It's nearly too late to avoid the worst of climate change, according to a major new United Nations report. If humans don't speed up the shift from fossil fuels to clean energy, our chance to keep heat rise below the international target of 2 degrees Celsius (3.6 Fahrenheit) "will slip away within the next decade."

That warning comes from U.N. Secretary-General Ban Ki-moon, who was in Copenhagen this week to unveil the biggest, most comprehensive climate-change report in history. The 116-page overview is the fifth released since 1990 — and the first since 2007 — by the U.N. Intergovernmental Panel on Climate Change (IPCC), a collection of the world's foremost experts on climate science.

"Our assessment finds that the atmosphere and oceans have warmed, the amount of snow and ice has diminished, sea level has risen and the concentration of carbon dioxide has increased to a level unprecedented in at least the last 800,000 years," IPCC physicist Thomas Stocker says in a press release about the report, which describes humanity's role in climate change as "clear and growing."

To avert the worst-case climate scenarios, fossil fuels will need to be almost completely phased out by the end of this century, the report's authors conclude. That means the share of low-carbon electricity must rise from 30 percent to more than 80 percent by 2050, and to nearly 100 percent by 2100.

wind turbines

A quarter of global electricity will come from renewable sources by 2018, according to the International Energy Agency. (Photo: Getty Images)

Yet that isn't as daunting as it might seem. The cost of solar and wind power has been falling for years, Ban notes, helping make renewable energy the fastest-growing source of electricity on Earth. The tools for weaning humanity off fossil fuels are already available, and a quick transition is far more financially prudent than putting it off — despite some long-running arguments to the contrary.

"There is a myth, which is shared unscientifically and uneconomically, that climate action will cost heavily," Ban says. "But I am telling you that inaction will cost much, much more."

"We have the means to limit climate change," adds IPCC Chairman R.K. Pachauri. "The solutions are many and allow for continued economic and human development. All we need is the will to change, which we trust will be motivated by knowledge and understanding of the science of climate change."

Carbon dioxide in Earth's atmosphere has surged to around 400 parts per million (ppm) over the past two centuries, a level unprecedented in human history. CO2 is a vital part of our planet's air, but this recent excess — driven by emissions from burnt fossil fuels — lets the greenhouse gas trap too much solar heat, rapidly recreating a steamy atmosphere that hasn't existed since the Pliocene Epoch.

The fact that such conditions have existed before is little consolation for a species who has never had to endure them. If CO2 levels reach 450 or 500 ppm, the mix of heat and humidity in some places "is expected to compromise common human activities," the IPCC warns, "including growing food and working outdoors." Many coastal areas will become unlivable due to sea-level rise, crops will wither amid megadroughts and some diseases will spread more widely, among other catastrophic effects.

The new IPCC report, parts of which were leaked earlier this year, is meant to inform world leaders about climate science ahead of a major U.N. summit in 2015. Delegates will convene in Paris next December in an effort to reach a new global treaty that would rein in man-made climate change.

"We can't prevent a large scale disaster if we don't heed this kind of hard science," U.S. Secretary of State John Kerry says in a statement on the report. "The longer we are stuck in a debate over ideology and politics, the more the costs of inaction grow and grow. Those who choose to ignore or dispute the science so clearly laid out in this report do so at great risk for all of us and for our kids and grandkids."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.