Dan 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.

Schrag 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 Snowball Earth.

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.

1.04.01 Paul asked:
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.

Schrag's response:
Dear 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 Scientific 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 question.
Thanks for writing.

1.04.01 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 warming,"
2) believe that Earth's own systems can reverse anything humanly induced?
Thanks very much for your time with these queries.

Schrag's response:
A) 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 so.

C) 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.

D) 1) 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.

Thanks for your questions.