Wednesday, February 25, 2015

Are We Entering a New Period of Rapid Global Warming? - Bob Henson

Residents of New England may understandably look back at 2015 as the year of their never-ending winter.  For the planet as a whole, though, this year could stand out most for putting to rest the “hiatus”— the 15-year slowdown in atmospheric warming that gained intense scrutiny by pundits, scientists, and the public.  While interesting in its own right, the hiatus garnered far more attention than it deserved as a purported sign that future global warming would be much less than expected.  The slowdown was preceded by almost 20 years of dramatic global temperature rise, and with 2014 having set a new global record high, there are signs that another decade-plus period of intensified atmospheric warming may be at our doorstep.
Departures from average sea-surface temperature (degrees C) and wind (arrows) that typically prevail when the Pacific Decadal Oscillation is in its positive mode (left) and negative mode (right). (Credit: University of Washington) Click to Enlarge.

The most compelling argument for a renewed surge in global air temperature is rooted in the Pacific Decadal Oscillation (PDO).  This index tracks the fingerprint of sea surface temperature (SST) across the Pacific north of 20°N.  A closely related index, the Interdecadal Pacific Oscillation (IPO), covers a larger swath of the entire Pacific.  Both the PDO and IPO capture back-and-forth swings in the geography of Pacific SSTs that affect the exchange of heat between ocean and atmosphere (see Figure at right).  We’ll use PDO as shorthand for both indexes in the following discussion.
From the AMS meeting
The hiatus was discussed at length in a series of talks during the annual meeting of the American Meteorological Society last month in Phoenix.  Jerry Meehl, from the National Center for Atmospheric Research (my former employer), gave a whirlwind 15-minute overview of hiatus-oriented research conducted over the last six years.  Meehl’s talk can be viewed online.  More than 20 papers have studied the hiatus and its links to the PDO/IPO, according to Matthew England (University of New South Wales).  Most of the flattening of global temperature during the hiatus can be traced to cooler-than-average conditions over the eastern tropical Pacific, which pulled down global averages.  An emerging theme is that natural, or internal, variability in the tropical Pacific can explain at least half of the hiatus.  NCAR’s Clara Deser presented new modeling evidence along these lines (see video online). Other factors may be involved as well, including a series of weak volcanic eruptions that could explain a small part of the hiatus, according to a recent analysis by Ben Santer (Lawrence Livermore National Laboratory). 

One crucial point is that global warming didn’t “stop” during the hiatus:  the world’s oceans actually gained heat at an accelerated pace.  Trade winds blew more strongly from east to west across the Pacific, consistent with the tendency toward La Niña conditions, as described in this open-access article by NCAR’s Kevin Trenberth and John Fasullo.  Over parts of the central tropical Pacific, trade winds averaged about 3 mph stronger during 1999-2012 compared to 1976-1988.  These speeds are higher than for any previous hiatus on record, bolstering the idea that other factors may have joined this negative PDO/IPO phase.  The faster trade winds encouraged upwelling of cooler water to the east and helped deepen and strengthen the warm pool to the west—enough, in fact, to raise sea level around the Philippines by as much as 8 inches.  Other parts of the deep ocean warmed as well.  A new study led by Dean Roemmich (Scripps Institution of Oceanography) maps the areas of greatest ocean heating from 2006 to 2013 and finds that significant warming extended to depths of greater than 6600 feet. 

What next for the PDO?
The PDO index, as calculated at the University of Washington, scored positive values during every month in 2014, the first such streak since 2003.  By December it reached +2.51, the largest positive value for any December in records that go back to 1900.  The January value from UW was 2.45, again a monthly record. (NOAA calculates its own PDO values through a closely related methodology.)
When looking at global temperature over a full PDO cycle (1970s to 2010s), the overall rise becomes evident, despite the flattening observed in the last 15 years. (Credit: NOAA) Click to Enlarge.
“I am inclined to think the hiatus is over, mainly based on the PDO index change,” NCAR’s Kevin Trenberth told me.  While Matthew England isn’t ready to offer such a prediction, he emphasized that any post-hiatus global temperature rise is likely to be fairly rapid.  Trenberth also commented on an interesting NOAA analysis (see Figure left):  “If one takes the global mean temperature from 1970 on, everything fits a linear trend quite well except 1998.”

A record-strong El Niño occurred in 1998, providing an unusually powerful boost to global temperature and fueling years of subsequent declarations that “global warming stopped in 1998.”  The record warmth of 2014 made it clear that global warming has no intention of stopping, and the next few years are likely to reinforce that point.

Read more at Are We Entering a New Period of Rapid Global Warming?

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