Every time world leaders get together and talk about climate change, they tend to agree on one big thing: We shouldn't let global average temperatures rise more than 2°C above pre-industrial levels. The risks would be too great. It would be too dangerous.

Then those same world leaders fly back home and... don't do enough about it. The U.N.'s Environment Programme is out with its big annual report on this very subject: Even if you add up all the bright-eyed promises that countries have made to tackle climate change, global greenhouse-gas emissions are still on pace to rise too fast to avoid 2°C of warming.

Here's the giant chart showing the "emissions gap" — the gulf between climate ambitions and actual practice:

Let's unpack this. There are a few elements here:

The 2°C goal: If the world wants to limit global warming to below 2°C this century, the report notes, then the world's annual greenhouse-gas emissions will likely need to peak soon and decline roughly 14 percent by 2020 (and then, crucially, keep falling for decades thereafter). That's the shaded blue area.

More accurately, the shaded blue area shows the range of possible emissions scenarios that are likely to keep us below 2°C of warming — there's a range here due to scientific uncertainties in estimating climate sensitivity. We might have to cut more than 14 percent. We might be able to get away with cutting a bit less. But that's the ballpark figure.

The chart also compares this 2°C goal with a bunch of emissions scenarios. The dotted line is the "business-as-usual" case, showing what will happen if humans keep burning fossil fuels and emitting greenhouse gases at their current rate. In that case, global temperatures are likely to rise more than 2°C above pre-industrial levels this century. The World Bank has outlined reasons this would be bad news here.

Current climate promises don't come close to the 2°C goal: The U.N. report also looks at various pledges that countries have made to cut emissions between now and 2020.

Some of these promises are weak. Others are backed by strict laws. Others are in between. The Obama administration, for instance, has vowed to cut U.S. emissions 17 percent below 2005 levels by 2020. But it's unclear if that will actually happen — a lot depends on how stringent forthcoming carbon limits on coal and natural-gas plants will be.

A few wind turbines aren't going to cut it. (Ken James / Bloomberg) A few wind turbines aren't going to cut it. (Ken James / Bloomberg)

Now look at the line in the big chart above marked "Case 4." This is a scenario in which the world's nations actually follow through on their most ambitious pledges and obey strict greenhouse-gas accounting rules — in other words, no cheating with gimmicky offsets. Even then, the world wouldn't be on pace to meet its 2°C target by 2020.

The difference between meeting the 2°C goal and the world's actual policies is known as the "emissions gap." As the gap gets bigger each year, the U.N. report notes, the task of meeting that 2°C target gets harder and more expensive.

"Failure to invest today in best available technologies and options not only represents a lost opportunity to reduce emissions," the report argues, "it also curtails our ability to reduce them in the near future as high energy use and emission patterns are locked-in for several decades."

Could the world still meet the 2°C goal? It's not all gloom here. The report does look at how the world's nations could close the emissions gap by 2020 using existing technology.

Some examples: Vehicle fuel-economy standards could get ratcheted up. Energy-efficiency codes for buildings could get strengthened. Regulators could focus on cutting "short-lived" greenhouse gases like methane. Lower-carbon energy sources like wind or solar or nuclear could get scaled up. (The United States has reined in emissions by swapping out coal for cleaner natural gas.) Developing nations could cut back the $480 billion in subsidies for fossil-fuel consumption handed out each year.

There's also a new section in this year's UNEP emissions report exploring agricultural practices that could reduce emissions between now and 2020. That includes expanding the use of no-till farming (which churns up less carbon from soils) to improved water management in rice production (which would cut methane emissions).

The study, drafted by 44 groups of scientists around the world, argues that most of these near-term technologies are readily available and capable of being scaled up quickly. But, for now, the world is far from taking full advantage of them.

Note also that the report is mainly laying out a road map to cut emissions between now and 2020. But to stay below 2°C, global greenhouse-gas emissions would have to keep falling in the decades thereafter. That could prove even more difficult — especially since the world isn't even on track to meet its short-term targets.

Related: Global carbon emissions grew more slowly in 2012. But will they ever decline?