Biden promises big on climate change. Delivering will be much harder.

On Jan. 19, the United States was the only nation to have renounced the Paris climate accord after four years of an administration committed to rolling back every environmental rule it could.

The next day, President Joe Biden took the oath and reentered the country into the Paris agreement, making clear his intention of addressing global warming with ambition.

He is hosting a climate summit on Thursday.

The president is expected to quantify that ambition:

The Post reported that Biden would commit to reducing U.S. greenhouse gas emissions by at least 50% by the end of the decade.

Such promises are easy. Making good on them, and on this one is hard.

Biden’s pledge to the global community nearly doubles President Barack Obama’s

2015 Paris commitment of 26 to 28% by 2025. This would get the United States much closer to the kind of commitment it must make to keep global warming below 1.5 degrees Celsius, the line scientists warn humanity must not breach. It is also achievable.

Biden’s promise aligns not only with the wishes of major environmental groups, but also with the leaders of a wide range of major U.S. corporations, including Apple, General Electric and Walmart, 300 of whom called last week for a commitment of this size.

The quickly changing economics of power generation will aid this shift, as the costs of wind and solar power have plummeted.

Still, ramping up renewables while maintaining grid reliability is a major challenge.

The United States will require new transmission lines to send electricity from where the wind is blowing, and the sun is shining to where it is not.

Continuing research into large-scale energy storage is needed to enable utilities to bank intermittent renewable energy for later use.

Huge amounts of new wind, solar and, potentially, nuclear and hydropower infrastructure must be built in a short time, and fossil fuel plants must be forced offline before their natural retirement dates.

The electrification of vehicles is an even larger challenge. All this will require tremendous amounts of public and private investment.

Biden and other Democrats seek to make the pending infrastructure bill a vehicle, emphasizing massive government spending on specific clean-energy initiatives.

Much of Biden’s plan, such as his proposed new spending on energy research and building new power lines, is badly needed.

A clean energy mandate on the electricity sector would force the retirement of old plants and promote renewables, next-generation nuclear and other clean technologies.

But it is not clear this policy could meet the standard for passage by a simple majority vote in the Senate, putting its political viability into question.

What’s missing is an economy-wide policy that would cut demand for fossil fuels in every industry in every state.

A substantial, steadily rising carbon tax would ensure emissions reductions happened even if some of Biden’s government-funded green projects failed because it would dampen underlying demand for fossil fuels.

Cutting the first 50% of the nation’s emissions total is going to be far easier than cutting the second 50%.

Mr. Biden must show that big goals like his new Paris commitment can be met, and in an orderly and efficient manner. The planet depends on the administration and Congress getting this right.