SEPARATING
HUMAN AND NATURAL INFLUENCES ON CLIMATE
The issues involved here are both simple and complex, and of critical importance is distinguishing between climate change and (normal) climate variability. They are “(s)imple because we know that the unnatural rise in global average temperatures is largely caused by burning fossil fuels, as well as by agricultural practices and damage to or destruction of natural features that absorb and store carbon. Complex because it's not easy to predict the various consequences and effects of interacting elements of the earth's natural systems. Everything is interconnected. This is especially true of the planet's natural cycles and the ways in which atmosphere, land, oceans, ice, and living things interact”[1].
As an example of natural cycles, El Niño/La Niña is a natural phenomenon centred in the tropical Pacific Ocean which occurs roughly every 4 to 7 years[2]. Under ‘normal’ (or neutral) conditions, waters to the north of Australia are very warm. As a result, air rises here (flowing in from the trade winds), bringing steady rainfall to the region, including much of eastern Australia. This flow pattern is known as the Walker circulation.
During an El Niño event (also known as El Niño–Southern Oscillation (ENSO)), more fully explained here, waters in the central Pacific become abnormally warm, reversing the trade winds, shifting the rising air and bringing drought to Australia[3]. During a La Niña-Southern Oscillation, the opposite happens, the Walker strengthens, bringing floods to Australia, thus:
In other words, (normal) climate variability needs to be distinguished from (man-made) climate change. This interrelationship between natural and human-made factors, must therefore constantly be borne in mind as these issues are analysed, and in assessing the warming pattern in a particular area of the globe, it may be necessary to factor out the El Niño effect where that is relevant, in order to assess the extent of warming which is the product of climate change.
But the system has a memory with events playing out over a century or more. A lull in one phase can lead to more severe repercussions down the track, and tiny changes in initial conditions may well effect eventual El Nino/La Nina activity in years to come: the so-called "butterfly effect". "If the greenhouse effect now on the El Nino [is to suppress it], in the future the response to that subsequent greenhouse warming will be much more dramatic".
But the system has a memory with events playing out over a century or more. A lull in one phase can lead to more severe repercussions down the track, and tiny changes in initial conditions may well effect eventual El Nino/La Nina activity in years to come: the so-called "butterfly effect". "If the greenhouse effect now on the El Nino [is to suppress it], in the future the response to that subsequent greenhouse warming will be much more dramatic".
How a warming world is already affecting ENSO and what might lie ahead
For complex interactions between the air and oceans, the impacts of the greenhouse effect are difficult to model, not least because they overlay natural fluctuations that run into years and decades. One difficulty is assembling historical data that can help researchers peer long enough into the past to separate a climate signal from the background variability.
Despite the paucity of El Niños and La Niñas with precise data, there is evidence to suggest that the impacts of ENSO are being compounded by climate change even now simply because of the super-position of ENSO conditions on a warmer background state. Examples include the rainfall extremes in Australia during the 2010-11 La Niña, and the record year for tropical Pacific cyclones and mass coral bleaching on the Great Barrier Reef and beyond during the 2015-16 El Niño.
One implication is that La Niñas will be easier to produce under global heating. The difference between sea surface temperatures and those at depth will be greater facilitating the coupling of winds and upwelling water necessary to trigger an event. Already, it is evident that modern La Niña years are much warmer than La Niña years of the past, showing that cycles of natural climate variability, such as La Niña and El Niño, are operating against the tide of rising global surface temperatures. [4]
For complex interactions between the air and oceans, the impacts of the greenhouse effect are difficult to model, not least because they overlay natural fluctuations that run into years and decades. One difficulty is assembling historical data that can help researchers peer long enough into the past to separate a climate signal from the background variability.
Despite the paucity of El Niños and La Niñas with precise data, there is evidence to suggest that the impacts of ENSO are being compounded by climate change even now simply because of the super-position of ENSO conditions on a warmer background state. Examples include the rainfall extremes in Australia during the 2010-11 La Niña, and the record year for tropical Pacific cyclones and mass coral bleaching on the Great Barrier Reef and beyond during the 2015-16 El Niño.
One implication is that La Niñas will be easier to produce under global heating. The difference between sea surface temperatures and those at depth will be greater facilitating the coupling of winds and upwelling water necessary to trigger an event. Already, it is evident that modern La Niña years are much warmer than La Niña years of the past, showing that cycles of natural climate variability, such as La Niña and El Niño, are operating against the tide of rising global surface temperatures. [4]
Current model projections also suggest that the number of extreme El Niños would double in frequency from one in 20 years to once every decade by the end of this century. For La Niñas, extreme events will go from once every 23 years to once every 13, according to Cai Wenju, a senior CSIRO scientist. “The worst extremes for Australia may come from El Niños because they worsen the droughts and severe bushfire seasons that are already under way.”
[1] David Suzuki and Ian Hannington, “How to fix the future, Sun-Herald, 2 April 2017, 26-27; also at http://www.smh.com.au/environment/how-to-fix-the-future-saving-the-planet-from-climate-change-requires-action--now-write-david-suzuki-and-ian-hannington-20170327-gv75h8.html
[2] Box, op cit, 4.4.4
[3] Ibid.
[4] Ben Domensino@Ben_Domensino, 31 December 2020 (Twitter)
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