IPCC Fourth Assessment Report summary [567KB]
Boosting renewable energy, reducing deforestation and improving energy efficiency can all help, they said.
This is the third report this year from the Intergovernmental Panel on Climate Change (IPCC), and aims to set out the costs and benefits of various policies.
IPCC chair Rajendra Pachauri said the report was "stunning".
Human society as a whole has to look for changes in consumption patterns," he told reporters at a news conference in the Thai capital.
The report suggests that if major climate impacts are to be avoided, global emissions should peak and begin declining within one or two decades.
The IPCC has already this year produced the two other elements of this global assessment report - its fourth since 1990 - dealing respectively with the science of climate change and the potential impacts.
Greenhouse gas emissions have risen by 70% since 1970, and will rise by between 25% and 90% over the next 25 years under "business as usual".
That rise will mainly be caused by an expansion in the use of fossil fuels, which are set to continue as the world's dominant energy source.
"If we continue to do what we are doing now, we are in deep trouble," observed Ogunlade Davidson, a senior author on the report.
During the early part of the week, China had repeatedly tried to tone down some elements of the draft text, delegates said, being keen to remove references to scenarios which it feared could affect its short-term economic growth.
But in later sessions it played a more consensual hand.
The report assesses the likely costs to the global economy of stabilising greenhouse gases at various concentrations in the atmosphere.
Stabilisation at reasonable cost is possible, it concludes, commenting: "There is considerable economic potential for the mitigation of global greenhouse gas emissions over the coming decades, that could offset the projected growth of global emissions or reduce emissions below current levels."
The sharpest cuts, keeping greenhouse gas concentrations to levels equivalent to between 445 and 535 parts per million of carbon dioxide, might cost anything up to 3% of global GDP by 2030, while milder curbs could even enhance growth.
The current atmospheric concentrations are equivalent to about 425 parts per million of CO2.
Assessing the impacts of a given concentration is not an exact science, but many researchers believe that keeping concentrations below about 450ppm CO2-eq is necessary if the average global temperature rise is to be kept below 2C, and major impacts avoided.
The IPCC suggests that concentrations between 445ppm and 490ppm would keep the temperature rise to 2.0-2.8C. European Union policy is to avoid a rise greater than 2C.
"If you want to stabilise around 450ppm, that means in a decade or two you have to start reducing emissions far below the current level," said Dr Pachauri.
"So in other words, we have a very short window for turning around the trend we have in rising greenhouse gas emissions. We don't have the luxury of time.
Many technologies can play a role in cutting emissions, the IPCC says; but it singles out the building sector as a potential major contributor, and energy efficiency as something that "plays a key role across many scenarios for most regions and timescales".
Nuclear power and carbon capture and storage, where emissions from fossil fuel power stations are captured and buried underground, could play important roles; but with nuclear, "safety, weapons, proliferation and waste remain as constraints".
But, observed Dr Pachauri, economic incentives are needed to drive any technological transformation.
"Technology alone will not be enough," he told BBC News, "and in any case, technology has a clear connection with government policy.
"Incentives... must come from taxes or a carbon price."
Catherine Pearce, international climate campaigner with Friends of the Earth UK, said there was now no economic excuse for inaction.
"This report shows that many affordable technologies and effective policies are readily available to tackle climate change," she said.
"By introducing measures and investment that will stimulate sustainable renewable energies and energy efficiency, governments can help to achieve cuts in global emissions by 50% by 2050. Without this, we face devastating consequences."
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