Geoscience Reference
In-Depth Information
Time to Stop Waffling
A sound theory makes predictions that can be tested and, when they are, turn out to
be true. As we saw, the Hansen group's 1981 paper made a number of predictions.
In today's era of record-setting temperatures, it is easy to lose sight of the most
important one, which at that time even many scientists did not accept: that global
temperatures would rise. They have, and they will.
By the end of the first decade of the twenty-first century, other predictions from
the paper were also coming true. The Northwest Passage opened, Arctic sea ice
reached its lowest extent in the satellite era, global sea level rose, the West Antarc-
tic ice sheet began to melt, and the southwestern United States and other regions
sufferedsuccessive severedroughts.Hansen'smodelwasnotsufficiently clairvoy-
ant to predict when a major volcano would erupt, but one did.
In 1991, Mount Pinatubo in the Philippines produced the second-largest erup-
tion of the twentieth century. Pinatubo injected an estimated 20 million tons of sul-
fur dioxide into the atmosphere, where it oxidized into aerosols that spread round
the globe. On October 3, 1991, less than four months after the eruption,
Geophys-
ical Research Letters
received a paper from Hansen's group predicting that the
aerosols would cause “a dramatic but temporary break in recent global warming
trends.” Global cooling should develop by mid-1992, they wrote, and end “in the
In 1988, testifying before Congress on a sweltering June day, Hansen gave the
most dramatic warning any scientist had yet offered, saying that he was 99 per-
cent certain that greenhouse gases rather than a natural variation had caused the
observed warming trend. “It is time to stop waffling so much and say that the evid-
ence is pretty strong that the greenhouse effect is here,” Hansen said.
2
The NASA scientist made three projections of future temperature, each using
a different assumption for the growth of greenhouse gas emissions. Scenario A
assumed exponential growth—in other words, business as usual; Scenario B as-
sumed slowing and eventual constant growth; and Scenario C assumed draconi-
an emission cuts to eliminate growth by 2000. Actual temperature increases have
been a bit lower than Hansen's scenario B. With hindsight, we can see why. First,
actual greenhouse gas emissions have been slightly less than he projected. Second,
