Most people in New Zealand and Australia will be familiar with the terms El Niño and La Niña which describe the Pacific ocean-atmosphere oscillation and which influence local temperature and rainfall. For Australia, El Niño events tend to bring warmer temperatures and drought while La Niña often brings widespread rain and flooding. The floods of 1973-1974 occurred during a La Niña. La Niña events also tend to bring cooler temperatures. This oscillation between El Niño and La Niña creates natural variations in temperature from warmer El Niños to cooler La Niñas. This cycle does not cause long-term changes in temperature however as over time, the two tend to cancel each other out.
This cycle of El Niño/La Niña causes shifts in the temperature of the equatorial Pacific Ocean which lasts for decades or more. This is known as the Pacific Decadal Oscillation. Since 1998, the oscillation has been in a cooling phase so it should have been getting colder. Yet we know that the temperature over the last decade has actually increased by about 0.05°C. A paper published in Nature this month finds that this La Niña-like decadal cooling has actually lowered global temperatures by about 0.15°C for the last decade. This has cancelled out much of the warming due to human-made greenhouse gases. In an article published in The Conversation, the authors of this paper write,
When the climate cycle that governs that ocean cooling reverses and begins warming again, the planet-wide march toward higher temperatures will resume with vigor.
So the question should not be, has global warming stopped?, but rather, why isn’t it getting colder? The answer is because natural variation caused by the Pacific Decadal Oscillation is currently masking global warming which is still happening at a rate which is unprecedented in at least the last 11,300 years.
The bottom line is that those who claim that global warming has “stopped” or even “paused” are deluding themselves. The phrase “global warming” refers to climate change, including temperature increase, which is caused by mankind, and that has continued unabated. In fact, if it weren’t for the continued warming due to human activity, natural variations (like ENSO) would have brought about a notable cooling over the last decade or so. But earth hasn’t cooled during that period, not even at the surface where we notice it most immediately, and that’s because the man-made component — global warming — has continued.
Global warming is a consequence of rising atmospheric greenhouse gas concentrations that act to trap outgoing long-wavelength radiation and increase the amount of energy in the climate system. This energy is then distributed across the climate system, with the majority (90% or more) going into the oceans. How this energy is distributed across the climate system will, however, depend on the properties of the different parts of the system. It seems reasonable that there will be times when a larger fraction goes into the oceans and other times when this fraction reduces, increasing the fraction heating the surface and atmosphere.
No-one knows when the oscillation will swing back to a warming phase but should we be thinking about spraying the planet with sunscreen before this happens? Am I the only one who thinks we need to do something drastic before all of the ice melts?