In the coming years, the planet could experience a catastrophic climate change, so abrupt that no adaptation would be possible. The rapid melting of the continental ice sheets in the Northern Hemisphere is altering the system of ocean currents that regulate the global climate. This has already happened in the past, related to the disappearance of the Neanderthals about 40,000 years ago.
Seen from space, Earth is nothing more than a pale blue dot, as described by the renowned scientist Carl Sagan. If we get closer, we see water everywhere. Blue and liquid in the oceans, white and solid in the polar ice caps, and white again in the vapor of the clouds.
Seawater, which covers 75 percent of the planet's surface, appears placid. But up close, movement is evident, both the vapor of the atmosphere and the liquid of the oceans and continental ice sheets. It is a dance between the atmosphere and the oceans that circulates seawater around the globe, generating heat and nutrient circulation throughout the planet.
This movement has a direct impact on the climate system and biodiversity of our planet. An example of this is the Atlantic Ocean current system known as AMOC, for Atlantic Meridional Overturning Circulation. It is vital in regulating the climate of the entire planet, carrying warm water to the north and colder water to the south.
Research by scientists at Utrecht University in the Netherlands indicates that glacier melt could disrupt the Gulf Stream, which carries heat to much of the Northern Hemisphere. This would lead to a catastrophic freezing in Europe, North America and Asia, while increasing temperatures in the Southern Hemisphere.
The possibility of AMOC reaching a tipping point today is the result of how human activities have affected the global climate in the last 200 years. In the past, AMOC also reached that point, but due to natural causes, which have to do with climate cycles of thousands of years.
These past catastrophes, known as Heinrich events, led to drastic climate changes in Europe and America, which generated situations such as the extinction of the Neanderthals about 40,000 years ago in Europe, or the possibility of humans passing from Asia to America for the first time about 30,000 years ago.
A planet in motion
To understand why ocean currents exist, and how they affect us, we must be clear that the world ocean and the atmosphere form a single interconnected system. The two are linked by complex feedback loops.
The great ocean currents are a response to the flow of energy between the tropics and the polar regions of the planet. An exchange of heat transported both by the winds of the atmosphere and by the oceans, which play a leading role in characterizing the climate of each region of the Earth.
Our planet is a hemisphere that rotates on itself constantly. At its waist, the Equator, the atmospheric pressure is low and the hot air accumulates there as it is the region most affected by solar radiation. But that hot air does not stand still, it tends to rise in the atmosphere and move towards the north and south poles.
The same happens with the oceans, which cover two thirds of the planet's surface. Global circulation is generated by differences in water density, which are due to temperature and salinity. These variations lead them to move in time with the winds.
Due to a slight push that comes from the Earth's rotation, known as the Coriolis effect, both winds and currents tend to form circles. Hence the shapes and trajectories they tend to follow, since this effect causes objects that move on Earth to deviate to the right in the northern hemisphere and to the left in the south.
But that balance achieved by the dance between the winds and the waters, to the sound of the music of the Earth's rotation, is very delicate. It can collapse both due to natural causes, as has happened repeatedly in the last 100,000 years, or by the effect of human activities, as is believed could happen in the coming years.
Past abrupt cold spells
In the past, there have been six AMOC collapses in a span of 60,000 years. Six times the collapse of the North Atlantic ocean circulation has led to periods of abrupt and extreme cold in the North Atlantic and a rise in temperatures in the Southern Hemisphere.
A large number of scientific studies point to a possible culprit, related to what is happening today, which would be the freshwater from the melting ice in the North Atlantic.
During the last ice age, continental ice sheets were much more massive than they are today, to the point that sea level had dropped many meters due to the accumulation of water in glaciers.
The ice sheet that covered what is now the Labrador Peninsula, in the far north of Canada, became so massive during the most extreme cold periods that it ended up collapsing due to its own size. The result of this collapse were huge icebergs that joined the North Atlantic ocean currents and ended up melting there.
Such an injection of freshwater reduced the salinity of seawater, which led to a change in the density of the water. This cut off one of the main engines of the Atlantic Ocean current system, called thermohaline circulation, which is when the difference in water density, a product of its salinity, makes it move from one side to the other. This led to the collapse of AMOC.
There were six collapses that scientists were able to measure, and they all occurred during the last Ice Age. They happened within the framework of extremely cold periods known as Heinrich events, in honor of paleoclimatologist Hartmut Heinrich.
They were recorded in the North Atlantic seabeds as layers of sediment from the continent, carried there by icebergs detached from the continental ice sheets. Some of these icebergs traveled up to 3,000 kilometers before melting.
Heinrich events occurred during a period that scientists call MIS 3, for its acronym Marine Isotope Stage 3. It is a time of climatic instability during which many species were unable to adapt to the radical and constant changes.
It was a relatively warm period, within the last Ice Age, during which the short periods of extreme cold called Heinrich occurred, followed by longer periods of extreme heat known as Dansgaard-Oeschger (D-O). All of this happened from about 60,000 years ago to about 20,000 years ago.
It is a very important period in our history, since it was when the other human species, such as the Neanderthals, became extinct, leaving Homo sapiens as the only survivor. It was also when the climatic conditions were right for humans to reach the last continent on the planet to be populated: America.
Possibilities for some, the end for others
In the last glacial period, sea levels had dropped tens of meters due to the large amount of water that accumulated on the continents in the form of ice. This led to the disappearance of the Bering Strait, which separates Asia from America, and the sea that surrounds it, and they were transformed into what is known as Beringia. An immensely vast emerged territory equivalent to the whole of Europe, in square kilometers.
The hypotheses most accepted by the majority of experts suggest that Asians lived on Beringia for thousands of years until the conditions were right to enter America. This is because large continental ice masses blocked the passage in what is now Alaska.
Some believed that the conditions had only arisen about 15,000 years ago, but other studies such as that of Jorge Rabassa and colleagues published in the journal Quaternary International, suggest that the collapse of AMOC would have generated the conditions for Beringia and Alaska to live in a temperate climate of forests and grasslands.
In northeastern Asia and northwestern America, very warm conditions occurred during the MIS 3 period, which led to the retreat of the large glaciers that covered much of Alaska about 30,000 years ago. This enabled the southward migration of all kinds of animals, including humans. Starting the Settlement of America.
In Europe, the exact opposite was experienced, there were periods of extreme cold and aridity that came abruptly, so that no living being could adapt quickly to such changing conditions. This led to the extinction of many species, the disappearance of entire forests, and left our relatives the Neanderthals on the brink of extinction.
Their population was so small and dispersed that when our Homo sapiens ancestors arrived in more favorable times, they ended up absorbing them into their larger populations, which is why they still live within our genome.
On the way to collapse
Similar to what has happened many times in the past, today the melting of the ice in the northern hemisphere is altering the AMOC current system.
Based on computer models and data provided by samples of ancient ice and seabed, a group of researchers from the University of Utrecht in the Netherlands developed indicators that can alert when AMOC is approaching the point of collapse. What they discovered is that it is already on that course, and it is not something that can be avoided.
"While we cannot estimate the exact moment when the AMOC collapse will occur, we can say that it is approaching that point," René van Westen, a scientist at the University of Utrecht in the Netherlands, who was part of the study, told Muy.
The consequences of the collapse of AMOC today, according to the study, would lead to a climate change 10 times faster than what humans are currently causing. It would raise sea levels in the Atlantic by at least one meter.
The climate would fluctuate even more around the world. The Southern Hemisphere would become warmer, the Amazon drier, and Europe would have less rainfall while cooling rapidly and drastically.
"While similar events have occurred in the past, Heinrich events were different in that the climate was much colder and there were large continental ice masses over Europe and North America. The process is now substantially different in that the collapse of AMOC is influenced by human-induced climate change."
"At present we see it as a potential future scenario with severe climatic impact," added the researcher. "There are already indications that AMOC is on its way to collapse. Once this happens, the European climate will cool and the Arctic ice will begin to expand. On human time scales, it is impossible to reverse the collapse of AMOC."
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