Page 117 - Earth's Climate Past and Future

CHAPTER 5 • Greenhouse Climate 93

Over the slightly longer term of days to years, the
dust and soot that remained within the atmosphere
spread around the planet, blocking most incoming solar
radiation (see Figure 5-15). The debris injected into the
lower atmosphere (the troposphere) would probably
have been removed over a period of days to at most
weeks because rainfall clears debris from that level of
the atmosphere. In contrast, it would have taken
months or years for the dust and soot injected into the
stratosphere to settle out. The only means of removing
debris at those altitudes is the slow pull of gravity on
small particles. As a result, stratospheric particles (par-
ticularly dark soot) would have blocked significant
amounts of sunlight for a year or more and cooled
Earth’s climate. Another likely effect over the course of
a few years would have been the partial acidification of
the oceans due to the creation of nitric acid (a compo-
nent of acid rain) from atmospheric nitrogen, oxygen,
and water vapor by the heat of the impact.
On the longer term of decades to centuries, the ini-
tial injection of carbon biomass into the atmosphere by
Highest Average Lowest
gravity gravity gravity burning should have produced higher CO levels (see
2
Figure 5-15). The plants that recovered from the
FIGURE 5-14 65-Myr-old impact crater? Mexico’s Yucatán firestorm and began to grow would have pulled some of
Peninsula has a circular area more than 200 km in diameter the excess CO out of the atmosphere, but full recovery
2
that is a good candidate for the site of the asteroid impact 65 and the development of new forms of vegetation to
Myr ago. The pattern shown is a result of measurements of replace the ones that became extinct took longer. The
Earth’s gravity that can detect low-density pulverized rock warming induced by higher CO may have lasted for
(in blue) and higher-density rock (in green and yellow). centuries or longer. 2
(B. Sharpton, Lunar and Planetary Institute.)
By any standard used, this impact event was an enor-
mous short-term environmental catastrophe, but what

Climate scientists have concluded that asteroid
impacts could have affected Earth on time scales rang- Time Minutes Days Decades
ing in duration from instantaneous to as long as a few after or less to to
hundred or even a few thousand years (Figure 5-15). asteroid years centuries
The instantaneous effects were caused when the aster- impact
oid blasted a hole through Earth’s atmosphere. The Effects Shock waves Soot & dust in Higher
speed of the incoming asteroid, 20 km/s, created a on the stratosphere levels of
shock wave that moved outward, flattening objects for environment Water & rock CO 2
hundreds of miles around the impact site and heating vaporized Acidification in the
of lakes
Earth’s atmosphere. Seismic waves sent through Earth’s Tidal waves and ocean atmosphere
interior are thought to have been equivalent to those Firestorms
caused by an earthquake that would have measured 11
on the Richter scale, 100 to 1000 times stronger than
the strongest earthquakes in recorded human history. Climatic
Some of the water and rock in the vicinity of the effects Warming Cooling Warming
impact were instantly vaporized by the heat of the
impact and blasted back out into space through the hole FIGURE 5-15 Climatic and environmental effects of
created by the incoming asteroid. The rest of the hot asteroid impacts The asteroid impact 65 Myr ago is thought
debris remained in Earth’s atmosphere, heating it to have had major effects on Earth’s environment, including
still further. The combined heating caused large-scale the extinction of over two-thirds of the species then alive. The
(possibly global) wildfires that ignited much of the likely climatic effects vary with the amount of elapsed time
above-ground vegetation and sent a thick layer of soot after the initial impact and appear to have been restricted to a
into the atmosphere. few centuries.