A: because they are anomalies.
Climate sceptics want to be taken seriously. They want to hold joint conferences with climate scientists. It is not impossible for climate sceptics to do good research, though whether the odds are better than a monkey typing Hamlet is unclear on the evidence below.
A few weeks ago, Willis Eschenbach posted some CERES total solar irradiance (TSI) data at WUWT that showed that TSI varies from about 330 to 350 W m-2 over the course of a year. Eschenbach wondered why this >20 W m-2 difference was not obvious in the global temperature record.
… where is the effect of the ~ 22 W/m2 annual variation in the amount of sun hitting the earth?
He has some ideas. But not good ones.
To get an idea of the predicted effect of this variation in TSI, using IPCC figures this TSI change of 22 W/m2 is about the same change in forcing that we would get from six doublings of CO2 … that is to say, CO2 going from the current level (400 ppmv) to the extraordinary level of 25,600 ppmv.
In addition, again according to the IPCC, using their central value of 3°C warming per doubling of CO2 (3.7 W/m2 additional forcing), this change in forcing should be accompanied by a change in temperature of no less than 18°C (32°F).
Now, I can accept that this would be somewhat reduced because of the thermal lag of the climate system. But the transient (immediate) climate response to increased forcing is said to be on the order of 2°C per doubling of CO2. So this still should result in a warming of 12°C (22°F) … and we see nothing of the sort.
One would have hoped that Eschenbach had been interested in climate change long enough to remember some of the basic definitions. The transient climate response is not an immediate response. It is defined by the IPCC as.
… the change in the global surface temperature, averaged over a 20-year period, centred at the time of atmospheric carbon dioxide doubling, that is, at year 70 in a 1% yr–1 compound carbon dioxide increase experiment with a global coupled climate model.
The Earth’s climate will no more show an immediate 12°C response to the intra-annual variability in TSI than a kettle will instantly boil when plugged in. The climate system has an enormous thermal inertia, mainly in the oceans.
But shouldn’t there be some response to this variability in TSI? Yes, but the global temperature anomaly is an anomaly. For each January, the mean of the climate normal period (often 1961-1990) for January is subtracted. Ditto for February and the other months. This processing removes any tendency for Januaries to be warmer or colder than Julys. Eschenbach won’t find the pattern he is looking for because he is looking at the wrong data and he really ought to know this.
But what if he looked in the right place – the variability in absolute temperature as discussed by Jones et al (1999)
The annual cycle of global mean temperatures follows that of the land-dominated NH, with a maximum in July of 15.9°C and a minimum in January of 12.2°C.
Perihelion, the Earth’s closest approach to the Sun, currently occurs in early January. This timing probably mutes the annual cycle in global mean temperature by offsetting some of the hemispheric differences.
Not to be out done, Stan Robertson takes up Eschenbach’s theme without realising that it is nonsense.
… why don’t we see some significant annual cyclic variation of global mean temperature? This is a truly profound question! It ought to keep climate modelers awake all night, every night.
We do need a better class of climate sceptics.