Back in 2015, the world’s governments met in Paris and agreed to keep global warming below 2°C, to avoid the worst risks of a hotter planet. See here for background on why, but that’s the goal. For context, the planet’s warmed ~1°C since the 19th century.

One problem with framing the goal this way, though, is that it’s maddeningly abstract. What does staying below 2°C entail? Papers on this topic usually drone on about a “carbon budget” — the total amount of CO2 humans can emit this century before we likely bust past 2°C — and then debate how to divvy up that budget among nations. There’s math involved. It’s eye-glazing, and hard to translate into actual policy. It’s also a long-term goal, easy for policymakers to shrug off.

So, not surprisingly, countries have thus far responded by putting forward a welter of vague pledges on curbing emissions that are hard to compare and definitely don’t add up to staying below 2°C. Everyone agrees more is needed, but there’s lots of uncertainty as to what “more” means. Few people grasp how radically — or how quickly — we’d have to revamp the global economy to meet the Paris climate goals.

Surely there’s a better, more concrete way to think about this. So, in a new paper for Science, a group of European researchers try to do just that — laying out in vivid detail what would have to happen in each of the next three decades if we want to stay well below 2°C. Fair warning: It’s unsettling.

A simple (but daunting!) road map for staying below 2°C

They start with the big picture: To hit the Paris climate goals without geoengineering, the world has to do three broad (and incredibly ambitious) things:

1) Global CO2 emissions from energy and industry have to fall in half each decade. That is, in the 2020s, the world cuts emissions in half. Then we do it again in the 2030s. Then we do it again in the 2040s. They dub this a “carbon law.” Lead author Johan Rockström told me they were thinking of an analogy to Moore’s law for transistors; we’ll see why.

2) Net emissions from land use — i.e., from agriculture and deforestation — have to fall steadily to zero by 2050. This would need to happen even as the world population grows and we’re feeding ever more people.

3) Technologies to suck carbon dioxide out of the atmosphere have to start scaling up massively, until we’re artificially pulling 5 gigatons of CO2 per year out of the atmosphere by 2050 — nearly double what all the world’s trees and soils already do.

“It’s way more than adding solar or wind,” says Rockström. “It’s rapid decarbonization, plus a revolution in food production, plus a sustainability revolution, plus a massive engineering scale-up [for carbon removal].”

So, uh, how do we cut CO2 emissions in half, then half again, then half again? Here, the authors lay out a sample “roadmap” of what specific actions the world would have to take each decade, based on current research. This isn’t the only path for making big CO2 cuts, but it gives a sense of the sheer scale and speed required:

2017-2020: All countries would prepare for the herculean task ahead by laying vital policy groundwork. Like: scrapping the $500 billion per year in global fossil fuel subsidies. Zeroing out investments in any new coal plants, even in countries like India and Indonesia. All major nations commit to going carbon-neutral by 2050 and put in place policies — like carbon pricing or clean electricity standards — that point down that path. “By 2020,” the paper adds, “all cities and major corporations in the industrialized world should have decarbonization strategies in place.”

2020-2030: Now the hard stuff begins! In this decade, carbon pricing would expand to cover most aspects of the global economy, averaging around $50 per ton (far higher than seen almost anywhere today) and rising. Aggressive energy efficiency programs ramp up. Coal power is phased out in rich countries by the end of the decade and is declining sharply elsewhere. Leading cities like Copenhagen are going totally fossil fuel free. Wealthy countries no longer sell new combustion engine cars by 2030, and transportation gets widely electrified, with many short-haul flights replaced by rail.

In addition, spending on clean energy research increases by “an order of magnitude” this decade, with a sustained focus on developing new batteries, drastically reducing the cost of carbon capture and storage (CCS), and perfecting low-carbon processes for producing steel and concrete, plus improving smart grids, greener aircraft systems, and sustainable urbanization techniques.

Meanwhile, efforts to start pulling carbon dioxide out of the air start this decade. That means reforesting degraded land and deploying technologies such as direct-air capture or bioenergy with CCS to pull CO2 out of the atmosphere. By 2030, we’d need to be removing 100 to 500 megatons of CO2 each year and have a sense of how to scale up.

2030-2040: By this decade, hopefully, we’re reaping the fruits of major technological advances in clean energy. Leading countries like Denmark and Sweden should now have completely carbon-free grids and have electrified virtually all of their transport, heating, and industry. Cars with internal combustion engines “will have become rare on roads worldwide.” (Let that sink in.) Aircraft will be almost entirely powered by carbon-neutral fuels, say, biofuels or hydrogen. New building construction will be largely carbon-neutral, by using emissions-free methods for steel and concrete or through other techniques. And “radical new energy generation solutions will enter the market.”

Meanwhile, we’d need to be sucking about 1 to 2 gigatons of CO2 from the air each year, with a heavy R&D effort on expanding that further.

2040-2050: By the early 2040s, major European countries are close to carbon-neutral, and the rest of the world is moving toward that goal by the end of the decade. Electricity grids are nearly entirely carbon-free: “Natural gas still provides some back up energy, but CCS ensures its carbon footprint is limited. Modular nuclear reactors may contribute to the energy mix in some places.” Lower-income countries are still using some fossil fuels, and the world is still emitting a small bit of CO2 in 2050 (about one-eighth the amount of today), but work continues on eventually phasing that out.

Finally, by 2050, we’d need to be removing more than 5 gigatons of CO2 per year from the atmosphere. It’s possible this is simply impractical — if we tried to do that all by burning biomass for energy and sequestering the resulting carbon (a “negative emissions” process), we might well run into serious land constraints that hinder agriculture. If, in the 2020s, we realize this will be the case, then we’ll have to revamp the road map to cut CO2 emissions from energy and industry even faster.

The paper also notes that the precise details of any road map will be tentative — after all, the nature of unpredictable technological change means it’s difficult to say what the world will look like in 2030 or 2040 or 2050. So policymakers will need to meet regularly, take stock of where they are, and revise as needed.

This road map is staggering. That’s the point.

It’d be entirely understandable to look at this all and say, “That’s insane.” Phasing out sales of combustion engine vehicles by 2030? Carbon-neutral air travel within two decades? Cities going entirely fossil fuel–free in the next 13 years? Come on.

And fair enough. None of this is easy. It might well prove impossible. But this is roughly what staying well below 2°C entails — at least without large-scale geoengineering to filter out sunlight and cool the planet (a risky step). This is what world governments implicitly agreed to when they all signed on to the Paris accord.

“We wanted to show what meeting those Paris goals requires,” says Rockström. “Up until now, we felt that scientists haven’t been very effective in communicating what these carbon budgets actually mean in terms of concrete action.”

Rockström and his colleagues argue that future UN climate talks should strive to create a much more detailed decade-by-decade road map along the lines of their Science paper, in order to gain much more clarity on what needs to happen to stay below 2°C.

Rockström adds that the road map’s sheer difficulty doesn’t mean climate action is hopeless. “You could just as easily see this becoming a self-fulfilling prophecy,” he says. “Countries start taking these targets seriously and then begin pursuing the innovation needed to make this come true.” That’s what Moore’s law did for the semiconductor industry; the prediction that chip performance would double every 18 months helped guide firms in thinking what they needed to do to make that come true. A “carbon law,” Rockström argues, could do the same for countries and cities and companies.

Oliver Geden — a German climate policy analyst who wasn’t involved in the Science paper but who has criticized scientists and policymakers for obscuring what the 2°C target really requires — praised the broad approach here, though noted that some of the details were debatable.

“One thing I like is that this is not just another global calculation [on CO2 emissions] that doesn’t talk about actors or policies,” Geden told me by email. “I think this should be the way forward, translating [overarching climate goals] into ‘policy portfolios’ and then asking policymakers if they are going to do it or not.”

For example, the paper lays out a specific timeline for deploying technology to remove carbon dioxide from the atmosphere. Most modeling scenarios for staying below 2°C now envision massive CO2 removal efforts, but few policymakers have acknowledged this fact. Presenting them with a detailed proposed timeline could, hopefully, change that. If it turns out that scaling up bioenergy with CCS is logistically impossible (as it might be), then at least we’d come to terms with that sooner, rather than keeping it as an unspoken background assumption in broad climate plans.

Of course, it’s possible that if policymakers really grappled with what staying below 2°C entails, they might come away thinking it’s impractical or undesirable. They might decide to accept more global warming, say, 2.5°C or 3°C or more, and deal with the severe risks that result, from higher sea-level rise to droughts to crop failures. (I’ve written more on that here.)

But something has to force that conversation. If this 2°C climate goal is going to loom over every international climate meeting, every white paper and discussion, then the least people can do is take it seriously.

Further reading

— Here is a history of the 2°C global warming target — and what it would mean to miss it. And note that no country in the world is currently taking the goal seriously.

— This new paper by Jesse Jenkins and Samuel Thernstrom looks at the research around achieving deep decarbonization in the electricity sector. Note that while it might be physically possible to decarbonize the grid using only renewables, a number of studies suggest it’d be much more cost-effective to harness nuclear power or coal/gas with carbon capture and storage as well.

— The Science paper is also a good framework for thinking about Donald Trump. Trump, recall, wants to dismantle US climate policies and slash clean energy research. In the short term, that probably won’t hamper the incremental decline in US emissions already underway, as natural gas and renewables keep pushing out coal in the power sector. But Trump’s policies could easily hinder the push for deep decarbonization in the US — or at least delay it until well after 2020, making the 2°C goal all the harder.

— Over at Carbon Brief, Jocelyn Timperly dissects a new International Energy Agency report showing how we might stay on a 2°C pathway. It gets at the problem a little differently (and mainly focuses on short-term energy policies), but is basically compatible with the Science paper’s analysis.