The U.N. Just Gave Us an 85-Year Deadline on Global Warming

There's no doubt that climate change is real, and staving off its worst effects will likely require humanity to get its net greenhouse-gas contributions to near zero by the end of the century, United Nations scientists say in a new report.

The latest synthesis report released Sunday morning by the U.N. Intergovernmental Panel on Climate Change doesn't offer governments much in the way of specific action plans to cut greenhouse-gas emissions. The document wraps up thousands of pages of scientific findings produced by the scientific body over the past 14 months, laying out the "unequivocal" existence of climate change and its "severe, pervasive and irreversible impacts" for the planet.

The 116-page report, with an accompanying 40-page summary for policymakers, is meant as a road map as delegates prepare to meet in Paris to hammer out an international agreement on how to fight climate change. The document summarizes five previous reports by the working group that identified the science and impacts of climate change as well as possible mitigation strategies, while also giving policymakers a potential path forward.

The report builds on the IPCC's idea of a "carbon budget" measuring of the amount of emissions before the Earth warms to 2 degrees Celsius compared to preindustrial levels, a projected benchmark for triggering major climate impacts. But keeping that carbon budget in check and avoiding that barrier would require near-zero net emissions of carbon dioxide and other greenhouse gases by the end of the century, involving big cuts in emissions and offsetting measures for remaining ones. Renewable energy will have to grow past its current share of power generation—roughly 30 percent—to more than 80 percent by mid-century and 90 percent by 2100.

The report echoes previous IPCC findings that global warming is real and its impacts are being felt, saying that the period from 1983 until 2012 was likely the planet's warmest 30-year stretch in 1,400 years. Short of meeting the 2-degree goal, the IPCC found that the impact of global warming is already being felt and will continue to cause more heat waves, extreme storms, sea-level rise, and changes in precipitation patterns. The impacts, the report says, will be felt in a variety of ways, including lower crop yields, fires, disease outbreaks, and even conflicts like civil wars.

Implementing those required plans, the report summary says, would require "substantial technological, economic, social, and institutional challenges" that would grow more difficult with delays. Even waiting until 2030 for additional mitigation would require greater reliance on clean energy, carbon dioxide removal, and "higher transitional and long-term economic impacts."

The scientists do identify a number of possible strategies, ranging from clean energy sources and more efficient buildings and vehicles to a carbon tax, or even extreme measures like trapping and storing emitted carbon dioxide. But the panel stopped short of laying out a specific pathway for countries to follow, instead impressing the importance of a coordinated international plan as opposed to individual country-by-country strategies.

That leaves a tall order for officials heading into talks in Lima, Peru, this December and the final negotiations in Paris next year, where officials are hoping to see a legally binding agreement to fight climate change.

But even weeklong talks between scientists and government officials in Copenhagen this week to review the latest report exposed the complicated politics of climate-change adaptation; scientists told reporters that there was even a standoff over the inclusion of some text related to Article 2 of the U.N. Framework Convention on Climate Change because of the debate over developing the world's smaller economies versus fighting climate change (the box of the text was omitted, but the information remains elsewhere in the document).

In a statement, White House Office of Science & Technology Policy Director John Holdren said the report was "yet another wake-up call to the global community that we must act together swiftly and aggressively in order to stem climate change and avoid its worst impacts." The document, he added, "underscores the need to fully implement President Obama's Climate Action Plan, including continued engagement with other countries on ambitious emissions-reductions targets and the policies and technologies necessary to achieve them."

Despite the dire predictions from the panel, IPCC chairman Rajendra Pachauri has urged officials to stay optimistic and "avoid being overcome by the seeming hopelessness" of the climate fight.

"Tremendous strides are being made in alternative sources of clean energy," Pachauri said at the Copenhagen talks. "There is much we can do to use energy more efficiently. Reducing and ultimately eliminating deforestation provides additional avenues for action."