From 1980 to the year 2000, air temperatures at the surface of the earth warmed relatively rapidly, keeping in line with the theory of global warming as well as predictions of climate models. However, at the start of the 21st century, average global temperatures began to hold relatively steady.
This hiatus in global warming has caused plenty of controversy, doubt and head scratching. Over the past several years, scientists have proposed that aerosols blocking sunlight from increased volcanic activity or pollution from China could be the culprit, or even a decrease in solar output due to sunspots.
What’s Causing the Pause in Global Warming?
Based on the most recent work, it appears that the oceans are a more likely explanation for the pause than aerosols, since the amount of missing heat is too large to be explained by a decrease in incoming solar radiation alone.
It is more likely that the ocean is storing the bulk of the excess heat, since water has a much greater capacity to store heat than the air. Although there is agreement (for the most part) among atmospheric scientists that the ocean is responsible for the global warming pause, there is disagreement as to whether the Atlantic or the Pacific Ocean plays a greater role.
In early August, research was published that pointed to a trade wind anomaly in the Pacific Ocean as the cause of suppressed atmospheric warming. Then, later that month, researchers Xianyao Chen of the Ocean University of China in Qingdao and Ka-Kit Tung of the University of Washington, Seattle published a piece in Science saying that the Atlantic Ocean is more likely the missing heat sink, due to a naturally occurring cycle in the thermohaline circulation.
Global Warming Sink: The Case for the Pacific Ocean
Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research (NCAR) found that a particularly intense and long-lasting shift toward La Niña since the year 2000, as part of the Pacific Decadal Oscillation, could explain the global warming hiatus.
This unusual La Niña event produces extra strong trade winds that are pushing warm surface water towards Asia while drawing colder water from deeper in the ocean to the surface, cooling the atmosphere in the eastern and central Pacific Ocean. Additionally, as the warmer water glides over the surface, some of its heat gets transferred and stored deeper in the ocean.
Global Warming Sink: The Case for the Atlantic Ocean
The Atlantic Meridional Overturning Current (AMOC) is the part of the thermohaline circulation that occurs in the Northern Atlantic Ocean. Ocean water in the tropics is very salty because there is more evaporation than precipitation, unlike at higher latitudes.
The thermohaline circulation acts like a conveyor belt in the ocean, guiding the hot salty tropical water from the equator toward the North Atlantic. In the North Atlantic ocean, this water cools somewhat while maintaining its high salinity, making it much denser than the surrounding relatively fresher water. The dense tropical water sinks rapidly, effectively moving some of the heat into the ocean.
The AMOC naturally cycles back and forth between a stronger and a weaker flow of water. When the flow is stronger (as it is now), more heat is removed from the atmosphere and taken to the depths of the ocean.
According to the new study in Science, mentioned above, the amount of heat going into the North and South Atlantic oceans, between 300 and 1,500 meters beneath the surface, is enough to account for the missing heat from the atmosphere entirely.
This natural oscillation of the AMOC happens because the faster current that transports more heat northward melts more ice, which makes the subpolar surface water less salty. Although a faster current brings more salty water to subpolar regions, which enhances sinking and speeds up the flow, it also brings warmer water, which melts more ice, making the surface water fresher. Once there is enough melting, the salty tropical water is diluted enough that it becomes less dense, inhibiting sinking and slowing down the circulation.
In the past, this stronger current and extra storage of heat in the ocean would cause a cooling phase. But, since we are continuing to trap more heat at the earth’s surface, instead the temperature alternates between increasing and holding steady, rather than warming and cooling periods (as it would in the absence of global warming).
Is This Really a Pause – and Should We Still be Concerned?
It is worrisome news that oceans are mostly responsible for global warming hiatus, since that means the heat is still in the system. If aerosols from volcanoes or pollution were mainly the culprit, that would mean that energy from the sun was getting reflected back out to space. But, since this is not the case, it is not so much a hiatus from warming, as a redistribution of heat from the atmosphere to the ocean.
Besides showing up in ocean temperature measurements, this is also apparent in the melting of Arctic sea ice and sea level rise due to warming water.
Regardless of whether the missing heat is residing in the Atlantic or the Pacific oceans, either cycle responsible for suppressing atmospheric warming will reverse in the not-so-distant future, returning us to a path of climbing air temperatures.
If the pattern of the natural oscillation of the thermohaline circulation in the Atlantic continues as in the past, with a 20 to 35 year cooling period, we can expect the pause to last for about another five to 20 years.
The researchers warn, however, that this is not a certainty. Climate change tends to cause instability in the natural climate-driven cycles. For example, extra freshwater from rapid glacial melting could inhibit sinking in the North Atlantic sooner, bringing about the warming cycle sooner or for longer. Additionally, once trade winds in the Pacific weaken, warm water piled up in the western-Pacific Ocean will slosh back into place and bring about an end to the cooling and heat storage in this ocean as well.
Climate Change/Global Warming: What Will Happen?
It is as of yet uncertain whether temperatures will fall more in line with climate model predictions during the next warming period (when this salinity-driven natural cycle leads to a weaker THC), or if this cycle will cause additional warming that is not yet factored into current climate models.
One thing is for certain, that this “pause” will not last indefinitely, and it is not a reason to lose sight of the real threat of global warming.