US President Barack Obama has unveiled even steeper greenhouse gas cuts from American power plants than previously expected.
The cuts will set a challenge to the rest of the world to take serious action as a global summit to finalise a landmark climate change treaty approaches at the end of this year.
Mr Obama's actions on climate change have faced opposition from Republicans and even some Democrats, who fear the impacts on the US economy.
The issue is already being raised among candidates for next year's presidential election, as much of the work will lie with his successor. Democratic front-runner Hillary Clinton has already expressed support.
Opponents plan to sue immediately, and to ask the courts to block the rule temporarily. The National Mining Association yesterday asked the Environmental Protection Agency to put the rule on hold while legal challenges play out.
The Obama administration last year proposed the first greenhouse gas limits on existing power plants in US history, triggering a year-long review. These final changes aim to address concerns raised by both environmentalists and the energy industry.
Some changes further cut the heat-trapping gases blamed for global warming. Other changes delay implementation and eliminate certain options that states could use to show they're cutting emissions, making it harder to comply.
Some states will be given a more lenient target, while others will have tougher targets to meet. The Obama administration has yet to disclose those state-specific targets. Many states, however, have threatened not to comply.
Power plants will have to attain an even lower level of carbon dioxide pollution to be in compliance. Mr Obama's proposal from last year set the target as a 30pc nationwide cut by 2030, compared to the levels in 2005. His new plan calls for a 32pc cut in the same time period.
Left unchanged is Mr Obama's overall goal for US emissions cuts from all sources of pollution.
Power plants account for roughly one-third of all US emissions of the heat-trapping gases blamed for global warming, making them the largest single source.
Mr Obama's revised plan relies more heavily on renewable energy sources like wind and solar replacing dirtier coal-fired power plants.