The first item is the substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC), otherwise known as the Gulf Stream. The second is the rapidly accelerating melting of the Greenland Ice Sheet. And the third is the relentless increase in CO² levels. The worst bad news of all, though, is that all three are related and mutually reinforcing.
AMOC
The AMOC is a huge current of water originating near
the Equator off the western coast of Africa.
From its point of origin, this warm, salty water flows northwest to South America; from there, it flows toward the Gulf
of Mexico and along the eastern seaboard of the United States. Somewhere off Labrador, it crosses the North
Atlantic, warming up Western Europe and moderating our winter climate. During its long northward journey, the water gradually
loses heat. When the Gulf Stream reaches
Greenland, cooling rapidly increases, as does the density of the salty
water, which sinks to lower depths and takes the current back toward Africa. The
whole effect is that of a conveyor belt, or it was.
To see the conveyor belt in motion, understand its complexity in more detail, and read about the variety of scientific opinion on what is happening to the AMOC and why, see this terrific NYT Interactive.
Four years ago, a research paper entitled, “Overlooked possibility of collapsed Atlantic Meridional Overturning Circulation” was published by the Office of Scientific and Technological Information, United States Department of Energy. The OSTI report, discussing biases in climate modelling, stated:
Current climate models suffer from biases, and therefore, it is critically important to assess the potential impact of the model biases on future climate projections. One vital player for climate change is the Atlantic Meridional Overturning Circulation (AMOC).
There is evidence that the AMOC has slowed down since the early 20th century, although this long-term declining trend of AMOC strength is subject to great uncertainty. Under future global warming, the AMOC is predicted to further weaken, but the degree of the change is uncertain, ranging from a moderate slowdown to a complete shutdown.
The point OSTI was making is that climate models had assumed the AMOC was stable, but in fact, it isn’t. Just how unstable it is has been difficult to measure. Because direct measurements have been taken only since 2004, scientists use “proxy evidence”—seabed core samples, ocean temperature patterns, fish migrations, and the like-- to supplement their AMOC prediction modelling.
Since direct measurements began and the OSTI research paper was published, predictions about AMOC weakening have been borne out. The Washington Post reported recently that scientists confirmed the Gulf Stream has weakened by 15% since 1950 and is now weaker than it has been in 1,000 years. The weakening has led to harsher winters in Northern Europe, odd temperature patterns in the Atlantic, more (and more severe) hurricanes in the U.S., rainfall migration out of arid regions, and the disturbance of fisheries, such as Maine lobster and cod, which are moving north as their native waters warm. Understanding the cause of the AMOC’s weakening and its potential future collapse is key to understanding why scientists are alarmed.
It appears that what may be causing the conveyor belt to slow down is something called “the cold blob (below).” This where the melting of the Greenland Ice Sheet comes in.
GREENLAND ICE SHEET MELT
As the Washington Post reports:
...a recurrent “cold blob” has been observed in the ocean to the south of Greenland — a large region that is bucking the overall global warming trend and instead showing a marked cooling pattern. Scientists think this is evidence that less warm water is reaching this region than previously, and that it may also be a result of runoff from the melting ice sheet.
Greenland’s
ice sheet [above] has melted completely before, as these ice core findings published in the Proceedings of the National Academy of Sciences
last month recount (PNAS ) (emphasis added):
We
analyzed sediment at the bottom of the Camp Century ice core [above], collected 120 km
from the coast in northwestern Greenland. The sediment, frozen under nearly 1.4
km of ice, contains well-preserved fossil plants and biomolecules [below] sourced from
at least two ice-free warm periods in the past few million years. Enriched
stable isotopes in pore ice indicate precipitation at lower elevations than present,
implying ice-sheet absence. The similarity of cosmogenic isotope ratios in the
upper-most sediment to those measured in bedrock near the center of Greenland
suggests that the ice sheet melted and re-formed at least once during the past
million years.
If that language seems denser than the ice cores, one of the Academy's scientists who examined the plant fossils explains their significance in this WaPo Ice Sheet article (emphasis added):
Early analysis suggests that the plants are no older than a million years, which suggests they must have grown during the epoch of repeated ice ages known as the Pleistocene. During that time, greenhouse gas concentrations in the atmosphere were far lower than current levels, and the Earth was rarely as hot as it is now.
“If we had found a much older age, it would have been impressive, but it might not have been as scary,” Christ said. “Because what we found means the ice sheet melted away and raised sea level within a climate system kind of like ours. That, as a climate scientist, has more gravity.”
Not good, but should we panic? According to the UN International Panel on Climate Change, the Greenland Ice Sheet is "very unlikely" to collapse in the immediate future. The IPCC defines "very unlikely" as a less than 10% chance; immediate future is not defined. Whether or not those odds keep you up at night, the fact is that the last time the ice sheet disappeared, temperatures were cooler and greenhouse gas concentrations were not nearly as high as they are now. And this is where the recent spike in CO² levels comes in.
CO² LEVELS ARE MORE THAN HALF WAY TO DOUBLING
Again, the Washington Post has the story (WaPo CO2):
For the first time in recorded history, the concentration of atmospheric carbon dioxide, or CO2, was measured at more than 420 parts per million at the Mauna Loa Observatory on the Big Island of Hawaii [below]. It’s a disconcerting milestone in the human-induced warming of the planet, around the halfway point on our path toward doubling preindustrial CO2 levels.
The anticipated doubling of atmospheric CO2, which is likely by the year 2060, has been connected to a predicted three-degree [Celsius] or greater warming of the planet. A study released last year found that doubling CO2 levels will probably lead to a temperature rise between 4.1 and 8.1 degrees Fahrenheit (2.3 and 4.5 degrees Celsius), ruling out more modest warming scenarios.
To put this into context, pre-industrial CO² levels (1850-1900) were around 275 ppm, so they would double at 550 ppm. We are already clocking in at 420 ppm. If you refer to the title graphic, CO² emissions show no signs of slowing down. Their growth is a classic 45° linear curve. As CO² increases in the atmosphere, so do global mean temperatures. The World Meteorological Organization's annual State of the Global Climate published at the end of 2020, found that global mean temperatures are already 1.2° Celsius above pre-industrial levels (Bloomberg).
You don't have to be a scientist or a mathematician to understand that continuing to pump CO² into the atmosphere will continue to heat up the planet, which will continue to melt the Greenland Ice Sheet, which will continue to interfere with the AMOC. Were we to double the 1990 levels of CO² and reach 700 ppm, modelling predicts the AMOC will collapse. Per the OSTI report:
...we show that the AMOC collapses 300 years after the atmospheric CO2 concentration is abruptly doubled from the 1990 level....[T]he AMOC collapse brings about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas, sea ice increases over the Greenland-Iceland-Norwegian seas and to the south of Greenland, and a significant southward rain-belt migration over the tropical Atlantic.
We can take ironic comfort in the fact that the AMOC will eventually recover when glaciers return to the North Pole. The Gulf Stream needs a mere 200 years to get its conveyor belt moving again, per this research paper published in Information from Paleoclimate Archives (IPPC):
the interglacial mode of the Atlantic Ocean meridional overturning circulation (AMOC) can recover from a short-term freshwater input into the subpolar North Atlantic. Approximately 8200 years ago, a sudden freshwater release occurred during the final stages of North America ice sheet melting. Paleoclimate observations and model results indicate, with high confidence, a marked reduction in the strength of the AMOC followed by a rapid recovery, within approximately 200 years after the perturbation.
These paleo-glacial periods may seem long, long ago and the next glacial period may seem far, far away. But in terms of geologic time, the weakening of the Gulf Stream, the melting of the Greenland Ice Sheet, and the increase in CO² levels happening simultaneously during the current interglacial period are occurring in the blink of an eye. If you're a Maine lobster man, you're already seeing the effects. And if you're young and have your whole life ahead of you, you're wondering why we're sitting around, tinkering at the edges, not doing nearly enough.
Keep it real! And wear your damn mask!
Marilyn
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