The world has a likely chance of meeting the UN's warming limit of two degrees Celsius (3.6 degrees Fahrenheit) if it cuts annual greenhouse gas emissions 40-70 percent by 2050, especially from energy, a top expert panel said yesterday.
The longer it takes to switch from carbon-polluting fuels to cleaner energy sources, the harder and more expensive the target will become to reach, it said.
It warned that on present trends the planet will be 3.7-4.8 C warmer by 2100 -- a level scientists say could be catastrophic.
"There is a clear message from science: to avoid dangerous interference with the climate system, we need to move away from business as usual," said Ottmar Edenhofer, co-chairman of the UN expert group that compiled the document.
"Many different pathways lead to a future within the boundaries set by the two degrees Celsius goal. All these require substantial investments."
The report is the third and final chapter of a mammoth overview by the UN's Intergovernmental Panel on Climate Change (IPCC) -- its first since 2007.
Compiled by hundreds of experts over four years, it collates the available science on climate change, seeking to inform national policies and the faltering global effort to formulate a pact by 2015 on curbing climate-altering emissions.
For a 66 percent or "likely" chance of meeting the 2C target, the report said, the concentration of greenhouse gases in the atmosphere should be contained at about 450 particles per million of CO2 equivalent (ppm CO2eq) in 2100 -- compared to 430 ppm CO2eq in 2011.
This would entail a 40-70 percent emissions reduction from 2010 to 2050, nearing zero by 2100.
There would also be a "tripling to nearly a quadrupling" in the share of energy from renewable and nuclear sources and from traditional fossil or new biofuel sources whose emissions are captured -- what the report described as "large-scale global changes in the energy supply sector".
This will come at a cost, which the report estimated would clip some 0.06 percentage points annually off growth in global consumption -- which would otherwise have been about 1.6-3.0 percent per year over the century.
The calculation did not factor in the potential cost benefits from tackling emissions, including healthier humans and ecosystems and energy security.