A new study published in the journal Nature has an explanation for the recent slow down in global warming: La Niña-like conditions.
La Niña is a natural cooling of the eastern Pacific ocean in the tropics which can last for one to several years. When a La Niña occurs in the Pacific, the ocean generates less heat than normal and global temperatures cool. Over the last 10-15 years, La Niña-like conditions have occurred frequently. Conversely, El Niño-like conditions, associated with warming of eastern tropical Pacific waters, have seldom occurred. So not as much heat has been injected into the climate system.
The Nature study “Recent global-warming hiatus tied to equatorial Pacific surface cooling“, concludes the dominance of La Niña-like conditions have dampened the rise in global temperatures driven by manmade greenhouse gas.
“Our results show that the current hiatus [in global warming] is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling,” the study, by scientists Yu Kosaka and Shang-Ping Xie, says.
The Kosaka and Xie study essentially re-iterates the hypothesis put forward by Texas state climatologist John Nielsen-Gammon last year.
“[W]e see a couple of recent La Niñas have caused the recent global temperature trend to level off,” Nielsen-Gammon wrote.
Some scientists are not convinced the La Nina-focused hypothesis holds up. See Andrew Freedman’s piece for some different perspectives: Study Ties Global Warming ‘Hiatus’ to Pacific Cooldown
Why is the slower warming a surprise?
The warming slow down has perplexed scientists, as climate model projections projected a swift and steady rise in temperatures over the last one to two decades.
An independent commentary published today in the journal Nature Climate Change proposes an explanation: computer models have a poor handle on the cycle of La Niñas and El Niños and other possible factors which, along with greenhouse gases, have governed the course of recent temperatures.
The commentary “Overestimated global warming over the past 20 years” notes observed warming is much less than projected by models used by the United Nation’s Intergovernmental Panel on Climate Change (IPCC).
How should this discrepancy between models and real-world warming be reconciled? The commentary recommends scientists work to better understand the climate system and look out the window.
“Ultimately the causes of this inconsistency will only be understood after careful comparison of simulated internal climate variability and climate model forcings with observations from the past two decades, and by waiting to see how global temperature responds over the coming decades,” the commentary says.
Of course, the Kosaka and Xie study discussed above – which finds La Niña-like conditions are the best explanation for the warming hiatus – takes a step in this direction. The authors develop a model that integrates actual ocean temperatures in the tropical Pacific and, by doing this, are able to successfully simulate the recent warming slow down. They recommend their method used for diagnosis be applied to improve predictions.
Warming has slowed, not stopped
Despite the slow down in warming, temperatures have not ceased climbing. This fact is especially apparent when examining recent trends in temperatures binned according to whether it’s a La Niña, El Niña or neutral year (neither La Niña or El Niño).
Look at this chart which shows the indisputable rise in temperature for the different phases of this Pacific ocean cycle:
Post editorial: Slow down in warming no reason to put off action on greenhouse gases
In an editorial published Sunday, the Post discusses the warming slow down, a topic prominently addressed in the forthcoming IPCC review of climate science, parts of which have been leaked:
The IPCC admits that it doesn’t have a sure answer to a vexing question: Why has warming slowed a bit in the past decade or so? With medium confidence, the draft suggests that the explanation lies in a mix of natural variations and things such as the oceans absorbing more heat or more volcano debris reflecting sunlight back into space. It’s also possible, the scientists admit, that the planet’s sensitivity to greenhouse emissions is lower than middle-of-the-road projections.
In my opinion, the Post’s Editorial Board smartly argues the slow down is not a reason to stop pursuing policies to curb greenhouse gas emissions:
Some uncertainties are inevitable when humans try to comprehend an incredibly complex climate system. Scientists might not be able to answer some questions for years, until they can look back at what changed after so much carbon dioxide entered the atmosphere so quickly. Those inevitable uncertainties are all the more reason for governments, starting with the United States’, to head off the ample risks of continuing to release huge amounts of greenhouse gases into the air and to set about it with speed and ambition.