"Unveiling the latest United Nations assessment of climate science, the experts cited a litany of changes that are already under way, warned that they are likely to accelerate and expressed virtual certainty that human activity is the main cause," reports Justin Gillis.
“'Climate change is the greatest challenge of our time,' said Thomas F. Stocker, co-chairman of the Intergovernmental Panel on Climate Change, the United Nations-sponsored group of scientists that produced the report [PDF]. 'In short, it threatens our planet, our only home.'”
With a look at rising atmospheric carbon levels, land and ocean temperatures, and sea levels, the synopsis presents a litany of evidence of a changing planet. And for the first time, the world’s top climate scientists set "a 'carbon budget' for humanity — a limit on the amount of the primary greenhouse gas, carbon dioxide, that can be produced by industrial activities and the clearing of forests."
"To stand the best chance of keeping the planetary warming below an internationally agreed target of 3.6 degrees Fahrenheit (2 degrees Celsius) above the level of preindustrial times, the panel found, no more than one trillion metric tons of carbon can be burned and the resulting gas released into the atmosphere," says Gillis.
"Just over half that amount has already been emitted since the beginning of the Industrial Revolution, and at the rate energy consumption is growing, the trillionth ton will be released somewhere 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 three trillion tons of carbon are still left in the ground as fossil fuels."