This week, the world’s nations are slogging through yet another round of U.N. climate negotiations in Durban, South Africa. The task at hand is daunting. In 2010, greenhouse-gas emissions made their largest one-year leap ever. That means it’s becoming increasingly difficult for the world to meet its professed goal of limiting global warming to 2°C (3.6°F) above pre-industrial levels. There’s been a lot of discussion lately about what that goal means, whether the world still has a shot at meeting it, and whether we’re doomed or not. Here’s a FAQ for the discussion.

A concrete wall holds back the rising seas in Tokelau, New Zealand. (MICHAEL FIELD/AFP)

So what can we expect if the world warms 2°C? So far, the world has warmed about 0.7°C above preindustrial levels, and the IPCC warns us that we’re already seeing extreme weather impacts: floods, droughts, killer heat waves, shifts in rainfall. Things will likely get worse as we reach 2°C (we’re already locked in for another 0.7°C). A 2010 report from the National Research Council tried to quantify a few of the risks for the United States. For example, a 2°C rise could mean a 400 percent increase in wildfires in the western United States, a 15 percent reduction in corn yields, and an increased risk of severe droughts. Low-lying island nations such as Tuvalu would likely get swallowed up by rising seas. Even 2°C could prove costly to adapt to.

That doesn’t sound good. Could 2°C be too much? Countries like Tuvalu certainly think so. Meanwhile, some scientists, like NASA’s James Hansen, have argued that even 2°C is far too dangerous. Paleoclimate records suggest that when the world last had enough carbon dioxide in the air to surpass that 2°C mark, much of the world’s ice had melted, pushing sea levels up four to six meters higher than today. One problem is that it’s not exactly clear how much warming will trigger destabilizing feedback loops — like Siberia permafrost thawing and releasing methane into the air, warming the planet further. The hope has always been that the climate (and things like Greeland’s ice sheet) would remain stable if we stay below 2°C, but it’s not entirely certain.

Are we currently on pace to meet the 2°C goal? Nope. Ecofys, a consulting group, tracks and calculates the various pledges made by the world’s nations. If all of the cuts currently promised actually get carried out (a big if, since many pledges are voluntary), then we can expect 3.5°C of warming by 2100. A few extra degrees might not sound like much, but many scientists think that much warming would be unmanageable. Dave Roberts cites some truly dire research on the subject by the Tyndall Centre’s Kevin Anderson, who warns that “a 4 degrees C future is incompatible with an organized global community, is likely to be beyond ‘adaptation,’ is devastating to the majority of ecosystems, and has a high probability of not being stable.”

Is 4°C really that catastrophic? It seems so. Lately, scientists have been racing to pin down what a 4°C (7.2°F) rise would look like, and it’s not a pretty picture. A batch of papers in the Philosophical Transactions of the Royal Society found that 4°C warming could mean things like the total collapse of agriculture in sub-Saharan Africa and widespread desertification. We run the risk that the Amazon rainforests could die off and the Asian monsoon cycle could vanish. Plus, there are those destabilizing feedback mechanisms to worry about. The University of Southampton’s Eelco Rohling presented data at the American Geophysical Union conference this week suggesting that the sorts of CO2 emission levels we’re talking about here have, in the distant past, been associated with a 25-meter sea-level rise (though that would take awhile to unfold).

Never mind, let’s go back to 2°C. How do we do that? The world’s emissions would need to peak in the next five years or so. According to the U.N.’s Environment Programme, we’d have to limit global emissions to about 44 billion metric tons of CO2 per year by 2020 — right now we’re on pace for 50 to 66 billion tons. One way to close the gap, the UNEP argues, is by deploying clean energy more aggressively, drastically boosting energy efficiency and slashing some of the lesser-known pollutants causing warming, such as methane and soot from inefficient cookstoves in the developing world. Doing all that could give us a decent shot at staying below 2°C.

Okay, meeting the 2°C target is possible. Is it politically realistic? Not right now. At the moment, the world shows no signs of letting global emissions peak before 2020 (some scientists think emissions would have to peak as early as 2015). U.S. officials have said the world might not even have a legally binding treaty by then. “We’d have to put our economy on a war footing,” says Saleemul Huq, a enior fellow at the International Institute for Environment and Development. “The same way the United States reoriented its economy during World War II.” Yet no one’s talking about an all-hands-on-deck approach to global warming right now. The topic barely even comes up in the United States.

Does that mean the world is doomed? That’s the big question. Dave Roberts offers some forceful reasons to think only drastic action will avert catastrophe. Many advocates, meanwhile, have urged the world not to abandon the 2°C goal, since even if we miss it, ending up at 2.5°C, while potentially dangerous, is much preferable to hitting 4°C or 5°C. (And yes, it’s possible that we’ll get lucky and stay under the safe limit even with a not-quite-good-enough effort — climate models do have uncertainties — but that’s a real gamble, since, as Hansen notes, things could easily turn out worse than expected, not better.) All of which points to a compelling case for ramping up mitigation efforts, especially since, as the International Energy Agency has detailed, it’s a lot cheaper to act now than dither until it’s much too late.