Natural factors: Internal variability
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Professor Iain Stewart explains how some changes in climate have no external trigger. These changes are instead caused by interactions within the climate system itself, often involving positive feedbacks. One example of such ‘internal variability’ is the El Niño–La Niña cycle, which can cause temporary warming and cooling. But these changes are short-term, only lasting a few years. No known natural climate cycles cause the type of long-term global warming trend observed since 1970.
One example of internal variability is El Niño. The cycle begins when the trade winds blowing from east to west over the Pacific Ocean become slightly weaker than usual. This reduces the flow of warm surface water being pushed westwards, resulting in a warming at the surface of the eastern Pacific and a cooling in the west. An El Niño is often followed by the opposite conditions, known as La Niña, in which the surface waters in the eastern Pacific cool. Both phenomena affect atmospheric circulation patterns and influence global climate. While El Niño increases global temperature, La Niña decreases it. This cycle repeats itself on a timescale of about five years.
One example of internal variability is the Arctic oscillation (AO), which is associated with changing patterns of air pressure in the northern hemisphere. The atmospheric pressure in the polar regions is typically high, in contrast to subpolar latitudes where it is low. One phase of the AO increases this pressure difference, giving the poles even higher pressure than usual and the subpolar regions lower pressure. This brings warmer weather to parts of Europe and North America, leaving the Arctic colder than usual. The other phase of the AO brings the opposite conditions, resulting in a warmer-than-usual Arctic and colder weather in the subpolar regions. Because of this seesaw effect, the AO has little effect on global temperatures, but can significantly influence local and regional weather.
Many external factors affect our planet’s climate, including variations in the Sun’s output and slight shifts in the Earth’s orbit. Volcanic eruptions also have an effect, and since they’re driven by processes beneath the Earth’s surface that are mostly independent of the other, interconnected components of the climate system, they’re considered an ‘external’ factor. Scientists use the term internal variability to refer to any change in climate that occurs without an external change to trigger it. Positive feedbacks within the climate system can magnify tiny initial changes. But such variability usually lasts just a few years. No known natural climate cycles cause the type of long-term global warming trend observed since 1970.