Many countries have publicly gone on record as supporting the goal of limiting future climate change to two degrees Celsius. Although that creates some risks for future generations, it avoids some of the potentially catastrophic changes that would come with a four degree Celsius rise.

Given what we know about the sensitivity of the climate to added greenhouse gases, it's possible to calculate how much more carbon dioxide we can admit while still having a reasonable chance of staying within the two degree Celsius envelope. What's striking about these calculations is how many large changes we'll have to make in order to get there. According to Jeffrey Sachs of Columbia University's Earth Institute, the per-capita emissions would have to drop from five tons annually (where they are now) to 1.6 tons by 2050.

To accomplish this, Sachs says that all nations will have to undergo a process he calls "deep decarbonization," which is part of the title of a report he's helped organize and deliver to the UN today. Pathways to Deep Decarbonization, prepared by researchers in 15 different countries, looks into what's needed to achieve sufficient cuts in our carbon emissions. The report finds that current government pledges aren't sufficient, and the technology we need to succeed may exist, but most of it hasn't been proven to scale sufficiently.

The 15 countries involved are Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Japan, Mexico, Russia, South Africa, South Korea, the UK, and the US. Collectively, they currently produce 70 percent of the annual greenhouse gas emissions. They also represent economies at various stages of development, each with distinctive challenges in cutting their carbon emissions to the required levels. Groups from each country analyzed what would be needed to do so.

Achieving this, the report's authors argue, will have to come with a normal pace of economic growth: "There is no prospect of winning the fight against climate change if countries fail on poverty eradication or if countries do not succeed in raising the living standards of their people." Although this may add to the challenge of lowering carbon emissions, the report concludes that "Robust economic growth and rising prosperity are consistent with the objective of deep decarbonization."

The report identifies what Sachs called "three pillars" of emissions reductions: low-carbon electricity, massive efficiency gains, and a greater electrification of transit and infrastructure. (Sachs also added that land use changes could also have a major impact.)

There are some differences in the precise implementation approaches among the countries involved. For example, the researchers from India focus on renewable and nuclear power, in part because they have less of a commitment to a fossil fuel infrastructure and in part because the potential for carbon capture and storage in India hasn't been examined. In contrast, the plans for Canada, China, Indonesia, Mexico, Russia, and the UK all include a heavy reliance on carbon capture and storage.

But the most important takeaways are the general lessons: every country will have to rely on technologies that aren't yet fully developed. Decarbonization strategies for things like freight and air transport aren't well developed. It's possible that some of this could be electrified. But air travel especially is likely to end up relying on advanced biofuels, which are still in the research stage. Pilot carbon capture and storage projects exist, but we have no real sense of the challenges of scaling them to a nationwide effort.

The list identified by the report's authors goes on. Electric vehicles are just starting to be scaled up, and they don't currently have the range to handle as many use cases as we'd need them to. Flexible load management on the electric grid, needed for high levels of intermittent renewable power, is also just in the testing phase. We'll also need a form of advanced nuclear power that's safe enough that it "sustains public confidence and support."

All of these require some combination of further research and government incentives to get them up to scale as quickly as possible. The report is careful to argue that the incentives have to be focused on emissions reductions, rather than any particular technology for achieving them: "Directed technological change should not be conceived as picking winners, but as making sure the market has enough winners to pick from."

The report also calls on governments to start acting like they're serious about achieving their stated goals. "Decisions made today with regards to, say, power generation and transport infrastructure," it states, "will have a long-term impact on future [greenhouse gas] emissions, which must be mapped out carefully and understood quantitatively."

Sachs was a bit more forceful as he discussed the report with the press, saying he wished that governments would get serious about cutting into carbon emissions and "stop promoting little gimmicks as a solution." More generally, Sachs said, the fact that this report was so far ahead of what government planning has achieved shows just how limited their efforts to tackle the challenges has been.

And Sachs really drove home how urgent the effort has to be, saying the goal of keeping things under two degrees Celsius is "achievable, but just barely." He pointed out that we do have the resources—he said we devote over $30 billion dollars a year toward biomedical research, but less than a tenth of that to research into low-carbon energy. It's just that we're choosing not to use them at a pace that will help us achieve our goals.