Climatologists and environmental scientists have been cataloging the global warming trend for decades, examining multiple fields of data that show fluctuations over a period of eons. And despite what appears to be a consistent trend warming that has been taking place since the 18th century – when levels of atmospheric CO2 began to climb steadily – there have been anomalies in the data.

One period was the three decades that fall between the 1940’s and 1970’s when no significant terrestrial warming took place, and the Pacific Ocean was anomalously cold. The Pacific is somewhat of a wild card when it comes to our climate, since it is responsible for the weather patterns known as El Niño and La Niña that can swing global average temperatures by as much as 0.3 degree Celsius.

For the past decade or so the tropical Pacific has again gone cold and a new study suggests that it may once again be related to the recent “pause” in global warming of average temperatures. Although the past decade also qualifies as the hottest on record, the trend has been milder than expected, with average surface temperatures plateauing for many years.

This is in stark contrast to the end of the 20th century, when rising concentrations of greenhouse gases in the atmosphere accelerated warming to new heights. To explain this, climate scientists Shang-Ping Xie and Yu Kosaka of the Scripps Institution of Oceanography at the University of California looked to the Pacific Ocean, using observable data and an advanced computer model.

The latter came from the US Department of Commerce’s Geophysical Fluid Dynamics Laboratory computer model of the oceans and atmosphere. By adding in sea-surface temperatures of an oceanic area covering roughly 8 percent of the globe, the researchers were able to mimic the recent hiatus in global warming as well as weather phenomena like the prolonged drought in the southern US.

The results were published in the Aug. 29th edition of Nature Magazine. In it, Xie observed that the “tropical Pacific is the engine that drives the global atmosphere and climate. There were epochs of accelerated and stalled warming in the past.” This included the pause in a global warming trend between the 1940s and 1970s, which has often been attributed to sunlight-blocking air pollution from Europe, the Soviet Union and the US.

Other factors have also been considered – volcanoes, an unusually weak solar cycle, air pollution from China – when looking at restraining trends in global warming. Some of the observed climate effects may also stem from other ocean dynamics such as variations in the mixing of surface and deep ocean waters. And the meltdown of significant ice from Greenland or Antarctica might even cool oceans enough to offset the extra heat trapped by rising levels of greenhouse gases for a time.

What is less clear at this point is what is driving cycles of cooling and heating of tropical Pacific Ocean waters. But it is clear that the cool Pacific pattern cannot persist forever to cancel out the extra heat trapped by rising CO2 concentrations, Xie notes. As climate modeler Gavin Schmidt of the NASA Goddard Institute for Space Studies recently stated:

We need updates to the forcings and a proper exploration of all the different mechanisms together. This has taken time but will happen soon-ish.

And despite any pause in the trend toward hotter temperatures, the first decade of the 21st century was still the hottest recorded decade since the 1880s, and it included record heat waves in Russia and the US as well as a precipitous meltdown of Arctic sea ice and surging sea level rise. Atmospheric concentrations of CO2 touched 400 parts per million on Mauna Loa in May, a first in the time line of human existence.

A cooler Pacific due to prolonged La Niña activity may have restrained global warming for the past decade or so, but it is unlikely to last. As Xie noted:

This effect of natural variability will be averaged out over a period of 100 years. and cannot argue away the threat of persistent anthropogenic warming that is occurring now.

These warnings are key since any changes or anomalous readings are often seized upon by Climate Change deniers as evidence that the problem does not exist, is not man-made, or is at least not as severe as otherwise predicted. But in the coming decades, even the most benign scenarios are still fraught with peril. If the worst is to be averted, extensive and positive changes need to be made now.

Source: news.cnet.com, nature.com