• Global ocean warming started later in th

    From ScienceDaily@1:317/3 to All on Mon Aug 9 21:30:50 2021
    Global ocean warming started later in the 20th century than previously estimated

    Date:
    August 9, 2021
    Source:
    University of California - Santa Barbara
    Summary:
    In estimations of ocean heat content -- important when assessing
    and predicting the effects of climate change -- calculations have
    often presented the rate of warming as a gradual rise from the
    mid-20th century to today. However, new research could overturn
    that assumption, suggesting the ocean maintained a relatively
    steady temperature throughout most of the 20th century, before
    embarking on a steep rise.

    The newly discovered dynamics may have significant implications
    for what we might expect in the future.



    FULL STORY ==========================================================================
    In estimations of ocean heat content -- important when assessing and
    predicting the effects of climate change -- calculations have often
    presented the rate of warming as a gradual rise from the mid-20th century
    to today. However, new research from UC Santa Barbara scientists Timothy DeVries and Aaron Bagnell could overturn that assumption, suggesting
    the ocean maintained a relatively steady temperature throughout most of
    the 20th century, before embarking on a steep rise. The newly discovered dynamics may have significant implications for what we might expect in
    the future.


    ========================================================================== "There wasn't an onset of an imbalance until about 1990, which is
    later than most estimates," said DeVries, an associate professor in the Department of Geography, and a co-author on a paper that appears in the
    journal Nature Communications. According to the study, the period from
    1950 to1990 saw temperature fluctuations in the water column but no
    net warming. After 1990, the study continues, the entire water column
    switched from cooling to warming.

    These findings are the result of the addition of a largely underexplored
    factor in ocean heat content (OHC): deep ocean temperatures.

    "Prior studies didn't consider the deep ocean," said Bagnell, a graduate scholar in DeVries' laboratory and the paper's lead author. Because of
    the challenges involved in getting temperature measurements in the deep
    ocean (below 2,000 meters) that region has gone largely unaccounted for,
    and data has been sparse. "There is some existing data, from research
    cruises and autonomous floats," he added.

    The researchers used an autoregressive artificial neural network (ARANN)
    and machine learning methods to connect the dots between data points and "produce a single consistent estimate of the top-to-bottom OHC change
    for 1946 to 2019." The result was a trend that delays warming by decades
    over previous models.

    There are two main possibilities for why the effects of global warming
    took so long to reach the ocean, De Vries said.

    "One is that anthropogenic warming might have been weaker than previously thought during the 20th century, perhaps due to the cooling effects of
    aerosol pollution," he said. The other is that the deep ocean may still
    be exhibiting the effects of climate events long past.

    "It can take centuries for climate signals to propagate from the surface
    to the interior," he said. Thus, the effects of a cooling event such as
    the Little Ice Age might be deep history to us on the surface, but the
    echoes of the event may have continued to resonate in the deep ocean
    into the 20th century, providing a buffer to the warming Earth.

    The delayed cooling effect ended in 1990, after which ocean temperatures, according to the study, have been accelerating upward.

    "The lag is catching up and the ocean is warming more strongly now,"
    Bagnell said. The Atlantic Ocean and Southern Ocean are currently where
    most of the warming is, with the Pacific Ocean and Indian Ocean not
    far behind.

    Ocean warming is a concern on many levels, as it can cause changes in circulation, reduce its ability to absorb carbon and fuel more intense
    storms, in addition to causing sea level rise and creating inhospitable environments for undersea life. If the trend continues, the effects
    might last centuries, thanks to the same lag that kept the oceans cool
    until the last 30 years.

    "The ocean remembers," DeVries said.

    ========================================================================== Story Source: Materials provided by
    University_of_California_-_Santa_Barbara. Original written by Sonia
    Fernandez. Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. A. Bagnell, T. DeVries. 20th century cooling of the deep ocean
    contributed to delayed acceleration of Earth's energy
    imbalance. Nature Communications, 2021; 12 (1) DOI:
    10.1038/s41467-021-24472-3 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2021/08/210809162742.htm

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