natural processes and human activities.

In addition to being chemically active in the stratosphere and troposphere, several of the afore-mentioned and other gases serve as "greenhouse gases," thereby significantly adding to the climate warming caused by C02. On the other hand, aerosol particles, in particular, sulfates derived by the oxidation of largely anthropogenic SO, from oil and coal burning, have a cooling effect on climate.

The estimation of the impact of various kinds of human activities on atmospheric chemistry and climate clearly requires a good understanding of the natural and anthropogenic sources of large number of trace gases, as well as particulate matter, and the biological processes creating them. This research not only deals with the present and future, but also profits much from the vast amount of information regarding climate parameters and chemical composition of the atmosphere cores that is deposited in sediments and in ice. The latter data clearly show that the biosphere does not counteract climate change in some Gaian fashion. On the contrary, during earlier glacial periods all greenhouse gases were less abundant in the atmosphere than during the interglacials. This research has received special international, political attention in connection with the proposed Kyoto protocol to reduce the emissions of C02 caused by fossil fuel burning and deforestation. As was the case with the CFC regulations, effective C02 emission control measures will rely also on a strong scientific base. It was the realization of these strong needs, requiring improved knowledge especially about the biogeochemical cycles of C, N, S, P, and trace compounds such as iron, that led to the creation of a Max Planck Institute for Biogeochemistry. During the initial discussions, involving the cream of the international, biogeochemical, and climate community, the proposal received enthusiastic endorsement, emphasizing the uniqueness of the institute on the global scence. The proposal was also well received by the scientific members and the senate of the Max Planck Society. The search for directors and key scientific personel of the institute proved highly successful, with several key recruitments coming from overseas, clearly showing the enthusiasm accompanying the creation of the institute.

The MPI for Biogeochemistry in Jena is one of several Max Planck Institutes involved in global change research. This book, based on the presentations given to celebrate the first anniversary of the institute shows many important examples of the breadth and excitement of Global Change research around the world, in cluding legal/political aspects. I hope that the so successful creation sets an example and promotes initiatives elsewhere to enhance bio-geochemical research efforts, and its connections to ecology, climate and atmospheric chemistry. Many Happy Returns.

Dr. Paul Crutzen

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