Top climate scientists say in a new report that industrial carbon emissions need to be kept below a cumulative total of 1 trillion tons to avoid dangerous climate change — and they note that humanity has already used up more than half that allotment.

The Intergovernmental Panel on Climate Change said in the report that it is “extremely likely” human beings are the main drivers for the rise in temperatures recorded around the world over the last 50 years — the strongest words the panel has used to describe the effect humans are having on the planet.

“Continued emissions of greenhouse gases will cause further global warming and changes in all components of the climate system,” the IPCC report said. “Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”

More rain could pour down over tropical areas, and monsoon seasons could expand to include larger areas, striking earlier and lasting longer, according to the report. It is “virtually certain” that sea levels will continue to rise through the end of this century and beyond, and they said it is likely that the heat waves have become more frequent across swaths of Europe, Asia, and Australia.

“Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia,” the researchers wrote in the report. “The atmosphere and ocean have warmed, the amount of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.”

And while the evidence is clearer than ever that the planet’s getting toastier, the report said that people now are living through a truly historic period of uncomfortable warmth — at least as far as the last millennium is concerned.

“Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850,” according to the report. “In the Northern Hemisphere, 1983-2012 was likely the warmest 30-year period of the last 1400 years.”

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The amount of greenhouse gases including carbon dioxide, methane, and nitrous oxide is at an 800,000-year high, according to the report, with concentrations of CO2 showing a 40 percent spike over pre-industrial levels. About a third of the gas emitted by humans has been absorbed by the ocean, making the waters acidic.

A ceiling on carbon emissions?

The climate situation could get much worse, the scientists said: Past studies have suggested that a mean global temperature rise of 2 degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels would have dangerous, irreversible effects on sea levels and weather patterns. In the new report, experts estimate that 2-degree target would be reached when industrial carbon dioxide emissions add up to 1 trillion tons.



They say the running total, going from the 19th century to 2011, amounts to 531 billion tons. Thus, even at the current emission rate of roughly 10 tons per year, humanity would hit the 1-trillion-ton ceiling by 2060. And if carbon emissions continue to rise, as expected, the ceiling could be reached by the 2040s. Such projections are likely to spark calls for renewed negotiations on global greenhouse-gas limits.

The scientists said other measures to head off global warming — for example, sequestering carbon underground or pumping aerosols into the atmosphere to reduce sunlight — would have limited or unpredictable effects.



Explaining the slowdown in warming

The climate panel’s report rebuts claims made by people who point to a so-called “hiatus” in warming over the past 15 years — a claim that experts say is just blip compared to the long-term trends of climate change.



“The hiatus is a denier-manufactured diversion,” Kevin Trenberth, climate scientist with Colorado’s National Center for Atmospheric Research, told NBC News in an email before the report’s release. “The 2000s are the warmest decade on record by far.”

Part of the seeming slowdown in warming may be because of the amount of heat trapped by the ocean, said John Reilly, co-director of the MIT Joint Program on the Science and Policy of Global Change. “The hiatus, if anything, may just fool us because while the ocean is taking up more heat this past decade, sometime in the future it may take up relatively less heat, and then we’ll see the atmosphere warming just that much more,” Reilly said.

Other factors may include the transitory effects of volcanic eruptions and a downturn in the sun's 11-year activity cycle, scientists said in the report.

'Another wakeup call'

The IPCC report was received warmly by the Obama administration. In a statement, Secretary of State John Kerry called it “yet another wakeup call.”



“Once again, the science grows clearer, the case grows more compelling, and the costs of inaction grow beyond anything that anyone with conscience or common sense should be willing to even contemplate,” Kerry said in the statement. “This isn’t a run of the mill report to be dumped in a filing cabinet. This isn’t a political document produced by politicians.”

“It’s science.”

The effects of global warming are likely to continue to unfold over centuries, scientists said, even if human beings take immediate action to limit their role in changing the environment, causing ocean patterns to change as the rising water heats up and Arctic sea ice gets thinner.

“Scientists have confirmed what farmers in poor countries around the world have been telling us or years, that changes to their climate are destroying their livelihoods, ruining crops, hitting incomes, food quality and often their family’s health,” said Winnie Byanyima, executive director of Oxfam, according to Reuters.

NBC News’ John Roach and Alan Boyle contributed to this report. The Associated Press and Reuters also contributed. This story was updated at 12:45 p.m. ET Friday.