While
the upper part of the world’s oceans continue to absorb heat from
global warming, ocean depths have not warmed measurably in the last
decade. This image shows heat radiating from the Pacific Ocean as imaged
by the NASA’s Clouds and the Earth's Radiant Energy System instrument
on the Terra satellite. (Blue regions indicate thick cloud cover.)
Deep
sea creatures, like these anemones at a hydrothermal vent, are not yet
feeling the heat from global climate change. Although the top half of
the ocean continues to warm, the bottom half has not increased
measurably in temperature in the last decade.
The cold waters of Earth’s deep ocean have not warmed measurably
since 2005, according to a new NASA study, leaving unsolved the mystery
of why global warming appears to have slowed in recent years.
Scientists at NASA's Jet Propulsion Laboratory (JPL) in Pasadena,
California, analyzed satellite and direct ocean temperature data from
2005 to 2013 and found the ocean abyss below 1.24 miles (1,995 meters)
has not warmed measurably. Study coauthor Josh Willis of JPL said these
findings do not throw suspicion on climate change itself.
"The sea level is still rising," Willis noted. "We're just trying to understand the nitty-gritty details."
In the 21st century, greenhouse gases have continued to accumulate in
the atmosphere, just as they did in the 20th century, but global
average surface air temperatures have stopped rising in tandem with the
gases. The temperature of the top half of the world's oceans -- above
the 1.24-mile mark -- is still climbing, but not fast enough to account
for the stalled air temperatures.
Many processes on land, air and sea have been invoked to explain what
is happening to the "missing" heat. One of the most prominent ideas is
that the bottom half of the ocean is taking up the slack, but supporting
evidence is slim. This latest study is the first to test the idea using
satellite observations, as well as direct temperature measurements of
the upper ocean. Scientists have been taking the temperature of the top
half of the ocean directly since 2005, using a network of 3,000 floating
temperature probes called the Argo array.
"The deep parts of the ocean are harder to measure," said JPL's
William Llovel, lead author of the study published Sunday in the journal
Nature Climate Change. "The combination of satellite and direct
temperature data gives us a glimpse of how much sea level rise is due to
deep warming. The answer is -- not much."
The study took advantage of the fact that water expands as it gets
warmer. The sea level is rising because of this expansion and the water
added by glacier and ice sheet melt.
To arrive at their conclusion, the JPL scientists did a
straightforward subtraction calculation, using data for 2005-2013 from
the Argo buoys, NASA's Jason-1 and Jason-2 satellites, and the agency’s
Gravity Recovery and Climate Experiment (GRACE) satellites. From the
total amount of sea level rise, they subtracted the amount of rise from
the expansion in the upper ocean, and the amount of rise that came from
added meltwater. The remainder represented the amount of sea level rise
caused by warming in the deep ocean.
The remainder was essentially zero. Deep ocean warming contributed virtually nothing to sea level rise during this period.
Coauthor Felix Landerer of JPL noted that during the same period
warming in the top half of the ocean continued unabated, an unequivocal
sign that our planet is heating up. Some recent studies reporting
deep-ocean warming were, in fact, referring to the warming in the upper
half of the ocean but below the topmost layer, which ends about 0.4 mile
(700 meters) down.
Landerer also is a coauthor of another paper in the same journal
issue on 1970-2005 ocean warming in the Southern Hemisphere. Before Argo
floats were deployed, temperature measurements in the Southern Ocean
were spotty, at best. Using satellite measurements and climate
simulations of sea level changes around the world, the new study found
the global ocean absorbed far more heat in those 35 years than
previously thought -- a whopping 24 to 58 percent more than early
estimates.
Both papers result from the work of the newly formed NASA Sea Level
Change Team, an interdisciplinary group tasked with using NASA satellite
data to improve the accuracy and scale of current and future estimates
of sea level change. The Southern Hemisphere paper was led by three
scientists at Lawrence Livermore National Laboratory in Livermore,
California.
NASA monitors Earth's vital signs from land, air and space with a
fleet of satellites and ambitious airborne and ground-based observation
campaigns. NASA develops new ways to observe and study Earth's
interconnected natural systems with long-term data records and computer
analysis tools to better see how our planet is changing. The agency
shares this unique knowledge with the global community and works with
institutions in the United States and around the world that contribute
to understanding and protecting our home planet.
For more information about NASA's Earth science activities in 2014, visit:
For more information on ocean surface topography from space, visit:
More information on NASA’s GRACE satellites is available at:
For more information on the Argo array, visit:
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