Globally speaking, it’s a rather hot day.
According to GFS model runs and observational data, the past 24 hour period has shown temperatures in the range of +0.72 C above the already hotter than normal 1979 to 2000 average. A hot day in a hot month that is likely to be among the hottest on record, if not an all-time record-breaker itself.
A couple of days ago, hourly CO2 levels rocketed from 396 ppm to 399.5 ppm. A rather odd and somewhat ominous jump back toward the 400 ppm level at a time of year when atmospheric CO2 should be just starting a slow rebound from lowest ebb. A bottom that this year hit about 395 ppm during mid September. A measure already more than 2.2 ppm above last year’s low. To say the least, an hourly upward swing of 3.5 ppm isn’t exactly normal, especially when one considers the fact that the world hasn’t seen near 400 ppm CO2 levels for about three million years (this year peaked near 403 ppm during late spring).
And all that extra CO2, when combined with other greenhouse gasses, is having an increasingly obvious impact on climate. We see it in the record global average temperatures. We see it in the rising oceans which have come more and more to threaten the cities, lands and isles upon which so many of us reside. We see it in increasing instances of extreme weather around the globe — in the extraordinary and often persistent droughts, floods, storms and wildfires. And we see it in the form of a rather strong temperature amplification at both poles.
(Global temperature anomaly maps provided by GFS and the University of Maine shows no regions of the world cooler than average with the highest abnormal warm temperature departures concentrated, as usual, at the poles.)
Greenhouse Gasses as Primary Driver of Polar Amplification
Today, the Arctic is 1.60 C above the already hotter than normal 1979 to 2000 average. Meanwhile, the Antarctic boasts the highest departures for any global region at +2.09 C. Taking a closer look at the Antarctic Continent, we find an angry red splotch featuring temperature anomalies in the range of +12 to +20 C above average. A region associated with a tropics-to-pole transfer of airs we’ll discuss more in depth later.
What causes such a powerful and visible polar amplification? In short, it can best be described as the general impact of added greenhouse gasses on the global climate system.
Because most of the sun’s radiation falls on the equatorial regions, temperatures there are governed to a greater degree by direct solar insolation. But move toward the poles where sunlight hits the earth at a much lower angle, if at all, then the impact of the greenhouse effect holds greater sway. There, the ability of a gas like CO2 to trap and re-radiate long wave solar heat radiation can have a rather extraordinary impact.
On an Earth with no atmosphere, the temperature differential between poles and equator, between night and day, would be even more extreme than the variance we see today. But as the atmosphere thickens and the greenhouse gas overburden intensifies, the temperature difference grows less. For Earth’s present climate the temperature difference between the Equator and the Arctic averages about 42 degrees C. For the Antarctic, the average is about 71 degrees C.
On a world like Venus, where a kind of super greenhouse is in force and much of the atmosphere is composed of CO2, there is practically no difference in temperature between the equator and the poles. The reason for this is that greenhouse gasses trap the sun’s long wave radiation and recirculate it around a planetary system. And on Venus, a ray of long wave sunlight that comes in has very little chance to get out. So its heat recirculates many times within Venus’s atmosphere before it finally escapes.
On a place like Earth, where greenhouse gas levels are increasing, we would expect the temperature difference between the equator and the poles to drop as the poles warm faster due to the added impact of the increased greenhouse gasses. And since about the mid 20th Century, this is exactly what we’ve seen.
(Top frame shows North Pole to Equator temperature difference since 1948. Bottom frame shows South Pole to Equator temperature difference from 1948 to 2011. Note the approximate 3 C temperature swing indicating a faster warming at the poles in both graphs. Data is from the NCAR-NCEP reanalysis model.)
Lowering differences in Equator to polar temperature on a warming world also denotes a much faster warming of the polar zones. Hence the term polar amplification.
Now, for the Arctic, polar amplification has also become synonymous with loss of sea ice, loss of snow cover, increased land darkening due to changes in vegetation, and local release of greenhouse gasses via feedbacks from the Arctic environment. Each of these changes has the potential to add increased warming on top of the warming already being driven by global greenhouse gas increase even as such changes likely also drive changes to local and Northern Hemisphere weather. But as important as these additional changes may be, the larger driver remains an increase in global greenhouse gases driven by human emissions.
How Polar Amplification Drives Changes to the Jet Stream
In the end, such a polar amplification is a strong driver for changes to the world’s weather. Primarily, by reducing the difference in temperature between the poles and the Equator, we tend to see weaknesses forming in the circumpolar wind field known as the Jet Stream. At times, the Jet will slow and meander, allowing for the formation of ridges that extend far into polar zones and for troughs that dip deep into the middle and lower latitudes. Rather than a west-to-east flow of wind and weather, such a shift generates more of an Equator-to-pole flow:
(Triple tendrils — meridional flows converge on Antarctica. Note the massive highs sitting in the ridge systems driving the poleward wind flows. Image source: Earth Nullschool.)
And today we see two large north to south flows issuing from the 20 degree south latitude region, traversing thousands of miles of ocean in a poleward flood and terminating at the great ice sheets of Antarctica in the region of 70 to 75 south latitude.
Note that the flow originating off the west coast of South America terminates at the vulnerable West Antarctic Ice Sheet — a region that has been warming at an extraordinary pace of 0.25 to 0.5 C each decade. The second flow, originating from the South Atlantic and terminating over East Antarctica is heavily involved in the +12-20 degree C temperature anomalies ongoing there today.
Looking at these massive flows of air and the related spikes in temperature anomalies, it is easy to become confused over the issue of cause and effect. But it is simple to recall if you understand that first, added greenhouse gasses warmed the pole which in turn weakened the Jet Stream, which in turn allowed an amplification of the north-south meridional flow transporting yet more heat into this southern polar region.
For the southern polar region, today, we see some extraordinary high temperature departures for mid-to-late spring. At this time, polar amplification should be fading as more sunlight streams in. And yet we have a still strong positive temperature anomaly.
And as for the northern polar zone with its numerous additional polar amplification vectors, we shall see to what degree, if any, a potentially emerging El Nino tamps down the extraordinary meridional flows and polar vortex disruptions seen during just this past year’s freakish winter of 2013-2014.