I . WHAT' S HAPPENIN G TO GLOBA L CLIMATE ?
Warm current i reduces Peruvian > fish catch by
45 per cent
criticise the GCMs of the early 1990s for their over-prediction of the average surface temperature rise. What Singer and others such as Frederick Seitz, past President of the US National Academy of Sciences, patently failed to mention, however, was the modellers' awareness that the 'extra' greenhouse gases were not the whole story and that their models, to be one step closer to reality, need ed to take on board the effect of 'offending' atmospheric aerosols. By realising that the sulphur dioxide emitted with the burning of fossil fuels has had a cooling effect on the Earth's surface through reflecting incoming light back out to space, the modellers at the UK's Met Office's Hadley Centre are now able to get good corre lations with the records of past surface temperatures. Once again the discrepancy has been cleared up: the model shows a warming of little more than 0.5°C since 1860, just as has been found from measurements on the ground.8
In fact, the upward trend in temperature over the past 130 years has been in fits and starts rather than being a steady increase. The reason for the jerkiness becomes clear once the industrially-gen erated sulphate aerosols are included, which, in sharp contrast to the greenhouse gases, with an atmospheric lifetime of roughly 50 to 200 years, have an atmospheric lifetime of two weeks at most, together with a distribution that is extremely patchy. When indus trial activity is high, for instance during the two World Wars, the emissions of sulphur go up, and since their effect on the atmos phere is immediate but short-lived they tend to dominate in the short-term. When high industrial activity is followed by a slump, as in the Great Depression, the concentration of atmospheric sul phur rapidly falls and the impact of the greenhouse gases comes shining through. We therefore have the paradoxical situation that cooler periods in the past resulted from greater industrial activity and warmer periods from economic and industrial recession. Clearly, as we institute sulphur-scrubbing to reduce sulphur emis sions on an international basis, in accord with the Helsinki Proto col, the skies will become clearer and the full warming impact of the added greenhouse gases will be revealed.
Even as the theories of the small band of climate change scep
tics are being demolished, discrepancies or lack of correlation between carbon dioxide levels and climate over the past few hun dred years are still being manipulated as evidence that our current greenhouse gas emissions cannot be correlated with global warm ing. One notable claim is that the Sun is largely responsible for such 'natural' fluctuations in climate through variations in sunspot activity. Thus, a shorter cycle of around nine years, compared with the average 11-year cycle, is generally associated with greater sunspot activity and there is evidence that those periods coincide with warmer surface temperatures, such as in late Roman times and in the Middle Ages. By the same token, periods of cool sur face temperatures, such as between AD 1400 and 1510, a period known as the Sporer minimum, and the Maunder Minimum of the seventeenth century - when the Sun's brightness fell by at least 0.4 per cent - coincided with low sunspot activity.9
As various scientists have pointed out, the sunspot cycle is now months shorter than it was one century ago, implying more solar activity and presumably a warming. But, far more important than the actual length of the solar cycle is the number of sunspots in evidence at any one time, and they have been declining since 1960 - an indication that the Earth should be getting cooler, at least on the surface. Hence, the only possible remaining reason for the warming is the rise in greenhouse gases which are now swamping fluctuations in sunspot activity.
Still, climate change sceptics argue that climatic changes we may be witnessing today are a consequence of natural phenomena - such as El Nino. Whilst El Nino is normally a natural phenom enon, its recent extreme manifestation is highly likely to be the consequence of severe aggravation by human activities, including human-induced global warming and tropical forest destruction. In fact, according to some climatologists, i f natural variability were the overriding factor, far from causing warming, it would current ly be leading us into a period of cooling — a glacial. Writing 20 years ago, those climatologists were basing their argument on what was known of the Earth's orbiting around the Sun - known as the Milankovitch Wobble. The Earth's orbit shifts from being
The Ecologist, Vol. 29, No 2, March/April 1999 WE'R E CHANGIN G OUR CLIMATE ! WHO CA N DOUB T IT ?
The Rise of Greenhouse Gas Concentrations Atmospheric concentrations - the accumulation of emissions - of greenhouse gases have grown significantly since preindustrial times as a result of human activities. Carbon dioxide concentrations - the most important greenhouse gas apart from water vapour - has increased more than 30 per cent from 280 ppmv (parts per million by volume) in the pre-industrial era to 365 ppmv by the late 1990s. The current rate of increase is around 1.5 ppmv per year. Unfortunately, a large proportion of the carbon dioxide we put into the atmosphere remains there, warming the planet, for around 200 years. Methane - on a weight-per-weight basis some 20 times more powerful as a greenhouse gas than carbon dioxide - has
more than doubled its concentration, from 700 to 1,720 parts per billion, by volume, (ppbv), primarily because of deforestation and the growth in rice and cattle production. Natural gas leaks are another source. Methane's residence time in the atmosphere is relatively short approximately 12 years. Nitrous oxide, associated with modern agriculture and the heavy application of chemical fertilisers, has increased from preindustrial levels of 275 ppbv to 310, with a current annual growth rate of 0.25 per cent. On a weight-per-weight basis it is more than 200 times more powerful as a greenhouse gas compared with carbon dioxide. Its residence time in the atmosphere is around 120 years. The chlorofluorocarbons, CFC11 and
CFC12, both with growth rates of 4 per cent per year during the past decade, have now reached levels of 280 parts per trillion by volume (pptv) and 484 pptv respectively. They have a 'greenhouse gas potential' that is many thousands of times greater than carbon dioxide on a weight-per-weight basis, and they remain in the atmosphere from several thousand years.
When we take the residence time in the atmosphere of the different gases and their specific effectiveness as greenhouse gases into account, carbon dioxide's contribution is some 55 per cent of the whole, compared with 17 per cent for the two CFCs and 1 5 per cent for methane. Other CFCs and nitrous oxide account for 8 and 5 per cent respectively of the changes in radiative forcing.
circular to elliptical over the course of 100,000 years. Its tilt varies too, from 21.8 to 24.4 degrees over a 40,000-year period and is currently tilted at 23.44. The more tilted the Earth the greater the impact of the seasons. Which hemisphere is closest to the Sun dur ing its summer or indeed winter varies over a 25,000-year cycle. The northern hemisphere is now closest to the Sun during its win ter and furthest away in the summer, which means that it receives approximately 5 per cent less summer sunshine than it received 12,000 years ago. The Earth's current trajectory is one which has more in common with a cooling period and therefore we should be heading towards another ice-age. Recent history of the Earth sug gests that ice-ages last 90,000 years with 10,000 years of interglacial. On that basis the timing is right for the development of another ice-age. The current spate of warming is therefore indica tive that new factors - human emissions of greenhouse gases and mass deforestation - have been introduced which are counteract ing and even overwhelming the consequences of a natural process.10
Waiting fo r 'more certainty ' cannot be an optio n The handful of climate change sceptics enjoy repeating the mantra that too many uncertainties exist in the science of climate change and that these must be eliminated before we take economically 'costly' mitigating action. Such arguments are false and in leading to prevarication they are extremely dangerous: all the evidence of the IPCC has been properly peer-reviewed by the best climatolo gists in the world and it shows without doubt that global warming is a human-induced phenomenon that has a significant statistical base. The only elements of uncertainty concern the precise effects global warming will have on the rest of the Earth's climate-stabil ising systems, and the speed with which changes will occur. But that must not be used as a reason for delaying action. Quite the opposite, for such uncertainty encompasses the possibility of high ly disruptive, extremely long-lasting climatic change. The longer we delay reducing our greenhouse gas emissions, the more likely it is that the warming we have set in motion will increase to the extent that it causes new factors to come into play - such as the collapse of the planet's natural greenhouse-gas-absorbing sinks, which will in turn feed back on the warming process, causing cli matic changes that are potentially catastrophic and effectively irre versible for centuries i f not millennia to come (see 'How Climate Change Could Spiral Out of Control', p.68).
I f such effects were unleashed, we would not be able to return rapidly to where we were by simply switching off the emission of greenhouse gases and deforestation that caused the impact in the
first place. For, once carbon dioxide is in the atmosphere, between 40 and 60 per cent of it remains there for a historically long period - some 200 years when the carbon sinks are in healthy operation.
Waiting for 'more certainty' or more damage to occur is an extremely dangerous and irresponsible position to take for anoth er reason. The radiative thermodynamic physics of the greenhouse effect are such to cause a long delay between the emission of car bon dioxide into the atmosphere and the time when the effects on the climate actually manifest themselves. Hence, the CO: that we emit and accumulate in the atmosphere now will only act on the climate 50 to 80 years in the future. Conversely, climatic changes, such as temperature increase, extreme weather events and damage to crop yields that we are experiencing today, are occurring in response to the CO: that we emitted half-a-century or more ago when atmospheric concentrations were much lower than they cur rently are. It therefore follows that in 50 to 80 years from now, we will experience incomparably more damage than today.
Our politicians should therefore understand that i f they only take action proportionate to the damage they see now, they will dramatically and catastrophically underestimate the damage that will actually take place, and they will hence underestimate the degree of action that is needed to avert it. Measures to prevent such severe climatic disruption cannot therefore be taken soon enough. The reality of climate change and the need for preventive action is now inescapable - no one should doubt it.
Simon Retallack is guest editor of this special issue of The Ecologist. Peter Bunyard Science Editor of this special issue is the author of Gaia In Action: Science of the Living Earth. His forthcoming book on climate change is called The Impact of Global Warming.
References: 1. IPCC's Second Assessment Report, Summary for Policymakers, Cambridge
Univeristy Press, 1995. 2. Ross Gelbspan, The Heat is On, Addison Wesley, 1997. 3. Ross Gelbspan, Climate change: local and global, article 1998. 4. Climate Change, The IPCC Scientific Assessment, Processes and Modelling,
WMO/UNEP, 1990. 5. Stephen Hume, The Vancouver Sun, December 30, 1998. 6. Frank Wentz, Matthias Schabel, Nature, Vol. 394, p.661, August, 1998. Also see
James Hansen et ai, Science, Vol. 281, p.930 and Jeff Hecht, New Scientist, August 15, 1998. 7. Martin Jarvis, British Antarctic Survey, Journal of Geophysical Research, Vol. 103,
p.20 774. 8. UK Climate Impacts Programme: Technical Report No. 1, The Met Office, October
1998. 9. John Eddy, Solar History and Human Affairs, Human Ecology, Vol. 22 Nol , 1994. 10. David Waugh, Geography: an Integrated Approach, Nelson, second edition, 1995.
The Ecologist, Vol. 29, No 2, March/April 1999