
Okay, so we know the concentration of greenhouse gases has increased in the atmosphere over the last 50 years (Keeling curve and other observations) so what has happened with the global temperature over that time period? It has increased but here is a difference in the increase between the land and the ocean?

[Video opens with two glasses of soda. The one on the left has very little air bubbles rising to the top. The one on the right is bubbling at a much higher rate.]
Okay, so now we realize it is a complex issue. Other causes have been suggested, such as increased solar activity warming the planet, which warms the oceans and reduces the amount of CO2 that can be held in the water. (huh????)
The important fact is that the temperature of the planet has increased by about 1 °F during the last 100 years. Phew, I thought we were in trouble for a minute! Why all the fuss over about 1°F?
If we look at longer timelines we can determine that this rate of change is very rapid! The planet goes through these changes anyway, but very slowly, usually. After all, the geography of large areas of North America is a result of ice ages and glaciers scouring the earth. So how do we know what the temperature was hundreds to thousands of years ago, or what the CO2 levels were?
How do we know what the temperature and CO2 concentrations were?
Look at tree rings.

Some of the trees in the world live long, long lives and by looking at the thickness of the rings as well as the number, the relative climate can be determined. This technique was used to show that the first few years of the Jamestown colony were very dry which contributed to their near starvation. While this technique can only go back a few hundred years it is still very useful for looking back at climate history over those years. The image shown indicates a little trouble with a forest fire in this tree's history. Here at Penn State, there is a tree ring laboratory, where this image was obtained (with permission I might add!). Recall that trees grow because of CO2 (the carbon cycle). They also grow faster with more CO2 so some believe that we should treat CO2 not as a pollutant but rather as a fertilizer for the trees. More CO2 may well produce quicker growing forests and other crops!
Listen to an audio track about wood from the lake.
Wood from the Lake Video: Text Version (click to reveal)
Dr. Mathews: When sailing ships from Great Britain found the continent of the United States they were very happy to find one thing that was desperately needed, trees. Very, very large trees. Trees, remember, are very important for military reasons. Remember England is an island, so if you want to go out and conquer the world, you need a navy. A navy requires wood. And the bigger the trees there are, the bigger the boats you can build with larger guns. And so we were very happy to find very large trees in American which we could take back and use in our shipbuilding processes. The trees you see here now in Pennsylvania are essentially the third generation of clear-cutting. Originally the forest would have been very different. And in fact there is a company out, I think, in the lakes of Minnesota, where they found large submerged logs. And they would bring them up with cranes and sell them at market. What makes this wood so valuable and worth the effort, is it was part of the original forest, the first clear-cutting. In those days a canopy was very very thick. And so the trees tended to grow slower because they received less light. Therefore, the rings were much closer together. And the grain or the beauty in the wood in the furniture making process is much richer and highly more valued. And so these trees could be worth a hundred thousand dollars. A hundred and twenty-five years later after laying in the bottom of a lake.
From Ice Cores
The air temperature influences the size of snowflakes. Warmer snow has larger flakes. Thus, when you look at snow after digging a hole to sit in you will see layering. This is useful information for the prediction of avalanche conditions. As the snow ages and more snow is deposited on top, the snow will form ice. There are a few locations in the world where the ice is very thick and hence there is data for thousands of years in the ice cores that they drill and bring back. You can also see the volcanic dust from long since past eruptions and date the ice with known volcanic events, or simply count the ice layers as you would rings on the tree (size of the snowflake influences the optics of the ice, which is related to the temperature).

Another very useful aspect of the ice is small air bubbles trapped within. These are miniature time capsules that contain a sample of the air from when the snow fell. Thus, we can determine the levels of CO2, methane, or other gases in the atmosphere. We have some faculty here at Penn State that do this. Dr. Richard Alley!
From Coral

In the same way that ice cores can reveal both temperature and a measure of the gas concentrations, coral can indicate the temperature and the concentration of CO2 in the ocean, which is related to the concentration of CO2 in the atmosphere. Coring the coral looks like a much better job than coring the ice! As the coral has a slow growth rate there are again thousands of years of information available in certain locations. Unfortunately for the coral, the temperature sensitivity will cause their loss in some locations, if it gets much warmer.
Here is some data from a longer time period