Atmospheric levels of carbon dioxide would be much higher now if it weren't for a centuries-old trend toward the use of lower-carbon fuels that has largely gone unnoticed.

• For nearly a century and a half, carbon-rich fuels have progressively been replaced by fuels lower in carbon.

• First wood -- which is high in carbon -- was eclipsed in the late 19th century by coal, which contains less.

• Then oil -- which has a still lower carbon content -- overtook coal in the 1960s.

• Now analysts say that natural gas, lighter still in carbon, may be entering its heyday, and that the day of hydrogen -- with no carbon at all -- may be about to dawn.

Experts say that the world burns less than two-thirds as much carbon per unit of energy produced as it did in 1860. Since 1950, the amount of carbon released per unit of production in the U.S. has been cut in half, scientists believe.

But because of the growth in population and the economy in the U.S., the concentrations may well have doubled by the year 2100.

Proponents of global warming theories link carbon dioxide emissions to increases in atmospheric temperatures worldwide.

Source: William K. Stevens, "Global Economy Slowly Cuts Use of High-Carbon Energy," New York Times, October 31, 1999.