(Bloomberg) -- With climate legislation passed through Congress on Friday, the US is on the brink of reviving a long-shot global hope: to limit warming temperatures to 1.5° Celsius (2.7° Fahrenheit).

The 1.5°C warming limit was enshrined into the Paris Agreement in 2015 as a shared goal of world governments. It’s a crucial threshold beyond which heat waves, rain, drought, flooding and sea level rise become increasingly intolerable. But it’s a goal that has become seriously imperiled over the past seven years — in no small part due to the limited efforts of the US, one of the largest single sources of greenhouse gas.

“The 1.5 goal is going to be tough,” said Samantha Gross, director of the Brookings Institution’s energy security and climate initiative. But thanks to this bill, “we’re putting the US on a path to keep that goal possible. That ain’t nothing.”

Pollution reductions the bill helps make possible — a roughly 40% drop in emissions by the decade’s end, according to researchers — represent a substantial individual contribution to the global effort. It equals as much as a gigaton, or one billion tons, of annual carbon dioxide emissions avoided by 2030. That’s the equivalent of France and Germany’s combined annual emissions.

Another way of putting it: “2.5% of current global emissions, just by one country and one law,” said Zeke Hausfather, climate research lead at the payments company Stripe and a contributor to international and US climate assessments. “It’s hard to get much bigger impact than that.”

President Joe Biden is expected to sign the Inflation Reduction Act into law next week, deploying some $374 billion into climate- and energy-related spending in what is the largest US climate law ever. “It’s a piece of legislation that backs up the US government’s commitments to net zero and its commitments to reduce emissions by 2030. And if implemented effectively, it will make a difference,” said Rachel Kyte, dean of the Fletcher School at Tufts University. “It keeps the US in the game and in a leadership position.”

The Biden administration had previously pledged to cut US emissions by at least 50% by 2030 compared to 2005 levels, following scientific guidance. In reality, though, the US has been on track to cut emissions only about 30% in that timeframe, according to modeling by the climate and energy research firm Rhodium Group.

Nearly 200 countries agreed in 2015 to reduce greenhouse gas emissions such that warming since pre-industrial times stays well below 2°C, and set out a preferred target of 1.5°C. As recently as April, however, the UN Intergovernmental Panel on Climate Change warned that the 1.5°C limit was officially on life support. In fact, scientists warned that heating of nearly 3°C remained the likeliest trajectory.

Today, the worldwide average is about 1.2°C above pre-industrial levels.

To stave off far more dangerous levels of warming, the UN-backed scientists made the sharpest call yet for government policies that would drive an immediate shift away from fossil fuels, in order to reach the global peak of new carbon dioxide pollution before 2025. Accomplishing such a sudden shift would require far more action from the US, the largest historical greenhouse gas emitter and the current second-largest annual emitter behind China.Now, for the first time since Paris, the full weight of the US government is starting to move in the direction of 1.5°C. By implementing a climate law instead of an executive order from the White House, the US is also taking an approach that can’t easily be undone by future administrations. “This isn't just a regulation or an executive order, it's a law,” said Gross. “This has more staying power and that's really important.”

But the path to limiting warming to 1.5°C remains uphill and will likely mean overshooting the target at first. In overshoot scenarios, the worldwide average temperatures rise to 1.6°C or higher; later — maybe within a decade — human efforts to deploy widespread nature-based solutions like planting trees and technological tools to capture carbon help pull temperatures back below 1.5°C.

Overshooting is the most likely trajectory, according to the IPCC’s most recent assessment, and it remains to be seen if the new US law will be enough to change those projections. 

Hausfather notes that only nine of the 230 scenarios considered by the UN-backed scientists in their most recent report saw warming stop at 1.5°C without any overshoot. That was “the real nail in the coffin for me,” he said. “You start getting less and less realistic assumptions the longer we take to cut emissions, and that also depends on your willingness to believe that the world might come together and spend trillions of dollars on bringing temperatures back down in the future.”Of course, until a few weeks ago most observers had given up hope that the US would enact a sweeping climate effort. Now the world is more likely to come together on climate spending if the US is leading by example. “Nothing happens in a vacuum,” said Hausfather. “The US embracing more ambitious climate policy makes it easier for other countries to do the same politically.”Research backs this up. In November 2014, the US and China announced a bilateral climate pact that created global momentum, culminating in the Paris Agreement the following year. By comparing countries’ pledges to the global accord with their prior baselines, Rhodium Group researchers were able to estimate the return on the US climate pledge. They found that for every ton of CO₂ the US pledges to curb, other countries commit to reduce pollution by as much as seven tons. This “deliberately conservative estimate,” the authors wrote, grows much larger if they assume that emissions will fall beyond 2030, which is extremely likely. 

If the bill fulfills the high-end projections of reducing a gigaton of CO₂ a year, the Rhodium Group methodology suggests it might lead to seven gigatons worth of cuts in other countries. That makes 1.5°C maybe remotely attainable, even if the US would need to do much more to make it a reality by spending more at home and helping finance the clean-energy transition by other countries. 

Even if the new US bill alone does not satisfy the Paris pledge made by Biden, the IRA “will provide a clear impetus for technology and innovation that will subsequently accelerate emissions reductions,” said Joeri Rogelj, director of climate research at Imperial College London’s Grantham Institute. “And can benefit the wider world when it’s exported.”

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