Wendy Koch

USA TODAY

A rapid shift to less-polluting energy will be needed to avoid catastrophic global warming, because global emissions of heat-trapping greenhouse gases have accelerated to unprecedented levels, the United Nations reports today.

These emissions — largely from the burning of oil, gas and coal — grew more quickly between 2000 and 2010 than in any of the three previous decades and will need to be slashed 40% to 70% by mid-century and almost entirely by century's end to keep global temperatures from spiraling out of control, according to a landmark report by the U.N. Intergovernmental Panel on Climate Change.

Even those emissions cuts might not be enough. The IPCC report, striking a particularly urgent tone, says countries might even need to enlist controversial technologies that remove carbon dioxide from the atmosphere.

"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 Germany's Ottmar Edenhofer of the Potsdam Institute for Climate Impact Research, who co-chaired the IPCC report, the third in a series released in the past year. The Working Group III report, written by 235 scientists from 57 countries, looks at myriad ways to fight climate change and serves as a potential road map for policymakers who plan to negotiate a new climate treaty next year in Paris.

"If we do nothing, temperatures will continue to rise," co-author Leon Clarke, a scientist at the Pacific Northwest National Laboratory, said from Berlin after wrapping up a week of discussions there to finalize the report's wording. "It's not necessarily a phaseout of fossil fuels," he said, but rather "a phaseout" of power plants and other facilities that don't capture the carbon they emit.

Holding emission increases to 3.6 degree Fahrenheit (2 degrees Celsius) above pre-industrial levels — a goal sought in international agreements — will require "heroic efforts" and a "massive" shift in the energy sector, says another U.S. co-author, David Victor, professor of international relations at the University of California, San Diego. "It's doable in theory ... but it will be extremely difficult."

Despite efforts to mitigate climate change, the report says global greenhouse gas emissions rose 2.2% annually in the past decade — nearly twice the annual rate of 1.3% from 1970 to 2000. It says fossil fuel combustion and industrial processes, which rose as the global population and economy grew, accounted for 78% of the emissions' increase between 1970 and 2010. It says about half of cumulative man-made carbon emissions since 1750 has occurred in the last 40 years.

The IPCC report says delaying action will only escalate the costs of transitioning to energy that emits less or zero greenhouse gases. It doesn't endorse any single approach but cites the value of planting forests, boosting energy efficiency and — by 2050 — at least tripling the share of energy from zero-carbon sources such as nuclear, solar and wind.

It also points to more ambitious measures such as "bio-energy with carbon capture and storage" or BECCS, in which power plants produce fuel by burning biomass — trees, plant waste, wood chips — then capture and store the CO2 emissions underground. Victor says BECCS holds appeal for the future because it produces energy while actually reducing emissions.

The IPCC acknowledges that some measures could slightly slow economic growth while others may face financing and technical challenges. It notes, for example, that not a single BECCS plant has been built or tested on an industrial scale. Also, industries that could be affected by these changes will seek to hold on to market share.

Exxon Mobil, the largest U.S. oil and gas company, said in a March 31 report that global climate policies are "highly unlikely" to stop it from producing and selling fossil fuels. It said these fuels will continue to meet about three-quarters of global energy needs through 2040.

"The risk of climate change is clear and the risk warrants action," William Colton, the company's vice president of corporate strategic planning, said in a statement. He called for greater energy efficiency and research funding for emission-cutting technologies.

Since 1990, the IPCC has released a major assessment every six or seven years that summarizes the latest peer-reviewed science on climate change. As part of its current multi-part fifth assessment, its first two reports said there's near certainty that man-made greenhouse gas emissions are exacerbating global warming and affecting every part of the planet.

Since its last major assessment in 2007, IPCC co-authors say they've learned a lot about how to mitigate climate change, For today's report, 31 modeling teams analyzed 1,200 scenarios to focus on potential solutions.

"We need to finally take on board what the IPCC is telling us" from a risk management and business development perspective, says Mark Way,who works on sustainability issues for insurer Swiss Re. "The transition to a low carbon future is the only sensible choice."