If countries hope to achieve their economic aspirations without pushing the global climate system deep into uncharted territory for humans, they have a rapidly closing window for taking extensive action at the lowest cost, according to a report issued Sunday by the UN-sponsored Intergovernmental Panel on Climate Change.

The report, released initially as a 37-page summary, notes that to stand a 50 percent of holding global average temperatures to an increase of about 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels by the end of this century, greenhouse gas emissions must peak and at least stabilize at levels that would be reached as soon as 2020, based on current emissions rates.

"The high-speed mitigation train would need to leave the station soon and all of global society would have to get on board" if the goal is to hew to the 2-degree marker, said IPCC chairman Rajendra Pachauri during a press conference in Berlin Sunday.

If that happens, the price of the ticket could be as low as shaving 0.06 percent a year from the annual growth in global consumption, the report indicates, yielding an overall slowdown of about 4.8 percent by 2100. At assumed growth rates of 1.6 to 3 percent per year as a baseline, that would delay by about 2 years the level of growth in consumption the world might otherwise see.

This doesn't include the avoided costs of adaptation measures that would be required for temperature increases above the 2 degree goal, nor does it include the economic value of public-health or ecosystem-health benefits from moving down a low-carbon energy path, the report acknowledges.

Still, the challenge in meeting the 2-degree goal is enormous, the report notes.

The most cost-effective path the IPCC identified is based on assumptions that are "quite optimistic," acknowledged Ottmar Edenhofer, deputy director and chief economist at the Potsdam Institute for Climate Impact Research. Dr. Edenhofer co-chaired Working Group III, the IPCC panel that produced the report.

It assumes policies are adopted and implemented quickly. It assumes that the full range of low-carbon energy options – including controversial ones such as expanded use of nuclear energy, natural gas as a bridge fuel, and the deployment of carbon capture and storage – scale up rapidly, in addition to other emissions-limiting approaches. And it assumes an unprecedented level of global cooperation.

The report "provides modest hope" that the world can achieve a 2-degree goal, currently an aspirational goal at the center of ongoing international talks for a new global agreement, Edenhofer said.

The report summary, released Sunday, is tied to one of three main volumes that form the centerpiece of the IPCC's periodic review of climate science, the implications the science holds for vulnerability to change and for adaptation measures, as well as the efforts needed and costs associated with reducing emissions. The full report from Working Group III, which runs more than 1,000 pages, is set for release April 15.

At the root of the challenge greenhouse-gas emissions pose rest population growth and economic growth with fossil fuels as the primary energy sources, the authors write.

The rising demand for energy from a growing population with rising economic aspirations has led to an explosion in emissions, particularly during the past decade.

Greenhouse-gas emissions, dominated by carbon dioxide, grew at an average rate of about 400 million tons a year between 1970 and 2000. From 2001 to 2010, the rate more than doubled to 1 billion tons a year. The increase came despite an increase in the number of efforts from local to regional efforts to curb emissions. And it overwhelmed progress the global economy had been making to reduce the amount of greenhouse gases it emitted per unit of gross domestic product, a broad measure of economic activity.

In its report, released last September, the IPCC's Working Group I noted that since the dawn of the Industrial Revolution, humans have pumped 515 billion tons of CO2 into the atmosphere. A review of existing scientific literature suggested that to hold to a 2-degree target, the accumulated human emissions to the atmosphere would have to stop at the point when 800-1,000 gigatons of CO2 had accumulated.

Working Group III noted that by 2010, global emissions had reached 49 billion tons a year. If sustained, this rate would put accumulated emissions at about 1,000 gigatons by 2020, now a scant six years off.

A great deal of debate swirls around many of the technological approaches to following a low-carbon path, as well as over how quickly and extensively many of these technologies can be deployed. Indeed, the IPCC authors note that each carries risks to varying degrees as well as advantages in reducing greenhouse-gas emissions.

Putting the world on a solid path toward the 2-degree goal "is technologically viable," says Leon Clarke, a coordinating lead author for one of the report's chapters and a senior scientist at the Joint Global Change Research Institute based at the University of Maryland at College Park. The institute is a collaboration between the university and the US Energy Department's Pacific Northwest National Laboratory in Richland, Wash.

"A mixed portfolio that matches the character of a particular country or particular region is certainly possible," he says.

But "equally important is this notion of addressing a range of factors that are more societal, political, and geopolitical," he says, and not just in rich countries.

Countries tagged as developing or as emerging economies "often have a totally different set of priorities, and often, climate change is not near the top," Dr. Clarke says. Dealing with global warming per se can take a back seat to food security, water issues, access to energy, or air quality, among other priorities, including economic growth in general.

Working Group II, whose report came out March 31, illustrated how climate change touches on each of these in affecting a society's resilience.

That approach has carried over to this new report as the authors tried to show how mitigation also can support efforts to meet these other social or political goals.

Still, he says, he's concerned that mitigation could have a hard time gaining traction unless the links to other national goals are clear or unless a country opts to make mitigation a high priority.

Looking ahead to global talks in September under the aegis of UN Secretary General Ban Ki-moon and on to talks under the UN Framework Convention on Climate Change in Lima at the end of the year and in Paris at the end of 2015, the report issued Sunday "is certainly a good wake-up call to the scale of global action that is required," says Kelly Levin, a senior associate at the World Resources Institute in Washington who focuses on helping developing countries set up and maintain records of emissions reductions.

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In particular, the meeting in Paris aims to wrap up a new global climate treaty that would take force in 2020.

"This has to be on the reading list of every decision maker going into Lima and into Paris," she says, adding, "at the same time, there is an ongoing conversation in the negotiations about how to ramp up pre-2020 ambitions. As the report noted, we are not on a pathway to the 2-degree target right now."