Donald Trump’s presidency begins today, giving him the ability to act on numerous pledges he has made related to global warming. They include “canceling” American involvement in the Paris climate accord, reviving the coal industry and rolling back federal environmental regulations. If Trump follows through, scientists say it could have a profound long-term effect on the planet.

The public may assume the U.S. and the rest of the world can delay climate action for a presidential term or two, and catch up later. But such a scenario is highly unlikely because cumulative greenhouse gas emissions control temperature rise and effects such as sea level rise and extreme weather. The longer the world waits to reduce emissions, the lower its chances are for limiting warming to 2 degree Celsius, acknowledged as the upper safe limit for the planet. “It’s not like you shift the consequences eight years down the line, and you just hit your target eight years later than planned,” explains Ben Sanderson, a scientist at the National Center for Atmospheric Research. “A decade of reduced ambition would have permanent consequences.”

Average global temperature has already risen at least 0.8 degree C above preindustrial levels, and Sanderson estimates the most current number is likely between 0.9 to 1.3 degrees C. Under the Paris agreement nearly 200 countries agreed to limit the global temperature rise this century to less than 2 degrees C as well as consider ways to reach an even tighter 1.5-degree C target. There is little margin left between where the world is now and where it does not want to go. If the U.S., the world’s second-largest greenhouse gas emitter, fails to act, the world will find it incredibly difficult to meet the 2-degree C goal. “The consequences of a delay in action,” Sanderson says, “are that we permanently miss our target.”

To hit the 2-degree C mark, Sanderson estimates global emissions would have to peak in the next decade, decline to zero by 2060–70, then go negative. In order to help make that happen the U.S. has pledged to cut emissions 26 to 28 percent from 2005 levels by 2025, which would reduce the country’s climate pollution down to between 4.6 billion and 5.5 billion metric tons annually. The latest EPA estimate put U.S. greenhouse gas emissions around 6.9 billion metric tons in 2014.

Reaching the 1.5 degree C goal would be even harder. “What you need to do to achieve 1.5 degrees is incredibly aggressive,” Sanderson says. To get there the world would need to pull off zero emissions by mid-century, and after that countries would have to start removing a huge amount of greenhouse gases from the air. Both of these scenarios assume people will have invented the technology to economically remove those gases on a large-scale. (Currently such a capability does not exist.)

Even if the Trump administration retains all of the U.S.’s current emissions-reducing policies, and carries out all of the proposed ones, there is a good chance the U.S. will still miss its Paris agreement pledge. The nation may still emit an excess 1.6 billion metric tons of carbon dioxide annually over its Paris target in 2025, according to one study in Nature Climate Change. That is because current and proposed policies do not go far enough to reduce emissions. Experts note the rest of the world has not planned enough emissions cuts yet either to reach the 2-degree C goal; part of the Paris agreement is that nations will ratchet up their pledges over time. According to one estimate, nations’ current mitigation policies would still result in a 3.6-degree C increase in average global temperature by the end of the century.

If Trump pulls the U.S. out of the Paris climate accord and follows through on his other declarations, however, he will drive the odds of achieving 2 degrees C even lower. The largest blow to U.S. mitigation efforts will be if Trump rescinds or weakens the Clean Power Plan—a rule that requires power plants to reduce their carbon emissions, which was finalized in 2015 but is currently tied up in court. “I can’t see us reaching our Paris commitment without adopting some form of the Clean Power Plan,” says Rob Jackson, a professor of Earth sciences at Stanford University. The Nature Climate Change study estimates the Clean Power Plan accounts for about half of emissions cuts from the U.S.’s current and proposed policies.

The Trump administration could possibly roll back other climate rules that would increase U.S. emissions, such as automaker fuel-efficiency standards or regulations that limit methane leaks from the oil and gas industry. Trump has also promised to “lift restrictions on the production” of shale, oil, natural gas and clean coal—such a move would increase the market share of fossil-fuel power, and could drive emissions up. Of course, market forces have an immense influence on U.S. emissions, and they are currently favoring natural gas and renewables, and shutting down coal plants. “U.S. emissions have been dropping slowly but steadily for more than a decade,” largely because of those market forces, Jackson explains, “A very plausible scenario would be for us to continue with modest reductions in emissions, but fall far short of the Paris agreement.”

A U.S. withdrawal from the Paris agreement could have further global consequences. Other countries may reduce their ambitions as well—especially because Trump has said he would “cancel billions in payments to U.N. climate change programs.” As Sanderson explains, “Historically, some countries have been unwilling to act on climate without very comprehensive and clear goals from the U.S.”

Sanderson and his colleagues put together an analysis for various global scenarios under an eight-year Trump presidency. They found that if the U.S. delays its emissions goals for eight years and other nations follow suit—so that emissions increase for that period and there is less investment in low-carbon tech—it would result in about 1,300 billion additional metric tons of CO2 emissions globally. That would drop the chances of the world achieving a 2-degree C ceiling by 2100 from 66 percent (with aggressive mitigation from all nations) to about 10 percent, and move the likeliest warming scenario up to 2.5 degrees C. “There’s no wiggle room in the latter half of the century,” Sanderson says. “We can’t compensate later on, because we’re assuming the world is already doing as much as physically possible.”