40 ways to change a world climate

As the adage goes, forecasting is very difficult, especially if it is about the future. Yet concern about humanity's newly discovered and inadvertent capacity to alter the global climate forces governments, organisations and individuals to take a much longer-term view of the future than is conventional. Politicians think mostly about a single electoral cycle of four to five years; businesses for the most part are concerned with the annual profit margin or with short-term returns on investment; and we individuals are concerned mostly about events over the coming year, only occasionally raising our horizons to think perhaps about our children's education or our retirement. Decisions that we take now, and in the next few years, may well have profound effects on the climate inherited by our grandchildren and by generations beyond, and therefore greatly influence the ability of such future societies to prosper.

Current thinking suggests that the likely range of global warming during the coming century is 1.4-5.8C, with a rise in average sea level of 9-88cm. Between about a third to a half of this range originates from the unknown future rather than from any deficiencies in our climate models. In what direction will global society move in the decades to come? Greater globalisation or Balkanisation? Stabilising of global population or doubling to 12 billion or more? Greater consumerism or a reduction in the material intensity of our lifestyles? A prolongation of our carbon-based economy or the rapid decarbonising of energy systems? It is uncertainties about these trends that underlie an important systemic source of uncertainty about future climate predictions.

This latest report from the Intergovernmental Panel on Climate Change, the Special Report on Emissions Scenarios (SRES), edited by Nebojsa Nakicenovic and Rob Swart, matures the way we think about the future and about the climate that the future will deliver.

The IPCC is the body established in 1988 by the United Nations to provide for governments' periodic and authoritative assessments of knowledge about climate change. Previous IPCC scientific assessments have based their climate predictions on a small number of alternative greenhouse-gas emissions scenarios. In the 1990 assessment, four such scenarios were proposed - a "business-as-usual" case and three variants. These scenarios were rather hurriedly constructed and were designed to ensure doubling of pre-industrial carbon dioxide concentrations in the atmosphere by certain fixed dates rather than being based on any coherent underlying vision of the future. The 1996 IPCC assessment relied upon six new emissions scenarios - labelled IS92a to IS92f - each of which was associated with different assumptions about future population growth, gross domestic product per capita and carbon intensity of energy supply. Although none of the six was proposed by the IPCC as normative, the de facto standard rapidly became the IS92a scenario around which a large majority of climate modelling and impacts, adaptation and policy analysis has been conducted in recent years.

These previous scenarios, and the way in which they were used by climate modellers, were flawed for a number of reasons. This new emissions report from the IPCC has, however, done the climate-change debate a great service in three main ways: by undermining the concept of the "business-as-usual" scenario; by adopting for each scenario an underlying narrative vision of the future; and by adopting an open process of review and adjustment.

The SRES report clearly lays to rest the notion, still favoured by some climate modellers, of a "business-as-usual" emissions scenario and, by association, a "business-as-usual" climate future. As the report makes clear: "There is no single most likely 'central' or 'best guess' scenario, either with respect to SRES scenarios or to the underlying literature." The future will not be like the past, certainly not in terms of the energy, political and cultural paths that the world takes in the decades to come. To limit our thinking, and our climate modelling, to one "preferred" future is not only the height of arrogance, it is downright dangerous. That is why Shell, and a number of other transnational corporations, routinely consider several alternative scenarios in their long-term business planning. Indeed, the SRES report benefited considerably from the intellectual input provided by one of the pioneering Shell scenario planners.

The SRES emissions scenarios are founded on storyline narrative rather than on otherwise disconnected quantitative assumptions about future population growth, GDP per capita and energy intensity. Yes, these numbers are articulated for each scenario (and comprehensive tables of these and other numbers are usefully provided in the report), but each scenario holds together because of the underlying thinking about the sort of world being described. Thus the contrast between the A1 and B1 worlds stems from the different perspectives on materialism embodied in these two scenarios. They have identical populations, and both have institutions for global governance and are technologically progressive. But the difference in values between these two worlds makes a huge difference to the demand for energy and in the uptake of different energy technologies. Consequently, the B1 world yields just over 5 billion tonnes of carbon emitted per year by 2100, compared with the A1 world with up to 30 billion tonnes - more than four times current levels. The difference for climate beyond the 21st century of these two scenarios, although not within the scope of the SRES report to comment on, is considerable. Of course, one can argue over whether our society will or will not reduce its material intensity in the decades to come, but that is the joy and essence of scenarios - they force us to think about the future we really want. If we do not know where we want to go, we are never going to get there.

The third laudable attribute of the SRES report has been the openness of the three-year process that fashioned the scenarios. The core writing team of 28 experts from 12 countries was supported by six modelling teams from North America, Europe and Japan. These modelling teams provided alternative qualifications of each of the narratives, a diversity that yielded an eventual set of 40 different emissions scenarios being spawned by the four core storylines. These quantifications were in turn subject to an open review process lasting nine months, whereby any research group or interested organisation was able to comment on the underlying assumptions of the quantifications themselves.

This volume is crucial contextual reading for understanding the forthcoming Third Assessment Report of the IPCC - due out in the summer - and it also provides the context for the next generation of vulnerability and adaptation studies on climate change now under way. It is essential reading for anyone who is researching in the area of future climate change and its implications. The report also has a wider relevance still in that it exemplifies an inclusive, systematic and structured approach to thinking about the global future of the 21st century. And in relation to climate change, it demonstrates that the future, and therefore future climate, is not given. The next time you read or hear of a climate prediction being made for the year 2100 or beyond, make sure you seek out the underlying future worldview or narrative on which that prediction has been made. The chances are it will be represented by one of the 40 scenarios in this SRES report.

The challenge to climate-change scientists is not so much to predict future climate but to provide society with the options and tools it needs to choose its own climate future. The IPCC SRES report begins to sketch out what some of these choices are, and their implications.

Mike Hulme is executive director, Tyndall Centre for Climate Change Research, University of East Anglia.