"Between now and 2020 we estimate increases of some 3.5 billion tonnes per year of additional carbon emissions, so it's definitely increasing," Broiles said at an energy conference sponsored by accounting and consulting firm Deloitte.

He said about 7 billion tonnes of carbon dioxide, which is a byproduct of burning fossil fuels, go into the earth's atmosphere each year from power plants, cars and other sources.

Experts say the United States, which has the world's largest economy and 4 percent of its population, is responsible for about 25 percent of so-called "greenhouse" gases now produced, but Broiles said most future growth in output will come from developing countries.

"Eighty percent of that number, 80 percent of 3.5 billion tonnes, is going to be driven by those developing countries, those economies that are growing at the 4 to 5 percent range, so that's where it's coming from," he said.

A huge increase in the number of cars will cause part of the pollution growth.

Broiles said there are now 15 cars for every 1,000 people in the world, but ExxonMobil expects that number to rise to 50 cars per 1,000 by 2020.

He said ExxonMobil foresees a 40 percent increase in energy demand even though humans are boosting their energy efficiency by about 1 percent a year. Despite advances in technology most energy will still come from fossil fuels, and in particular oil and gas, of which there remain very large reserves, he said.