Environmental conditions are constantly in flux. Many of these changes may escape our notice: Temperatures rise and fall throughout the day, humidity and air pressure fluctuate, and clouds form and dissipate. However, these same variables can combine to create phenomena that are readily observable, such as wind, rain, snow, and thunderstorms. These relatively short-term environmental changes, which might occur over periods of hours, days, weeks, or seasons, are collectively referred to as weather.

Climate describes environmental conditions over much longer periods of time than weather forecasts and reports. These long-term environmental analyses characterize a specific geographic location's temperature and precipitation averages and ranges. Anomalous high and low readings are absorbed by these averages, resulting in a reliable estimate of expected normal conditions.

Indeed, the global climate is, by definition, more stable than local weather. But climate is also constantly changing. In fact, research conducted over the last 20 years or so describes dramatic shifts in climate in Earth's distant past. These shifts occurred over a period of a decade or less, rather than over thousands of years as scientists once thought was necessary.

Scientists began studying evidence of climate change, especially the role of ice ages in Earth's geologic history, more than a century ago. During the most recent ice age, the Pleistocene, average global temperatures were about 5°C or more below present temperatures. This and other ice ages detected in the geological record were set in motion by gradual changes in the Earth's tilt, rotation, and orbit over thousands of years. Despite the gradual nature of these changes, Earth's climate appears to respond rapidly once certain boundary conditions are set in place.

Layers of ice analyzed from Greenland ice cores provide a chronology detailing the rapid onset of ice age conditions. They show average continental surface temperatures rising and falling dramatically in just a few years, rather than over the course of hundreds or thousands. For example, between 43,000 BC and 8,000 BC, average global temperatures fluctuated periodically by as much as 20°C (36°F) or more. In contrast, climate changes since 8,000 BC have been characterized by temperature shifts of just 4°C (7°F) or less.

Many climatologists think these events resulted from changes in heat energy transfer by ocean currents from the tropics to the higher latitudes, caused by a decrease in salinity. For example, computer models suggest that around 13,000 years ago, the Gulf Stream waters, which warm northwestern Europe, might have been altered or halted dramatically by influxes of fresh water from melting glaciers. However, scientists do not understand the specifics of how a decrease in the rate of energy transfer by the ocean currents from the tropics to the higher latitudes translates to changes in regional and global climate.

Print Background Essay