Earth's Changing Climate

The course begins with a look at Earth's average temperature over the past century and a half, which shows an overall warming trend. How do scientists take Earth's temperature, and how do they interpret the pattern of variation?

This lecture looks at more subtle indicators of climate change and shows how statistical analysis reveals clear "fingerprints" of change on a host of natural systems.

Thermometer-based temperature rec­ords go back only 150 years. This lecture explores techniques that scientists use to push the global temperature record back millions, even billions of years.

Stable climate entails a balance between incoming sunlight and outgoing infrared radiation. Infrared-absorbing greenhouse gases in a planet's atmosphere alter the details of this balance, causing the planet's surface to warm.

How do we know that greenhouse gases such as water vapor and carbon dioxide are associated with the warming of Earth's surface? Nature provides a climate "experiment" on neighbor planets Venus and Mars.

Cycling of materials plays a role in climate, with the most important cycles being those of water and carbon. Carbon added to the system stays for centuries to millennia and adds to the atmospheric carbon content, enhancing the greenhouse effect.

Fossil fuel burning by humans has in­creased the concentration of carbon di­ox­ide in the atmosphere by nearly 40 per­cent since the start of the Industrial Revolution—to levels the planet has not seen in at least a million years.

Climate models are mathematical descriptions, exploring how climate be­haves in response to human-induced changes and natural factors. Most models pro­ject a global temperature rise of several de­grees Celsius over the next century.

A temperature rise of only a few degrees will have significant effects. The rise will be more substantial particularly in the polar regions and over almost all land.

Energy use is the dominant reason for our increasing influence on Earth's climate. Per capita energy consumption in the United States is more than 100 times our own bodies' energy output, meaning that we have the equivalent of about 100 "energy servants" each.

The fossil fuels that supply most of the world's energy have many deleterious environmental impacts, one of which is the emission of climate-changing greenhouse gases. This lecture surveys alternative energy resources.

Avoiding disruptive climate change in the future probably means keeping atmospheric carbon dioxide to at most a doubling of its preindustrial level. This final lecture discusses several possible paths to a stable climate.