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Running AMOC in Pictures
“[D]ifferent kinds of human disturbance act synergistically so that the sum total of their ecological
consequences exceeds that of their effects in isolation.” Jeremy Jackson [1]:
Tony Noerpel
Figure 1 shows the thermohaline ocean circulation in the Atlantic. The worldwide interconnectivity of
this circulation and its causes were discovered by Wallace Broecker. It is driven by variations in
temperature and salinity in different parts of the ocean [2]. The red flows are on the surface and the
blue flows are in the deep ocean. The Atlantic Meridional Overturning Circulation (AMOC) [3] running
along the US East coast is better known as the Gulf Stream. This great flow of warm water warms
Europe. In the movie “The Day after Tomorrow” this flow stops due to climate change and the Northern
Hemisphere is thrown into an ice age. The part that is true is that human-caused climate change does
slow down this flow in many climate models and indeed this is happening now as shown in the right
hand side of Figure 2. It is also true that if AMOC flow stops an ice age could be initiated. The part that
is fanciful besides the abrupt timing is ignoring the overwhelming influence of increased carbon dioxide
and other greenhouse gases in the atmosphere. Some regions of the planet may cool down in
opposition to the worldwide trend because of ocean current changes but eventually the whole planet
surface warms quite dramatically. An ice age cannot occur when atmospheric carbon dioxide is above
400 ppmV as it is now [4-5].
The slowdown does have negative consequences. Figure 2 shows a simulated model forecast of the
AMOC flow slowdown on the left [6]. Note the cold water blob forming south of Greenland (blue) and
the consequent warm water pooling off the East coast (red). The picture on the right in Figure 2 from
NASA [7] shows that this is indeed happening. The huge red stuff in the Pacific Ocean is the current el
Nino which we’ve discussed a few weeks ago [8]. Now let’s discuss some of the consequences.
Figure 3 shows that between 1900 and 2000 the once productive Northwest Atlantic Cod (Gadus
morhua) fishery in Newfoundland had collapsed by an order of magnitude due to over fishing [9]. In fact
this was observed in 1992 and in 1993 stakeholders agreed to a moratorium on Atlantic Cod fishing
which is still in place. Unexpectedly the population never recovered. From a 2014 NOAA press release:
“In August, scientists declared that the abundance of Gulf of Maine cod was at an all-time low. Scientists
estimated that the population was at a mere 3-4 percent of a sustainable abundance level. They also
found very few young cod in the population, which means recovery of the population is not going to
occur anytime soon.” [10-11]
Several reasons for this lack of recovery of the cod population have been proposed including increased
predation by seals, but a paper published in the journal Science last November by Andrew Pershing and
colleagues [12] hypothesizes that global warming is preventing the cod recovery despite the
moratorium. They make the point that The Atlantic Ocean on the east coast of the United States has
been warming twice as fast as the average global ocean temperature since 1980. The have simulated
the impact of a warmer ocean on the cod population and their simulation results are shown in Figure 4.
They are in agreement with observation.
Many biologists believe that the collapse of the cod biomass might be permanent because of a shift to a
different stable food web. Cod are an apex predator for all smaller fish. Without cod the entire food
web is disrupted. Large mammals such as seals eat cod. Cod eat smaller fish which in turn eat
zooplankton which in turn eat the primary producers or phytoplankton. What we would expect to
happen is that the smaller pelagic fish (mackerel, herring, capelin,…) population would increase,
decreasing the population of zooplankton resulting in an increase in the population of phytoplankton.
Except that fisheries are now overfishing the smaller fish allowing the zooplankton to increase thus
reducing the population of the primary producers. Some observers point out that all trophic levels are
in decline [13-14].
The important observation is that collapse and lack of recovery of the cod fishery (See figure 5) is not
due to any single human activity but to the synergistic impacts of multiple human misbehaviors. Thus
human-caused climate change is not the primary cause but its effect is to further stress organisms and
ecosystems which have already been stressed possibly preventing them from recovering even after
other mitigation strategies such as moratoriums have been employed.
[1] Jeremy B. C. Jackson, The future of the oceans past, Phil. Trans. R. Soc. B 2010 365, 3765-3778, doi:
10.1098/rstb.2010.0278
[2] Tony Noerpel, The Great Ocean Conveyor, September 16, 2010, http://brleader.com/?p=2103
[3] Andreas Schmittner, John C.H. Chiang, and Sidney R. Hemming, Introduction: The Ocean’s Meridional
Overturning Circulation, Ocean Circulation: Mechanisms and Impacts, Geophysical Monograph Series
173, Copyright 2007 by the American Geophysical Union,
http://people.oregonstate.edu/~schmita2/pdf/S/schmittner07agu_intro.pdf
[4] Sybren Drijfhout, Competition between global warming and an abrupt collapse of the AMOC in
Earth’s energy imbalance, www.nature.com/scientificreports/| 5:14877 | DOI: 10.1038/srep14877
[5] A. Ganopolski, R. Winkelmann & H. J. Schellnhuber, Critical insolation–CO2 relation for diagnosing
past and future glacial inception, Nature, vol 529, 14 January, 2016, doi:10.1038/nature16494
[6] V.S. Saba, S.M. Griffies, W.G. Anderson, M. Winton, M.A. Alexander, T.L. Delworth, J.A. Hare, M.J.
Harrison, A. Rosati, G.A. Vecchi, and R. Zhang, "Enhanced warming of the northwest Atlantic Ocean
under climate change", J. Geophys. Res. Oceans, pp. n/a-n/a, 2015.
http://dx.doi.org/10.1002/2015JC011346
[7] http://www.realclimate.org/index.php/archives/2016/01/blizzard-jonas-and-the-slowdown-of-thegulf-stream-system/
[8] Tony Noerpel, El Nino Evolution, January 17, 2016 http://brleader.com/?p=19173
[9] Villy Christensen, Sylvie Guénette, Johanna J Heymans, Carl J Walters, Reginald Watson, Dirk Zeller
and Daniel Pauly, Hundred-year decline of North Atlantic predatory fishes, Article first published online:
22 APR 2003, Fish and Fisheries, Volume 4, Issue 1, pages 1–24, March 2003 DOI: 10.1046/j.14672979.2003.00103
[10] http://www.huffingtonpost.com/carl-safina/near-collapse-of-gulf-of-_b_6172258.html
[11] http://www.nefsc.noaa.gov/press_release/pr2014/other/MA1402/
[12] Andrew Pershing et al., slow adaptation in the face of rapid warming leads to collapse of the Gulf of
Maine cod fishery, Science, vol. 350, issue 6262, 13 November, 2015.
[13] Hare JA, Morrison WE, Nelson MW, Stachura MM, Teeters EJ, Griffis RB, et al. (2016) A Vulnerability
Assessment of Fish and Invertebrates to Climate Change on the Northeast U.S. Continental Shelf. PLoS
ONE 11(2): e0146756. doi:10.1371/journal.pone.0146756
[14] http://www.fisherycrisis.com/index.html
Figure 1
Figure 2
Fgiure 3
Figure 4
Gloucester, Massachusetts fishing port. One of the most important cod fishing ports in the Gulf
of Maine. Photo Credit: NOAA.
Figure 5