Natural Climate Cycles

Climate varies without human influence, and this natural variation is a backdrop for the human-caused climate change occurring now. These patterns hold important lessons for understanding the magnitude and scope of current and future climate changes.

Cyclical variations in the Earth’s climate occur at multiple time scales, from years to decades, centuries, and millennia. Cycles at each scale are caused by a variety of physical mechanisms. Climate over any given period is an expression of all of these nested mechanisms and cycles operating together.

Millennial Climate Cycles

Major glacial (cold) and interglacial (warm) periods are initiated by changes in the Earth’s orbit around the Sun, called Milankovitch cycles. These cycles have occurred at different intensities on multi-millennial time scales (10,000 – 100,000 year periods). The orbital changes occur slowly over time, influencing where solar radiation is received on the Earth’s surface during different seasons (NASA 2000).

By themselves, these changes in the distribution of solar radiation are not strong enough to cause large temperature changes. However they can initiate powerful feedback mechanisms that amplify the slight warming or cooling effect caused by the Milankovitch cycle. One of these feedbacks is caused through changes in global surface reflectivity (also called albedo). Even a slight increase in solar radiation at northern latitudes can increase ice melt. As a result of ice loss, less sunlight is reflected from the bright white surface of the ice, and more is absorbed by the Earth, increasing overall warming. A second feedback mechanism involves atmospheric greenhouse gas concentrations, such as carbon dioxide. The slight warming initiated by changes to Earth’s orbit warms oceans, which allows them to release carbon dioxide. As we’ve seen, more carbon dioxide in the atmosphere causes more warming, creating an amplifying effect (Hansen 2003). Distinct feedbacks in atmospheric CO2 concentrations may lag warming or cooling caused by orbital changes by as much as 1000 years.

In this way, what begins as fairly minor changes in orbit can produce the glacial and interglacial cycles of the last 800,000 years. A major concern with current climate change is that similar feedback mechanisms will cause a ‘runaway’ warming effect in modern times that will be extremely difficult to halt or reverse.

Century-scale Climate Cycles

In addition to multi-millennial glacial and interglacial cycles, there are shorter cold-warm cycles that occur on approximately 200 to 1,500 year time scales. The mechanisms that cause these cycles are not completely understood, but are thought to be driven by changes in the sun, along with several corresponding changes such as ocean circulation patterns (Bond et al. 2001, Wanner et al. 2008). The Medieval Warm Period (900-1300 AD) and the Little Ice Age (1450 to 1900 AD) are examples of warm and cold phases in one of these cycles. Some of these cycles, such as the Medieval Warm Period, may be regional, not necessarily reflecting large changes in global averages. Understanding and reconstructing the regional patterns of climate change during each of these periods is considered very important in accurately analyzing future regional impacts such as drought patterns (Mann et al. 2009).

Interannual to Decadal Climate Cycles

Ocean-atmosphere interactions regularly cause climate cycles on the order of years to decades. One of the most well-known cycles is the El Niño-Southern Oscillation (ENSO), an interaction between ocean temperatures and atmospheric patterns (commonly known as El Niño or its opposite effect, La Niña). ENSO events occur every 3 to 7 years, and bring different weather conditions to different parts of the world (NASA 2009). For example, in the U.S., El Niño events can result in a flow of warm dry air into the Northwest, but above average rainfall  in the southeast (NASA 2009).

Many other cyclical changes due to oceanic and/or atmospheric processes have been described, such as the Pacific Decadal Oscillation (PDO) which occurs in cycles of 25-45 years (Mantua et al. 1997), and the Atlantic Multi-decadal Oscillation (AMO), occurring on approximately 65-85 year cycles (Deser et al. 2010). Scientists are studying how each of these reoccurring cycles might interact with the enhanced greenhouse effect. There is some evidence that global warming may be intensifying ENSO events (Li et al. 2013).

Implications

Natural climate cycles can help to understand what climate patterns are expected, and how the recent increase in greenhouse gas emissions is causing deviations from these expected patterns. They can offer insight into amplifying effects that may intensify warming as greenhouse gas concentrations rise (Wolff 2011). They may also provide insight on regional impacts of climate change, which will be veryimportant for developing adaptation strategies for human and ecological communities. However, it is important to recognize that current rates of global climate change are extremely rapid compared to past changes (IPCC 2013 Ch.5), and may produce conditions that have not been anticipated.