10-15-07 SHORT HILLS: The weather forecaster on ABC-TV yesterday said that so far this fall every day has been above normal temperature, and that brings us back to climate change or global warming. October 15, 2007 is the day for all the Google Blogs to feature a climate change entry. I thought I would try to give an answer to the question people ask about ancient climate, “How do we know what the temperature was before people were around to measure and record it?”
One technique, used to estimate ocean temperatures in ancient Eras, measures the ratio between oxygen isotopes. Most of the world’s oxygen has an atomic weight of 16. Some small percent of naturally occurring oxygen has an atomic weight of 18. The heavier oxygen molecules have two extra neutrons in the nucleus. The ratio of O16:O18 is fixed and constant for all the earth’s oxygen and has the same ratio in water, H2O, as in the air. The ratio is reflected and preserved in seashells from ancient times. In other words, a brachiopod fossil, say, from the Devonian Period will have the same ratio of O16:O18 that the ocean did when the creature lived and made the shell.
The O18 containing water molecule is about 10% heavier than an O16 water molecule, and being heavier is less likely to evaporate from the surface of the ocean. Ordinarily water evaporates from the ocean and condenses in the atmosphere and then precipitates as rain. Some of the rain falls on land and is ultimately returned to the ocean by rivers and streams. This circulation of water from ocean to atmosphere to land and back to the ocean is called the Hydrologic Cycle. In the winter, snow remains frozen on land until the spring thaw and then the melt water runs off in the streams.
During periods of glaciation, cold periods obviously, the snow doesn’t fully melt in the spring, and each year more of the snow accumulates on land, and that frozen water is locked up in the glaciers. That ice has more of the, lighter and more easily evaporated, O16 water. With more of the O16 water out of circulation, the ocean becomes richer in O18 water and the ratio of O16:O18 is shifted. The change will be reflected in any fossil shell of the period, and lets us figure out what the average temperatures were at that time.
Climate change is happening now because the main greenhouse gas, carbon dioxide, CO2, is rapidly accumulating in the atmosphere. The CO2 comes from the burning of fossil fuels by humans. The CO2 levels have risen rapidly over the last 100 or so years. The CO2 acts like an atmospheric quilt and prevents the earth from losing heat. Now, the earth has been much hotter and much colder at different times in the distant past, but when those changes have occurred, they have happened slowly, over thousands or millions of years. The plant and animal species living during those periods of change have been able to adapt to the slow changes and evolve and thus survive. Rapid changes like this one will lead to many species extinctions. Here is a picture of the melting Arctic Sea Ice:
