OK, I know picking on climate pseudoskeptics is like… well, shooting fish in a barrel.  (Not that I’ve ever shot fish in a barrel – but Mythbusters have shown it is easy to do, and that’s good enough for me.)

But this example illustrates an important general point.

One of the most basic, widespread and damaging thinking errors is: failure to make a relevant comparison.*

Consider this comment by someone I’ll call “Peggy Balfour,” since that’s what she calls herself.

The Europen Union has been carbon trading since 2005. All are industrialised nations.

According to the Mauna Loa air quality measurements Global CO2 was rising at average 1.67ppm per year prior to 2005.

7 years later, 2012, CO2 is still rising at 1.67ppm per year.

Check out ‘Full Mauna Loa CO2 record’ on this site. http://www.esrl.noaa.gov/gmd/ccgg/trends/

E.U carbon trading hasn’t made one whit of difference.

To show that E.U. carbon trading “hasn’t made one whit of difference” Ms Balfour compares C02 levels in 2005 with those in 2012.

Of course, the relevant comparison is CO2 levels in 2012 with what CO2 levels would have been in 2012 had European Union carbon trading not been in place but all else remaining the same.

The relevant comparison for this kind of causal inference is between an actual reading and a hypothetical value, one which can only be estimated using the kinds of complex quantitative global climate models that pseudoskeptics are wont to cavalierly dismiss.

I personally have no idea how much difference EU trading has made.  Maybe its not very much.  But I do know that you can’t properly answer that question, or a host of others, without making relevant comparisons.

* I think Robyn Dawes made something like this point in one of his excellent books.