Climate research review

Climate research review

Climate research review This column is being introduced to provide readers with a regular review of developments in atmospheric and oceanic research which may have implications for energy policy and planning. The focus will clearly be on the issue of anthropogenic climate change, although future columns will also look at other areas of interaction, such as emerging links between shortterm weather forecasting and the power sector, and between oceanic circulation research and waste disposal. Wherever possible, articles referred to will be accessible to the interested non-specialist.

c o n c e n t r a t i o n - i n c r e a s e , is that it makes the apparent relative significance of current emissions heavily dependent on what has been emitted in the past. Part of the reason so much of the CO2 being emitted at present is re-absorbed is that, because CO2 production has been going on for decades, the global balance of carbon flows has already been disturbed from its pre-industrial equilibrium. In an extreme case, if current CO2 emission rates were held constant indefinitely, atmospheric concentration of CO2 would eventually stabilize (at a much higher level than the present one), giving the decision makers of that hypothetical future decade the imProposed index of global pression that their CO2 emissions were warming effect puts C02's no longer having any effect, despite share to over 70% the fact that, by sustaining that higher Estimates of the relative significance concentration, they would be conof the different anthropogenic green- tinuing to heat up the planet. house gases (GHGs) typically place A n alternative approach, which has the contribution of CO2 at 50-60°/o, l been used in the calculation of the with methane, nitrous oxide, CFCs etc relative global warming impact of making up the remainder. These fi- different C F C s , 3 is to calculate expligures are, in general, calculated by citly the total warming effect of a taking the current or projected rates given quantity of G H G released, takof increase in atmospheric concentra- ing into account the proportion of tions of each G H G , and then multi- those emissions remaining in the plying them by a factor representing atmosphere as a function of time, and the effect on the earth's radiation integrating over all time. The result is budget of injecting a given quantity of expressed as a ratio with some referthat gas into the atmosphere. 2 No- ence gas, to give a relative global where are the G H G ' s different atmos- warming potential (GWP): an indicapheric residence times, which vary tor of the gases' relative greenhouse from about 15 years in the case of potency. Calculations for CFCs have methane, to over a century (eg CFC- generally assumed that the warming 12), explicitly considered. Thus the effect of a G H G varies linearly with its total cumulative effect, over all time, atmospheric concentration, and have of l o n g - l i v e d G H G s is g e n e r a l l y used a single characteristic residence underestimated. This is particularly time for each gas: assuming therefore important in calculating the relative that the rate at which the gas disimportance of CO2, since it is the appears (either through reactions in longest-lived G H G : although a signifi- the atmosphere or by being reabcant proportion of CO2 emissions are sorbed by the ocean or biosphere) is absorbed by the oceans relatively soon directly proportional to the amount after they are released, most of the remaining in the atmosphere. CO2 which remains in the atmosphere As Daniel Lashof and Dilip Ahuja stays there (in effect) for several hun- point out in a recent letter to Nature, 4 dred years, and a small fraction sur- CO2 satisfies neither of these criteria: vives indefinitely. its effect on global warming is nonAnother disadvantage of this 'tradi- linear, and CO2 removal from the tional' approach, based on rate-of- atmosphere, involving a number of

ENERGY POLICY June 1990

different interacting carbon sinks, cannot be described in terms of a simple exponential decay. They propose an even m o r e sophisticated index of GWP, based on that described above for CFCs, but using a more complex (and realistic) CO2 removal scheme. Using this approach, they have calculated relative GWPs for the principle G H G s , using CO2 as the reference gas. They find that the contribution to global warming resulting from CO2 emissions in 1985 represented 71.5% of the total from all anthropogenic G H G emissions in that year. In contrast, recent studies based on the 'traditional' approach, using the rates at which G H G concentrations have increased over the 1980s, suggest a much lower relative contribution from CO2 (about 57%). 5 Lashof and Ahuja, somewhat optimistically, suggest that their G W P index could be used as a basis for formulating emission-reduction policies. While policy d e v e l o p m e n t should clearly take into account the retarded effect of CO2 emissions, this may be p r e m a t u r e . First, their calculated 'effective residence time' for CO2 depends on how we model its removal, and essential parameters governing ocean CO2 uptake and release, particularly those relating to biological activity, are still very uncertain. A more fundamental problem is that CO2 is the only G H G with, strictly speaking, an infinite effective residence time, since current models predict that a proportion (albeit quite a small one) of emitted CO2 will remain in the atmosphere indefinitely: thus the logic of their index suggests that all the other G H G s should be assigned a relative G W P of zero. In order to obtain meaningful results, Lashof and Ahuja are obliged to neglect all effects after a certain time, which they arbitrarily set at 1000 years. Unfortunately, their results are quite sensitive to this choice of cutoff time, making their relative GWPs correspondingly arbitrary. In an interesting analysis for economists, they also consider the impact on their results of changes in the discount rate. The above figure of 71.5% corresponds to a zero discount rate:

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Climate research review~Conference report equal weighting for warming effects at all times up to 1000 years. Increasing the discount rate to 1% pa, for example, halves the significance of CO2 relative to that of methane, as longterm residence times become less imp o r t a n t than i m m e d i a t e w a r m i n g effects. It is interesting (and perfectly credible) that the relative importance which we should assign to the different G H G s may be affected by our choice of discount rate (roughly, a low discount rate gives greater priority to long-lived CO2). This possibility is clearly not brought out by the 'traditional' approach to calculating relative GWPs. Given, however, that we are only beginning to be able to model the time-dependent response of the climate system to a given warming effect, it seems difficult to motivate this sort of detailed quantitative analysis of the timing (on a scale of centuries) of the warming effects resulting from a given G H G emission. The final choice of G W P index for policy formulation will, almost certainly, be determined primarily by the time-scales which political decision makers are

p r e p a r e d to c o n t e m p l a t e . E x p e c t more candidates for the G W P index over the coming months: the most useful will probably be those which bring out this dependence on timeframe explicitly in their formulation.

Myles R. Allen Department of Atmospheric Oceanic and Planetary Physics University of Oxford, UK

1See, for example, J.M. Mitchell, 'The greenhouse effect and climate change', Reviews of Geophysics, Vol 27, No 1, February 1989, pp 115--139. 2p. Okken 'Methane leakage from natural gas', Energy Policy, Vol 18, No 2, March 1990, pp 202-204. 3D.A. Fisher et al, 'Model calculations of the relative effects of CFCs and their replacements on global warming', Nature, Vol 344, No 6266, April 1990, pp 513-516. 4D.A. Lashof and D.R. Ahuja, 'Relative contributions of greenhouse gas emissions to global warming', Nature, Vol 344, No 6266, April 1990, pp 529-531. 5j. Hansen, A. Lacis and M. Prather, 'Greenhouse effect of ehlorofluoroearbons and other trace gases', Journal of Geophysical Research, Vol 94, 1989, pp 1641716421.

Conference report Environmental solutions 9th International Congress on Energy and the Environment, Clean Energy Institute, University of Miami, USA, 13-15 December 1989 In his welcoming address to the conference, John Sheffield of the Clean Energy Institute of the University of Miami, outlined the problem. He said that the biosphere was under attack from the production and utilization of energy sources, which was causing the greenhouse effect, acid rain, ozone depletion, chemical waste, oil spills and nuclear waste. This was resulting in deforestation, desertification, rising sea levels and poisoning of the atmosp h e r e and seas. The cost of this environmental damage had been estimated at 15% of the gross world product. H o w e v e r , m a n y a l t e r n a t i v e energy sources existed, so the outlook

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was not all gloom. But he warned that the alternatives were not without their problems and that the best minds would be required to find solutions. By contrast, Derek Lyth, General Manager of Texaco, giving the Invited Lecture, said that for the future Texaco look to providing energy in the form of petroleum and gas unless a cheaper form became apparent. Lyth went on to say that the petroleum Industry Research Institute was responding to environmental pressure, eg by developing cleaner burning motor fuels. A t present, petroleum supplied 47% of the energy in the Western free market world, and was

growing again back up to the previous high level of 1979. He said that though renewable sources such as biomass might supply energy for rural populations, nuclear energy may be the only solution for cities. The fundamental issue was neatly captured in the opening addresses; the academics believe that the greenhouse effect is happening now, and requires a radical shift in policies. But the industrialists and politicians do not seem fully persuaded, preferring a 'business as usual' policy with some environmental measures thrown in. Delegates from over 40 countries took part in this conference which covered a very wide range of topics from the greenhouse effect and climatic changes, to nuclear and renewable energy, conservation, recycling, and afforestation. Particularly relevant to the greenhouse effect were the papers on means to reduce carbon dioxide (CO2) emissions. Several presentations were concerned with pricing the costs of greenhouse effects, such as the cost of coastal defence strengthening, examples from the US and California being cited. Other papers gave estimates of future energy use after climatic changes. For instance, it was calculated that California's energy demand would rise significantly if the climate became warmer. T h e i n t e r n a t i o n a l d i m e n s i o n is evidently crucial, as the energy policies of several different countries, including Canada, the USA, Pakistan and T u r k e y showed. A paper on Canada, CO2 and the greenhouse effect by Larry Hughes and Sandy Scott pointed out that despite the pious announcements of the government to reduce CO2 emissions by 20%, their policies were actually going in the opposite direction. In fact, it was estimated that by 2005, there would be a 33% increase in emissions. Even greater CO2 emissions would come from the developing countries as they industrialized. However, strong energy demand from rapidly industrializing countries is leading to renewed interest in nuclear and renewable energy. Progress is also slow in the US, where four bills to reduce CO2 emissions by promoting nuclear, renew-

ENERGY POLICY June 1990