If we want to avoid severe global warming, we'll have to stay within a strict carbon budget in the decades ahead. That won't be easy. There are already 14 major fossil-fuel projects being planned worldwide with the potential to blow past that budget, a new report argues.

The key word here, however, is "potential." The report (pdf), from Greenpeace and Ecofys, identifies 14 of the largest fossil-fuel projects in development. That includes China's big coal-mining plans for its western provinces; proposed oil and gas drilling in the Arctic; the revival of Iraq's oil industry; and the current tar-sands boom in Canada. It also includes slated increases in coal exports from Indonesia, Australia and the United States.

In theory, the climate implications could be quite significant. The International Energy Agency estimates that the world can only burn about a third of its proven oil, gas, and coal reserves to have a good chance of keeping global warming below the 2°C threshold. And according to Ecofys, these 14 projects alone have the potential to burn through 30 percent of those reserves by 2030 — making those climate goals extremely difficult to reach.

But in the real world, things are likely to be much messier. Some of the fossil-fuel projects listed will likely displace other, existing fossil fuels. For instance, the report notes that new shale-gas production in the United States could add as much as 280 million tons of carbon-dioxide by 2020. That's a lot. But if natural gas is replacing U.S. coal consumption — as is currently the case — then it's actually helping to cut carbon emissions. (If that gas displaces renewable energy, however, then it's adding new carbon)

What's more, some of these planned projects could even undercut each other. The report highlights proposals for five new coal export terminals in the U.S. Pacific Northwest. Up to 190 million tons a year of coal could get shipped through these ports by 2020 — containing as much carbon as 100 million cars. But as Michael Levi has argued, the precise carbon impact depends on whether U.S. coal exports adds to or replaces coal from China, Indonesia, and Australia.

Still, setting aside the debates over exact numbers, there's a broader point embedded within the report. Many climate scientists have argued that the world needs to stick to a strict carbon budget to keep future warming below 2°C  — the National Research Council estimates that we can only emit another 500 gigatons of carbon total. (The world emitted about 32 gigatons in 2011 alone.) Every new fossil-fuel project that comes online has the potential to chew through more of that budget.

In the old days, the dominant climate strategy was to set up some sort of international treaty to manage that carbon budget in an orderly fashion. That's what the Kyoto Protocol and its successors was supposed to do — countries would set caps on emissions that gradually declined over time. But those climate talks have faltered in recent years.

So, nowadays, environmentalists are focused instead on trying to limit new expansions of fossil-fuel production. That's the logic behind the opposition to the Keystone XL pipeline, which would boost Canadian tar-sands production. It's the logic behind the protests over the coal-export terminals in the Pacific Northwest. With cuts to carbon consumption off the table for now, the green focus has shifted to keeping as much coal, oil and natural gas in the ground as possible.

Further reading:

--This post by the University of Chicago's Raymond Pierrehumbert is a good introduction to the idea of carbon budgets. (He discusses it in the context of Keystone XL.)

--Michael Levi has written a couple of interesting posts arguing that both environmentalists and fossil-fuel companies have good reason to be unhappy with the current climate strategy (no policies to cut carbon consumption, lots of protests over any new production of fossil fuels). He's sketched out some possible areas of compromise.