Climate Letter #1690

I think I’ve found a better way to explain the big changes we are seeing in the 500hPa air pressure reading over the high latitudes of the Northern Hemisphere. Starting from scratch, we assume that any air pressure reading is nothing more nor less than a gravitational measure of the total weight of every molecule of matter within a vertical column of air all the way from the measuring spot to the very top of the column. If you place an effective scale, like a barometer, at the planetary surface you get a reading of around 1000 millibars, equal to about 14.7 pounds per square inch. Raise the scale off the surface, to any height, and you keep getting lower and lower readings, because there will be fewer and fewer molecules pressing down gravitationally on the scale.

If you then hold the scale in place at any upper altitude and find a way to inject more molecules into the air column above it the scale will at once give you a higher reading. If the molecules you injected were all extracted from spots directly below, still within the same column, then a second scale placed at the bottom of the column would detect no change in air pressure. If the injected molecules came from somewhere else this column would instead have a higher reading and one or more other columns would be lower.

On the high-altitude maps we have been looking at, someone has been nice enough to move the scale around through every one of Earth’s air columns until each of them reads one specific selected weight, in this case 500hPa (the same as 500 millibars), and then record on a map the altitude that results for each column. Altitude measurements of this same weight are in fact taken every day, and can be compared. The comparisons normally show some shifting of positions, but within a normal set of parameters. What we are currently getting, day after day, is decidedly abnormal with reference to one large part of the global surface, the higher latitudes of the Northern Hemisphere, but nowhere else. The change reflects a considerable upward movement in the altitude of 500 hPa readings over almost the entire area north of 30 degrees latitude, just in just the last month or two, with the greatest changes occurring over the Arctic region that is farthest north and coldest of all.

What we are sure of is that a lot more molecules have been injected into the air above the level where 500 hPa was resting a month or two ago, so that old level would now (almost certainly) have a higher reading than 500 today if we dropped the scale back to where it was back then. What we still want to know is where the newly injected molecules all came from and then what prompted them to move—a move generally judged to be strange by historical standards. It seems likely that practically all of them moved up and out of the air space that still exists below the old 500 level, leaving that space with air that is less dense than before, all of the space above it more dense, and the combined sum of all air pressure readings at the surface unchanged because there has been no change in the total number of molecules. So what caused so many to move up?

One likely answer to that question stands out in my mind, bringing heat into play.  We know that air temperatures in this particular region have been rising considerably faster than the remainder of the planet for several decades, indeed exponentially faster at times.  We’ve all been taught that air volume tends to expand with added warmth, making it lighter in weight and thus likely to move upward as well.  In this situation the only option available for accommodating the expanding volumes may be to move up, and that is where the molecules are likely to remain unless or until there is a reversal back to more cool conditions down below.

Lastly, a word about the specific set of consequences assumed to be directly associated with this activity, as described in recent climate letters. We have seen how jetstream pathways have been disturbed in ways that allow streams of water vapor and its products, having originated from the evaporation of tropical seawaters and sent aloft, to more easily navigate toward the highest reaches of the Arctic, which is their natural inclination.  These streams exude a strong greenhouse effect on the air below, having great leverage because that air below is progressively so much colder and drier on the way north.  We see resulting anomalies on a large scale at the surface every day.  One may then be led to wonder whether the new degree of extra warming will have a further tendency to expand air volume that leads to an even greater upward shift in the 500hPa level, and thereby a potential for further disorganization and weakening of jetstream behavior and its demonstrated effect on water vapor movement as a result.  Unless the full picture I have presented is wrong, this looks like the perfect definition of a feedback loop of possibly extended duration, and perhaps a greatly unbalanced outcome for the climate system as a whole.

Carl