The International Energy Agency said Tuesday it expected the share of renewable energy on the global grid to expand more than initially thought.

"We are witnessing a transformation of global power markets led by renewables" IEA Executive Director Fatih Birol said in a statement.

In its latest market report, the IEA said it expected the share of renewable energy to increase 13 percent more over the six-year period ending in 2021 than its estimate last year. The agency said much of the growth came from strong policy support and lower costs for solar power and offshore wind resources, which combined for more than half of the new power that came online last year.

Regionally, the IEA said the United States was accountable for a good deal of its revised growth rate in part because of federal tax credits for solar and onshore wind energy. China, however, holds the lead in terms of renewable energy expansion.

"About half a million solar panels were installed every day around the world last year," the report read. "In China, which accounted for about half the wind additions and 40 percent of all renewable capacity increases, two wind turbines were installed every hour in 2015."

After issuing pledges to back the Paris climate agreement, the United States and China, the two leading global economies, said their collaboration would serve as an enduring legacy of the partnership necessary to combat climate change.

The IEA's report follows an assessment from the World Meteorological Organization that, in the 15 years since 1990, there was a 37 percent increase in the warming impact on the climate because of the atmospheric influence of greenhouse gases like CO2, methane and nitrous oxide. The WMO said at least some of that was because of a reliance on fossil fuels.

Nevertheless, the IEA said renewable energy will be the fastest-growing source of new electricity over the next five years.

"I am pleased to see that last year was one of records for renewables and that our projections for growth over the next five years are more optimistic," the IEA director said. "However, even these higher expectations remain modest compared with the huge untapped potential of renewables."