The Earth's climate system depends on many factors, and until the nineteenth century all of them were natural. But over the last two centuries exponential growth in human population and consumption has introduced an anthropogenic factor. How significant are anthropogenic effects? Can they alter the natural system, and if so, in what ways? To address these questions, we will consider the physical origins of climate change in Chapters 2 through 4. If you find some of the material challenging, please bear with me. It is fundamentally important to understanding the essence of the problem. Let's first clarify some terms.

Weather and climate

Although there is overlap at the margins, there is a difference between what is meant by weather and climate. Weather characterizes our immediate environment in terms of atmospheric conditions such as temperature, wind speed, cloud cover, and precipitation. As we commonly use it, the word describes conditions at the locale in which we reside or to which we are going and the time we are or will be there. At any locale, weather can vary significantly over small (hourly) time changes, as well as from one season to another. In contrast, climate represents long-term averages of atmospheric conditions for a particular region.

We can describe in general terms winter and summer climates of regions such as the American Southwest and Northeast, Siberia, and sub-Saharan Africa. The climate of any region can also experience statistically significant variations, but unlike weather, changes to climate have historically occurred over much longer time frames, commonly measured in millennia. It is in reference to such variations that we use the term climate change. Another way to frame the distinction is to view weather as more variable over much smaller time scales.

It is tempting to draw conclusions about climate change from local weather patterns, particularly if they involve extreme weather events. The temptation should be resisted. Of course, climate change affects weather. But it can be misleading to draw conclusions about climate change from recent weather as, for example, last winter's conditions in South Bend, Indiana, or last summer's weather in Moscow, Russia. If climate change is inferred from changing weather patterns, the patterns should be examined globally and over an extended (decadal or longer) time frame.