Amid the glittering skyscrapers of Doha, capital of the arid, oil-rich Arab emirate of Qatar, 17,000 diplomats, delegates, nongovernmental organizations, and environmentalists are converging this week and next in the conference halls and backrooms of the 18th annual United Nations climate-change summit. Their goal: pave the way toward a world treaty, to be signed in 2015, aimed at slowing global emissions of heat-trapping fossil-fuel pollution enough to keep the planet’s temperature from rising by 2 degrees Celsius (3.6 degrees Fahrenheit).

That’s the point, scientists say, at which the Earth’s polar ice sheets will melt and many of the hottest and driest regions will no longer be able to grow food. The 2-degree mark will set off a chain of extreme reactions, starting with rapid sea-level rise, widespread flooding, more extreme weather events, food shortages, and price spikes.

But no matter what the diplomats in Doha decide over the next week, it now appears inevitable that the world will indeed hit that 2-degree mark and could well shoot past it to average global increases of 4 degrees or 6 degrees—points at which scientists predict even worse catastrophes.

A scientific study published Sunday in the journal Nature Climate Change concluded that the world’s rapid increase in fossil fuel emissions now makes a global average temperature increase of 2 degrees Celsius all but inevitable. A report released last week by the U.N. Environment Program concluded that given the rapid projected increase in pollution from burning coal, oil, and gas around the world, nations’ current pledges to cut carbon emissions won’t be enough to stave off that 2-degree rise sometime before the end of the century.

Moreover, a November report from the International Energy Agency found that if action isn’t taken to significantly cut carbon emissions by 2017, the existing power plants, factories, and buildings will be enough to push temperatures past the 2-degree mark. Yet another sobering report last month from the accounting firm PricewaterhouseCoopers warned that the only way the world can prevent the 2-degree rise is if the global economy cuts its carbon intensity by 5.1 percent every year from now to 2050, essentially slamming the brakes on growth starting right now—and keeping the freeze on for 37 years.

Nobody expects that to happen. And even while the U.N. climate-change process is working toward a 2015 global deal in which the world’s biggest polluters agree to cut their carbon pollution, the terms of the agreement won’t be enforced until 2020. That means countries won’t even be required to start cutting their emissions for another eight years.

“The necessary rate of decarbonization is so high that it’s never been seen before, never been done before,” said Jonathan Grant, author of the PricewaterhouseCoopers report. “And the level of ambition that we’re seeing at Doha is far below what is required to stick to 2 degrees.”

So what needs to happen? Nations must begin to prepare now for the effects of a 2-degree temperature increase. In the U.S., that means starting immediately to plan and build for the inevitable consequences of more-destructive storms and rising sea levels, particularly in coastal cities such as New Orleans; Norfolk, Va.; and, as superstorm Sandy so frighteningly illustrated, New York. Those preparations are going to cost taxpayers a lot of money.

“When it comes to the worst-case scenarios of sea-level rise, I’m not sure $100 billion will even scratch the surface,” said Brian Murray, director of economic analysis at Duke University’s Nicholas Institute for Environmental Policy Solutions.

It also means that nations—first and foremost, the world’s two biggest polluters, the United States and China—must develop real, enforceable, and aggressive policies to cut their own carbon emissions, with or without a global agreement. Scientists say that once the world hits that 2-degree mark, the urgency of reducing carbon pollution to avoid a catastrophic tipping point becomes even greater.