While I have been educated in English for most of my life, and even my iPhone settings are completely in English, I have always preferred reading the news in Dutch. As I was scrolling through NU.nl on a particularly cold Saturday morning, an interesting article named “Freezing cold in March: ‘we can expect these kind of periods more often’” (roughly translated), caught my attention. While the article does state that the future winters will know more (cold) extremes due to the melting of the Arctic, it lacked an explanation of why/how this occurs. Out of curiosity I continued to further investigate the topic with one question in mind, is our cold bitter winter season in the Netherlands ultimately caused by global warming?
Initially I hoped some other Dutch media source had shown an interest in the cold weekend as well. Unfortunately, only RTL news had published a video to show that it was 21 degrees 13 years ago, which was “newsworthy” due to the freezing temperatures experienced on the same day in 2018. How insightful. Naturally this post caused some climate skeptics to question global warming on twitter. If it was 21 degrees thirteen years ago and freezing today, global warming must be in reverse, right?
Just because we still experience cold episodes does not mean the climate isn’t warming. And for those of you who still misconceive a cold day as an excuse that global warming does not exist, feel free to read one of the many articles explaining the difference between weather and climate. Nevertheless, as this blog post will show, global warming may be able to explain both the cold and warm days we are experiencing.
Fast Warming Arctic
It is no secret that human induced climate change is gradually warming our planet. NASA as well as many other research institutes, have indicated that notably the poles are warming much faster compared to the rest of the world. The American Meteorological Society states that sea ice loss is one of the main drivers warming the Arctic via the ice-albedo feedback (figure 1: feedback loop #1). Albedo refers to the reflectivity of a surface. A high albedo indicates that more energy is reflected back to space, while a low albedo indicates that more energy is absorbed by earth. Ice has a higher albedo compared to open ocean, therefore, when ice melts more energy is absorbed and temperatures increase. However, data indicates that there is a greater warming in the winter compared to summer, which NASA attributes to energy transportation. However, NASA does highlight that further research is required to understand this energy transportation, which is a common conclusion in climate sciences.
Additionally, WFF notes that the Arctic’s permafrost (frozen soil) contains a significant amount of methane, a potent greenhouse gas. As the Arctic continues to melt, this methane is released from the soil, leading to additional warming. This is referred to as the Arctic permafrost feedback loop (figure 1: feedback loop #2), and is one of the many means by which the warming Arctic affects the global energy budget.
Warm Arctic –Cold Continent
The Warm Arctic – Cold Continent Hypothesis, a notion that fast warming Arctic causes the displacement of cold air to lower latitudes is, according to Cohen et al., increasingly being discussed within the scientific community. It is a rather controversial topic considering that warming has dominated the global temperature trends over the past decades. However, in winter, cooling trends have been observed across Eurasia and the eastern United States, while the Arctic rapidly warms.
In order to comprehend the relationship between the warm Arctic and the cold winter episodes, it is important to understand the polar vortex. According to the New York Times, the polar vortex is a low-pressure system that rests over the North and South Pole. The boundary between the northern hemisphere vortex and the mid-latitudes is known as the polar jet stream, a fast counter clockwise moving wind. When the polar vortex is well defined (figure 2; stable polar vortex), it ensures that the cold air in the Arctic is well-contained. However, occasionally, the polar vortex weakens, which allows the cold air to escape to lower latitudes (figure 2; wavy polar vortex). This can be compared to opening and closing a freezer. When the freezer is closed there is no interaction between the temperature inside the freezer and the room temperature outside the freezer. When the freezer is opened cold air escapes from the freezer allowing warm air to come in, hence the room temperature decreases while the temperature in the freezer increases. However, when I leave my freezer open it starts to frantically beep causing me to close it, while a weakening in the polar vortex does not have such a warning system and can last weeks. Speculations have indicated that the strength of the polar vortex depends on the temperature gradients between the Arctic and the mid-latitudes. Due to the fact that the Arctic is warming much faster compared to anywhere else on earth, the temperature gradients is decreasing. This causes the jet stream to meander, weakening the vortex, moving cold air south and warm air north.
A very recent article published in Nature does note that “correlation does not mean causation”. Research concerning this phenomena is still in its early stages, hence, as long as this debate continues, this hypothesis is yet to be crowned as a theory.
Sudden Stratospheric Warming
It is plausible to presume that weakening of the polar vortex caused the extreme cold spell that hit Europe mid-February till early March. While this is definitely not a wrong assumption, the polar vortex did not just weaken, it actually split! A unique event known as Sudden Stratospheric Warming (SSW) was responsible for the split (and ultimately our ice skating adventures). SSW is defined by the The Irish Meteorological Service as a rapid increase in temperature in the stratosphere. The SSW is triggered by a disruption of the normal westerly flow due to natural weather patterns, or a disturbance in the lower atmosphere. This disturbance can lead to the wobbling of the jet stream as discussed earlier. If the “wobbles” break (like waves on a beach) they can be strong enough to significantly weaken or even reverse the westerlies to easterlies! The reverse causes the air in the stratosphere to collapse and warm due to adiabatic compression.
The 3 animations clearly represent what happened during the cold spell in February 2018. The animations were obtained from the data visualizer Earth, an interactive platform that allows you to observe the winds of the polar vortex in great detail at different times across different altitudes. Animation 1 shows the wind patterns at 10 hPa on the 1st of February before the polar vortex split. Animation 2 shows the wind patterns at 10 hPa on the 27th of February, clearly showing how the westerlies have shifted to easterlies. Animation 3 demonstrates the surface temperature on the 27th of February, confirming that higher latitudes were indeed warmer compared to most of Europe! I highly recommend you to play around with Earth between the 1st and 27th of February, it is pretty amazing to see the polar vortex split! You may also want to check out 250 hPa to see the polar jet in action.
Of course the question remains whether SSW events will become more frequent due to global warming. While, a recent paper published in the Journal of Climate does insinuate that this may be the case, they do so very cautiously as such an idea is even more uncertain compared to the Warm Arctic –Cold Continent hypothesis.
Now you can go home for Easter and impress your family with stories about the polar vortex that NU.nl lacked to mention. I speculate that NU.nl did not provide more detail about why the warming Arctic will cause more chilly periods in the mid-latitudes because the topic is still poorly understood. Yet they weren’t afraid to make one final bold statement in their conclusion; within the next 8 years weather reporter Piet Paulusma expects an elfstedentocht (eleven city tour, a Dutch ice-skating competition). Fun fact about the Netherlands; whenever “cold” weather is discussed by the Dutch, the term “elfstedentocht” has to be coined. Therefore, I too feel obligated to make a final statement about the future of the elfstedentocht.
Being born mid-February 1997, I have never experienced an elfstedentocht. If these extreme cold periods caused by a warm Arctic are similar to the one we experienced in February, the intense cold wind will hinder ice formation (let’s not pretend like the ice we skated on last month wasn’t shady). While there were fifteen elfstedentochten in 20th century in total, the Royal Dutch Meteorological Institute (KNMI) expect only about four in the 21st century. Bottom line is: I wouldn’t count on this Warm Arctic – Cold Continent phenomenon to make an elfstedentocht happen. The fact is, the average temperature in 2018 will likely be another record breaking year despite our cold winter. Sorry to disappoint, but we have to continue to take a long hard look as ourselves in the mirror in order to change our habits and keep our elfstedentocht dreams alive.