UK summer temperatures since 1933

Have summer temperatures in the UK increased since the 1930s? If so is CO2 to blame? Well it turns out that just cloud cover mostly determines summer UK temperatures while CO2 plays only a  minor role.  1975 and 1976 were the warmest summers since 1933 across the UK just because there was more sunshine and less cloud. There is barely any evidence of a long term warming trend in the data. If CO2 forcing is included in the calculation then the Transient Climate Response(TCR) to a doubling of CO2 works out to be at a modest 1.4C.  See below modelled results without AGW

Average of maximum temperatures (Tmax) across 22 weather stations all over UK. Tcalc is the calculated temperatures based only on  cloud forcing,  ignoring all CO2 forcing. See description of the model given in the text.

In June I received an email from Euan Mearns concerning UK temperature data and their interpretation. The historic data from 22 stations are available from the Met Office http://www.metoffice.gov.uk/climate/uk/stationdata/ He had noticed a striking correlation between monthly Hours of Sunshine and the Maximum Temperatures after about 1956. He interpreted hours of sunshine as being a proxy for cloud cover. His hypothesis was that perhaps average monthly cloud cover determined the maximum recorded temperature. He asked me for help to develop  a simple radiative forcing model to explain the data and we started a collaboration. This started nearly a 3 month study of cloud forcing compared to CO2 forcing. The previous two posts describe the annual average temperatures compared to a combined cloud & CO2 forcing model. The results show a modest CO2 effect and a strong cloud effect on UK annual temperatures. One criticism of the cloud forcing is that clouds behave differently with season. So in summer cloud cover is more likely to have a net warming effect whereas in winter clouds may have a net warming effect as daylight hours reduce. For this reason we looked at summer months only – June July & August to compare with the model on a yearly basis. This is more complex as for each station we need to calculate summer daylight hours and the incident insolation as these vary with latitude.

To proceed I  used the NASA climatology to obtain the clear sky solar radiative flux S0 for each latitude and longitude and month. The number of daylight hours for each station (dayhrs) were calculated and then used to derive the average cloud cover from the sunshine hours(sunhrs) each month.:

CC = (dayhrs-sunhrs)/dayhrs

For each station we then calculated the forcing due to the measured values of CO2 and Clouds averaged over the 3 months JJA for each year using the model described previously. The calculation was started  in 1930 and done indpendently for each station. Tcalc was normalised to the individual 1930 temperature and then  for each successive year Tcalc(y) was simply set to Tcalc(y-1) + DT.  The average value of Tcalc over all stations was then compared to the average Tmax over all measured values.

Variations in cloud cover explains all the yearly variations in summer temperatures. Including a CO2 term gives a reasonable fit based on R2 using NCF=0.69 (Net Cloud Forcing) and TCR = 1.4C – see Figure 1.

Fig 1: Comparison of calculated temperatures to measured temperatures for 22 stations across UK.

Fig3: Correlation plot of Tcalc versus Tmes. y=x is a perfect fit.

The fit is remarkably good. The clean air act was introduced in 1956 which helped reduce smoke across the UK and the correlation of the annual temperatures was not good before 1956. However the summer months were free of such problems because there were no open fires in towns so the correlation between sunshine hours (cloudiness) and temperatures is clear throughout the time series.

Is there any strong evidence for AGW in this data ? To check this I set TCR=0 and got the following result using cloud forcing only and NCF=0.69.

Fig 2: Comparison to cloud forcing only by setting CS=0 – i.e. ignoring CO2 forcing.

The fit is also very good which shows just how unimportant AGW has been for UK summers so far. The most important factor governing climate in the UK is changes in cloud cover – or seasonal weather. We kind of knew that already ! Another interesting observation is that good summers tend to come in runs  2 or 3 successive years.

So as far as the UK summer temperatures are concerned global warming is pretty much a non-issue thus far. Yearly variations are still much greater. So if we have a warm summer in 2014 no doubt the BBC will tell us global warming is to blame – but it won’t really be true. It will have been because we have had less cloud than normal. 1976 was the hottest summer experienced in the UK since 1933. I got married in 1976 in Wales and it actually rained that afternoon  for the first time in over 2 months !

References:

1. Myhre, G., E.J. Highwood, K.P.Shine,, F.Stordal, New Estimate of Radiative Forcing Due to Well Mixed Greenhouse Gases, Geophys. Res. Lett. 25, 2715-2718, 1998