WASHINGTON (AP) — The Environmental Protection Agency says global warming is one of the most significant public health threats of our time.

That's according to EPA Administrator Gina McCarthy. McCarthy is defending a plan to curb carbon pollution in a speech Friday morning. She says global warming is not just about melting glaciers.

The proposal is the first significant step in President Barack Obama's climate plan.

But the plan only deals with future power plants. That means there is a limit to how much it will dent the emissions blamed for global warming.

The existing fleet of power plants in the U.S. is the largest source of heat-trapping pollution. A proposal to deal with them is due next summer.