Geology / Climate Change
In Class With Robert Gaines
The following is an edited excerpt from a classroom discussion in the Climate
Change course taught by Assistant Professor of Geology Robert Gaines during the spring semester of 2009.
Gaines: ... We shifted gears on Monday, and we’re moving away from talking
about CO2 changes over long periods of time—tens to hundreds of millions of years -
—resulting in icehouse-greenhouse cycles. Now it’s time to look at the influence of
solar radiation on climate. We have a very good record of recent changes in global ice
volume back to four million and perhaps longer. Over long-time scales, one orbital
parameter has varied. What is that?
Andrew: Speed of the earth’s rotation.
G: How has that changed through time?
Andrew: It’s slowed down.
G: How do we know that?
Andrew: From the coral record.
G: Coral has this particular quality of adding daily growth bands. It grows calcium
carbonate skeletons with the aid of photosynthetic bacteria, called xoxanthellae that
live inside the coral. The bacteria photosynthesize and take up carbon dioxide from the
immediate environment, which allows the coral to precipitate calcium carbonate. We
can place this record of growth bands we see in coral onto the seasonal sine wave of
diminishing radiation in the winter and increasing radiation in the summer. We
know the earth spun 11 percent faster 440 million years ago.
When we get back after spring break, we’re going to walk through how these
changes in insolation affect ice sheet dynamics, how ice sheets grow in response
to solar forcing. Do you think that ice sheets are going to map precisely onto forcing—
that the growth of ice sheets will correspond with insolation maxima and minima
and vice versa? Do you think the ice sheets respond immediately?
Mark: No. There are other factors involved as well. It just takes time for things like the
ocean or the air to warm up and cool down. Just because it’s hotter doesn’t mean the ice
sheets will respond immediately.
G: That’s exactly right. What do we call the delay between forcing and response?
Sam: The lag time.
G: There is a significant lag time and, in the case of ice sheets, there is a particular
feedback that becomes important to their development. It was first mathematically
described by Milankovitch and is called the ice albedo feedback. Fresh snow and ice are
extremely reflective—the most reflective material found on the surface of the earth. Water, on the other hand, is the least reflective
material found on the surface of the earth. When ice and snow are formed, they begin to reflect radiation back into the atmosphere. The
larger the ice sheets grow, the more enhanced this effect becomes and that can buffer the ice sheets against changes.
One of the test systems that your book focuses on is the monsoonal cycle. Would someone walk through how monsoon circulation
works in the summertime and how it’s different than in winter?
Ben: It’s different in the summer because precipitation occurs over land. In the summer the atmosphere is rising because of the water’s
G: The land loses its heat quickly and heats up quickly, whereas the temperature of the ocean is much more buffered by its high heat
capacity and is more constant than temperatures over the land. Strong summer heating, as Ben pointed out, drives the atmosphere
upward and that creates precipitation. It also sucks moist air off the adjacent oceans in areas where monsoon circulation is prevalent and
continues to drive that air upwards, so it’s a conveyor belt of moist air over the land that is driven up in the atmosphere, cools and rains
out as precipitation. The opposite is true in the wintertime. Radiation decreases and the ocean is warmer than the land surface.
Unlike the monsoonal circulation in Southern California or around the Himalayas, which are topographically driven, the North African
monsoon results simply from the blunt force of summer heating. If Milankovitch is right, we should see a response in the monsoon system
to any change in seasonal temperature, and indeed, that is exactly what we see in the geologic record.
When summer radiation is increased, the monsoons become stronger, and the big lakes in the Sahara fill with water. During times
when the monsoons are suppressed because the differences in seasonality are suppressed, the lakes become dry and the sediments in
those lakes blow out into the ocean. Diatoms, the microfossils that are dominant in the oceans today, also live in freshwater lakes and
provide a marker in the geologic record. They have tiny shells, and when the lakes dry out, they’re blown away from Africa and out into
the Atlantic Ocean.
It’s actually our plankton record from the oceans that tells us a
huge amount about oceanic cooling and warming—in this case, the
oceanic record contains both the plankton that live in the ocean and
also the intermittent presence of species that lived on land in aqueous
environments and were blown away to the oceans, deflated,
essentially, by changes in climate.
We’ll talk about that after spring break when we discuss the effects of insolation on these massive continental–scale
ice sheets. ...
A Brief Glossary of Terms
Insolation: The term climate scientists used to describe the
fixed amount of energy that is received per square meter at the surface
of the atmosphere from the sun.
Milankovitch Cycles: Cyclic variations in the Earth’s
orbit that influence the amount of solar radiation striking different parts
of the Earth at different times of year. They are named after a Serbian
mathematician, Milutin Milankovitch, who explained how these
orbital cycles cause the advance and retreat of continental ice sheets.
Albedo: a measurement of a surface’s ability to reflect incoming
radiation. The albedo effect, sometimes referred to as the albedo
feedback effect, refers to positive feedback loops acting on the surface
of the Earth (increased snow cover promotes cooling and the
development of new snow cover).
Diatoms: Single-celled phytoplanktons that are the primary
producers in today’s oceans. Their cells are enclosed in minute shells
made of silica.