This is a deeper cut than called for by most climate experts and policymakers, including President-elect Barack Obama, who favors an 80 percent drop in U.S. carbon emissions by mid-century.
Limiting carbon emissions aims to keep global mean temperature from rising more than 3.6 degrees F (2 degrees C) over what it was before the Industrial Revolution -- but one Worldwatch author said even this is too dangerous.
"Global warming needs to be reduced from peak levels to 1 degree (Celsius, or 1.8 degrees Fahrenheit) as fast as possible," co-author William Hare said at a briefing on the "State of the World 2009" report. "At this level you can see some of the risks fade into the background."
Global mean temperature has already risen 1.4 degrees F (0.8 C) since 1850, so drastic cuts in emissions of climate-warming carbon dioxide are needed, according to Hare, now working at the Potsdam Institute for Climate Impact Research in Germany.
Hare said that global greenhouse gas emissions would need to hit their peak by 2020 and drop 85 percent below 1990 levels by 2050, and keep dropping after that. He said carbon dioxide emissions would have to "go negative," with more being absorbed than emitted, in the second half of this century.
The burden of cutting greenhouse emissions should fall more heavily on rich countries than poor ones, Hare said, with industrialized nations reducing emissions by 90 percent by 2050, allowing developing nations to let their economies grow and develop new technologies that will ultimately reduce climate-warming gases.
2009: A PIVOTAL YEAR?
Even with these dramatic changes, the world may face an additional rise of nearly 1.8 degrees F (1 degree C) because the impact of past greenhouse emissions hasn't yet been felt on surface temperatures, the report said.
This year could be pivotal in the movement against climate change, said co-author Robert Engelman, with "scientists more certain and concerned, the public more engaged than ever before, an incoming U.S. president bringing to the White House for the first time a solid commitment to cap and then shrink this country's massive injections of greenhouse gases ... into the atmosphere."
Engelman also noted this year's deadline for a global agreement to craft a successor pact to the carbon-capping Kyoto Protocol. This is set to happen in December at a meeting in Copenhagen.
Engelman said the Copenhagen meeting could put in place a new "financial architecture" that discourages greenhouse emissions and rewards actions that take these emissions out of the atmosphere.
This could take the form of a cap-and-trade system or a carbon tax, he said, and could also include "the best terms of trade, investment and credit" for countries that make the transition to a low-carbon economy.
"However this turns out, we still have some precious time and a clear shot at safely managing human-induced climate change," Engelman said. "What's at stake is not just nature as we've always known it, but quite possibly the survival of our civilization. It's going to be a really interesting year."
Commenting on the report, environmental chemist Stephen Lincoln of the University of Adelaide in Australia said in a statement: "The strongest message from State of the World 2009 is this: if the world does not take action early and in adequate measure, the impacts of climate change could prove extremely harmful and overwhelm our capacity to adapt."
The Washington-based Worldwatch Institute is an independent research organization.