Greenhouse gases prevent heat escaping from Earth

A draft report by many of the world's leading climatologists says emissions of greenhouse gases could rise hugely over the next 100 years.

The report, which comes from the highly influential Intergovernmental Panel on Climate Change (IPCC), envisages one scenario in which carbon dioxide (CO2) emissions at the end of the next century are five times what they are today.

This would almost certainly have dire consequences for the Earth's climate system, with much higher global temperatures and sea levels. Such a scenario would inevitably cause social and economic upheaval as populations migrated from flooded coastal communities.

The report, leaked to BBC News Online, is the summary of a much bigger document prepared for the world's politicians and policy makers.

It comes from one of the IPCC's working groups on emission scenarios and looks at how much greenhouse gas might be put into the atmosphere.

The report's 40 scenarios are "based on an extensive literature assessment, six alternative modelling approaches, and an 'open process' that solicited worldwide participation and feedback".

The authors emphasise that "scenarios are neither predictions nor forecasts, but alternative images of how the future might unfold". Their work incorporates several variables, including population increase, economic growth, energy use and land use change.

They designate four of the 40 scenarios as "markers", each intended to characterise one of four scenario families. The four markers present a summary of what are thought to be possible emission levels in 2100 of the main greenhouse gases, with sulphur dioxide also included. The range of possibilities for emissions of CO2, the main greenhouse gas caused by human activity, is large, and not reassuring. The lowest marker scenario foresees an annual CO2 emission level in 2100 of about 5.7 Gt (gigatonnes - a billion tonnes). The range suggested by the scenarios in the same family stretches from 4.0 Gt to 8.2 Gt.

Scientific consensus

The median figure is slightly less than present annual emissions of about 6.0 Gt from the burning of fossil fuels (deforestation probably accounts for another 20% of that figure). The two middle marker scenarios envisage emissions roughly double today's, at 13.3 Gt and 13.5 Gt annually. But the "family" range is vast - from a low of 4.3 Gt to a high of 36.7 Gt. The highest marker suggests CO2 emissions could be just over 29 Gt in a century's time. The range goes from 19.6 Gt to 34.5 Gt.

On the face of it, those scenarios which suggest emission levels around today's, or even lower, appear to offer hope of improvement. But there is widespread scientific agreement that dealing with climate change will involve not just preventing an increase in emissions, but trying to achieve some sort of decrease that will pull them back to near where they are today - if not further.

Uncharted territory

The greenhouse gases take decades to accumulate and some of them have very long lifetimes. Like a bath, the atmosphere is gradually filling up with the gases. So reducing concentrations must mean cutting the flow from the tap - in other words, big cuts in annual emissions. There is no question of returning concentrations to their pre-industrial levels. But many scientists say that averting serious climatic consequences must mean reducing emission levels by up to 80%, or even more.

None of the report's marker scenarios foresees the possibility of anything remotely approaching that. The best they can offer is an annual addition to the atmosphere only little less than today's, which would do scarcely anything to dent concentrations.

And the highest scenarios suggest a world that has moved into completely uncharted territory, with consequences that would themselves trigger further climatic disturbances.

Mass death of forests

An annual emission level of 29 Gt of CO2 would probably mean the mass death of forests, with the trees releasing the CO2 they had stored up, adding to global warming instead of restraining it. It would be likely to make the eventual collapse of the Ross Ice Shelf in Antarctica inevitable. That, in turn, could trigger a significant global sea-level rise, and the loss of huge and densely-populated coastal areas.

Some scientists have challenged the IPCC's work, insisting either that global warming is not a certainty, or that human activity is not a significant factor in causing it. But the IPCC represents the best consensus the world's leading climatologists have been able to achieve. (BBC News Sep 1999).