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Rapid Acceleration in Sea Level Rise — From 2009 Through October 2015, Global Oceans Have Risen by 5 Millimeters Per Year
02/04/2016
The
evidence that a human-forced warming of the globe is hitting a much
higher gear in terms of both added heat and ramping impacts just
keeps streaming on in. Today, an
update in the satellite monitor tracking global sea level rise
provides yet one more ominous marker.
The world’s oceans are rising at an unprecedented rate not seen
since the end of the last Ice Age. A rate that appears to be rapidly
accelerating.
(Surface
melt visible across the Zachariae Isstrom Glacier in Greenland on
July 20th of 2015. Melt like that occurring on this glacier has
become more and more widespread over Antarctica and Greenland. It’s
an ongoing heat accumulation in the world’s great ice mountains
that is contributing to increasing melt water outflows into the
rising world ocean system. Image source: LANCE
MODIS.)
It’s
a tough bit of evidence that the world is swiftly accumulating heat.
For aside from atmospheric temperature readings, the rate of sea
level rise is probably the best marker for how fast the world is
warming. It’s a sign of heat build-up that’s thermally expanding
the ocean. And, far more ominously,it’s
a sign that the great glaciers of the world are starting to
accumulate enough heat to go into a more and more widespread melt and
destabilization.
Ocean
Rise Begins with Ramp-up in CO2 Emissions
Ever
since the Holocene climate era began about 10,000 years ago, ocean
levels and shorelines have remained remarkably stable. At the close
of the 19th Century, and in conjunction with a build-up of
heat-trapping gasses in the atmosphere through the extraction and
burning of fossil fuels, sea levels began a rise that would start to
mark a departure from the stable coastlines human civilizations had
enjoyed for so long.
(Global
sea level rise has ramped higher and higher — an upward curve that
follows increasing volumes of CO2 in the atmosphere and rising global
temperatures. Image source: Dr.
James Hansen.)
At
first the rise in global waters, driven by a then slow accumulation
of heat in the world ocean system, was slight and gradual. Beginning
in 1870, and continuing on through 1925, sea levels across the world
increased by about 0.8 millimeters per year. The increase was likely
driven by heat accumulating in the atmosphere and then transferring
to the surface waters of the oceans. From 1870 through 1925,
atmospheric carbon dioxide levels had increased from around 280 parts
per million to 305 parts per million — into a range about 25 parts
per million above the typical interglacial peak CO2 level of the last
2 million years. A volume of heat trapping gasses that began to
slowly upset the Holocene’s relative stability.
If
scientists and researchers at the time were paying closer attention,
they would have noted this mild but consistent increase in the height
of global surface waters as the first hint that the human emission of
greenhouse gasses was starting to alter the Earth environment. Sadly,
it took many more decades to begin to understand the profound changes
that were starting to take place.
The
First Acceleration — 1925 to 1992
While
climate science was still in its infancy during 1925, a human forced
warming of the globe was starting to kick into higher gear. A signal
of atmospheric warming since the 1880s was beginning to develop.
Though unclear, it was becoming apparent that the airs of the world
were building up heat. But the waters of the world were providing a
strong signal that the Earth was accumulating that heat more and more
rapidly.
Sea
level rise, at that time driven by thermal expansion and by a later
small but growing contribution from glacial melt, took its first leap
higher. And from 1925 through 1992, the average rate of sea level
rise more than doubled to 1.9 millimeters per year. It was a sign
that the Earth was warming more and more rapidly and that the heat
was showing up in still more thermal expansion of the world’s
waters.
(Globally,
CO2 began to increase in the atmosphere starting with the widespread
burning of coal in England during the 17th and 18th Century. As new
fossil fuels like natural gas and oil were added to the mix and as
fossil fuel based burning greatly expanded during the 19th, 20th, and
21st Centuries, concentrations of this key greenhouse gas
sky-rocketed. By the decade of the 2010s, the rate of atmospheric
greenhouse gas accumulation was about 6 times faster than at any time
in the geological record. A human emission that, if it continues for
just a blink in geological timescales, is the equivalent to multiple
clathrate guns firing off at the same time. Image source: The
Keeling Curve.)
During
the same period, atmospheric greenhouse gasses increased from 305
parts per million in 1925 to around 350 parts per million by 1992.
This jump by 45 parts per million in just 67 years pushed the Earth’s
climate well outside the range of past interglacials — exceeding
the previous peak of 280 parts per million CO2 by more than 70 parts
per million overall. Atmospheric temperatures, by 1992, had also
increased into a range about 0.5 C above 1880s values.
We
had started to enter a period where the context of the human-driven
warming (primarily enforced by a monopolization of energy markets by
fossil fuels) was being pushed far outside the range of the Holocene
and into time periods tens of thousands to hundreds of thousands of
years in the geological past. The Earth System, in other words, was
entering a period of increasingly dangerous imbalance.
The
Second Acceleration 1992 to 2009
During
the 17 years from 1992 through 2009, atmospheric carbon dioxide
levels rose by 40 parts per million to about 390 parts per million in
total. That’s a rate of accumulation nearly four times faster than
the entire period from 1925 through 1992. An accumulation that by
2009 had pushed the world into a climate context more similar to the
Pliocene of 3 million years ago, than of the geological epoch in
which human civilization emerged and thrived. For the Holocene was
then starting to look like some fond memory fading off into an
increasingly murky and smoke-filled far horizon.
Rates
of sea level rise again increased — hitting a ramp up to around 3
millimeters per year. More ominously, scientific
studies were beginning to indicate that the Greenland Ice Sheet and
West Antarctica were starting to significantly contribute to the
rising waters.
The great glaciers were showing their first signs of a mass seaward
movement called a Heinrich Event. And with the world hitting 0.8
degrees Celsius above 1880s temperature values and rising, such an
event was starting to look more and more likely.
Sea
Level Rise at 5 Millimeters Per Year Since 2009
Now,
by early 2016, with the world at 1.1 C warmer than 1880s averages and
with CO2 levels likely to peak at around 407 parts per million this
year, it appears that rates of sea level rise have again jumped
markedly higher. For according to satellite altimetry data from
AVISO, global sea levels rose by 36 millimeters from the end of 2009
through October of 2015. That’s an annual rate of around 5
millimeters per year and one far above the longer term range of 3.1
mm per year established from 1992 through 2012.
(Global
sea level rise as measured by satellite altimetry hits a noticeably
higher ramp from 2009 through late 2015. Image source: AVISO.)
We
can clearly see the departure from the trend line starting post 2011
in the above graph. And if we were to cherry pick that particular
departure zone, the rate from trough-to-peak would be 7 millimeters
per year. However, since a La Nina occurred during 2011-2012 and a
record strong El Nino is occurring now, that particular trend line is
probably a bit exaggerated. The reason being that La Nina tends to
dampen rates of sea level rise through variable cooling and El Nino
tends to spike rates of sea level rise as world surface waters warm
during such events.
However,
even when correcting for La Nina and El Nino variation, it appears
that sea level rise since 2009 is tracking in a range of 4 to 5
millimeters each year — which is yet another significant departure
from the trend. A rate that, if it were to further solidify, would be
5 to 6 times faster than initial rates of sea level rise at the start
of the 20th Century or two and a half times faster than the sea level
rise rates from 1925 through 1992.
(Open
water and no snow in Southern Greenland on February 2 of 2016. Zero
sea ice and no snow in southern Greenland during Winter is a strong
sign that the island is falling deeper and deeper into the grips of a
severe warming event. Image source: Greenland
Today.)
Spiking
rates of heat accumulation and related thermal expansion of the
world’s oceans is likely playing a part in the current increase.
But, all-too-likely, the numerous destabilized glaciers now rushing
seaward — which in total contain at least enough water to raise
seas by 15-20 feet — are also starting to add greater and great
contributions. And, unfortunately, with global temperatures now
pushing into a very dangerous range between 1 and 2 degrees Celsius
above 1880s averages, we are likely to see more and more of these
glaciers go into a rapid seaward plunge. It looks like we’ve
already locked in a ramping rate of sea level rise for decades to
come and at least 15-20 feet long term. But that pales in comparison
to what happens if we keep burning fossil fuels.
Links:
Hat
Tip to Catherine Simpson
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