Natural variations in climate
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The Earth’s climate has exhibited marked "natural" changes, with time scales varying from many millions of years down to a few years. For example:
- Changes in land and ocean floor topography have had major influences on global climate at time scales of 50 million to 150 million years. (These changes influenced the patterns of absorption of incoming radiation from the sun, and affected circulation patterns in the atmosphere and oceans).
- Over the last two million years the onset and recession of the great Ice Ages were probably influenced by changes in the earth’s orbit and the tilt of its axis, which caused systematic variations in the amount and distribution of solar radiation. The difference in globally averaged temperatures between glacials (“ice ages”) and interglacials is about 5°C.
- Since the end of the last ice age (14,000–10,000 years ago) globally averaged surface temperatures have fluctuated over a range of up to 2°C on time scales of centuries or more. Factors influencing these changes probably included fluctuations in the radiation output from the sun, and changes in circulation and overturning in the oceans. These temperature variations through the Holocene (the last 10,000 years) have been small compared to those during ice ages. In fact the Holocene has probably been the longest “warm stable” period over the past 400,000 years.
- Over periods of a few years, fluctuations in global surface temperatures of a few tenths of a degree are common. Some of these are related to the El Niño–Southern Oscillation phenomenon, and major volcanic eruptions have also had some effects.
The temperature fluctuations discussed above were often accompanied by variations in wind patterns, rainfall, drought, and sea level. Rapid and large changes in temperature occurred during the last glacial period and the deglaciation that ended it, generally associated with changes in the circulation of the atmosphere and ocean. For example central Greenland temperatures increased by 7°C or more in a few decades about 11,500 years ago. The Royal Society of New Zealand’s Alpha 120 – Unlocking the Ice House – Antarctica and its Role in Climate Change (PDF) – provides an interesting perspective on past variations in climate, including abrupt changes.
These natural fluctuations must be taken into account in any search for human influences on global climate (for example due to enhanced greenhouse gas emissions). This is because estimates of such human influences on temperature over the past century based on physics calculations are of a similar size to the expected natural fluctuations. The detection of human-induced climate change was addressed by the Intergovernmental Panel on Climate Change (IPCC) in their 2007 assessment: Climate Change 2007: The Physical Science Basis. This assessment states that the warming over the past 100 years is very unlikely to be entirely natural in origin. It concludes that “while many factors continue to influence climate, scientists have determined that human activities have become a dominant force, and are responsible for most of the warming observed over the past 50 years”.
Part of the evidence is from reconstructions of the Northern Hemisphere temperature over the past 1,000 years using various paleoclimate data, shown in this figure from the IPCC assessment.
Researchers are making substantial progress in understanding seasonal to interannual anomalies in climate, including El Niño. Various organisations (including NIWA) now provide seasonal climate outlooks which exhibit useful skill.
NIWA’s research on climate variability concentrates on identifying Southern Hemisphere climate variations which affect New Zealand (including the El Niño), understanding their causes, using this knowledge to improve seasonal forecasting methods, and investigating trends and variability in New Zealand and South Pacific climate records over the last 150 years.
Forster, P.; Ramaswamy, V.; Artaxo, P.; Bernsten, T.; Betts, R.; Fahey, D.W.; Haywood, J.; Lean, J.; Lowe, D.C.; Myhre, G.; Nganga, J.; Prinn, R.; Raga, G.; Schulz, M.; Van Dorland, R. (2007). Changes in atmospheric constituents and in radiative forcing. In: Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.B.; Tignor, M.; Miller, H.L. (eds). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, pp. 129-234. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Prepared by David Wratt and Brett Mullan<