Schrag is Professor of Geochemistry in the Department
of Earth and Planetary Sciences at Harvard University.
A native of New York City, he graduated from Yale University
in 1988, where he studied geology as well as political
science. He received his Ph. D. in Geology from the
University of California at Berkeley in 1993.
studies the history of oceans and climate using analytical
chemistry and modeling. His projects cover the widest
range of time scales. Currently, he is using corals
from the Pacific to study El Niņo and modern ocean circulation,
deep sea sediments to study the last ice age (20,000
years ago), and ancient sediments to study the Neoproterozoic
He and colleague Paul Hoffman, also of Harvard, have
helped draw attention to the mysteries of Neoproterozoic
glaciations, combining their expertise to articulate
the various components of the Snowball Earth hypothesis.
Their work with Namibian rocks and similar deposits
on other continents, as well as modeling work in collaboration
with a variety of other colleagues, will help them test
the hypothesis and further their understanding of the
climate dynamics of this unusual time in Earth history.
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How could the earth freeze over if the sun was vastly
closer to the earth "600 million years" ago? The Royal
English Observatory has proven that the sun is shrinking
an average of 5 feet/hr (8.3 miles/yr) due to the nuclear
conversion of hydrogen to radiant energy. Doing the
math, 600 million times 8.3 miles per year, the numbers
to support your guess do not stand.
Paul - I believe your information on the proximity of
the Sun to the Earth is incorrect. The distance between
the Sun and Earth have changed very little through Earth
history. In fact, most astronomers and planetary scientists
think that the Sun was approximately 30% dimmer when
the solar system began (4.55 billion years ago), and
has been getting brighter ever since. This has led to
a paradox known as the problem of the "faint young sun."
In essence, the question is how can Earth's climate
have stayed warm if it received so much less radiation
from the sun. The answer appears to be that carbon dioxide
was higher in the past, controlled by the chemical weathering
of rocks. There is a better explanation of this in our
American article that appeared last January. What
the faint young sun means for the Snowball Earth is
that it was probably easier to initiate a snowball glaciation
early in Earth history, and more and more difficult
to initiate one as time went on. Of course, the ones
we have studied are fairly recent... only 600 million
years out of 4.55 billion! I hope this answers your
Thanks for writing.
B. Rossi asked:
Dear Dr. Schrag: I wonder if you would clarify a few
points for me? The synopsis mentions that after the
glacial encrusting of the planet, the ensuing global
warming reached average, atmospheric temps of 90 deg.
in just a few years.
A) Is there any idea of what near surface temps during
the complete glaciation period might have been? Can
a rough estimate of volcanism during that period be
given, relative to what we have today?
B) Can the levels of CO2 be estimated with regard to
what would have been necessary to trigger the change
with the environment in that state?
C) For what period of time would you postulate that
Earth's temps averaged 90 degrees?
D) Finally, what are your views regarding contemporary
climate change....i.e., do you :
1) believe that humans can or cannot, induce "global
2) believe that Earth's own systems can reverse anything
Thanks very much for your time with these queries.
Average surface temperature at the start of the glaciation
would be in the vicinity of -50 degrees C. When the
ice melted, the average temperature had risen to approximately
-15 degrees C. The rate of volcanism is highly uncertain,
but it was unlikely to be more than a factor of two
different from today. The key to escaping the snowball
glaciation is not extreme volcanism, but just slow and
steady release of carbon dioxide over millions of years.
B) Ken Caldeira and Jim Kasting have estimated this
number to be approximately 120,000 ppm CO2, but this
number is highly uncertain. Because we don't know many
aspects of what the Earth was like during the glaciation,
including how white the ice was on the surface, that
number is probably accurate within a factor of 4 or
The greenhouse aftermath passed much more quickly than
the glaciation, but the exact timing is uncertain. The
peak temperatures probably lasted thousands to hundreds
of thousands of years.
Humans are causing our climate to warm. There is clear
evidence that this is happening, and the vast majority
of climate scientists believe that this view is correct.
It is of great concern for human society over the next
100 years. 2) This is a question of time scale. Over
millions of years, the Earth can overwhelm any climate
perturbation brought on by humans. But that may not
be of much comfort to us... 50 million years ago, we
believe that carbon dioxide was between 4 and 10 times
higher than present. At that time, sea level was 100
meters higher, the deep ocean was 12 degrees C (compared
with 2 to 4 degrees today), crocodiles lived on Greenland,
and palm trees lived in Canada. We all hope that human-induced
climate change will not push the Earth that far! But
even more subtle changes can have disastrous impacts
on people, especially in developing countries, and on
ecosystems such as tropical rainforests.
for your questions.