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CGD Climate Highlights: Past Climate Variability

What do we know about past climate variability?

Climate varies naturally on many different time scales. Some variations are regular such as the heating and cooling from day to night and summer to winter. Other variations are less regular. The variations in the tropical Pacific, associated with the El Nino Southern Oscillation phenomenon, typically occur every 3 to 5 years. They can lead to droughts in Australia and flooding in the U.S. Gulf Coast states. Climate also varies on much longer time scales from centuries and millennia to tens of thousands of years to million of years. Climate variations on shorter time scales are superimposed on these variations occurring on longer time scales.

Global surface temperatures have warmed by about 0.6°C over the past century. Most climate scientists agree that the warming in the latter half of the 20th century is primarily due to human-induced changes in greenhouse gases. Before the industrial era, variations in climate were forced by natural mechanisms. Variations over the past 2000 years occurred in response to changes in the sun's brightness and explosive volcanic eruptions. Much larger climate variations also occurred as the Earth varied between glacial and interglacial periods. These orbital variations are related to oscillations in the Earth's orbit due to the gravitational pull with the Sun, Moon, and other planets. Over hundreds of millions of year, the Earth's surface temperature has also varied as the continents have drifted and the composition of the Earth's atmosphere has changed.

Past climate variability gives us a longer-term perspective to test our understanding of the climate system and the interactions among the atmosphere, oceans, and land surface. At NCAR, we also use information about past variability to see how realistic the climate models being used to project future climate change are when the large climate forcings of the past are imposed.