Natural Climate Cycles
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 (IPCC 2007a Ch.6).
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, 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 conditions to different parts of the world (IRI 2007). For example, in the U.S., El Niño events can result in dry weather in the pacific northwest and southeast, but wet weather in the southwest (IRI 2008).
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, and the Atlantic Multi- decadal Oscillation (AMO), occurring on approximately 65-85 year cycles (Deser et al. 2010). It is not yet well known how each of these reoccurring cycles might interact with the enhanced greenhouse effect.
For more information about natural climate cycles and their implications, see a presentation by paleoecologist Connie Millar.
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 exacerbate warming as greenhouse gas concentrations rise (Wolff 2011). They may also provide insight on regional impacts of climate change, which will be enormously important 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 2007a Ch.6), and may produce conditions that have not been anticipated.
Need more information?
See the following primers and resources for more introductory information on climate change.
Climate Change Resource Center:
United States Global Change Research Program:
Global Climate Change Impacts in the United States
The US EPA
Climate Change Science Facts
Anderson A.; Bows, A. 2011. Beyond 'dangerous' climate change: emission scenarios for a new world. Philosophical Transactions of the Royal Society. 369: 20-44.
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Carbon Dioxide Information Analysis Center (CDIAC). 2011. Recent Greenhouse Gas Concentrations. (Accessed 12-8-2011)
Center for Climate and Energy Solutions (C2E). 2012. Long Term Trends in Carbon Dioxide and Surface Temperature. (Accessed 1-9-2012).
Deser, C.; Alexander, M.A.; Xie, S.P.; Phillips, A.S. 2010. Sea Surface Temperature Variability: Patterns and Mechanisms. Annual Review of Marine Science. 2: 115-143.
Global Carbon Project. 2011. Carbon budget and trends 2010. (Accessed 12-8-2011)
Hansen, J.E. 2003. Can we defuse the global warming time bomb? (Accessed 12-8-2011)
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The International Research Institute for Climate and Society (IRI). 2007. Overview of the ENSO System. (Accessed 12-8-2011)
The International Research Institute for Climate and Society (IRI). 2008. Global Effects of ENSO. (Accessed 12-8-2011). Formerly available at: http://iri.columbia.edu/climate/ENSO/globalimpact/temp_precip/index.html
Mann, M.E.; Zhang, Z.; Rutherford, S.; Bradley, R.S.; Hughes, M.K.; Shindell, D.; Ammann, C.; Faluvegi, G.; Ni, F. 2009. Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science. 27 (326): 1256-1260.
NASA - Goddard Institute for Space Studies. 2011. NASA Research Finds 2010 Tied for Warmest Year on Record. Research News.
IPCC, 2000: IPCC Special Report: Emissions Scenarios. Nakicenovic, N.; Swart, R. (Eds.) Cambridge University Press, Cambridge, UK. 570 pp.
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IPCC, 2007b: Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K.; Reisinger, A. (eds.)]. IPCC, Geneva, Switzerland, 104 pp.
IPCC, 2011: Summary for Policymakers. In: Intergovernmental Panel on Climate Change, Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C. B.; Barros, V.; Stocker, T.F.; Qin, D.; Dokken, D.; Ebi, K.L.; Mastrandrea, M. D.; Mach, K. J.; Plattner, G.K.; Allen, S.; Tignor, M.; Midgley, P. M. (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA.
Ramanathan, V.; Feng, Y. 2009. Air pollution, greenhouse gases and climate change: Global and regional perspectives. Atmospheric Environment. 43: 37-50.
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United States Global Change Research Program (USGCRP). 2009. Global Climate Change Impacts in the United States. Karl, T.R.; Melillo, J.M.; Peterson, T.C. (eds). Cambridge University Press.
Wanner, H.; Beer, J.; Butikofer, J.; Crowley, T.J.; Cubasch, U.; Fluckiger, J.; Goosse, H.; Grosjean, M.; Joos, F.; Kaplan, J.O.; Kuttel,M.; Muller, S.A.; Prentice, C.; Solomina, O.; Stocker, T.F.; Tarasov, P.; Wagner,M.; Widmann, M. 2008. Mid- to Late Holocene climate change: an overview. Quaternary Science Reviews. 27: 1791-1828.
Wolff, E.W. 2011. Greenhouse gases in the Earth system: a palaeoclimate perspective. Philosophical Transactions of the Royal Society. 369: 2133-2147.