– The world needs to triple the energy it gets from renewables, nuclear reactors and power plants that use emissions-capture technology to avoid dangerous levels of global warming, United Nations scientists said.

Investments needed to keep climate change within safe limits would shave a fraction of a percent off annual global growth, the U.N. said Sunday in the third part of its most comprehensive study on warming. A delay in stemming rising greenhouse gases will cut chances to limit the global temperature increase, add to costs and lead to increasing reliance on unproven technologies, they said.

"The longer we wait to implement climate policy, the more risky the options we'll have to take," Ottmar Edenhofer, a co-chair of the 235-scientist panel that drafted the report, said in a phone interview from Berlin. "We need to depart from business as usual, and this departure is a huge technological and institutional challenge."

The U.N. said governments must accelerate efforts to build wind farms and solar parks and provide incentives to develop carbon capture and storage technology, or CCS, for fossil-fuel plants by making it more costly to emit carbon. The study aims to guide envoys from 194 nations next year as they devise a new accord to slash greenhouse gases.

The researchers said emissions growth accelerated to an average of 2.2 percent a year for the 2000-2010 period from an annual 1.3 percent the preceding three decades. That puts in jeopardy the target agreed upon by climate treaty negotiators to stabilize warming since preindustrial times to below 3.6 degrees Fahrenheit.

The possible situation in 2100 is "either you'll have some fossil-fuel power generation with carbon capture and storage, or a complete switch-over to renewables and smart energy storage," Jonathan Grant, director of climate change at consultants PwC in London, said. "The problem with some of those scenarios is the transition takes too long."

Global greenhouse gas emissions would have to be lowered by 40 to 70 percent by midcentury from 2010 levels, and to "near-zero" by the end of the century, the U.N. panel said Sunday in a statement handed out in Berlin.

Without extra effort to cut greenhouse gases, current trends may triple their concentration in the atmosphere this century, according to the report. That would raise the risk of melting glaciers and sea ice, lengthening droughts and heat waves and intensifying storms and flooding.

"This report brings out the need for an unprecedented level of international cooperation," Rajendra Pachauri, chairman of the panel, told reporters.

The assessment is intended as a reference for officials around the world as they devise emissions-curbing policies. Hundreds of scientists and government officials have spent the past week in Berlin editing a draft line-by-line to put it into clearer wording understandable to policymakers.

When the panel finished its last study seven years ago, it won the Nobel Peace Prize and the prospect its findings might spur a globally binding treaty to cut greenhouse gases.