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How's this for a New Year's resolution: Act quickly to reduce greenhouse gas emissions—while there's still time to prevent a level of global warming that would make parts of the planet too hot to inhabit, melt glaciers that provide water to billions, flood many of the world's coastal cities and push mass migration to a full blown crisis.

It can't be accomplished in a single year, of course. But there isn't much time.

We have about three years left to bend global greenhouse gas emissions to a downward trajectory if we hope to meet the goals of the Paris climate agreement, a group of leading climate experts warned in the journal Nature last June. In an article that was both urgent and optimistic, they argued that the daunting task can be met using technologies that are already at hand.

"When it comes to climate, timing is everything," the group, including former UN climate chief Christiana Figueres and Hans Joachim Schellnhuber, head of Germany's Potsdam Institute for Climate Impact Research, wrote. "If we delay, the conditions for human prosperity will be severely curtailed."

So, as 2018 begins, here are some of the goals for 2020 that the optimists presented:

Boost the world's renewable electricity generation to 30 percent of total supply.

Begin to retire all remaining coal-fired power plants.

Expand sales of electric vehicles to 15 percent of new cars.

Provide $1 trillion in financing each year—public and private—for climate action.

Those efforts can help reach a peak in global emissions by 2020, but it's only a start.

To have a good chance of avoiding dangerous warming, scientists say, emissions of greenhouse gases must effectively hit zero within the next few decades—at the latest, sometime in the second half of this century. If we can't halt the burning of fossil fuels that rapidly, and many people think we won't, we'll likely need widespread use of technologies that capture carbon dioxide from smokestacks and bury it in the ground, or find other ways of removing it from the atmosphere.

We Are Not on Track

An International Energy Agency analysis, which assumes all nations will meet the pledges they made as part of the Paris climate agreement, projects global CO2 emissions in 2040 will be slightly higher than today.

The most recent "gap report" of the United Nations Environment Program, an annual estimate of how far we are falling short, found that without further cuts, the world would likely warm 3 degrees by 2100. The Paris goal is to limit warming to well below 2 degrees Celsius since the start of the industrial era.

According to the World Resources Institute, the United States is very likely to fall short of its targets, which the Trump administration has abandoned. It would have fallen short even under President Obama's policies, which Trump is now working hard to undo, including its centerpiece, the Clean Power Plan, which was designed to regulate emissions from power plants.

Signs of Hope

There are glimmers of hope, however. In the United States, many states, cities and companies have pledged that they are "still in" for meeting the Paris goals.

Global emissions from fossil fuels, cement and other industrial sources, which make up about 70 percent of all greenhouse gas emissions, had held steady for three consecutive years—before increasing again last year by an estimated 2 percent.

Anthony Hobley, chief executive of Carbon Tracker, a British research and advocacy group, said we're in the midst of an energy "paradigm shift" as wind, solar and natural gas become cheaper than coal.

"I think the good news is the direction of travel is clear," said Hobley, one of the authors of the Nature paper. "The challenge is to speed up that transition and make it go faster, and that's where all the efforts on policy and advocacy should be focused."

Kelly Levin, a senior associate at the World Resources Institute, said that while there are some modeling scenarios suggesting it's still possible to limit warming to less than 2 degrees even if we delay action, it would require more drastic cuts that would imply much greater costs and risk of economic disruption.

That would probably involve a greater reliance on what are called "negative emissions technologies," such as carbon capture and sequestration, or far-reaching changes in agriculture and forestry. It's a tempting idea—for the first time, it got a chapter in the UN's gap report. There's just one problem: despite decades of research and pilot projects, no one has yet been able to make this technology work on a larger scale at a cost that's practical.

So, watch closely to see if emissions continue rising this year.

"We are going to have to deal with a lot of disruption, and of course the people who will get hit the hardest are the poorest and most vulnerable," Hobley said. All the other pressing global issues—poverty, migration, disease—he said, "all of those things will suffer, and we'll lose progress in all those areas if we have an unstable and disruptive climate."

For now, it's not too late.