Geoscience Reference
In-Depth Information
Republican candidate denied global warming. According to one analysis, in 2013,
55 percent of U.S. House of Representatives Republicans, 65 percent of U.S. Sen-
ate Republicans, and 77 percent of Republican members of the House Committee
cians and the public who agree with them be right and scientists be wrong?
To answer, let us begin by reviewing what we mean by global warming. Here
is one way to summarize the concept:
By burning fossil fuels, humans have added
ancient carbon to the atmosphere and, through the greenhouse effect, caused glob-
al temperature to rise
. How do we know that each part of that sentence is true?
Humans have been burning increasing amounts of fossil fuels since the Indus-
trial Revolution. Fossil fuels are mostly carbon, which combustion converts in-
to carbon dioxide. Since 1800, carbon dioxide emissions have risen steadily, with
shortupticksanddownticksaswarsandeconomicrecessionshaveaffectedoutput.
Between 2009 and 2010, global CO
2
emissions grew by 5.9 percent to reach 33.5
billion tons. At that rate, by the early 2020s CO
2
emissions will have doubled over
the preindustrial level.
As we saw, in addition to keeping track of the amount of CO
2
emitted, scientists
measure the amount accumulating in the atmosphere. They find that CO
2
concen-
tration has risen in nearly exact parallel with emissions. At the end of the last ice
age, atmospheric CO
2
was at about 280 ppm. When Charles David Keeling began
his measurements in 1958, it had climbed to 315 ppm. CO
2
rose inexorably after
that until, for a brief moment in the spring of 2013, it reached 400 ppm before fall-
ing back in obedience to the seesaw cycle of the seasons. But sometime in 2014,
atmospheric CO
2
will reach a
yearly average
of 400 ppm. After that, it will keep
on rising, perhaps not to return to 400 ppm for centuries, or millennia.
How do we know that the added CO
2
derives from human activities? Coal, oil,
and natural gas come from plants, whose carbon-12-carbon-13 ratio differs from
thatoftheatmosphereoverall.Ashumanshaveburnedmorefossilfuelsandadded
more CO
2
, the carbon isotopic composition of the atmosphere has shifted steadily
toward the distinctive plant ratio. The same shift occurs in the bodies of athletes
who take synthetic testosterone, which, like fossil fuels, comes from plants.
The added carbon comes not just from plants but from ancient plants. How do
we know? As noted earlier, carbon-14 is radioactive and dies away to undetectable
levels in 50,000 years or so. Fossil fuels, being millions of years old, have no C-14
left. Correcting for the amount of C-14 from nuclear tests, as the amount of car-
bon in the atmosphere emitted has risen, the proportion of C-14 has fallen in direct
proportion. This tells us that the added carbon is coming not from some modern
