Over the past 15 years, a strange thing has happened. On one hand, carbon dioxide concentrations have kept on shooting up thanks to humans burning fossil fuels—in May, we passed 400 parts per million for the first time in human history.
On the other hand, despite certain regions experiencing drastically warmer weather, global average temperatures have stopped increasing. Climate change deniers have seized upon this fact to argue that, contrary to the conclusions reached by major science academies (PDF) around the world, greenhouse gas emissions do not cause global warming.
As it turns out, the truth is much grimmer. A pair of scientists from Scripps Institution of Oceanography have determined that the underlying process of global warming has merely been masked by natural decade-scale variations in the temperature of Pacific Ocean surface waters, related to the El Niño/La Niña cycle. Once that’s finished, our planet’s warming will march onward as usual.
Climate scientists have speculated about the possibility that ENSO (the El Niño-Southern Oscillation, the proper term for the cycle) was behind the apparent hiatus in warming for some time, but the scientists behind the new study—Yu Kosaka and Shang-Ping Xie—are the first to take a quantitative look at the role of Pacific surface temperatures in pausing global warming as a whole. Their paper, published today in Nature, uses climate models to show that the abnormally cool surface waters observed over the Pacific since 1998 can account for the lack of recent warming entirely.
Why has the Pacific been abnormally cool for the past 15 years? Naturally, as part of ENSO, a large swath of the ocean off the western coast of South America becomes notably warmer some years (called El Niño events) and cooler in others (La Niña events). Scientists still don’t fully understand why this occurs, but they do know that the warmer years are related to the formation of high air pressures over the Indian Ocean and Australia, and lower pressures over the eastern part of the Pacific.
Because winds move from areas of high pressure to low pressure, this causes the region’s normal trade winds to reverse in direction and move from west to east. As they move, they bring warm water with them, causing the El Niño events; roughly the reverse of this process happens in other years, bringing about La Niña. As it happens, colder surface temperatures in the Pacific—either official La Niña events or abnormally cool years that don’t quite qualify for that designation—have outweighed warm years since 1998.
That, say Kosaka and Xie, is the reason for the surprising lack of increase in global average temperatures. To come to this conclusion, they developed a climate model that, along with factors like the concentration of greenhouse gases over time and natural variations in the solar cycle, specifically takes the ENSO-related cycle of Pacific surface temperatures into account.
Typically, climate models mainly use radiative forcing—the difference between the amount of energy absorbed by the planet and the amount sent back out to space, which is affected by greenhouse gas emissions—as a data input, but they found that when their model did so, it predicted that global average temperatures would increase much more over the past 15 years than they actually have. However, when the abnormally-cool waters present in the eastern Pacific were taken into account, the temperatures predicted by the model matched up with observed temperatures nicely.
In models, the presence of these cooler waters over a huge area (a region within the Pacific that makes up about 8.2% of the Earth’s surface) serves to absorb heat from the atmosphere and thus slow down the underlying warming process. If the phenomenon is representative of reality, the team’s calculations show that it has caused the planet’s overall average temperature to dip by about 0.27°F over the past decade, combating the effects of rising carbon dioxide emissions and causing the apparent pause in warming.
This isn’t the first localized climate-related event to have effects on the progression of climate change as a whole. Last week, other researchers determined that in 2010 and 2011, massive floods in Australia slowed down the global rise in sea level that would have been been expected from observed rates of glacier melting and the thermal expansion of sea water. In many cases, it seems, the subtle and complex dynamics of the planet’s climate systems can camouflage the background trend of warming, caused by human activity.
But that trend is continuing regardless, and so the most obvious impact of this new finding is a disconcerting one: the Pacific will eventually return to normal temperatures, and as a result, global warming will continue. The scientists don’t know exactly when this will happen, but records indicate that the Pacific goes through this longer-term cycle every decade or so, meaning that the era of an abnormally-cool Pacific will probably soon be over.
Perhaps most distressing, the study implies that the extreme warming experienced in recent years in some areas—including much of the U.S.—is actually less warming than would be expected given the amount of carbon dioxide we’ve released. Other regions that haven’t seen much warming yet, meanwhile, are likely in line for some higher temperatures soon.