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Under a Green Sky by
Peter Ward. Published in 2007 by Smithsonian Books. 242p.
Peter Ward, a paleontologist
at the University of Washington, has devoted his career to traveling our Earth
finding and interpreting fossils. In this book he focuses on the causes of mass
extinctions. Five major extinctions are recorded in fossil records. Was it an
asteroid that struck the Earth or was it climate change that triggered each of
these?
In 1980 the
scientists, Luis and Walter Alvarez, presented evidence that the Cretaceous
Tertiary extinction was triggered by an asteroid striking the Earth. Chemical
evidence, for example, iridium particles, remain after such a catastrophe. Later
the impact site was found near the Yucatan peninsula in Central America. The
impact layers from this collision at many locations contain many particles of
fine soot, indicating that fires raged over most of the Earth consuming most
vegetation. Animals without food die, so this extinction killed most dinosaurs.
After the Alvarez’s discovery, many assumed that all extinctions resulted from
collisions by meteorites. Peter examines the fossil and chemical records to
evaluate these. Impressively, modern chemical methods can identify biomarkers
even from some types of bacteria. He describes for a public audience, although
in a wordy style, details of other extinctions. The evidence documents that all
other mass extinctions resulted from climatic conditions on Earth. Thus we have
extensive records of climate changes from our past.
The giant conveyor belt system of ocean currents, resembling a huge roller
coaster, now moderates Earth’s climate. Peter suggests that changing or halting
this has contributed to climate change in the past and will in the future.
By burning more fossil fuels, people today continue to increase our carbon
dioxide output. In fact,. it is increasing at a faster rate now than 20 years
ago. In 1860 the atmosphere contained 290 ppm of carbon dioxide. This rose to
315ppm in 1960; and to 390ppm in 2010. Methane from both bacteria and volcanic
eruptions functions as a greenhouse gas that’s 23 times as strong as carbon
dioxide. The Greenland ice sheets are melting at an unprecedented rate. Volumes
of fresh water are pouring into the conveyor belt current. This alteration
brings another mass extinction closer.
When waters become anoxic, depleted of oxygen, they cannot support plankton,
shrimp, corals or fish. Only certain bacteria flourish without oxygen. Three
groups inhabit anoxic ocean waters: purple bacteria, green bacteria, and
bacteria that produce toxic hydrogen sulfide gas. This gas accumulates, bubbles
up to the surface and escapes killing most organisms there. Hydrogen sulfide has
contributed to extinctions in the past.
Our current global warming threatens to cause another mass extinction. The
devastation depends on how high the carbon dioxide level rises. High levels
produce purple oceans devoid of animal life. No birds soar overhead.
An awareness of the threat
of global warming to our human civilizations causes some people to despair. They
have done little to correct our situation. This graphic account triggers fear.
Will fear motivate actions to ameliorate this cascade of events?
Written February 17, 2011
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