UN scientists urge carbon tax to fight global warming

· Temperatures may rise 6C by 2100, says panel
· Report marks effort to find post-Kyoto consensus

Graphic: How the earth will heat up by 2099 (pdf)

• • David Adam, environment correspondent
  • The Guardian, UK

All sources of carbon pollution - from flights to inefficient light bulbs - must become more expensive if the world is to tackle global warming, an influential panel of scientists and government officials will say today.

Putting a price on harmful emissions from goods and services would require a fundamental shift in the world's economy, but "could realise significant mitigation potential in all sectors" according to a report from the UN's Intergovernmental Panel on Climate Change (IPCC).

The report will be launched today in Valencia by Ban Ki-moon, the UN secretary general, and marks the start of an international effort to agree a worldwide treaty to regulate greenhouse gas output.

The IPCC, which won this year's Nobel peace prize jointly with Al Gore, will confirm it is 90% sure that recent global warming is down to human activity, and warn that the impact of future temperature rise will be severe. It will say action to cut emissions is needed in the coming decades to stop global temperatures rising by as much as 6C by 2100, and that most of the technology needed already exists. Even deep cuts in carbon production would have only a marginal effect on economic growth, the IPCC will say.

Today's report comes two weeks before a UN meeting on climate in Bali, when countries will try to map out the timescale and structure of an agreement to replace the Kyoto protocol, the existing treaty which regulates greenhouse gases, when it expires in 2012. Analysts say the post-Kyoto deal needs to be finalised by 2009 for it to start in time.

Today's report combines and summarises three other papers from the IPCC earlier this year, which discussed the science of global warming, its likely impact, and possible ways to tackle the problem. Scientists and officials were still working yesterday on the final wording of the document. "This report is the blueprint for Bali and every country wants their own position represented. They know ... they can't ignore what it says," said one source close to the process.

The IPCC does not recommend specific policies, but a draft summary obtained by the Guardian highlights the introduction of an "effective carbon price signal". It says a carbon price of between $20-$80 (£10-£40) per tonne by 2030 should be enough to limit the expected temperature rise. How such a worldwide price could be introduced will be one focus of the Bali talks. Europe favours cap-and-trade systems, which place a mandatory limit on pollution from countries, companies and even individuals, who must buy the right to pollute more.

Despite reports that the US delegation was unhappy with the report, one of the senior authors told the Guardian yesterday there had been few serious disagreements. The draft report also restores the phrase "dangerous anthropogenic interference with the climate system" which was removed from the key summary chapters of the IPCC reports earlier this year after US protests.

Main points

A draft of today's synthesis report from the UN Intergovernmental Panel on Climate Change presents the challenge in five key areas.

Observed changes Warming of the system is visible in increases in air and sea temperatures, melting snow and ice, and rising sea levels. All regions are getting hotter. The extent of Arctic sea ice is shrinking by 2.7% a decade. More areas are affected by drought.

Causes of change Human greenhouse gas emissions grew by 70% from 1970 to 2004. Carbon dioxide output rose by 80%. Carbon dioxide in the atmosphere is highest for 650,000 years.

Projected change Greenhouse gas emissions are projected to almost double by 2030. Depending on emissions, temperatures to 2100 could increase by 1.1C to 6.4C. With no policies to curb pollution, the most likely increase is 4C. Sea levels could rise 0.18m to 0.59m by 2100. Heatwaves and hurricane strength will increase. Hundreds of millions more will suffer water shortages, up to 30% of species will risk extinction and food production will be hit.

Adaptation and mitigation options There are lots of cost-effective ways to mitigate greenhouse gas emissions using existing technology. An effective carbon price signal from governments could bring significant emission cuts across all sectors.