Investors seeking greener energy stocks will find it difficult to reproduce the returns offered by oil and natural gas producers, according to a report from Bloomberg New Energy Finance.

With a market value of $4.9 trillion, oil and gas investments offers a combination of scale, growth and dividends that can’t be readily found in other industries, the London-based research company said Aug. 26. Coal, which has already fallen out of favor with institutional investors, can be more easily replaced with bets in other industries.

Environmentalists have proposed fossil fuel divestment, modeled on a campaign that targeted companies in apartheid-era South Africa, as a way to curb climate change by shifting investments away from polluting technologies. Stanford University is among 13 colleges and universities to join the campaign this year, committing in May to divest from coal—but not oil and gas.

“Oil and gas stocks have outperformed other major sectors over the past five years,” according to the report. “Coal stocks, on the other hand, have been striking underperformers, reflecting a fall in international coal prices as the U.S. shale boom caused generators to switch into gas-firing.”

The divestiture movement is backed by Bill McKibben, an environmental writer and co-founder of 350.org, a group that has identified 200 companies with the largest reserves of coal, oil and gas. McKibben warns that fossil fuel companies hold undisclosed financial risks as governments move to limit emissions blamed for global warming.

“If you are investing in fossil fuels, you are essentially betting that we won’t ever take climate change seriously,” Jason Kowalski, U.S. policy director for 350.org, said in an email. Smart investors are moving “their money sooner rather than later.”

With a combined market capitalization of $230 billion, the coal industry is less than 5% of the total value of oil and gas equities, according to the paper. Even the largest coal companies are fractions of the size of oil and gas majors such as Exxon Mobil Corp. and Royal Dutch Shell Plc.

“Institutional investors are much less exposed to coal than to oil and gas—and as a result, divesting from coal would be much easier than divesting from oil and gas,” according to the paper.

Stanford’s $18.7 billion endowment is the fourth-largest among U.S. universities, according to data from the National Association of College and University Business Officers. Slowing climate change may persuade other endowments to unload all fossil fuel companies, according to David Turnbull, campaign director for the environmental advocacy group Oil Change International.

“They shouldn’t be investing in companies that are contributing to climate crisis,” Turnbull said.