Monday, April 13, 2020

No Noticeable Decrease in Atmospheric CO2

Despite many economies being partially shut-down due to Covid-19, there doesn't seem to be a decline in atmospheric CO2 beyond the usual gradual increase -- in fact, if anything, there's been a slight increase. Here are the changes in atmospheric CO2 compared to 52 weeks earlier:

Why would there be a slight increase over the last few months, when electricity use is down in the US and several other countries, and driving and public transportation surely are too? There are of course many variables and many things going on at once, so maybe no one knows. The only idea that comes to me is that we're on the border of short-term El Nino conditions (ONI of about 0.5), and El Ninos increase atmospheric CO2 because they bring heat the atmosphere, and especially the tropics, which causes more drought and fire there that kills plants that were taking up CO2. (Though it's not that simple.)

For example, here's the 52-week change in atmo CO2 over several decades:

The strong El Ninos of 1997-98 and 2015-16 clearly stand out.

Any other ideas?

PS: I saw a tweet the other day saying if a blood alcohol level of 0.08% is enough to impair driving and get you arrested, how can a CO2 level of 0.04% be considered a meaningless trace gas?


Ned said...

This is really bugging me. Roy, who should know better, and others on Twitter (who shouldn't) have been harping about this. The unstated implication seems to be "if you can't immediately see the effect of a sudden reduction in emissions on the total atmospheric CO2 concentration, maybe humans aren't responsible for the increase in atmospheric CO2" but that's obviously wrong so people prefer to just kind of sidle around it rather than saying it flat out.

The reduction in global emissions so far this year is less than 10% (back-of-envelope calculations). Anthropogenic emissions raise total atmospheric CO2 by about 2.4 ppmv per year or 0.6 ppmv per three-month window, so that less-than-10% cut translates to less than 0.06 ppmv.

But during this mid-Jan to mid-Apr window, the phenology of the NH biosphere causes CO2 to increase by an average of 3 ppmv plus or minus 1.1 ppmv 2-sigma. That pseudo-random variability in the seasonal cycle of the biosphere -- related to ENSO, as David Appell points out -- is, during these 3 months, 18x the magnitude of the emissions reduction from our economic crisis.

In other words, at this time of year, random unpredictable departures from the normal seasonal cycle of the biosphere could range from 18x larger than our emissions reduction to 18x smaller than it. There's no way to detect our reduction underneath that noise.

If that reduction were sustained for multiple years it would certainly become visible at some point. But not after mere weeks or months.

David Appell said...

Ned, good analysis. Thanks.