Climate Panel Says Upper Limit on Emissions Is Nearing

STOCKHOLM — For the first time, the world’s top climate scientists on Friday formally embraced an upper limit on greenhouse gases while warning that it is likely to be exceeded within decades if emissions continue at a brisk pace, underscoring the profound challenge humanity faces in bringing global warming under control.

A panel of experts appointed by the United Nations, unveiling its latest assessment of climate research, reinforced its earlier conclusions that global warming is real, that it is caused primarily if not exclusively by human emissions, and that it is likely to get substantially worse unless efforts to limit those emissions are rapidly accelerated.

“Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes,” the report said. “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

Going well beyond its four previous analyses of the emissions problem, the Intergovernmental Panel on Climate Change endorsed a “carbon budget” for humanity — an upper limit on the amount of the primary greenhouse gas, carbon dioxide, that can be emitted from industrial activities and forest destruction.

To stand the best chance of keeping the planetary warming below an internationally agreed target of 3.6 degrees Fahrenheit above preindustrial levels and thus avoiding the most dangerous effects of climate change, the panel found, only about 1 trillion tons of carbon can be burned and the resulting gas spewed into the atmosphere.

Just over half that amount has already been emitted since the beginning of the Industrial Revolution, and at current rates of energy consumption, the trillionth ton will be released around 2040, according to calculations by Myles R. Allen, a scientist at the University of Oxford and one of the authors of the new report. More than 3 trillion tons of carbon are still left in the ground as fossil fuels.

Limiting the warming to the agreed-upon target “is technically doable, but at the moment we’re not going in the right direction,” Dr. Allen said in an interview. “I don’t think we’ll do it unless we bite the bullet and start talking about what we’re going to do with that extra carbon that we can’t afford to dump into the atmosphere.”

To keep using fossil fuels beyond the trillionth ton of emissions, companies would have to develop potentially expensive technology to capture carbon dioxide from emissions sources like power plants and store it underground. Such efforts have been lagging badly; only last week, Norway scaled back one of the most ambitious such projects because of soaring costs.

But a considerable body of research suggests that in principle it could be done, and in the United States, …read more