Global Temperatures – April 2021

My calculation of April’s temperature (anomaly) was 0.64C, which is down 0.04C from March. This is based on V4C and HADSST3 data using spherical triangulation. It is the coldest April since 2015.

It was particularly cool across Europe, Central/Western US and in Australia and the Southern Polar region.


2021 so far is significantly cooler, so  roughly the same as 2014.

 

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Do CMIP6 models match the data?

Here is a comparison between my calculation of  global temperature data (spherical triangulation) and a sample of CMIP6 models blended so as to to match SST and land data (see here)

Comparison of monthly temperature anomalies and a sample of “blended” models with a common baseline of 1961-1990

The model data are computed as temperature anomalies on the same baseline as the data (1961-1990), so they all align themselves during this 30 year timespan. The real comparison  is  just on the temperatures trends post 1990.

It seems clear that low sensitivity models are more favoured.  This result implies that we can probably expect an average  rise in temperature since 1960 of about  1.5C  in 2050.

 

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CMIP6 temperature cycles

Nick Stokes has an interesting post comparing a blended version of CMIP6 tos/tas  to compare models to data. The blending is intended to correct for the fact that  Ocean temperatures are measured at the surface (tos) while land temperatures are measured 2m above the surface (tas). He made available the CMIP6 model data in an easy to use csv file. So I thought it would be easy to compare CMIP6 model results to my global temperature monthly data. However I discovered another effect.

CMIP6 monthly temperatures (Univ. Melbourne). Note the regular monthly oscillation in all models . For example the yellow post 2100 model signal (MRI ESM2)

Each model produces a monthly cycle of changing temperatures like a sine wave. The models work in absolute temperatures and you typically find a variation of 4 or 5 degrees during each year. The reason for this is probably 2 fold. Firstly there is an asymmetry between the land/ocean areas in the  Northern Hemisphere to that in the Southern Hemisphere. Secondly there is a slightly elliptical orbit of the earth (perihelion/aphelion). This causes the Southern Hemisphere summer to be about 4.1 million miles closer to the sun than the Northern Hemisphere summer. So why don’t we see this in the measured data?

The  fact is that none of the global temperature data show any sign of such an oscillation, although it is observed in Meteorological ‘reanalysis’ data. The main reason for this is that everyone always work in temperature ‘anomalies’ so they just calculate deviations from a ‘normal’ monthly average. I always take a 30 year normalisation period for weather stations of 1961-1991 which is the same as HadSST3/4. Let’s compare all 102 CMIP6 models with the data from Nick’s csv file where I calculate model “anomalies” on the same baseline.

Global temperature anomaly data compared to 102 CMIP6 model runs with tos/tas blending. The purple arrow shows the normalisation period for both.

The data are up to March 2012. The agreement appears reasonable good partly because they have been normalised to the same baseline. However more recent temperature data favours those models running slightly cooler.

What I find interesting though is that global temperatures actually change by ~5 degrees every year.  This is then superimposed onto an overall global “warming” of just 1 degree over the last 180 years. It is only by using temperature anomalies that this small effect can even be measured.  The above monthly model  temperature data are derived form monthly temperature data that actually looks like this !

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