"Hark! The faint bells of the sunken city Peal once more their wonted evening chime! From the deep abysses floats a ditty Wild and wondrous, of the olden time". - Wilhelm Mueller, "The Sunken City", translation from the German

We need to ask several questions: What happened? Why did it happen? Has it happened before? Will is happen again? How do we know about it in the first place? Click the image to the right to explore the hypothesized changes in ice cover and vegetation.

The speed at which climate can change has also recently become clear: Transitions between fundamentally different climates can occur within only decades. In order to understand these variations, we need to reconstruct them over a wide range of temporal and geographical scales. The importance of this task is underlined by the growing awareness of how profoundly human activity is affecting climate. As with so many other complex systems, the key to predicting the future lies in understanding the past

From abundant geological evidence, we know that only three hundred and fifty years ago the world was in the depths of a prolonged cold spell called the "Little Ice Age," which lingered for nearly 500 years. Twenty thousand years ago, in the middle of the last glacial period, large continental scale ice sheets covered much of North America, Northern Europe, and Northern Asia. Fifty million years ago, global temperatures were so high that there were no large ice sheets at all.

One of the most pressing concerns for humans on Earth today is climate change, and what will happen in the future. Given that the climate is definitely warming, it is logical to ask two questions: First, have such changes happened before in the history of the Earth? And second, what is causing this change? In the next two lectures we will examine past climates (paleoclimates) and the forces that caused them to change. This information will set the stage for asking if the same forces that caused past changes are causing climate warming today, and for making predictions about what will happen in the future (upcoming lecture on climate models).

Figure 1 . Vegetation during glaciation in North America in the last Ice Age. The geologic record indicates several ice surges, interspersed with periods of warming, called "interglacials." (Source: Oak Ridge National Laboratory )