Responding to the CO2 Coalition's "Fact #27" on Post WWII Temperatures

CO2 Coalition's "Fact #27" depends almost entirely on cherry picking. Here we're told that for 33 years, global temperatures cooled while CO2 emissions increased. "We have seen that the beginning of the modern increase in CO2 emissions began in the post-WW II industrial boom. Yet that great rise in CO2 was accompanied by a significant 33-year span of global cooling from 1944–1976. If CO2 is the primary driver of modern temperature change, why did temperatures actually fall during this time span?" Here's the graph they posted.
I'll answer their question after clarifying the issues here. The graph above contains numerous problems. First, they posted monthly temperature anomalies from HadCRUT4 (which is out of date) and annual emission CO2 emission values for 1944-1976. The decision compare monthly temperatures to annual emissions appears to be an attempt to exaggerate the differences between the two. CO2 Coalition should have used annual values for each. Beyond this, the correlation between GMST and CO2 is not specifically between temperature and emissions. Rather, there's a log correlation between GMST and CO2 concentrations, a linear correlation between GMST and CO2 forcings, and a linear correlation between GMST and cumulative emissions. CO2 Coalition didn't plot any of these. Let's fix the graph by plotting GMST from HadCRUT5 and CO2 concentrations.
In this graph you can see that CO2 increased by about 20 ppm, while there's not much of a discernable trend in temperatures. The HadCRUT5 trend for 1944-1976 is actually -0.03 ± 0.037°C/decade (2σ). Since the 2σ CI is larger than the trend, we can't rule out the possibility of warming to 95% certainty, so it's not accurate to call this a cooling trend. I do think that since we have 30+ years without a statistically significant trend, we can call this a "pause" while CO2 increased from 310.2 ppm to 332.03 ppm. So the real question from this would be, why did temperatures pause while CO2 increased? Does this present a problem for climate science?

To answer these questions, we need to do a little math. The radiative forcing increase for this increase CO2 can be calculated as RF = 5.35*ln(332.03/310.2) = 0.36 W/m^2. If all other forcings are stable, this would lead to about 0.2 C warming (assuming TCR is ~2 C). Since temperatures were flat, we should consider whether there was some other forcing that offset the warming influence of CO2. It turns out that there was a large ramping up of aerosol pollution following World War II that exerted a cooling influence on the climate. This was quantified in Gillet et al 2021[1].


The graph above shows the cooling influence of aerosol following WWII in the blue line with a large confidence interval. There are significant uncertainties associated with aerosol forcings, but needless to say it's quite obvious that this forcing can easily explain why GMST paused while CO2 increased.
And CO2 Coalition is clearly cherry picking. If they had shown a graph of CO2 concentrations with GMST since 1850, the correlation between CO2 and temperature would be undeniably obvious. There's a reason why CO2 Coalition doesn't show you either the graph above or the graph below.
The correlation between GMST and CO2 forcings can be quantified to about r^2 = 0.88. In other words, even with the 30-year pause in GMST, CO2 forcings continue to be a very good predictor of global temperatures.


References

[1] Gillett, N.P., Kirchmeier-Young, M., Ribes, A. et al. Constraining human contributions to observed warming since the pre-industrial period. Nat. Clim. Chang. 11, 207–212 (2021). https://doi.org/10.1038/s41558-020-00965-9







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