When we are told that temperatures have risen by 1.2C since pre-industrial times, most of us assume this simple means that daytime temperatures are increasing. So summers are getting hotter and droughts are getting more extreme, but this is not true. The global temperature data that climate science relies upon is all based on daily measurements across thousands of weather stations, some going back to the 1700’s. All these stations record the average temperature for each day based on the minimum (night-time) and maximum (day-time) temperatures. Originally these were measured by physical Min-Max thermometers, whereas today they are all automated digital recordings. The daily average temperature though is always simply
Tav = (Tmin + Tmax) / 2
The monthly average of <Tav> is calculated for each 12 months of the year and a 30 year climatology derived for each station by averaging <TAV> over 30 years. These are called “normals” and the 30 year period a “baseline”. CRU uses a 1961-1990 baseline and NASA uses a 1951 – 1980 baseline. Finally all the monthly temperatures at each station are subtracted from the monthly “normals” to give monthly temperature “anomalies”.
All results are based on temperature “anomalies” which are averaged over the earth’s surface to give annual temperature series relative to the chosen baseline. These series then show a net warming of roughly 1.2C since pre-industrial.
Comparison of major temperature datasets (combined land and SST)
However they don’t tell you how that warming is happening on land. To investigate that process we must return to the daily values. GHCN-Daily is a huge archive (33600 stations) of measurements dating back to 1750. Each station recorded the maximum temperature and minimum temperature each day of operation. Essentially this also measures the diurnal temperature range because Tmin always occurs at night and Tmax occurs after midday.
Tav ( average Daily temperature) = (Tmax+Tmin)/2
Trange (Daily Diurnal Range) = Tmax – Tmin
These are the results I find for the average temperature and the temperature range analysing GHCN Daily
Temperature and Trange anomalies for full time range- Annual averages.
The temperature range has fallen since 1880 whereas the average has increased. This means that minimum temperatures (night) are increasing faster than maximum temperatures (day). Berkeley Earth gives a similar result
Tav and Trange anomalies (Berkeley Earth)
We can estimate the changes in Tmax and Tmin from these results. The average land temperature has risen by ~ 1.7C since 1880 whereas the range has fallen by ~ 0.7C so it appears that on land maximum temperatures have risen by ~1C since preindustrial times whereas minimum temperatures have risen by ~ 1.7C
This corresponds to our everyday experience. The UK sees fewer frosty days than it did in the past and in my experience there is less overnight snow than there was say 40 years ago. Night-time temperatures in my experience are warmer but the same cannot really be said about maximum temperatures. There have been no heatwaves to match that say of 1976. However the UK is affected by a maritime climate but we can see an even stronger effect in an arid continental climate like Australia.
Average maximum land temperatures in Australia compared to average minimum temperatures
Annual maximum temperatures across Australia have hardly changed over the last 100 years whereas it is annual minimum temperatures that have risen by over 2 degrees. This explains the observed rise in average temperatures.
So why should the minimum temperatures (night, winter) rise faster then maximum temperatures? The greenhouse effect provides an explanation. During daytime in summer with little cloud, solar directly radiation heats the surface. This initiates strong convection currents which transport heat up through the atmosphere more efficiently than radiative transfer. Increasing CO2 increases the radiative emission height. However at night solar radiation is zero and the surface mostly cools by radiation only. That is also what causes dew and frost in winter.
Figure from Richard Lindzen. Pure radiative equilibrium would be the temperature gradient without convection. The surface temperature would be >20C warmer than today ! Thermodynamics drives the lapse rate towards the moist adiabatic lapse rate
As a result increasing CO2 increases the night-time temperatures more than the midday temperatures. This reduces the diurnal temperature difference by warming minimum temperatures faster than maximum temperatures. So the first noticeable effect of global warming is less severe winters rather than more heatwaves in summer. So it is winter sports tourism which suffers more than the summer holiday industry. I would argue that this has already happened. There is no doubt that we have less snow and ice in the UK than what I remember 60 years ago, and the ski season in Europe has certainly got shorter with lower lying resorts suffering the most.
