Kerry said climate change is at the top of the world’s energy challenges, and “one of the biggest threats facing our planet today.”

“With the right choices at the right speed, you can actually prevent the worst effects of climate change from crippling us forever,” Kerry said in a speech hosted by the Atlantic Council.

In the speech, he discussed the efforts that U.S. diplomats and their counterparts around the world are taking toward reaching an international agreement to reduce greenhouse gas emissions, which they hope to finalize in December in Paris.

The agreement will likely formalize each country’s individual commitments to take their own measures to reduce greenhouse gas emissions and help other countries adapt to the effects of a warmer Earth.

But both as part of that effort and to go beyond it, Kerry said the world needs to use far more renewable energy like wind and solar, and to cut back dramatically on oil, coal and other “outdated” forms of energy.

“Coal and oil are only cheap ways to power a nation in the very near term. But if you look a little further down the road, you begin to see an entirely different story,” he said.

Kerry said the real costs of oil and coal “actually pile up very quickly” in the effects of climate change and pollution, both of which are much greater with fossil fuels than with other energy sources.

That includes lower crop yields that will hurt farms and exacerbate hunger and malnutrition, along with health effects of pollution, like asthma.

“The bottom line is that we can’t only factor in the price of immediate energy needs; we have to include the long-term cost of carbon pollution,” he said.

Even without those harms, clean energy can be a great driver of economic activity and innovation, Kerry argued.

“And we can reach an agreement in Paris,” he concluded. “We can carve out a path toward a clean-energy future. We can meet this challenge.”