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Global warming's 'false pause' linked to Pacific Ocean, may mask serious concerns

The 'warm phase' of the Pacific Decadal Oscillation. Credit: Wikimedia Commons


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    Scott Sutherland
    Meteorologist/Science Writer

    Friday, February 20, 2015, 8:33 - Researchers may have found the culprit responsible for the recent 'slowdown' or 'pause' in the rise of global temperatures - a combination of long term cycles in the Atlantic and Pacific oceans, known for having a big influence on northern hemisphere temperatures.

    Over the past 15 years or so, global surface temperatures have been trending rather flat with time, this contrary to the fact that rising carbon dioxide emissions had been causing an alarming rise in global temperatures leading up to the new millennium. While it has been suggested by some that this so-called 'pause' shows that the warming evidenced since the mid-1800s was simply a natural process, which is now swinging back in the other direction, new research is showing that it is the pause that is due to natural variations, and the masked signal from human-caused warming could return with a vengeance once this cycle swings back the other way.

    What could this mean for the future?

    If the human-caused warming trend is largely being masked by this recent strong 'down-trend' in the Pacific Multidecadal Oscillation (PMO), an upswing in the oscillation would not only remove the mask, allowing the true warming trend to show through, but it could augment the trend to a dangerous extreme. We can hope for a possible down-swing in the Atlantic Multidecadal Oscillation (AMO) to offset this, but seeing as there's no reliable way to forecast these long-term trends, that would not be a very wise choice.

    (The wise choice would be to act, to lower carbon emissions and enact adaptations to the coming changes, while this 'pause' - although as pointed out below, a better name would be 'speed bump' - lasts.)

    Identifying & explaining the factors

    Which natural cycles are acting as this 'mask'? According to new research out of Penn State University, it's a combination of patterns in the North Atlantic and North Pacific oceans, known as the Atlantic Multidecadal Oscillation (AMO) and the Pacific Multidecadal Oscillation (PMO).

    According to Penn State:

    The Atlantic Multidecadal Oscillation (AMO) describes how North Atlantic sea-surface temperatures tend to oscillate with a periodicity of about 50 to 70 years. The Pacific Decadal Oscillation (PDO) varies over a broader range of timescales. The researchers looked only at the portion of the PDO that was multidecadal - what they term the Pacific multidecadal oscillation (PMO).

    As the AMO and PMO shift the patterns of ocean temperature back and forth in the Atlantic and Pacific oceans, the oscillations aren't synced up, but they do combine to produce an overall shift in northern hemisphere temperatures, known as the Northern Hemisphere Multidecadal Oscillation (NMO). The graph below gives an estimate of how the pattern fluctuates over time.


    The overall NMO pattern that results from the combination of the AMO and the PDO. Credit: Michael E. Mann/RealClimate.org

    According to what Mann wrote on RealClimate.org:

    "It is true that Earth's surface warmed a bit less than models predicted it to over the past decade-and-a-half or so. This doesn't mean that the models are flawed. Instead, it points to a discrepancy that likely arose from a combination of three main factors. These factors include the likely underestimation of the actual warming that has occurred, due to gaps in the observational data. Secondly, scientists have failed to include in model simulations some natural factors (low-level but persistent volcanic eruptions and a small dip in solar output) that had a slight cooling influence on Earth’s climate. Finally, 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'."

    Mann's latest study, published in the journal Science with fellow researchers Byron A. Steinman, from the University of Minnesota-Duluth and Sonya K. Miller from Penn State, looked at climate model runs along with real-world observations. The results of their research show how there's been very little contribution from the AMO in recent years, but a big shift into the 'cool' phase of the PMO is having a major impact on northern hemisphere temperatures.

    Thus, the 'flattening' of global surface temperatures observed over the past 15 years or so is due to the cooling caused by the shift in the PMO, balancing out the the artificial warming from excess carbon dioxide emissions.

     

    Credit: NOAA. Edits by Scott Sutherland. 'T' indicates years that are tied.

    This trend is even somewhat visible in the shorter-scale Pacific Decadal Oscillation, shown in the graph above. Several of the warmest years in the past two decades were during times when the PDO was more neutral or in its positive 'warm' phase. Even those short-term upticks allowed global temperatures to climb enough to rank those years into the top ten warmest years on record.

    Sources: PSU | RealClimate | NOAA


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