Humans are playing Russian roulette with Earth's climate by ignoring the growing risk of tipping points that, if passed, could jolt the climate system into "a new, less habitable 'hothouse' climate state," scientists are warning ahead of the annual UN climate summit.

Research now shows that there is a higher risk that "abrupt and irreversible changes" to the climate system could be triggered at smaller global temperature increases than thought just a few years ago. There are also indictations that exceeding tipping points in one system, such as the loss of Arctic sea ice, can increase the risk of crossing tipping points in others, a group of top scientists wrote Wednesday in the scientific journal Nature.

"What we're talking about is a point of no return, when we might actually lose control of this system, and there is a significant risk that we're going to do this," said Will Steffen, a climate researcher with the Australian National University and co-author of the commentary. "It's not going to be the same conditions with just a bit more heat or a bit more rainfall. It's a cascading process that gets out of control."

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The scientists focused on nine parts of the climate system susceptible to tipping points, some of them interconnected:

Arctic sea ice , which is critical for reflecting the sun's energy back into space but is disappearing as the planet warms.

, which is critical for reflecting the sun's energy back into space but is disappearing as the planet warms. The Greenland Ice Sheet , which could raise sea level 20 feet if it melts.

, which could raise sea level 20 feet if it melts. Boreal forests , which would release more carbon dioxide (CO2) than they absorb if they die and decay or burn.

, which would release more carbon dioxide (CO2) than they absorb if they die and decay or burn. Permafrost , which releases methane and other greenhouse gases as it thaws.

, which releases methane and other greenhouse gases as it thaws. The Atlantic Meridional Overturning Circulation , a key ocean current, which would shift global weather patterns if it slowed down or stopped.

, a key ocean current, which would shift global weather patterns if it slowed down or stopped. The Amazon rainforest , which could flip from a net absorber of greenhouse gases to a major emitter.

, which could flip from a net absorber of greenhouse gases to a major emitter. Warm-water corals , which will die on a large scale as the ocean warms, affecting commercial and subsistence fisheries.

, which will die on a large scale as the ocean warms, affecting commercial and subsistence fisheries. The West Antarctic Ice Sheet , which would raise sea level by at least 10 feet if it melted entirely and is already threatened by warming from above and below.

, which would raise sea level by at least 10 feet if it melted entirely and is already threatened by warming from above and below. Parts of the East Antarctic Ice Sheet that would also raise sea level significantly if they melted.

Just looking at Arctic changes shows how the links between parts of the climate system susceptible to tipping can amplify global warming and its effects, said Tim Lenton, director of the Global Systems Institute at Exeter University and an author of the commentary.

Shrinking sea ice increases ocean heat because it no longer reflects as much of the sun's energy back to space and it enables the darker water to absorb more warmth. The ocean heat extends over land and, combined with other heating effects, thaws permafrost, which releases more heat-trapping gases to the atmosphere, Lenton said.

Research shows that melting of the Greenland Ice Sheet may also be slowing the Atlantic Meridional Overturning Circulation, a key Atlantic Ocean current that transports heat between the Northern and Southern Hemispheres and directs rainfall around the planet. That could disrupt monsoon rains critical to agriculture in developing countries.

"If you transfer less heat from south to north, it warms up the Southern Hemisphere, which affects the Antarctic ice sheets. When you start thinking about this, heading to a new climate state becomes very plausible indeed," he said.

The ocean circulation slowdown was documented in a 2015 study by researchers including Michael Mann and Stefan Rahmstorf, a co-author of the new commentary. Cold water from the melting Greenland Ice Sheet is likely slowing the current, showing "how these tipping point responses are actually interrelated, not independent," Mann said. "If one goes early then so too may the others, like dominoes."

Lack of Policy Urgency as Permafrost Thaws

Despite increasingly urgent warnings about the effects of rising greenhouse gas emissions, two new reports published this week by the United Nations show that international efforts to slow global warming are falling far short of what scientists recommend.

The Emissions Gap Report—an annual assessment of global pledges to cut greenhouse gas emissions—shows that countries' current pledges under the Paris climate agreement will still raise global temperatures 3.2°C (5.8°F) by the end of the century, well beyond the Paris goal of limiting warming to well below 2°C (3.6°F). The Production Gap Report shows that the amount of oil, gas and coal that countries already plan to produce will lead to 50 percent more fossil fuels produced by 2030 than would be allowable to stay under 2°C warming.

The Earth is now warming faster and CO2 levels "are increasing at rates that are an order of magnitude higher than at the end of the last ice age," when rapid climate change destabilized the climate quickly, the scientists wrote in the Nature commentary.

"To err on the side of danger is not a responsible option," they wrote.

Scientific observations of melting ice sheets and glaciers, thawing permafrost, and changes to oceans and forests also show ominous signs that the risk of rapid and extreme sea level rise and runaway greenhouse gas emissions are higher than identified in major climate reports, including the most recent Intergovernmental Panel on Climate Change assessment.

"Permafrost is already thawing quite rapidly in response to ongoing warming," said Merrit Turetsky, who studies the Arctic's frozen soil at the University of Guelph in Canada. She recently published research on abrupt permafrost thaw that suggests a tipping point may be closer than scientists thought.

"Meters of permafrost can warm and thaw in a matter of months to years," she said. Such sudden thawing releases more methane than a more gradual process, suggesting that scientists are likely underestimating how the feedback from the melting Arctic will amplify global warming, she said. Her research suggests abrupt permafrost thaw could double the warming from greenhouse gases released from tundra.

Underestimating Risks of 'Irreversible Changes'

"We must admit that we have underestimated the risks of unleashing irreversible changes, where the planet self-amplifies global warming," said Johan Rockström, director of the Potsdam Institute for Climate Impact Research and a co-author of the commentary. "This is what we now start seeing, already at 1 degree Celsius global warming."

Those risks are a key reason climate scientists have urged policymakers to try to keep global temperatures from warming more than 1.5°C or 2°C compared to pre-industrial times, said University of Michigan climate researcher Jonathan Overpeck.

"The risks of triggering tipping points go up fast if we warm the planet more, meaning it might not be possible to limit warming to just 3 or 4 degrees Celsius if tipping point thresholds are crossed," he said. "If we push the climate system beyond hard-to-predict thresholds, or tipping points, climate change and its impacts get much bigger, faster."

The Western U.S. may already be seeing a forest dieback tipping point, Overpeck said.

"Much of our western forest cover is either dying or burning, leading to irreversible changes in vegetation that, in turn, impact water supplies and natural carbon storage," he said.

Some Tipping Points Are Already Close

One of the biggest concerns is that some tipping points, like the meltdown of alpine glaciers and near-total loss of coral reefs, will be reached even if the world meets the goals of the Paris climate agreement, said Katherine Richardson, a climate researcher at the University of Copenhagen and co-author of the commentary.

If multiple tipping points are reached, it's questionable whether emissions reductions will be enough to stabilize the climate system, she said.

"Another problem we have is that we have taken a cost-benefit approach," she said. "Economists assume high-impact tipping points are low probability events, but we may already have passed some of them—which completely changes the way we should be doing our cost-benefit analyses.

"In 2001, IPCC said not to worry (about tipping points) until there is 5 degrees Celsius warming, now they're saying 1 to 2 degrees. Economists have to stay on top of the science."

Richardson said she had just explained that morning to the Danish Parliament that some of the common tools for cutting emissions weren't proving to be effective enough, including the European Union's carbon trading market at its current prices.

"There are so many emissions credits floating around that using them won't have any effect on total emissions or atmospheric concentrations on a meaningful time scale. We can't wait 30 to 40 years," she said. The UN Emissions Gap Report released this week "basically says we've done nothing so far."

The new Nature article reinforces other recent similar warnings, said climate scientist Katharine Hayhoe, director of the Texas Tech University Climate Science Center. That includes a chapter in the latest U.S. National Climate Assessment on "potential surprises" in the climate system, "The scariest thing that you'll ever read that's not by Stephen King," Hayhoe said.

It explores climate impacts and feedback systems that we don't fully understand, she said, and "that may be far more worrisome than what we do know."