This post addresses the question: How “well mixed” is carbon dioxide in Earth’s atmosphere?
Here are some images of surface CO2 concentration for yesterday (June 23 in Australia) from nullschool.
Darker areas show lower CO2, lighter areas are higher. Very nifty.
Fig. 1: Northern Hemisphere CO2:
The dark areas with low CO2 are the northern forests and farm land, now growing strongly. Note the cold, dry North Pole has high CO2.
Fig. 2: Southern Hemisphere:
Cold, dry Antarctica has high CO2, whereas a broad area of inland Eastern Australia, which recently has had some decent rain, has lower CO2.
Fig. 3:The East:
Fig. 4: The West:
The contrast in South America is interesting!
Fig. 5: The Pacific (a hemisphere on its own):
Note the northern Pacific (north of 5 degrees north) is predominantly above 400ppm, while a broad band from about 5 degrees north to about 20 degrees south is about 395ppm.
Note also a tiny area in southern California pluming into the Pacific with a very high reading of 437ppm. Los Angeles.
The IPCC and climate scientists generally refer to data from Mauna Loa in Hawaii. The CSIRO in Australia also measures CO2 concentration at Cape Grim in Tasmania. The next few charts compare Cape Grim data with that of Mauna Loa.
Fig. 6: Comparison Mauna Loa and Cape Grim CO2 1976-2016
Here is a closer look at the most recent years:
Fig. 7: Comparison Mauna Loa and Cape Grim CO2 2010-2016
There are several points to note:
Cape Grim CO2 concentration is increasing at the same rate as Mauna Loa.
There are massive swings in Mauna Loa’s data, while Cape Grim fluctuates gently. In 2016 there was no “bottom” at all.
Cape Grim is much lower- in fact the annual high points are at about the same level as Mauna Loa’s low points.
The records are out of phase. Mauna Loa peaks in northern spring and bottoms out in northern autumn, whereas Cape Grim peaks in southern Spring and “bottoms out” in southern Summer.
Now I look at the seasonal change in concentration.
Fig. 8: Seasonal rises and falls at Cape Grim
Fig. 9: Seasonal rises and falls at Mauna Loa
Notice at Mauna Loa the annual rises from bottoms to peaks are getting larger, but so are the falls, while at Cape Grim there are slower rises but falls are lessening. I compare rises and falls separately in the next two plots:
Fig. 10: Seasonal increases compared
Fig. 11: Seasonal decreases compared
I would interpret this as follows:
As emissions increase, carbon dioxide sinks (mainly growing plants) consume more and more. However this is not enough to remove all of the additional CO2, so each year the growth continues.
In the Northern Hemisphere, sinks completely overwhelm sources in summer.
In the Southern Hemisphere there is a much less pronounced annual peak in spring, perhaps because there is less land, especially from 30 to 70 degrees south, and much of it is dry. CO2 concentration has increased to the level at which vegetation CO2 sinks are becoming unable to make an impression (at least in El Nino years).
The bulk of CO2 increase originates in the Northern Hemisphere. In northern winter as the Inter-Tropical Convergence Zone shifts south of the Equator, the north east trade winds move CO2 to the Southern Hemisphere where it is gradually mixed. In northern summer (now), the ITCZ is north of the Equator, and the image of the Pacific in Figure 5 above shows trade winds crossing the Equator with less CO2 concentration than just to the north.
We know there are large changes to CO2 concentration following ENSO events. This may be due to the changing circulation over the tropical Pacific as more or less CO2 is shifted by trade winds north and south. Or perhaps changing ocean currents, upwelling, or downwelling warm or cool large ocean areas.
Drier areas of the globe (deserts, Polar regions) have higher CO2 concentration than wetter areas. Few growing plants, more CO2. More and greener plants, less CO2.
And finally: CO2 is not “well mixed” globally, and an average concentration is as elusive as an average temperature. There is a range of concentrations between areas of sources and sinks approaching 80ppm.