Multimode mean correlation plot between low-frequency AMOC at 26°N and SST (12). The stars numbered 1 through 15 indicate the location of the site.
(Image source: Lapointe et.al., 10.1126/sciadv.abi8230)
The cause of the Xiaoice period is a problem that has long plagued historical climatology, environmental historiography and earth science. A new study from the University of Massachusetts Amherst shows that the answer is a paradox — warming.
The Xiaoice period was one of the coldest periods of the past 10,000 years, during which the cooling in the North Atlantic region was particularly pronounced. The exact timeline of this ice age remains controversial in academic circles, with it widely believed to have begun about 600 years ago. It is the main culprit in Europe's food failures, famines and epidemics, which have caused millions of deaths. The mechanism by which this harsh climate was created has so far been inconclusive. However, a new paper recently published in the journal Science Advances paints an up-to-date picture of the causes of the Xiaoice issue. Surprisingly, this cooling appears to have been triggered by an unusually warm period.
When the paper's lead authors, Francois Lapointe (a postdoctoral fellow and earth science lecturer at UMass Amherst) and Raymond Bradley (distinguished professor of earth sciences at Umass Amherst), began scrutinizing the results of their previous reconstructions of 3,000 years of north Atlantic sea surface temperature changes (the findings were published in the Proceedings of the National Academy of Sciences (PNAS) in 2020). They noticed something surprising: it took only 20 years to suddenly change from very warm climatic conditions at the end of the 14th century to unprecedented cold climatic conditions in the early 15th century.
(Image source: pixabay)
Using many detailed records of ocean data, Lapointe and Bradley found that there was a very strong northward movement of the warm current at the end of the 14th century, a phenomenon that peaked around 1380. As a result, greenland and northern Europe become exceptionally warm with the waters of the South China Sea. Lapointe said, "No one noticed this before. ”
In general, warmer waters always flow from the tropics to the Arctic. This famous process is called the Atlantic WarpEdiring Circulation (AMOC), and it acts like a planetary conveyor belt. In general, warm water from the tropics flows north along the Nordic coast, and when it reaches the higher latitudes and encounters colder Arctic waters, the warm current loses heat, lowers the temperature and becomes more dense, causing the water body to sink to the bottom of the ocean, and then, in this way, the deep water flow flows south along the North American coast and continues to circulate around the world. But at the end of the 14th century, AMOC increased significantly, which meant more warm currents moving north than usual, which in turn led to the rapid melting of Arctic sea ice. In the decades from the late 14th to the beginning of the 15th century, large amounts of sea ice washed into the North Atlantic, causing the North Atlantic to lower the temperature and dilute the salinity, eventually leading to the collapse of the AMOC. It was this collapse that triggered the drastic cooling that followed.
Fast forward to our time: Between the 1960s and 1980s, we also observed a significant increase in AMOC due to the persistent high pressure of Greenland's atmosphere. In Lapointe and Bradley's view, this is the same as the atmospheric conditions before the Xiaoice period. So what triggered the persistent high pressure around 1380?
Lapointe found that the answer was hidden in the tree. Once the researchers compared their findings with new records of solar activity revealed by radiocarbon isotopes preserved in tree rings, they found that there were unusually strong records of solar activity in the late 14th century. This solar activity tends to cause high pressure over Greenland. At the same time, there were few volcanic eruptions during this period, so the amount of volcanic ash in the air was low. The "cleaner" atmosphere makes Earth more sensitive to changes in solar activity. Lapointe notes, "Therefore, the effect of intense solar activity on atmospheric circulation in the North Atlantic is extremely significant. ”
Lapointe and Bradley have been wondering whether this sudden cooling event will happen again in the face of global climate change. They point out that existing Arctic sea ice has been drastically reduced by global warming, so events like those involving sea ice transport in the early 15th century are unlikely. "But we have to pay close attention to the increase in freshwater in the Beaufort Sea (in northern Alaska), where the accumulation of freshwater has increased by 40 percent over the past 20 years." The fresh water it transports to the subpolar North Atlantic subpolars of the subpolars can have a strong impact on ocean circulation. "In addition, the persistent high pressure over Greenland in the summer has become more frequent in the past decade and is closely linked to unprecedented snow and ice melting." Since climate models cannot reliably capture these events, we may be underestimating the loss of future ice sheets. As more and more freshwater flows into the North Atlantic, AMOC is likely to weaken or even collapse. Lapointe said. The authors ultimately conclude that measures need to be taken as soon as possible to resolve these uncertain events.
Translation: Rong Jiyan
Reviewer: Zhang Chu
Source: University of Massachusetts Amherst
This article is from: China Digital Science and Technology Museum