Climate negotiators gathering in Germany this week are still flush with the success of the Paris Agreement two years ago. But as they begin assembling a rule book for ensuring that the national pledges made in Paris are fulfilled, there comes a hard dose of reality. Those pledges, which constrain greenhouse gas emissions from now to 2030, will only deliver a third of the cuts needed to put the world on track to keep warming below the promised 2 degrees Celsius.

And as for 1.5 degrees? Forget it, says a report from the United Nations Environment Programme released last week.

The Paris deal was the end of a long negotiation to find a formula for halting arguably the biggest threat to human society in the 21st century: climate change. It was a victory for climate diplomacy. But the climate responds to actions on carbon emissions, not hot air in conference halls.

So how are we doing? Do White House attempts to pull the U.S. out of the deal threaten to bring it crashing down? Is the deal itself too weak to forestall future warming? Or – with wind turbines and solar panels spreading across the planet, and electric cars zipping from test rigs to showrooms – could we be on the verge of fixing the climate?

Nobody imagined the pledges made by nations in Paris would on their own deliver the declared temperature promise. The imperatives were more diplomatic than climatic. After past failures to create a hard deal with fixed emissions targets, canny French diplomats took the soft route. Like organizers of a charity fundraiser, they cajoled rather than hectored, asking countries what they would be willing to pledge.

The fundraiser went surprisingly well. The conference agreed to keep global average temperatures to well below 2 degrees above pre-industrial levels, and strive to stop at 1.5 degrees. As a down payment on delivery by 2030, Brazil promised a 37 percent cut in its greenhouse gas emissions, the United States 27 percent, and the European Union 40 percent. China promised to peak emissions by 2030, while lowering carbon emissions per unit of GDP by 60-65 percent; and India agreed to get 40 percent of its electricity from non-fossil fuels, such as solar.

Hardened veterans were pinching themselves as they stepped blinking into the Parisian winter sunshine. But last week, the United Nations Environment Programme (UNEP) pronounced with brutal precision just how far those national pledges fall short of delivering the temperature targets.

Even if all the pledges are fulfilled, they will deliver “only approximately one-third of the emissions reductions needed to be on a least-cost pathway for the goal of staying well below 2 degrees C,” a report from the UNEP, one of the Paris deal’s sponsors, concluded. The gap between the climate promise and the emissions pledges was, it said, “alarmingly high.”

Once emitted, most carbon dioxide, the main gas behind the warming, stays in the air for centuries. So to stop warming, the world has to bring emissions down to zero as soon as possible. And how much we emit between now and then will determine where global temperatures end up.

Precise calculations of what is needed come with wide error bars. But the UNEP report concludes that to keep warming below 2 degrees, we have to limit future global emissions to about 1,000 billion tons of CO2. Keeping below 1.5 degrees sets the limit at just 300 billion tons. The report says that if nations stick to their 2030 Paris pledges, 80 percent of the budget for 2 degrees will be used up by that date – and we would already be out of time for keeping below 1.5 degrees.

Without tougher targets, “it is extremely unlikely that the goal of holding warming below 2 degrees C can still be reached,” says the report. We would be on course for at least 3 degrees of warming by the end of the century.

Some researchers quibble with these numbers. And, given the error bars on the precise sensitivity of the climate to CO2, other interpretations are possible.

In September, Oxford University climate physicists Myles Allen and Richard Millar argued that the world is heating more slowly than predicted by most climate models. Adjusting for that, they suggested we might be able to emit as much as 880 billion tons of CO2 and still have a good chance of staying below 1.5 degrees. It made the Paris 1.5-degree aspiration “doable,” Allen said, provided CO2 emissions could be brought to zero by mid-century.

This new analysis set off a storm. Climate skeptics claimed it showed the mainstream climate models were wrong. (It didn’t: Allen’s conclusions remained within the models’ error bars.) Some climate scientists, such as Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, argued that Allen and Millar had underestimated warming to date and so overestimated the future emissions budget.

Even given this lingering uncertainty, however, the findings of the UNEP’s Emissions Gap Report are sobering. The world has been making good progress toward decoupling economic growth from carbon emissions. Since 2014, the global economy has grown by about 8 percent, but CO2 emissions have not risen. We might even have reached “peak carbon.” But the report is a reminder that peak carbon does not mean peak temperature. Power stations and other industrial activities continue to add around 36 billion tons of CO2 to the atmosphere each year. Every ton will be responsible for further warming.

Nature has been rubbing home the message in recent months. Last year, despite no rise in human emissions, there was a record surge in the CO2 content of the atmosphere. It leapt from 400 parts per million in 2015 to 403.3 ppm. The extent of the increase was almost certainly a one-off response to an El Nino in the Pacific Ocean. El Niño’s cause droughts and high temperatures across the tropics that stress forests and grasslands, causing a net release of carbon from these ecosystems.

To add to our woes, the slowing in the rate of warming that had held since 1999 – also a result of natural heat cycles in the Pacific — is now over. Following three consecutive years of record temperatures in 2014-2016, “the long-term rate of global warming has now returned to the level seen in the second half of the 20th century,” says Stephen Belcher, chief scientist for the Met Office, the U.K.’s national weather service. The world is now, by most measures, 1 degree or more warmer than in pre-industrial times. We are already at least two-thirds of the way to the 1.5-degree target.

That said, there is good news. A combination of political will, technological advances, and the changing economics of the energy industry are driving dramatic changes that offer the prospect of heading off climate change.

Investments in new, cheaper sources of low-carbon electricity are now twice those in fossil fuels. And there is potentially much more to come.

The Emissions Gap Report finds that faster switching to solar and wind energy, and more energy efficient appliances and cars, coupled with a switch from deforestation to reforestation, could together cut emissions by 22 billion tons of CO2 a year – enough to close the “emissions gap” and put the world back on track to halt warming at 1.5 degrees.

This is not pie in the sky. Witness Morocco’s huge solar complex covering nearly 10 square miles of Sahara desert where they once filmed “Lawrence of Arabia,” which may soon be sending grid power to Europe. Or the world’s first floating wind farm now under construction off Scotland. Or the first electric airplane, which could be flying short-haul passengers between European capitals within a decade.

Meanwhile, the burning of coal, the dirtiest of the major fuels, may already have peaked. China, which drove a global surge in coal for a decade, is burning less than four years ago. The U.S. coal industry is in a two-decade-long decline that few believe even President Trump’s enthusiasm can reverse. India, which was a reluctant signatory in Paris, is now switching to massive investment in renewables, adding 9 gigawatts of solar capacity since Paris. Britain, which was the crucible of the coal-fired industrial revolution two centuries ago, will burn no more coal in power stations after 2025.

The Emissions Gap Report says there are still more than 6,600 coal-fired power stations, most of them in Asia. By the time they close, they could have added 190 billion tons of CO2 to the atmosphere. But they may not. Political will and market signals could banish coal “remarkably quickly,” it says.

We must hope so. The report says that “between 80 and 90 percent of coal reserves worldwide will need to remain in the ground” for targets to be met, compared to 50 percent of natural gas reserves and 35 percent of oil.

For every climate pessimist today, there is a super-optimist. The DNV–GL, an international energy consultancy based in Norway, in September predicted that the global economy was becoming less energy-intensive. According to DNV-GL’s projections, global energy demand could peak as early as 2025 and halve by 2050, as coal, oil, and gas burning all enter long-term decline.

And many optimists pin further hopes on tackling non-industrial contributors to global warming. One of these is deforestation.

Last week, the Woods Hole Research Center in Massachusetts said replacing deforestation with an era of reforestation, especially the revival of forests degraded by piecemeal logging and clearance, “could help get the world at least a quarter of the way to… limiting warming to 1.5 degrees C.” The center’s senior scientist, Richard Houghton, says up to half a billion hectares of degraded forests – an area half the size of Canada – are ripe for restoration without impinging on existing farmland.

The turnaround could take 370 billion tons of CO2 out of the atmosphere by 2100, as trees absorb CO2 to grow, according to Woods Hole researchers. That is the equivalent of a decade’s annual emissions from industrial activity, said the center’s director, Philip Duffy.

Any carbon bonus from reforestation would be a one-off benefit, as forest regrew. Once mature, the new forests would be carbon-neutral. But locking up carbon in forests would buy time to reduce industrial emissions. Houghton has called this “providing a bridge to a fossil fuel-free world.”

Another way of buying time is to limit other greenhouse gases emitted by human activity. Hopes are highest for curbing methane, which currently contributes nearly one-fifth of global warming and has increased sharply in the past decade. The main cause of the increase is thought to be the rising output of methane-generating microbes from tropical agriculture, especially rice paddies and the stomachs of livestock.

Methane only lasts in the atmosphere for about 10 years. So it doesn’t accumulate in the same way as CO2. But tackling those emissions – by breeding livestock that emit less methane and changing rice farming techniques – would give a sharp downward tug on the planetary thermostat. In total, strict controls on methane and other short-lived greenhouse gases could avoid 0.6 degrees of warming by mid-century, according to the Emissions Gap Report.

That could be a big help in keeping warming to 1.5 rather than 2 degrees. But the one thing all agree on is that even if plugging methane and growing forests can buy time, stabilizing climate will require bringing industrial emissions of CO2 to zero as quickly as possible. Until that happens, the world will keep on warming.