Carbon dioxide indirectly causes up to 50 per cent more global warming than originally thought, a finding that raises questions over targets for stabilising carbon emissions over the long term, according to a study.
In a paper published in the journal Nature geoscience, British scientists said a tool commonly used in climate modelling may have badly underlooked the sensitivity of key natural processes to the warming caused by CO2.
As a result, calculations for man-made global warming on the basis of carbon emissions may be under-pitched by between 30 and 50 per cent, they said.
The study was published on the eve of the12-day UN conference in Copenhagen aimed at providing a durable solution to the greenhouse-gas problem.
The authors stressed that the more-than-expected warming would unfold over a matter of hundreds of years, rather than this century.
The findings do not mean that the predictions for temperature rise by 2100, established notably by the UN Intergovernmental Panel on Climate Change, should be rewritten, they said.
"We don't want to be overly alarmist here," said lead author Dan Lunt of Britain's University of Bristol.
"But if people are thinking about stabilising CO2 at a certain atmospheric level, or putting together a treaty, or having a debate about what the levels should be, it really is important to know what the long-term consequences of those emissions are going to be, because CO2 hangs around for so long."
Dr Lunt and colleagues decided to test a widely-used climate model on an epoch called the mid-Pliocene warm period, about three million years ago, when earth heated up in response to natural processes.
Cores drilled from ocean sediment provide a good idea about atmospheric carbon levels and temperature at the time.
What the team found, though, was that the CO2 levels in the Pliocene - around 400 parts per million (ppm) - were not consistent with the warming, which was around three degrees Celsius higher than today.
The difference could only be fully explained by the long-term loss of icesheets and changes in vegetation, says the paper. These changes cause earth's surface to absorb more solar radiation, which causes more warming, and so on.
When applied to what awaits us this century, the adjusted model suggests that nothing significantly different will happen compared to what has already been estimated.
"In that time scale, we don't think the Greenland icesheet is going to melt completely or that East Antarctica will melt. That was what we saw in the model for three million years ago, but it is unlikely to take place in the next century," said Dr Lunt.
Main options for tackling greenhouse emission
• Energy efficiency: Improve energy efficiency in transport, buildings, appliances, lighting. Advantage: Can make big, early gains in braking growth in carbon pollution. Disadvantage: Tougher standards may run into opposition from transport lobby, costs may meet consumer resistance.
• Clean renewables: Wind, solar and hydro are the main sources, followed by biomass, geothermal, tidal and wave power. Hydrogen and fuel cells are promising but still distant as a commercial source. Advantages: Non-polluting, safe, free from geopolitical risk as the energy is derived from within the country and not imported. Disadvantages: Entry costs still high, although those of wind have fallen fast and the efficiency of solar cells has improved; climate and geography may impose a niche role; large hydro projects contested because of environmental impact.
• Biofuels: Transport fuel derived from plants saw a spectacular boom three years ago, driven by the US efforts to reduce its dependence on foreign oil. Advantage: Relatively low pollution. The plant sucks CO2 from the air in order to grow, and this CO2 is released when the fuel is burned. Additional emissions, though, come from using machinery to plant, harvest and process the crop. Disadvantage: Present-generation biofuels are derived in part from corn, sugar and other crops which has helped drive up food prices. Next-generation biofuels, based on woodchips and fibrous plants, still only in pilot phase.
• Carbon taxes and emissions caps: Levies on coal, oil and gas or statutory limits on emissions from these fuels. Advantage: Helps set a "carbon price" to reflect the environmental cost of fossil fuels, thus encouraging quick switch to cleaner energy. With a carbon price of 20-100 dollars per tonne of CO2, renewables could account for 30-25 percent of global energy supply by 2030, according to the UN's Intergovernmental Panel on Climate Change (IPCC). Disadvantage: Could slow economic growth because of dependence on fossil fuels; resistance from vested interests, consumers.
• Carbon capture and sequestration (CCS): Entails taking CO2, the heat-trapping by-product of combustion, at source from a coal-fired power station or oil refinery and piping the gas underground, storing it in deep geological chambers such as disused oilfields, rather than letting it escape into the atmosphere. Advantage: Would smooth the transition from carbon to hydrogen energy, especially as coal will be a major fuel for many decades to come. Disadvantages: Still in pilot or small commercial phase; commercial costs unknown or high; difficult to retrofit to existing plants; long-term safety unclear - a chamber breach could send a dangerous bubble of CO2 into the atmosphere.
• Nuclear: Expansion of nuclear energy is being strongly promoted as a solution to the greenhouse-gas problem. Advantages: Very low greenhouse-gas emissions during operational lifespan; can reliably deliver very high volumes of electricity around the clock; geopolitical vulnerability low. Disadvantages: Safety still under scrutiny after Chernobyl and Three Mile Island; fears about nuclear proliferation and terrorism; long-term storage of highly radioactive waste still unresolved. (AFP)