Seasonal trends

It is almost comical how people will grasp at any straw they can come up with to claim that global warming isn't happening.  The most recent bit of hilarity?  A claim that the winter trend since 2002 is cooling, therefore we're in global cooling, not global warming.  Let's check that one out to see just how ridiculous it is.



First, here are the seasonal trends since 1979 using Berkeley Earth land + ocean data.  Why 1979?  Because that's the first full year of the satellite era, enabling readers to cross check me if they prefer satellite data instead of surface data.

DJF = December, January, February, MAM = March, April, May, JJA = June, July, August, SON = September, October, November

Looking at the trend lines, it's obvious that SON (September, October, November) has experienced the largest increase, DJF (December, January, February) the least.  The calculated trends bear this out:

Quarter	Trend	95% Confidence
DJF	0.137ºC/decade	± 0.043ºC per decade
MAM	0.155ºC/decade	± 0.038ºC per decade
JJA	0.160ºC/decade	± 0.036ºC per decade
SON	0.193ºC/decade	± 0.036ºC per decade

The overall trend?  0.161ºC per decade ± 0.034ºC per decade.  Yep.  Sure sounds like global cooling to me.  /sarcasm.

Now for a closer look at the data since 2002, which is far short of the 17 year minimum needed to reliably detect climate trends.

DJF = December, January, February, MAM = March, April, May, JJA = June, July, August, SON = September, October, November

It does indeed look like there's been quite a bit of cooling during the DJF quarter.  However, take a look at the 95% confidence interval, as appearances can be deceiving:

Quarter	Trend	95% Confidence
DJF	-0.090ºC/decade	± 0.189ºC/decade
MAM	-0.020ºC/decade	± 0.159ºC/decade
JJA	0.085ºC/decade	± 0.123ºC/decade
SON	0.045ºC/decade	± 0.104ºC/decade

The data shows a minor trend in each quarter but the confidence intervals tell us that those trends are not statistically significant.  That simply means that any trends since 2002 are due to random chance.  The overall temperature change since 2002 using all the data? 0.000872ºC per decade ± 0.107839ºC per decade. In other words, as flat as the myth of global cooling since 2002.

Update: The R code I used to generate the graphs and statistics has now been published by request.