The world can still meet the UN goal to limit global warming provided it cuts annual greenhouse gas emissions by 40-70 percent by 2050, a top expert panel said Sunday.
The cost will not be crippling, but the longer it takes to switch to cleaner energy sources, the harder and more expensive it will become to halt warming at the UN's targeted two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.
The Intergovernmental Panel on Climate Change (IPCC) warned starkly that on present trends the planet would be 3.7-4.8 C warmer by 2100 -- a level many 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 report.
While most scenarios for meeting the target "require substantial investments", he said, this "does not mean that the world... needs to sacrifice economic growth."
UN climate chief Christiana Figueres said the findings challenged governments to make the world carbon-neutral in the second half of the century.
"We cannot play a waiting game where we bet on future technological miracles to emerge and save the day," she said in a statement.
The report is the third and final chapter of a mammoth overview by the Nobel Prize-winning panel -- its first assessment 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, which Figueres oversees, to formulate a pact by 2015 on curbing emissions.
A summary of the document, issued Sunday, warns that delaying further cuts to 2030 would "substantially increase the difficulty" of reaching the 2C goal.
For a 66 percent or "likely" chance of meeting it, the concentration of greenhouse gases in the atmosphere must 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.
Unprecedented Emissions Rise
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.
This will come at a cost -- clipping about 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, according to the estimates.
The calculation did not factor in potential savings from tackling emissions, including healthier humans and ecosystems and energy security.
"The IPCC is clear that acting on climate change is possible, beneficial and affordable," said WWF climate representative Samantha Smith.
"If we act now, costs will be only a very small fraction of global economies."