On Monday the U.S. Environmental Protection Agency is expected to unveil proposed regulations that could be the biggest step the U.S. has taken yet toward dealing with climate change. The regulations would limit emissions from existing power plants, which currently account for 40 percent of all carbon dioxide emissions in the United States, by far the biggest source.

“There’s a good chance that when history is written, this will be seen as the moment when the U.S. fully committed to combating climate change,” says Michael Greenstone, professor of environmental economics at MIT. “It’s a tremendous step forward.”

Regulations on power plants have long been seen as the second-best option for reducing greenhouse gas emissions. Economists say the cheapest route would be comprehensive energy and climate legislation covering emissions across the entire economy, providing flexibility in how emissions are reduced, and incorporating funding for R&D to develop low-carbon technologies.

While regulations aren’t ideal, they may be the only way to move forward on U.S. climate policy. And the new rules are expected to be far more flexible than some past regulations, keeping down costs. The regulations might also help revive efforts at international cooperation on climate change. The U.S. could point to the EPA rules in international negotiations, promising stricter limits in the future if other companies also take steps to reduce emissions.

Although it remains unlikely that we’ll see the single, comprehensive global treaty to reduce emissions that U.N. negotiators have been attempting for 20 years, collaborations between even a few countries could have big results. Just 10 countries account for nearly 70 percent of total emissions. The EPA regulations could also help propel investment in low-carbon technologies, such as renewable energy, as utilities seek to reduce emissions.

“The regulations have been a long time coming,” says Megan Ceronsky, director of regulatory policy at the Environmental Defense Fund. Legal battles around whether the EPA has authority to regulate power plants under the Clean Air Act have been going on since at least 2005 (two Supreme Court rulings now say it does). Yet one of the biggest reasons the proposed regulations haven’t been released is that policy experts were holding out for comprehensive energy and climate legislation while using the threat of EPA regulations to get opponents to sign on.

Needless to say, that strategy failed. Congress came closest to success in 2009, when optimism for action on climate both in the U.S. and internationally may have been at an all-time high. The House passed legislation that would have established a cap on emissions and a system for trading pollution permits, and included funding for R&D. But the legislation stalled in the Senate, and that failure helped undermine efforts later that year to achieve a global treaty.

The new regulations will be relatively flexible, at least in comparison to another set of proposed regulations on carbon dioxide emissions from new power plants. While, technically speaking, the proposed regulations on new plants don’t mandate the use of a specific technology, in practice the only technology that can meet the emissions rules captures and stores carbon dioxide, much the way plants capture mercury or sulfur dioxide now.

In contrast, the new rules, which will address existing power plants, may not require power plant owners to add any new technology. States will decide for themselves how to keep below the EPA’s emissions limits for power plants, and depending on the wording of the rules, they may be able to do things like employ energy efficiency measures to decrease overall power use, install wind turbines to displace fossil fuel power, or change grid operation to use inefficient power plants less. States may also collaborate, trading credits for emissions reductions with each other.

When the proposed rules come out, policy experts will scour them to figure out just how much flexibility there is. The proposed rules will be open to public comment, with the final rules to be established next year. States will then have some time to draw up plans for how they will comply.

The regulations will almost certainly face legal challenges, and may need to be modified. But in light of the Supreme Court rulings upholding the EPA’s authority to regulate greenhouse gas emissions, “it’s very unlikely the courts will completely strike down the EPA efforts here,” says Jason Schwartz, legal director for the Institute for Policy Integrity at the New York University School of Law.

The EPA has already taken steps to reduce emissions from cars by establishing new fuel economy regulations. After the power plant regulations, it will likely move on to other sectors of the economy.

“If we needed to, we could get where we need to go with emissions reductions using a regulatory framework,” Ceronsky says. “It’s the single greatest opportunity under existing authority to drive down emissions of greenhouse gases.”