Written by Tom Di Liberto on Climate.gov
In recent years, a popular debate has been occurring within the climate community. No, it’s not whether climate change is occurring and caused by humans. (It is.) Instead, it’s regarding how much of an effect, if any, the rapid warming of Earth’s frozen cake-topper we call the Arctic, and the corresponding reduction in Arctic sea ice, has been having on winter across the mid-latitudes where most of us live.
The debate arose because, while winters have in general warmed during the last century, some regions have shown a weak cooling trend for certain periods. A cooling trend, from around 1990 through 2013 in landmasses across the mid-latitudes, has been noted in previous research (Cohen et al 2014). It seems these cooling trends have generally weakened or disappeared when data has been updated to the present, but recent winters with bouts of record cold keep the debate alive.
The answer to the debate could have large ramifications for our ability to forecast the winter year in and year out. And I know that is something that concerns all of you, based upon the plethora of winter-related comments you leave on our posts.
In new research from across the pond in England, a team of scientists found that even though a lack of sea ice occurs at the same time as cold mid-latitude winters, it doesn’t cause those cold mid-latitude winters. In scientific jargon-ese, “correlation does NOT mean causation.” The real driver, they say, is an atmospheric circulation pattern across higher latitudes, which drives both low amounts of sea ice and the spilling of cold air into the mid-latitudes. But in order to come to that conclusion, the scientists first had to untangle those multiple, messy climate influencers.
Untangling the options
The difficulty in most investigations of our planet is that a lot of things are influencing the climate all at the same time. It’s like getting your Christmas lights out of the attic and finding two strands of lights hopelessly intertwined. This research, led by Dr. Russell Blackport, first had to untangle those Arctic “lights” in order to see which one is the strand that is “working”, or influencing our mid-latitude winters.
Let’s briefly describe the two influencers at play here and how they could impact mid-latitude winters. First up, a below-average amount of Arctic sea ice. Sea ice is both a reflector and an insulator. When bright, white ice is replaced with the dark ocean in the summer and fall, the surface absorbs more sunlight, and the warmer ocean then warms and moisten the atmosphere above it. This boost of heat and moisture, the thinking goes, can then change the larger atmospheric patterns above the Arctic in winter, which can be the first in a cascade of atmospheric dominos that could potentially lead to impacts across the mid-latitudes.
The second potential influencer is changes in the atmospheric circulation. Circulation changes near the Arctic, even caused by changes as far away as the Tropics, can drive warm moist air into the Arctic and melt/slow the growth of sea ice (Baxter et al. 2019, Clark and Lee, 2019). These same circulation changes could also be simultaneously associated with colder weather in the mid-latitudes.
The critical difference between these two options is the direction heat flows at the surface:
(1). Anomalous “bottom up” heat flows (from ocean to atmosphere) would indicate that a lack of sea ice was driving the weather patterns; anomalous “top-down” heat flows would mean the atmosphere was running the show.
(2). These opposite heat flow patterns gave the scientists a way of testing whether, during winter, the amount of sea ice is driving the atmosphere or the atmosphere is driving the amount of sea ice.
Onto the pretty picture phase.
With a mechanism for sorting out the key players from the chaotic atmosphere determined, it was time for the authors to turn their attention to the full picture. The scientists ran two climate models many times to get a large sample of hypothetical winters, and also examined a dataset of historical observations. These representations of the atmosphere, specifically the sea level pressure and surface air temperature, were then linked to sea ice area in the Chukchi-Bering Sea to create a picture of the atmosphere during high and low amounts of sea ice.