Cold Waves and Global Warming
Each year the Earth’s average temperatures rises due to the rising concentration of greenhouse-gases, making Troposphere more opaque to long wavelength radiation and retaining more heat near Earth’s surface: that is Global Warming in short terms. The last four or five years were strikingly warmer, with average temperatures above the 1981–2010 climatological mean at almost any month.
How then, in the same period, we witnessed extreme cold waves (specially in North America, central and northeastern Europe and Siberia), severe blizzard and winter storms, even in “wrong times”? How, sometimes, a cold wave break records of low temperatures that persisted for so long time? In recent past we have ever seen snow in (northen) Saara, so how these events do not contradict Global Warming?
There is no contradictions. In truth, severe blizzards and certain kinds of cold waves strongly support the central thesis of Global Warming, and they are expected to occur in a scenario of rising Tropospheric temperatures in world-wide scale. How and why these events corroborate Global Warming? That is what this text is all about, for beginners.
Remembering some recent events of extreme cold and blizzards around the globe
It is known that 2016 was an atypical year, way warmer than the climatological average, specially around the North Pole. Between October and November, Arctic average temperatures reached 20°C above the climatological mean for these months, lingering at some 10°C warmer on December and January.
At the same time there were early blizzards at various locations. On the 11/10/2016 the Swedes woke up surprised by a 39cm record of snowfall. At Tokyo, on November 25th, the morning becames white by a thin layer of snow, the first in that date after more than fifty years of snowless November. Between December 17th-19th snow fallen beyond the Atlas Mountains’ slope, at northwestern Saara.
So 2017 started witnessing extreme cold and severe blizzards. East and southeastern Europe, Anatolia and Iran were struck by an extreme cold wave between January 5th and 7th, snowing on Greek Island at Egean Sea (e.g. Lesbos) and Athens’ temperature reached -15°C. Moscow’s temperature reached -30°C, the lowest in January in the last 120 years. Severe blizzards struck North America’s northwestern coast, snowing in Canada on April. Around a year later, on 03/21/2018, NY witnessed a bitter cold and an unprecedented strong blizzard.
The picture above shows, at left, the snow cover for the January 6th 201. At right, the climatological snow cover departure: the red areas are not covered by snow on that day, despite climatologically they are covered; the blue areas are covered by snow, despite it happens not so often. Note the uncommon blizzards striking central Europe and central and south North America.
Polar Vortex and Cold Waves
Along winter on Northern Hemisphere, the entire hemisphere receives less Solar radiation per unity of surface area, thus being less warmed than at another times of the year. The colder, densier air subsides, reaching ground around the geographic North Pole, where it usually encounter an enormous iced surface, that is flanked by frozen water and ice blocks. The ice reflects so much shorter wavelength radiation, cooling the air just above by radioactive cooling. That cooled air has already reached ground cooled, for that it sank due to its higher density and lower temperature.
So, at usually conditions, an enormous mass of cold, dry and dense air is formed just above the ice, in an almost circular area around the Pole. That High Pressure airmass is very stable and is maintained so by, aside other factors, the strong radioactive cooling from the circumpolar ice cover, letting the warmer, moister air at south. That is called Polar Vortex, that is usually strong and stable, lying at an icebed almost circular area around the Pole at most of the winter.
There are no big landmasses around the North Pole, but a frozen ocean acting as an ice layer just below the Polar Vortex. That ice is usually thinner than the landmasses’ ice sheets, and then it is prone to melt its borders. In warmer years the air temperature just above the sea is higher most of the time, and the sea surface’s temperature as well. In that situation the northern circumpolar ice sheet tends to retract its borders.
Ocean ice depends less on air temperature and more on surface sea temperature. The sea takes more time to cool. The Ocean’s heat capacity is way bigger than land’s, because water’s specific heat is big and Ocean’s mass is huge, even considering only the first meters below surface. These factors cause a “thermal inertia” in which Oceans delay their responses to thermal forcing.
In short, during warmer years the Arctic surface temperature is much higher than the climatological average, hardening the circumpolar ice sheet formation or even causing its retraction. With less iced areas around the Pole, the air just above the surface also becomes warmer.
Under these conditions the Polar Vortex becomes unstable, as it depends on the intense radioactive cooling caused by surface ice to stay stable. Warmer air just above the surface is less dense than the cold air above and around, than it rises and “pushes” the colder air to outside the usual border of a normal Polar Vortex. That brings moist and warmer air to the north, warming even more the surface and reforcing the process explained, weakening the Polar Vortex.
Therefore, when the circumpolar ice sheets retract, the Polar Vortex becomes weaker and divides itself in partitions that goes south, seeking stronger surface radioactive cooling. These cold, dense and dry airmasses are responsible for the severe cold waves at lower latitudes during setentrional winters, usually staying over mainland continental areas, specially over central Siberia and northern China, central and northeastern North America, and eastern Europe: the dense, cold and dry High Pressure airmasses find stability over cold, continental areas where the frozen soil has lower specific heat, relatively lower heat capacity and cools itself quicker than the Arctic Sea, giving conditions to occur a strong surface radioactive cooling and stabilizing these splitted portions of a former unified Polar Vortex.
The picture above shows the difference between normal and atypical winters. Darker areas indicate where colder, densier air lays over the surface, concentrated in an almost circular shape around the North Pole during a typical winter, at left. At right, during an atypical winter the Polar Vortex is weakened and splitted in portions laying over lower latitudes, specially over big landmasses.
Therefore, in warmer years the circumpolar ice sheet shrinks and makes the Polar Vortex unstable, splitting it in some nuclei that lay southward, specially over big landmasses: and that just causes the severe cold waves at winter, sometimes breaking records.
Blizzards at warmer years
It sounds as a paradox: warmer years can lead to more severe and earlier blizzards than most of colder years. That is even a strong prediction based on the main Global Warming hypothesis, but to explain why it is necessary to acknowledge some things about air humidity, its condensation, and its relation to air temperature.
When someone is talking about air humidity usually it refers to Relative Humidity (here called as R). The Air’s Relative Humidity (R) is the rate between the quantity of water vapour inside given volume of air and the maximal water vapour content that that ar volume can sustain before saturation (i.e. before the gasous water in the air turn itself in liquid water (droplets) or solid water (icelets), forming clouds and/or precipitation). The higher the air temperature, the higher the content of water vapour it can sustain before saturation occurs.
There is another kind of air humidity, though, the Specific Humidity (also called Absolute Humidity), here we call it q, expressed in the actual mass of water vapour per total mass of air (usually g/kg, but we will note that as m/M). The Specific Humidity tell us how much water there is in a given volume of air.
Therefore, the same volume of air, sustaining the same amount of water vapour (q), can get different Relative Humidity values (R) due to differences in the air temperature: warmer air sustains more water vapour until it condensates. Any volume of air sustaining water vapour can condensate if its temperature lowers enough: and it is how so many kinds of clouds are born.
As warmer air sustains more water vapour, there will be more vapour condensation (or even precipitation) when it cools itself. So, the larger the temperature gap between initial and final temperature of an air volume, the more condensation and precipitation it could form, what includes snow if cold enough.
The picture above tries to explain what was said about air humidity. The same volume of air has the same amount of water vapour (light-blue circles) in the first two panels, but different relative humidity (R) due to different temperatures. If the temperature drops until there is more water vapour than “room” to it stays on the air, the excess will turn to liquid (the dark-blue teardrop-like objects on the last panel).
When the winter comes in warmer years there is much more specific humidity in the air, and therefore there is more condensation and precipitation when the air cools out. Thus it is expected that, given the Global Warming, there will be more blizzards, even earlier ones. In an unusually cold year scenario, however, there is less specific humidity in the air when in cools out due to winter, therefore the snowfall is lower than in warmers years.
However the Global Warming shrinks the area where snowfall usually can happen (because warmer air takes longer to cool out, as it has more water in it, that has huge Heat Capacity), and the time period to snowfall also becomes shorter. The extra snowfall expected due to Global Warming occurs when moist, warmer of oceanic origin air infiltrates continental, colder areas (often around those splitted nuclei of Polar Vortex laying over big, frozen landmasses) and also due to frontal systems.
Cold and Snow in 2019
The second half of January witnessed blizzards and cold waves in central Europe, eastern and northern Great-Britain, and bitter cold and severe blizzard over North America in the last days of the month. The central portion of Siberia was also affected by intense drops in temperature, reaching -44°C around the mouth of Lena River, -42°C on Groenland and -28°C near Chicago.
The last year, 2018, was way warmer than the 1981–2010 climatology, as were the last few four to five years, as well. The current Arctic Ice Extent area is way smaller than the climatological average. These are conditions that usually lead to atypical winters, with splitted Polar Vortex causing strong, bitter cold waves and the yearly warmer, moist air causing severe blizzards when cooled out.
The regions around the Polar Circle has positive temperature anomalies and less snow deposition, while lower latitudes areas are under bitter cold waves and much more snow deposition: conditions that will be much more common in the future, due to Climate Changes, as the entire world becomes warmer.
Following the evidences, while the Southern Hemisphere stares severe heat waves (e. g. Brazil and Australia), bitter cold waves (Siberia, Europe, China and North America) show the pattern that probably will prevail in the next coming decades. And now the reader can understand why severe blizzard and unusual cold waves do not contradict Global Warming, but strongly support it.
Image Sources:
Rutgers University, Global Snow Lab
NSIDC
EarthSchool
Zack Labe’s Page
