When Exxon Mobil Corp.'s shareholders gather today for their annual meeting, the company will be doing battle with some of them - including nuns, pension funds, and heirs to its founder - over global warming and whether to separate the roles of chairman and chief executive officer.

The biggest U.S. oil producer is urging investors to vote against a resolution that would bar its chief executive from also serving as chairman. Supporters of the measure, including the California Public Employees' Retirement System and descendants of company founder John D. Rockefeller, want an independent chairman to work faster to combat climate change.

Rex Tillerson, the chief executive officer who led the Irving, Texas, company to a record $40.6 billion profit last year as crude prices rose 57 percent, should spend more to develop alternatives to oil, said Peter O'Neill, Rockefeller's great-great-grandson.

A similar proposal to split the leadership roles was supported by 40 percent of shareholders last year, up from 34.3 percent in 2006.

"I don't understand why they're not investing more in alternatives," O'Neill, who leads a Rockefeller family committee dealing with Exxon Mobil issues, said in a telephone interview from New York. "I want shareholder value to increase, and I think it can if they make incremental changes and diversify into other areas."

O'Neill said he had met with Exxon executives in the last two years to discuss petroleum alternatives. Citing a confidentiality agreement, he declined to disclose details such as research budgets or what type of alternative fuels the company was pursuing.

Alan Jeffers, an Exxon Mobil spokesman, said the company regularly conferred with shareholders and did not comment on individual meetings.

The company, which traces its roots to the 1880s and Rockefeller's Standard Oil Trust, failed to replace 24 percent of the oil and gas it pumped in 2007, its worst such performance in three years.

The 12 Rockefellers who have voting power based on Exxon Mobil stock and support the chairman-CEO resolution own 332,174 of the company's 5.4 billion shares. The 0.006 percent stake is worth about $30 million. John D. Rockefeller died in 1937.

Exxon Mobil's board would have to reconsider its rejection of the proposal if the nonbinding resolution wins more than 50 percent support. Tillerson, 56, is the only member of management on the board.

Sixteen of the 20 largest U.S. companies by market value have chief executives who are also chairmen, according to data compiled by Bloomberg.

ISS Governance Services and two other institutional-advisory companies urged support for splitting the positions. The proposal also received support from three state treasurers - New York City Comptroller William Thompson, California Controller John Chiang, and six British funds overseeing a combined $313 billion.

Exxon Mobil, in a May 12 statement sent to its 30 biggest U.S. shareholders, said: "The board believes very strongly that there is no single best organizational model that would be most effective in all circumstances. The board believes that the most effective leadership structure for Exxon Mobil at the present time is for Mr. Tillerson to serve as both chairman and CEO."

Meanwhile, Roman Catholic orders from New Jersey and Wisconsin and heirs to the Champion Spark Plug Co. Inc. fortune are among sponsors of 16 other resolutions on the environment, executive pay and gay rights.