With the United States, the world's second-largest emitter of greenhouse-gas emissions after China, walking away from the accord, other countries will presumably have to ramp up their ambitions still further if they want to avoid the prospect of dangerous warming.
"Avoiding a 2-degree warming was already hard when all of the key countries were rowing together," said Michael Oppenheimer, a climate researcher at Princeton University. "With the U.S. becoming a climate outlaw by withdrawing from Paris, that target becomes nearly impossible.
"It looks like Trump has condemned the U.S., the rest of the world, and future generations to live in the climate danger zone," he said.
It's only 2017, so projecting the greenhouse-gas emissions of disparate countries over many decades, and how they will change the planet's climate, is no exact science.
It's possible that other countries could find a way to compensate for the U.S. withdrawal — or even that a future U.S. administration would reverse Trump's action soon enough to avert a lot of climate damage.
It's also possible that actions by states such as California, or even individual cities and major corporations, could stabilize U.S. emissions, no matter what the Trump administration does on the federal level. Or a super-fast-moving renewable-energy transition, rather than the current slow and steady one, could come.
As Trump broke with the Paris agreement, New York Gov. Andrew M. Cuomo (D) rolled out a plan Thursday to invest $1.5 billion in renewable energy and energy efficiency.
Cuomo said the initiative would create 40,000 jobs in the next three years — or twice the number of mining and logging jobs in West Virginia, according to Bureau of Labor Statistics figures.
Other U.S. states and businesses are continuing to move ahead with efforts to cut emissions.
Still, there's no way to read the U.S. departure as good news for the 2-degree goal. The commitments made by countries in Paris, on their own, did not achieve sufficient reductions in greenhouse-gas emissions to meet the agreed-upon target.
Some scientists have been skeptical of the Paris target for some time, simply because there's only a finite amount of carbon dioxide that humans can put in the air before the Earth is consigned to a 2-degree rise in temperature.
That "carbon budget" gets narrower every year.
"There's so many things that need to go right for 2 degrees. Essentially, we've emitted too much, which makes the 2-degree challenge hard," said Glen Peters, a senior researcher at the Center for International Climate Research in Oslo.
Climate policy experts note that U.S. emissions cuts were set to make up a major part — more than a fifth — of the reductions envisioned under the Paris accord between now and 2030.
An analysis by the think tank Climate Interactive estimated that the U.S. pledges would account for 21 percent of the total expected emissions cuts out to the year 2030 under the Paris agreement.
That's assuming that the international community remains committed to the 2-degree goal.
Countries also agreed on a more aspirational goal at Paris to try to limit warming to 1.5 degrees, which was seen as a safer level for low-lying island states and other more-vulnerable countries.
"President Trump's withdrawal from the Paris Agreement, combined with the repeal of domestic actions resulting in halting the decline in U.S. emissions, will likely make it more difficult and costly overall to meet the Paris Agreement temperature goal of holding warming well below 2°C, and limiting it to 1.5°C," according to an analysis written by Bill Hare, a climate scientist and the chief executive of Climate Analytics, a group that analyzes climate change scenarios.
Warming of more than 2 degrees would have dramatic consequences: The planet's ice sheets would be far more likely to melt, triggering more sea-level rise, than at 1.5 degrees, which is considered the safer limit, according to Hans Joachim Schellnhuber, a physicist who heads the Potsdam Institute for Climate Impact Research in Germany.
"Particularly on sea-level rise, every tenth of a degree really matters," Schellnhuber said. "We find that with 1.5 degrees warming, you probably will get in the end something below a one-meter sea-level rise and then it stabilizes; with 2 degrees it just keeps on rising, because many of the ice sheets kick in."
Other major climate impacts at 2 degrees include severe threats to coral reefs across the globe, a greater risk of long-lasting heat waves and extreme rainfall events, and the risk of lower yields for key crops such as wheat in tropical regions.
Climate Interactive has estimated that the United States on its own would account for an additional 0.3 degrees Celsius (0.5 degrees Fahrenheit) of warming by the year 2100 if it leaves the Paris deal and takes no actions to reduce emissions for the rest of the century as other countries fully live up to their current pledges.
Schellnhuber similarly puts that number at a few tenths of a degree Celsius under this scenario.
However, it would be even more consequential if the U.S. departure from the Paris agreement caused other countries to weaken their commitments or lower their ambitions.
Schellnhuber thinks that that is unlikely for now — and that other countries will keep on pushing to cut their emissions.
If that is the case, those efforts may be able to offset the U.S. withdrawal from the agreement.
Niklas Hohne, a professor at Wageningen University in the Netherlands and a founder of the NewClimate Institute, agreed that the U.S. move worsens the prospects for meeting the 2-degree target. But he also said that he was seeing progress on emissions cuts in India and China that could be large enough to offset backsliding by the United States.
"We've looked at recent developments in China and India, and they are actually overcompensating for the potential increase in emissions in the U.S.," he said. "That's because renewables are happening much faster in China and India, they are replacing coal much faster, and that leads to significantly lower emissions."