The Pacific Ocean fills in another piece of the global warming puzzle

פורסם: 4 בספט׳ 2013, 6:39 על ידי: Sustainability Org   [ עודכן 4 בספט׳ 2013, 6:39 ]

Evidence continues to mount that the slowed warming of global surface temperatures is mainly due to changes in the oceans

Tuesday 3 September 2013
Jigsaw puzzle with gold piece
The global warming puzzle continues to be pieced together. Photograph: George Diebold/Getty Images

A new study published in the journal Nature incorporates temperature changes in the tropical Pacific Ocean into an advanced climate model, and finds that the model can reproduce observed global surface temperature changes remarkably well.

This graph shows the good match between model temperatures in the last few decades (in red) and measured temperatures (in black). Just accounting for radiative changes doesn't reproduce the recent surface warming slowdown (in purple).
This graph shows the good match between temperatures in the Nature paper model (in red) and measured temperatures (in black). Just accounting for human and solar climate influences doesn't reproduce the recent surface warming slowdown (in purple).

Importantly, as authors Yu Kosaka and Shang-Ping Xie from the Scripps Institution of Oceanography explain, accounting for the changes in the Pacific Ocean allows the model to reproduce the slowed global surface warming over the past 15 years. It also accurately reproduces the regional and seasonal changes in surface temperatures, which adds confidence that their results are meaningful.

"Our results show that the current hiatus is part of natural climate variability, tied specifically to La-Niña-like decadal cooling … For the recent decade, the decrease in tropical Pacific sea surface temperature has lowered the global temperature by about 0.15 degrees Celsius compared to the 1990s".

Despite only covering 8.2 percent of the Earth's surface, these results suggest that the tropical Pacific Ocean plays a major role in short-term changes in the average global surface temperature. And over the past 15 years, it's offset most of the global surface warming from the increased greenhouse effect.

These results are broadly consistent with several other important recent papers investigating the role of the oceans in global warming. For example, the model used in this study finds that the overall heating of the planet has not slowed when the warming of the oceans are taken into account, as studies led by John Abraham, myself, and several others have also concluded.

Research led by Gerald Meehl has similarly focused on the importance of the Pacific Ocean in short-term global surface temperature changes. His climate model predicts that there will be decades when surface temperature changes are relatively flat because more heat is transferred to the deep oceans, precisely as we have observed over the past decade. Meehl discussed the Kosaka & Xie study with Carbon Brief,

"This paper basically confirms, with a novel methodology, what we originally documented in our Nature Climate Change paper in 2011 and followed up with in our Journal of Climate paper ... We went beyond [the new paper] to show that when the tropical Pacific was cool for a decade ... more heat is mixed into the deeper ocean, something the new paper doesn't address."

Kevin Trenberth, who co-authored several of these important ocean studies, has likewise pointed to the important role of the Pacific Ocean in transferring more heat to the deep oceans.

"The cause of the shift is a particular change in winds, especially in the Pacific Ocean where the subtropical trade winds have become noticeably stronger, changing ocean currents and providing a mechanism for heat to be carried down into the ocean. This is associated with weather patterns in the Pacific, which are in turn related to the La Niña phase of the El Niño phenomenon."

Research by Masahiro Watanabe of the Japanese Atmosphere and Ocean Research Institute has also suggested that the transfer of heat to the deep oceans and corresponding slowed global surface warming is related to changes in the Pacific.

Thus the scientific picture is becoming increasingly clear that the Pacific Ocean has played a large role in the slowed surface warming in recent years, but the warming of the oceans and planet as a whole have continued unabated. Thus the slowed surface warming is very likely to be a temporary effect, similar to the flat global surface temperatures between 1940 and 1970 when the Pacific Ocean was in another cool cycle.

Although it may be a natural reaction to hope that the recent slowed surface warming suggests that climate change isn't an imminent threat, the scientific evidence simply does not support such optimism. Climate scientist Judith Curry, who I recently criticized for failing to grasp the concept of climate risk management, recently articulated this rosy view on her blog. By focusing at the model simulation data specifically from 1975 to 1998, Curry incorrectly argued that the study supports the position that global warming is mostly natural.

There are a few major problems with this argument. Between 1975 and 1998 when the Pacific Ocean was in a warm phase of its cycle, it accounted for some of the observed global surface warming (though less than Curry asserts; about 30 percent). But the thing about cycles - they're cyclical. Focusing on the warm cycles while ignoring the cool cycles is an example of classic cherry picking.

If we look at the full record, for both 1950–2012 and 1970–2012 (the Pacific Ocean temperature data are most reliable since 1970), according to the model used in this study, the Pacific Ocean has actually had a slight overall cooling effect on global surface temperatures. It's also important to note that this cycle is just moving heat around between oceans and air - the overall warming of the planet has remained steady and rapid.

Thus the body of scientific research consistently shows that global warming continues unabated, and is predominantly human-caused. The Pacific Ocean has likely played a significant role in the slowed global surface warming over the past 15 years by transferring more heat to the deep oceans, but that change appears to be a temporary one. When the Pacific Ocean enters its next warm cycle, we're likely to see a rapid warming of global surface temperatures. If we continue to use the temporary slowed surface warming as an excuse to delay climate action, we'll regret that decision when the surface warming kicks in with a vengeance.