Natural variability on these time scales is large, and at the same time, human activities are causing the atmospheric composition to change with the addition of radiatively important gases such as carbon dioxide. To understand the climate change, we need to understand how the earth's climate system works. Understanding climate has been important to human civilizations. Emmanuel Le Roy La Durie, in his classic book, Times of Feast, Times of Famine, shows us how the history of agriculture leads to a need for understanding the history of climate, since the two are closely related. Changes from year to year, the climate, makes the farmer sensitive to understanding how climate changes.
Some years are feast years, with bumper crops, some years are famine years.
The knowledge of climate that we gain helps us reduce the difference between feast and famine years. La Durie shows us how civilizations have learned to live with climate change, store food in bad times, anticipate climate change, and do the best they can to have a stable food supply. The Earth's climate system is complex. The overall system is driven by the sun, but it also involves many complex interactions among the atmosphere, the ocean, the land, ecosystems, sea ice, clouds, and greenhouse gases and aerosols. It is this very complexity of the climate system that makes it so interesting, and yet so subject to controversy.