In this lesson students look at two different kinds of data or evidence that scientists use to try to understand how the climate has changed in the past. The evidence is critically important to our understanding of climate change because it is only by working with data from the past that we can develop the models which help us predict the future. Lesson 7 will include a reading and activity which will discuss climate change modeling in more detail.

The evidence is also messy, since scientists often have to use a variety of “proxies” to estimate temperature and carbon dioxide at different time spans.

Students will look at data from the last century or so, using estimates of the temperature of the earth and carbon dioxide (CO2) data collected from Mauna Loa, a site remote enough from local and regional CO2 emissions to monitor global atmospheric changes. Students will also look at longer timescale data from the Vostok Ice Core in Antarctica. Scientists use a variety of other measures to try to link the climates from long ago (ice core measurements) with recent measurements. The intermediate range strategies include looking at tree rings (and fossilized tree rings), pollen samples, and fossilized shellfish and other animals. All of these estimates, even the most recent ones, are complex calculations, and subject to error.

The people who want to find out about ecosystems (and climates) in the past are called paleontologists (paleo = old, ontology = study of things), paleoecologists, or paleoclimatologists. All of these “paleo-people” have one thing in common – they really like deep lakes and thick glaciers.

Ice Core data is useful in reconstructing climate patterns over hundreds of thousands of years. A glacier forms because snow keeps falling in a really cold place. More snow falls than is able to melt each year, and the layers of snow keep piling up. The layers of ice can trap things like dust or dead insects. Even better, an ice core can sometimes trap bubbles of air. These bubbles can tell us how much oxygen or carbon dioxide was in the air at different times in the past. Moreover, the ratio between different kinds of oxygen can tell us the temperature of the air. This is because oceanic plants and animals tend to have different atomic isotopes of oxygen in their tissues at different water temperatures. It takes some very sophisticated measuring equipment to “read” this information in an ice core, but the result is an accurate record of sea temperatures in different places and at various times in the past.

The best places to look for thick ice sheets are in Greenland and Antarctica. Dozens of cores have been analyzed by different labs around the world, and they all tell roughly the same story. That story is that the temperature of the earth has varied in a fairly regular cycle for the last several million years. It goes up and down, from about 12-16 Fahrenheit degrees colder than at present to about 4-8 degrees warmer. The variation seems to be the result of a well-known cycle in sunlight intensity. This cycle is due to minor changes in the tilt of the earth and the shape of its orbit (Milankovitch Cycles).

Lake bottoms can provide a different kind of record. A deep lake is a rare geologic event. It is usually quite young (geologically speaking) because it was made by a recent extreme event of some kind – a volcanic explosion, earthquake, or glacier (or maybe just some people building a high dam!) A deep lake usually gets shallower every year. Dust from the air and mud from streams go into the lake, sink to the bottom, and slowly fill it up. Those layers of mud are a kind of file cabinet, because they can also trap things like fish skeletons, dead insects, or pollen grains from plants. For that reason, scientists can get a record of the past by digging a hole in the mud and recording what they observe in each layer. Deep lakes and pollen estimates, along with tree rings, are useful for trying to reconstruct climates over about the past 20,000 years.

The most recent atmospheric CO2 data set is measured on Mauna Loa in Hawaii. This record constitutes the longest record of direct measurements of CO2 in the atmosphere. The observations were started by Charles David Keeling of the Scripps Institution of Oceanography in March of 1958 and the plot of the Mauna Loa CO2 data is known as the Keeling Curve. There is confidence that the CO2 measurements made at the Mauna Loa Observatory reflect solid information about the global atmosphere. The main reasons for that confidence are:

• The Observatory near the summit of Mauna Loa, at an altitude of 3400 m, is well situated to measure air masses that are representative of very large areas.

• All of the measurements are rigorously and very frequently calibrated.

• Ongoing comparisons of independent measurements at the same site allow an estimate of the accuracy, which is generally better than 0.2 ppm. Carbon dioxide measurements at other locations in the world confirm the Mauna Loa trends.

Yearly analyses of global temperatures are carried out at four major climate centers: U.S. National Aeronautics and Space Administration (NASA), U.S. National Oceanic and Atmospheric Administration (NOAA), the University of East Anglia in the United Kingdom, and the Hadley Center UK MET Office. Thousands of land surface temperature-monitoring stations all over the world, measurements of sea-surface temperatures using ships, and satellites, which have been monitoring the global temperature for about 30 years, provide data for these analyses.

While there is some debate about the relationship between CO2 and temperature, and about whether climate change is a result of natural or human factors, there is even more debate about what to do about it!