Last year, quietly and without much fanfare from the mainstream press, the Pacific Ocean underwent a powerful shift. A shift from a state in which cooler surface waters absorbed atmospheric heat, to a phase in which surface water warming caused ocean heat to be transferred to the world’s already warming airs.
The shift was heralded by a powerful oceanic Kelvin Wave. One that brought warm water up from the depths and spread them across the Pacific Ocean surface. Ever since that time, warm Kelvin Waves have continued to refresh this surface water heat pool.
(Major Kelvin Wave that heralded a switch to strongly positive PDO values for the Pacific. Image source: NOAA/ESRL)
And so the Pacific Ocean surface warming continued throughout 2014.
By December, Pacific Decadal Oscillation (PDO) values, a measure directly tied to this warming, hit a new all-time record level of +2.51. That’s the highest and hottest PDO value since record-keeping began in 1900. One that only backed off slightly into January at +2.45 in the preliminary measure.
It’s a major swing in Pacific Ocean surface temperatures to a phase where more heat is dumped into the atmosphere. One that is causing some scientists to warn that a new period of rapid atmospheric warming may just be getting started.
(Sea surface temperature anomaly map shows very warm surface waters dominate much of the equatorial, eastern and northern Pacific in a signature that is strongly characteristic of a powerfully positive PDO on Thursday, February 26 2015. Image source: Earth Nullschool. Data Source: Global Forecast Systems Model.)
PDO and The Multi-Decadal Heat Pump
Pacific Decadal Oscillation (PDO) is a periodic change in sea surface temperature states in the Pacific. One that is thought by many oceanic and atmospheric researchers to have a strong influence on global weather and temperature variability.
In the negative state, PDO tends to encourage La Nina events which also coincide with a downswing in global temperatures as the vast Pacific waters take in more heat. In the positive state, PDO tends to encourage El Nino events which result in the great ocean belching heat into the air on a grand scale — pushing atmospheric temperatures higher.
All things being equal, this natural variability would pan out — marking upswings and downswings in a global average. However, human fossil fuel burning and related greenhouse gas emissions have bent this curve upward by trapping more and more heat in the lower atmosphere. So the case is now that during positive PDO phases, in which more El Ninos occur, atmospheric warming has tended to ramp drastically higher. And, consequently, during negative PDO phases, atmospheric warming has tended to merely slow down even as oceanic warming sped up.
You can see this speeding up and slowing down in the below graphic provided by Weather Underground:
(Global temperature change since 1900 vs phases of positive PDO [1925 to 1945 and 1975 to 1998] and negative PDO [1945 to 1975 and 1998 to 2014]. Image source: Weather Underground. Data Source: NOAA.)
In the era during which global temperatures have been increasingly driven by human greenhouse gas emissions, four phases of PDO have been recorded. Two were positive, two were negative.
In the first positive PDO phase during 1925 through 1945, global average temperatures jumped upward by about 0.5 degrees F (+0.25 F per decade). This initial surge in atmospheric warming abated as global temperatures remained roughly steady during the negative PDO period of 1945 to 1975 (+/- 0 F per decade). But by 1975, PDO values were positive again and the period through 1998 showed a rapid warming of about 0.8 F over 23 years (+0.4 F per decade). After the super El Nino of 1998, PDO values again trended negative as atmospheric warming continued at a somewhat slower pace of about +0.15 F per decade.
Global Warming and Related Ills to Ramp Up?
This underlying trend should be cause for serious concern.
The first issue is that we see warming now during negative PDO decades where we only saw cooling or zero warming before. Given the present rate of warming in the range of +0.15 F per decade during periods in which the Pacific Ocean is taking on atmospheric heat, one could expect the next positive PDO phase to see decadal warming in the range of +0.55 F or higher (or by about 1 C in 20-30 years).
Such a rapid pace of warming could challenge the fabled 2 C ‘point of no return’ before 2050 (for reference, we are at about +0.85 C above 1880s values now). And it is for this reason that some scientists are now starting to sound alarm bells.
In the recent Weather Underground commentary penned by Jeff Masters, Kevin Trenberth, an NCAR scientist and one of the world’s foremost experts on ocean temperature dynamics, noted:
“I am inclined to think the hiatus [in global temperature increases] is over, mainly based on the PDO index change. If one takes the global mean temperature from 1970 on, everything fits a linear trend quite well except 1998.”
(Global surface temperature anomalies. Image source: NOAA.)
Though Trenberth does not explicitly spell out the potential for an overall warming acceleration, he does point toward a return to the +0.29 F per decade trend line seen since 1970. Meanwhile, Matthew England of the University of South Wales warned in the same Weather Underground commentary that any post ‘hiatus’ warming would be likely to be very rapid.
Dr. Michael Mann, a climate scientist of considerable fame both due to his Hockey Stick tree ring study and due to his ongoing success fighting off smear campaigns launched by climate change deniers, recently put together climate model assessments that showed world temperatures exceeding the 2 C threshold by 2036 under business as usual greenhouse gas emissions. To reach such a high reading so soon would require in excess of 1 degree Fahrenheit warming over each of the next two decades. And such a rate of warming would be very rapid indeed, unprecedentedly rapid and well outside the linear trend line from 1970.
Michael Mann today made related comments at Realclimate on the more recent oscillations in Pacific Ocean sea surface temperature:
There is the possibility that internal, natural oscillations in temperature may have masked some surface warming in recent decades, much as an outbreak of Arctic air can mask the seasonal warming of spring during a late season cold snap. One could call it a global warming “speed bump”. In fact, I have… Given the pattern of past historical variation, this trend will likely reverse with internal variability, instead adding to anthropogenic warming in the coming decades. That is perhaps the most worrying implication of our study, for it implies that the “false pause” may simply have been a cause for false complacency, when it comes to averting dangerous climate change.
To these points, it is worth noting that any rate of warming above 0.3 F (0.2 C) per decade is extraordinary and terrifying. Such a rate is enough to achieve post ice age warming of 4 C in merely 2 centuries where it took 10,000 years to achieve such warming before. Warming at 0.4, 0.6 or 1 F per decade would be both drastic and devastating to current climates, geophysical stability, weather stability, glacial stability, water security, food security, and ocean health. In the current world, already warmed by about 0.85 C above 1880s levels any acceleration to current warming is a rather bad outcome on top of an already dangerous situation.
Hat Tip to Colorado Bob
Hat Tip to Bassman