Over the years, many nations have said they'd like to limit global warming to below 2°C (3.6°F). Past that point, the thinking goes, we're in the realm of "dangerous" climate change — although some experts worry that even 2°C is too risky, given the changes we've already seen, such as the collapse of Arctic sea ice.

A stock photo of some pollution and weather in the Netherlands because, at this point, we've run out of creative ways to illustrate climate change. (Washington Post)

But let's say 2°C is the goal. That looks daunting. After all, the world has already warmed about 0.8°C above pre-industrial levels. And the carbon dioxide we've put in the air has committed us to another 0.7°C of warming in the decades ahead. Plus, carbon emissions keep rising each year. So have we finally reached the point where it's too late?

(See further reading here.)

Not necessarily. At least, that's according to a new report (pdf) from the analysts at the Climate Action Tracker. They say it's still technically feasible for the world to stay below that 2°C target. But the world would need to start revamping its energy system now. As in, today. Because with each passing year, meeting that 2°C goal becomes significantly more arduous.

Right now, the world is still off-track. When the analysts added up all existing pledges to curb emissions and plugged them into the latest climate models, they found that humans are currently on pace to heat the planet somewhere between 2.7°C and 4.2°C by the end of the century. (There's a range because there's still some uncertainty as to exactly how sensitive the climate is to increases in atmospheric carbon.)

Yet the Climate Action Tracker analysts aren't ready to despair just yet. If the world's nations could somehow trim their emissions 15 percent below present levels by 2020, and then keep cutting, then there are a number of different scenarios in which global warming could stay below 2°C.

Various strategies for how to attain this goal can be found in a big new report from the U.N. Environmental Programme. Big cuts wouldn't be easy or cheap: Nations would need to invest in everything from improving the energy-efficiency of buildings to limiting deforestation to scaling up renewable energy and nuclear power. These efforts would cost about 1 percent of global GDP, assuming that the investments are spread out evenly over time.

Speed turns out to be critical. If the world starts making big cuts today, the reports conclude, there's still some margin for error. Even if some clean-energy technologies are simply unworkable — such as, say, carbon capture and sequestration for coal plants — there's still a chance to meet that 2°C target. (And if engineers could figure out how to suck carbon out of the air through biomass sequestration, 1.5°C might even be attainable.)

But with each passing year, that task gets harder and harder. Here's the key chart: 

If the world waits until after 2020 to start reducing its emissions, as is currently planned, then the cuts need to be deeper, the costs are potentially higher, and we'll have to hope that most or all of our low-carbon technologies pan out — or that something new and revolutionary gets invented in a pinch. The odds increase that the world heads for 3°C or 4°C of global warming, which, as a recent World Bank report detailed, could have ugly and unpredictable impacts.

Now, some observers might reasonably argue that the United Nations and other agencies are being way too optimistic about the prospects for staying below 2°C, even if the world did get started tomorrow. For instance, as Roger Pielke Jr. recently pointed out, the International Energy Agency's ambitious vision for curbing emissions would still leave 1.3 billion people with scant access to electricity. The idea that the world could just stop there isn't particularly realistic. 

What's more, there's little indication that countries are ready to start making ambitious cuts today. The Climate Action Tracker includes an appendix with an assessment of pledges made by various countries. And progress is slow. Japan, for instance, has an ambitious promise to cut emissions 25 percent by 2020, but the country is unlikely to meet the target now that it's shuttering its nuclear plants. China, meanwhile, will likely succeed in slowing down its rate of emissions growth, as promised — but China's pledge is still fairly weak.

That's a consistent theme in the report. The nations that have made big promises haven't yet fulfilled them. And the nations that have lived up to their pledges never promised that much in the first place. Unless that changes, the chances that the world stays below 2°C will soon vanish.

Further reading:

-- A World Bank report explains why the world should definitely avoid 4°C of global warming. Things get ugly fast.

-- The ongoing Doha climate talks, explained in three charts.

-- A new U.N. report looks at how the world can close the emissions gap and meet its climate targets.

--And if all else fails we can always try geoengineering to cool the Earth. Trouble is, it's awfully risky.