Today's view is entirely different. Sagan illustrated his point with slides of contemporary
Earth that showed a planet glowing with many illuminations. Although the aurora borealis
is still the brightest thing that can be seen, the remaining lighted areas are all the results of
civilization. Most are signs of the presence of cities, but some, in areas away from cities, are
different kind of reminders of technology: light from burning oil wells recalls how much we
depend on the burning of fossil fuels.
Throughout most of history, technology was weak and the number of humans grew slowly
because the birth and death rates were nearly equal. But now the human race numbers
about 5.4 billion and has spread to every continent. Our numbers and our worldtransforming
technology have reached such frightening proportions that we have become a danger to
ourselves.
Sagan talked about three environmental problems as aspects of this danger. One is the
destruction of the Earth's forests: we are destroying one acre of forest every second, and
if this rate continues there may be only one or two generations left before forests cease
to exist.
Another is the extinction of many species of life on the planet. Species are becoming
extinct at a faster rate than has occurred at any time since the catastrophe 65 million years
ago that destroyed most of the species then existing, indications that we are using
technology in the wrong way.
One of the most different to understand and difficult to fix of these probrems is global
warming, in which gases produced by the burning of fossil fuels create a "greenhouse
effect" that slows the release of heat from the Earth's atmosphere. Records of global
temperatures and projections into the future show that at present we are experiencing
a sudden and extreme rise in temperatures. If this increase continues, by the year 2000
the planet's average temperature will be the warmest in 150,000 years. The effects of
global warming could be very serious, including lack of rain in some areas, flooding
in other regions resulting from a rise in sea levels, and probrems in agriculture.