Some of the renewable sources with the greatest potential to deliver large amounts of energy, like certain kinds of solar power, remain expensive compared with burning fossil fuels, he said. And integrating a wide variety of renewable sources into existing power grids would be a huge technical and financial challenge, he added.

That caution was echoed by separate report released on May 24 by the International Energy Agency. While the agency found that biomass, geothermal and hydropower provide a steady stream of power and pose no greater challenge than conventional power to integrate into grids, other renewable sources — wind, solar, wave and tidal energy — fluctuate with the weather and are often in places that lack grids.

“When shares of variable renewables amount to just a few percent, a philosophy of ‘connect and manage’ will usually suffice,” said Nobua Tanaka, executive director of the I.E.A. Greater use of renewable sources means that “this will need to change,” he said.

A summary of the climate panel’s report was published on May 9, after 194 governments agreed to the text. The report was based on a comparison of 164 evaluations of the technology and provided the most comprehensive analysis to date of trends and perspectives for renewable energy. The panel was expected to publish a full report of more than 900 pages by mid-June, once scientists have completed final checks.

The report found that six sources — bioenergy, wind, solar, geothermal energy, hydropower and ocean energy — currently accounted for 13 percent of global energy supply. In one of the least optimistic outlooks for the sector examined by the panel, the world would generate 15 percent of its energy needs from those same six sources by 2050. But in one of the most optimistic projections, the world could generate 43 percent of its energy needs from those six sources by 2030 and 77 percent by 2050.

Renewable energy also would create jobs and help accelerate access to energy for 1.4 billion people without electricity.

The panel expected costs for many of the technologies to fall, though they still would be formidable. Reaching a point where renewable sources would contribute significantly more low-carbon energy than nuclear power and fossil fuels by midcentury will require investments by governments and the private sector of up to $5.1 trillion through 2020, and up to $7.2 trillion from 2021 to 2030.