The climate agreement reached in Paris last weekend has been hailed as a landmark in the global effort to reduce greenhouse gas emissions, and it could well turn out to be one.

But the accord’s lofty goals won’t be achieved without large corporations making big changes. And while many companies have welcomed the deal and voluntarily pledged to cut emissions, the sweeping reforms required to avert a sharp rise in global temperatures will almost certainly require substantial new government regulations.

In the United States, the goal is to reduce greenhouse gas emissions 26 to 28 percent from 2005 levels by the year 2025. To achieve that, the Obama administration is being forced to count mainly on several laws that are already on the books, rather than pursue new regulation.

That’s because the Republican-controlled Congress has vowed to block any climate legislation and to rescind the laws already in place. Senator Mitch McConnell of Kentucky, the Republican majority leader, said last week that President Obama was “making promises he can’t keep,” warning that the Paris agreement “is subject to being shredded in 13 months” if Republicans win the White House.