Paul Crutzen

Atmospheric Chemistry and Climate in the Anthropocene

Wednesday, 29 June 2005
15:00 - 17:00 CEST


From an evolutionary point of view homo sapiens has been a highly successful species. Its accumulated, catalytic brainpower, consisting of brains with weights of only 1200-1600 g per capita (with 1230 g Einstein scored on the low side), has led to great technological, agricultural, and medical advances. With plentiful available natural resources, mankind could expand tenfold in number to more than 6000 million over the past 3 centuries. This has happened despite frequent attempts at genocide and mass killings by the only species which is capable of doing so. Close to half of the human population now lives in cities, with rapidly rising tendency. During the past century alone, industrial output increased 40 times and energy use was more than ten times that during the entire proceeding millennium, mostly due to the high demands of only some 20 % of the entire human population. About 40 % of the continents have been modified by human action and 40 % of the land’s primary productivity is used my mankind. These and other human expansions have, however, come at a price: Yearly some 160 million tons of SO2 enter the atmosphere from coal and oil burning, at least two times the sum of all natural emissions. Releases of NO from fossil fuel and biomass burning are also larger than its natural inputs. The results are heavy air pollution, acid rain and high ozone levels over large tracts of the continents with substantial impacts on human health and the biosphere. Several climatically important so-called “greenhouse gases” have substantially increased in the atmosphere during the “anthropocene”, the current human dominated geological epoch, which started with James Watt’s design of the steam engine in 1874. During the past two centuries, atmospheric carbon dioxide (CO2) levels have risen by more than 30 % due to fossil fuel burning and deforestation, and methane (CH4) has doubled, produced for instance in the rumens of some 1400 million cattle (worldwide, on average, about one cow per family). As a consequence it was recently estimated by the Intergovernmental Panel of Climate Change (IPCC) that global average surface temperatures will increase by 1.4-5.8°C by the end of this century, making it substantially warmer than ever before during the existence of homo sapiens.

Only thirty years ago it was still generally believed that mankind could only cause urban and regional pollution, such as the Los Angeles ozone smog and the London smog of December, 1952 when some 4000 people died, mostly due to high sulphur dioxide emissions from home heating. Successful countermeasures, especially centralized power generation, has prevented a repeat of the London smog. However, the L. A. type of summertime photochemical smog, despite some local improvements in Los Angeles itself, now affects many urban regions around the world. In addition, extensive rural regions in the tropics and subtropics of Africa, South America, and Asia are affected by heavy loadings of ozone and smoke particles, especially as a consequence of extensive biomass burning during the dry season, for example tropical forest clearing.

A regional cross-boundary problem, which became an international issue by the early 1970’s was acid precipitation falling in West and Northwest Europe (and the Northeast of North America), due to sulphuric and nitric acids, coming from fossil fuel burning, and resulting in forest damage and fish death in the lakes of Scandinavia. After some relaxing of the problem in Europe and North America due to reductions in the SO2 emissions, the same problem has now reached extensive parts of Asia.

While by the 1960’s and 1970’s some types of local and regional air pollution thus had attracted the attention of scientists and policy makers, the possibility of global air pollution effects had not yet been raised. Then in the first half of the 1970’s, some scientists predicted catalytic depletions of stratospheric ozone due to nitric oxide (NO) emissions from proposed fleets of supersonic aircraft and reactive chlorine resulting from emissions of chlorofluorocarbons, thus thinning our shield against the harmful ultraviolet part of sun’s radiation. Especially the catalytic ozone destruction by chlorine turned out to be very important. In fact, its significance initially had even been much underestimated, after scientists of the British Antarctic Service in 1985 reported rapid ozone loss (~ 70%) during springtime. This discovery and the identification that it was caused by chlorine-catalysed ozone destruction, led to international regulations to phase out the production of CFCs and several other ozone-depleting halogen compounds. To reach this agreement was, however a slow legal process. The phase-out first became 100 % effective by 1996 that is about 10 years after the first reporting of the ozone hole and 20 years after the first warnings about the effect of the CFCs, otherwise totally benign compounds. Unfortunately, because of the longevity of the CFCs, the ozone hole will recur each springtime over Antarctica, at least until the middle of this century.

The development of the ozone hole came as a total surprise, also to the scientific community. Nobody had expected the largest loss of ozone to occur over Antarctica, the farthest away from where CFCs are released, the mid-latitude zone of the northern hemisphere. What other surprises may await us in the complex chemical / physical / societal web of interactions in the environmental system and where are its weak spots? Can they be predicted? These are particularly difficult questions.

Another issue is “global warming” caused by growing amounts of greenhouse gases in the atmosphere. Can we reduce the expected large climate warming and associated sea level rise, and societal and biospheric consequences for current and future centuries? The task is huge: to stabilize atmospheric CO2 levels requires a reduction in current burning of fossil fuel by 60 %, a criteria which seems unrealistic with some 80 % of the world population still living in poverty. Clearly, the industrial countries, which have caused most of this problem, should reduce their use of the world’s resources of fossil fuels, but, with great difficulty, only a very small step could be taken last year by acceptance of the Kyoto protocol by most nations with some exceptions, notably the U.S., the nation with the highest carbon dioxide emissions per capita. It is very important to note that it is not the energy production per sé which is heating earth’s climate. It is the production of the “greenhouse gas”, CO2, a by-product of energy production by the burning of fossil fuels, which is the problem. Without the release of this waste gas, the energy dumped into the atmosphere would only cause a percent of the climate heating by the “greenhouse” effect. The major task for the future is thus to find ways to produce energy without releasing CO2.

What many perhaps do not know, largely because of their poverty, the 3rd world is already polluting the atmosphere by primitive combustion technology and large emissions of pollutants from biomass burning. Recent research by scientists from India, the U. S., and Western Europe has identified a large regional scale pollution event covering most of South and Southeast Asia and much of the Indian Ocean during the dry, winter monsoon season. Most impressive was the large loading of partially scattering, partially absorbing, particulate matter causing a reduction in sunlight at the earth’s surface, including the northern hemisphere Indian Ocean, by more than 10 %. The radiative climate forcing near the earth’s surface caused by this major perturbation, regionally some ten times larger than the global greenhouse gas forcing of climate, may imply significant consequences for rainfall over the Indian subcontinent region. Similar effects, more due to biomass burning also occur over large parts of Africa and South America. There is little doubt that future climate, and I think also world peace, will be determined by the fate of the still increasing populations in the poor parts of the world. Strong attention should be given to this issue, in the first place by offering assistance to developing nations to reduce poverty and to involve more young scientists in research. Only then, their governments can be effectively informed about the multiple stresses that homo sapiens is exerting on an increasingly frail environment.

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