If greenhouse-gas emissions continue at their current rate, according to some estimates, average global temperatures could rise by as much as 8 degrees Fahrenheit by the end of this century.
What would it be like to live in a world that warm?
It's hard to say for sure, experts say, given the many uncertainties about how complex climate dynamics will play out. But the likelihood is high for a rising incidence of a variety of disasters – including deadly heat waves and damaging superstorms that would periodically flood coastal cities and low-lying regions, potentially displacing millions of people.
As the consequences of massive global warming reverberated through ecosystems and societies, other possible outcomes include higher rates of disease, a rising swell of environmental refugees, extensive crop failure and widespread species extinctions.
With talks ongoing among global leaders in Paris this week, the pressure of such a catastrophic future offers hope that better policies and technological advancements will slow the rate of warming, helping to avert at least some of the potential catastrophe.
"The idea of an 8-degree warmer world and what that would mean is just unfathomable to me. It would just be so horrible," says Richard Seager, a climate scientist at the Lamont Doherty Earth Observatory of Columbia University in Palisades, N.Y. "I really hope we don't get there. And I would remain relatively confident that we will avoid that."
The focus of some of the most intense climate-change speculation -- and the source of some of the biggest debate -- is the relationship between warming-induced melting of Arctic ice and rising sea levels. It's a complicated process that has evaded attention until recently, Seager says, so there are still many uncertainties.
But the worst-case scenario isn't pretty. In a controversial new paper that is still undergoing peer review, an international group of scientists theorized that even just 2°C (about 4°F) of warming above preindustrial levels would lead to rapid melting in Greenland and Antarctica, leading to a dangerous rise in sea levels and an increasing number of powerful storms.
Some scientists have already warned that the 2°C mark is a tipping point beyond which the Earth won't be able to recover. By then, the Greenland ice sheet will be melting at a rate that cannot be stopped, argued Mark Lynas in his 2008 book "Six Degrees: Our Future on a Hotter Planet."
Rising seas don't necessarily cause problems on a daily basis, at least not at first. Instead, problems occur when powerful storms coincide with high tides in full seas, says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. That combination ups the chances for disasters like Hurricane Katrina in 2005, Superstorm Sandy in 2012, and Super Typhoon Haiyan in 2013. During some such events, subways flood. Homes topple. Cities crumble. Thousands die.
"If you imagine someone on the beach and the sea level slowly rises to his toes, then his ankles, then his knees -- that's not how it works," Trenberth says. Instead, the devastation is periodic, scattered and unpredictable. "It's not the same community time after time."
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In the United States, areas most at risk include Miami, New York, New Orleans, Boston and Tampa, according to a 2013 analysis published in Nature Climate Change. Even more imperiled are low-lying coastal regions in the highly populated developing world. Places like Bangladesh and many Pacific islands lack infrastructure and resources to protect themselves.
With every degree of warming, moisture in the atmosphere increases by about five percent, Trenberth adds, contributing more power to storms that can then dump massive amounts of snow or rain. Chances for flooding rise, even in inland areas.
And it's not just storms we have to worry about. Warmer global temperatures bring more frequent summer heat waves that can turn deadly when nighttime temperatures fail to drop. Along with less cooling in the winter, higher temperatures also allow diseases like West Nile Virus and pests like the pine bark beetle to persist and spread into temperate regions that used to be inhospitable to them. There may even be a rise in social strife and civil unrest.
As seasons and growing zones shift, another major concern is what will happen to food crops, as well as wild species of plants and animals. Those are changes we (and other species) might be able to adapt to, Trenberth says, but only if we manage to buy a little extra time. And outcomes in Paris could make a big difference.
"The rates of change matter enormously," he says. Two or more degrees of warming in the next 50 years wouldn't be good. But "if we can slow it down to 100 years, we'll be so much better off. If we can slow it down to 150 years, that's even better. What happens in Paris will determine when that happens."