The latest comprehensive global scientific assessment of climate change, released on Sunday, sounds the direst warning yet about the need to drastically reduce greenhouse-gas emissions. But despite years of such reports, fossil-fuel use and human-caused emissions continue to rise, and renewable energy technologies have so far failed to make a significant difference.

The Intergovernmental Panel on Climate Change, a U.N.-convened panel of the world’s scientific community, estimates that in order to have a 66 percent chance of limiting total average warming to less than 2 °C relative to preindustrial levels—a goal widely seen as a threshold beyond which severe changes are far more likely—the world’s human population can emit no more than one trillion tons of carbon dioxide, and that we’ve already emitted more than half that much.

Avoiding going over one trillion tons would mean reducing greenhouse-gas emissions 40 to 70 percent by 2050 and slashing them to almost zero by 2100, the report estimates.  

Such estimates were first made in 2009 (see this Nature paper) without prompting much in the way of policy changes to reduce emissions. But this is the first time the IPCC has embraced the concept of a global carbon budget in one of its comprehensive sets of assessments, which the panel issues every few years. On Sunday, the IPCC released the synthesis of the fifth set of such reports since 1990.

The task ahead is now far clearer for countries that have signed on to the United Nations Framework Convention on Climate Change (UNFCCC), says Myles Allen, lead author of the 2009 paper, who heads climate research at the Environmental Change Institute of the University of Oxford’s School of Geography and the Environment. These nations will meet for the next round of climate talks in Paris in late 2015. 

With the IPCC having reviewed and endorsed the idea of a carbon budget, nations “haven’t got any excuse to ignore it now,” he says. “It’s not for the IPCC to recommend policy, but speaking personally, I very much hope [the countries] will now acknowledge the fact that their two-degree goal implies a cumulative limit on carbon emissions. And it is a limit we are rapidly approaching.”

At current rates, the “budget” would be spent in just 30 years. Reducing emissions below the threshold is a monumental task. It would require large-scale burial of carbon dioxide from many hundreds of existing coal power plants—but this effort has barely begun (see “Carbon Sequestration: Too Little, Too Late?”). In addition, it would require almost quadrupling the present supply of renewable energy and nuclear energy, the report estimates, as well as other vast efforts, including stopping deforestation and making widespread changes to agriculture practices.

And yet emissions keep rising. As one example, coal power plants already produce more than 14 billion tons of carbon dioxide emissions each year (that’s about four billion tons of carbon) and are becoming more numerous.

If we continue on the current path, heat-trapping gases will build up to produce a surge in average global temperatures of 3.7 °C to 4.8 °C by 2100. The result will be a dangerous rise in sea levels, more profound droughts and heat waves (greatly stressing world water and food supplies), and more powerful storms and floods.

The idea of a carbon budget could clarify matters for governments, says Tim Profeta, director of the Nicholas Institute for Environmental Policy Solutions at Duke University. “This concept might prove useful at the negotiating table, as it changes the question from one of annualized emissions of individual nations,” he says. “Negotiations could then focus on how to divide that budget amongst individual countries.”

The IPCC says it is at least 95 percent certain that human activities, led by the burning of fossil fuels, are the main cause of the climate change seen since 1950, up from 90 percent in the previous assessment in 2007 and 66 percent in 2002. Its report is based on 30,000 scientific papers studied by about 830 authors and 2,000 reviewers.