The Trump administration’s efforts could add billions of tons of climate pollution.

The plan: Federal agencies are marching ahead with long-expected plans to halt the rise in federal vehicle emissions standards, according to Bloomberg. The proposal, which is expected to be issued as early as this week, will also seek to withdraw Environmental Protection Agency waivers that have allowed California to implement its own, higher set of vehicle emissions rules, the news service reported. That could severely limit the ability of states to set and meet their own clean-energy goals.

How big of a difference could it make? The Obama administration’s decision to lock in rising fuel economy standards was a critical plank in the plan to meet the nation’s goals under the landmark Paris climate agreement, from which President Trump has vowed to withdraw. As MIT Technology Review  previously reported, the EPA estimated in 2012 that increasingly efficient cars and personal trucks would eliminate 140 million tons of greenhouse gases in 2025. That figure would climb to nearly two billion tons over the lifetime of vehicles made in the model years 2017 to 2025.

On the other hand ... Unraveling fuel economy standards and eliminating the California waiver will both entail an extended process of public notice and comment, Danny Cullenward, an energy economist and lawyer at the Carnegie Institution, previously told MIT Technology Review. California and 16 other states have already filed a lawsuit challenging the Trump administration’s efforts to roll back fuel standards, and other legal battles are likely to follow.