Graph from p3768 of J. Hansen et al.: Ice melt, sea level rise and superstorms.
Guest essay by Eric Worrall
The alleged weakening of the Atlantic Meridional Overturning
Circulation appears to be triggering a growing amount of speculation about
abrupt cooling, like the plot of the movie “The Day After Tomorrow”.
Crippled Atlantic currents triggered ice age climate change
The last
ice age wasn’t one long big chill. Dozens of times temperatures abruptly rose
or fell, causing all manner of ecological change. Mysteriously, ice cores from
Greenland and Antarctica show that these sudden shifts—which occurred every
1500 years or so—were out of sync in the two hemispheres: When it got cold in
the north, it grew warm in the south, and vice versa. Now, scientists have
implicated the culprit behind those seesaws—changes to a conveyor belt of ocean
currents known as the Atlantic Meridional Overturning Circulation (AMOC).
These
currents, which today drive the Gulf Stream, bring warm surface waters north
and send cold, deeper waters south. But they weakened suddenly and drastically,
nearly to the point of stopping, just before several periods of abrupt climate
change, researchers report today in Science. In a matter of decades,
temperatures plummeted in the north, as the currents brought less warmth in
that direction. Meanwhile, the backlog of warm, southern waters allowed the
Southern Hemisphere to heat up.
AMOC
slowdowns have long been suspected as the cause of the climate swings during
the last ice age, which lasted from 110,000 to 15,000 years ago, but never
definitively shown. The new study “is the best demonstration that this indeed
happened,” says Jerry McManus, a paleo-oceanographer at Columbia University’s
Lamont-Doherty Earth Observatory, and a study author. “It is very convincing
evidence,” adds Andreas Schmittner, a climate scientist at Oregon State
University, Corvallis. “We did not know that the circulation changed during
these shorter intervals.”
…
Another question is whether the AMOC—currently known to be in
decline—could drop off suddenly today, as depicted in the 2004 movie
The Day After Tomorrow, causing temperatures to plummet across northwestern
Europe. Schmittner says the past provides an eye-opener. “It’s evidence
that this really did happen in the past, on short time scales.” But McManus
says that studies looking deeper into the ice ages have found that the
1500-year climate oscillations tend not to be nearly as strong during
interglacial periods. “It would suggest that this kind of thing isn’t
so likely to happen today,” he says. On the other hand, he adds, “In most
interglacials, Greenland didn’t melt … and Greenland is currently melting.”
Read more: http://www.sciencemag.org/news/2016/06/crippled-atlantic-conveyor-triggered-ice-age-climate-change
The abstract of the study;
North Atlantic ocean circulation and abrupt climate change during
the last glaciation
The last ice age was characterized by rapid and hemispherically
asynchronous climate oscillations, whose origin remains unresolved. Variations
in oceanic meridional heat transport may contribute to these repeated climate
changes, which were most pronounced during marine isotope stage 3 (MIS3), the
glacial interval twenty-five to sixty thousand years ago. We examined climate
and ocean circulation proxies throughout this interval at high resolution in a
deep North Atlantic sediment core, combining the kinematic tracer Pa/Th with
the deep water-mass tracer, δ13CBF. These indicators suggest reduced Atlantic
overturning circulation during every cool northern stadial, with the greatest
reductions during episodic Hudson Strait iceberg discharges, while sharp northern
warming followed reinvigorated overturning. These results provide direct
evidence for the ocean’s persistent, central role in abrupt glacial climate
change.
Is the Atlantic Meridional Overturning Circulation slowing?
Models suggest it should be – but observation based studies have not found evidence of a slowdown.
Who else is speculating about abrupt cooling? One name which
might surprise you is former NASA GISS director James Hansen.
From Ice melt, sea level rise and superstorms: evidence from paleoclimate
data, climate modeling, and modern observations that 2 ◦C global warming could
be dangerous p3774;
… Global temperature becomes an unreliable
diagnostic of planetary condition as the ice melt rate increases.
Global energy imbalance (Fig. 15b) is a more meaningful measure of planetary
status as well as an estimate of the climate forcing change required to
stabilize climate. Our calculated present energy imbalance of ∼ 0.8 W m−2 (Fig.
15b) is larger than the observed 0.58 ± 0.15 W m−2 during 2005–2010 (Hansen et
al., 2011). The discrepancy is likely accounted for by excessive ocean heat
uptake at low latitudes in our model, a problem related to the model’s slow
surface response time (Fig. 4) that may be caused by excessive small-scale
ocean mixing.
Large scale regional cooling occurs in the North Atlantic and
Southern oceans by mid-century (Fig. 16) for 10-year
doubling of freshwater injection. A 20-year doubling places similar cooling
near the end of this century, 40 years ear- lier than in our prior simulations
(Fig. 7), as the factor of 4 increase in current freshwater from Antarctica is
a 40-year advance.
Cumulative North Atlantic freshwater forcing in sverdrup years
(Sv years) is 0.2 Sv years in 2014, 2.4 Sv years in 2050, and 3.4Sv years (its
maximum) prior to 2060 (Fig. S14). The critical issue is whether human-spurred
ice sheet mass loss can be approximated as an exponential process during the
next few decades. Such nonlinear behavior depends upon amplifying feedbacks,
which, indeed, our climate simulations reveal in the Southern Ocean. …
Naturally most of the climate scientists who make such
predictions expect the cooling to occur over a relatively short timescale,
before the ice melt forcing which causes the predicted cooling is overwhelmed
by our continued sinful emissions of CO2. But a fallback prediction of imminent
abrupt cooling does conveniently make it rather difficult to falsify
anthropogenic climate theories based on temperature alone, should global
temperatures suddenly drop.
