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.