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.
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
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
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.
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.
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.
“the ski season in Europe has got shorter with lower lying resorts suffering the most” is this really true? I’d be interested to see some reliable stats. I’ve been skiing in Europe almost every year since 1963 and the last few years have seen just as heavy, if not heavier, snow than I remember in 1963! I have a relative who skis in Scotland and he is of a similar opinion. Surely this is because the warming produces more evaporation which in turn produces more snow, not less.
You may be right. I went skiing every weekend for nearly 20 years in Italy. Snow depth varies from year to year. Certainly though UK winters have got milder. You see very little snow in southern England now.
” I’ve been skiing in Europe almost every year since 1963 and the last few years have seen just as heavy, if not heavier, snow than I remember in 1963! ”
Fact is that I live in Northeastern Germany since nearly 50 years, and that our last winter deserving the name was in… 2010: cold, lots of snow even in mid February. A huge shovel was heavily in use, but was no longer since then.
Since 2015, the winters are much too warm, except the one in 2017/18 with a tiny bit more snow than usual.
What we experience since two years are springs a lot cooler than usual.
Some say it’s due to La Niña’s current edition, some say it’s due to a cooling of the Northern Atlantic due to excessive sea ice and ice sheet melting, some think even it’s due to the lack of water vapor and carbon dioxide at 12 km height – because of the extreme lessening of flight activities since March 2020.
Boy do I wish the nighttime temperatures were a tad higher the first quarter of this year then they were here in Northern Ohio. The ski resort down the road was open more than I recall from back in the 60’s. I sure wish we had purchased more firewood early in the season as we burned through 2 cords before Feb 1st. Our fault for running out as we forgot to ask the previous owner how much wood it took for the entire winter.
Thanks for the detailed explanation of how the metrics are developed and reported.
Phil Toller, you may find this helpful. I’ve only glanced through it but it seems thorough. It is quite complicated. https://tc.copernicus.org/articles/15/1343/2021/
Thanks really interesting looks like there has been a downward trend but happily not much effect on skiing yet apparently, must be those snow cannons!
Thank you very much for this comprehensive, understandable text, which acts as a pleasant antithesis to the usual WUWT bullshit.
One or two years ago, UAH’s John Christy published a report about US stations’ daily maxima and minima temperatures in the US, based on USHCN daily data:
telling us that in the sum, ‘all was good’. No wonder.
I did the same using GHCN daily’s worldwide station set instead.
For CONUS there was a perfect match when using bars as did Christy, but lime plots are way better, and an additional plot for highest night temperatures was welcome too:
The same now for the Globe (with area weighting over a 2.5 degree grid, to prevent the Globe looking like CONUS’ backyard):
I was really surprised at this correlation between
– reduction of night minimums
– increase in day AND night maxima.
Very interesting and important observations. There is effectively no global warming in the Southern Hemisphere. The northern hemisphere nighttime warming can in part or in whole be attributed to replacing forest with car parks that continue to radiate into the night perhaps enhanced by increased water vapour levels from car exhaust, gas central heating, gas power generation and irrigation (garden hoses) etc. I don’t know if anyone has ever tried to quantify this. All that matters is the immediate environment surrounding the thermometer, for example a 1 km radius.
As for reduced snow fall. Alpine glaciers have melted completely about 10 times in the last 12000 years.
The Southern Hemisphere is mostly Ocean which “warms” slower than land. For sure there is an urban heating effect in cities and high density areas like the midlands but remote areas have warmed since pre-industrial times.
How much do natural cycles like the North Atlantic Oscillation play ? Perhaps they explain the rise in temperatures to 1920 to 1945. Superimposed on that is likely a ~ 1C rise due to CO2 increase. Nothing to justify the nihilist movement of XR though.
I would be surprised if CO2 is able to cause this change in diurnal temperature change. I think that we need to go back to first principals and think about heat sinks.
Oceans are enormous heat sinks, but unless there has been a change in insolation, it is hard to see why they would heat more at this time. My bet would be land activities are increasing the heatsink capacity of the surface. So what has changed in the last 170years and particularly what has changed since the late 70s?
much more land has come under the plough
reduction of forest area
irrigation – this allows soils to warm more since water retains heat much better than desert
irrigation also effectively increases the surface area of water for heating
across this time period:
cities are much more numerous to accommodate a much greater population. These populations are connected by roads and the roads have grown bigger and therefore increased their heat capacity.
Since the 70s there has been a huge increase in the built environment on all continents and a huge increase in the area of land given over to agriculture. Both of these factors could, I suspect, be enough to change the cooling profile of the night. The effect on day time temperature is more difficult to guess, since warmer morning air will lead to faster ocean evaporation and a change in cloud cover. Clouds are not a simple equation and they could be leading to cooler, warmer, or unchanged day temperatures.
About 20 years ago, returning from a walk in the hills one evening in Italy, I was surprised to hit a wall of heat. This intrigued me, since i could see no reason for this. After a while I realised that I was near a road and the heat was emanating from this source. I retraced my steps to get a feel of how far from the road I would be before I felt the heat difference. I was totally surprised to discover that this heat difference could be felt more than 100m from the road. The heat ribbon was therefore a couple of hundred meters wide. Imagine the heat ribbon of a trans continental motorway.
In my hang gliding days I used to love ploughed fields, since these were where you would get a great convection lift.
An old farmer in the north of Scotland recently told me that in his opinion the climate had not changed much, but that it was definitely more cloudy than it used to be.
If extra CO2 is causing warming at night, why has the minimum temperature over the Antarctic been decreasing since records have been available. This is despite a 30% increase in CO2.
How much of the observed overnight warming is due to the Urban heat Island effect?
Antarctica is a special case. Night (Winter) and day (Summer) each last up to 6 months. During winter the lapse rate disappears and the stratosphere essentially touches the ground (<-50C). In those conditions there is no greenhouse effect at all.
Thanks for your reply.
When I remove the winter temperature data for Mawson Station there is still a very slight reduction in minimum temperature.
The absence of greenhouse effect is probably due to the absence of moisture rather than the reduction in height of the stratosphere.
Are you saying that in the stratosphere there is no greenhouse effect, i.e. back radiation does not occur?
Even if the stratosphere is low over the Antarctic, the density of the atmosphere is not like the stratosphere and there would be plenty of air molecules to capture CO2 radiation/energy and thus there should still be a greenhouse effect from CO2.
The reason I chose Mawson station to analyse is that it has;
– a long history of observations
– No urban heat Island effect
– No moisture/humidity
– and by using the minimum temperature, this should correlate to night time, thus eliminating solar influences.
– the Antarctic Ocean should not be affected by El Nino/IOD cycles.
This only leaves CO2 as the climate change mechanism. This data so far shows that there is no correlation between temperature and CO2.
Antarctic average elevation is 8,200′ (2500m). The elevation at the South Pole is 9,300′ (2835m).
Mawson Station is basically at sea level
The larger increase in Tmin is supported by the evidence, but the causality by CO2 is an inference.
I agree that the increase in CO2 must necessarily have caused part of the warming, but your second Australian picture shows clearly that something else must be acting, as it shows two flat periods separated by a rapid raise. This is not how CO2 has increased.
The pattern is the same in global temperatures and many other climate phenomena. The 1976-1998 period was characterized by rapid change. The speed of change has greatly decreased since the early 2000s. In temperatures this is not so evident for the big El Niño of 2015.
Other causes are involved and they are being ignored because climate theory has become exclusively radiative, and dynamical changes are not considered. You would do well in leaving room for other causal factors in the increase in night temperatures.
I agree that natural cycles must play some role. The warming from 1900 to 1940 is an example of that. There is a warming signal though after 1976. This has been amplified since AR5 mainly by infilling regions with sparse data such as the Arctic which enhanced the global average. That would have been fine if the coverage had been constant in time, but of course it hasn’t. Coverage in the early decades was very sparse, especially in the oceans. Combine this with pair-wise homogenisation and you understand how the hiatus in global warming disappeared in time for AR6.
A warmer world is not necessarily a disaster. The tropics have their own thermostat to maintain temperatures. The water cycle increases. Arid regions though may suffer more.
Good to hear from you Euan,
Too bad your blog died with Roger Andrews. You missed the Peak Oil of late 2018 after so many years.
“A warmer world is not necessarily a disaster. The tropics have their own thermostat to maintain temperatures. The water cycle increases. Arid regions though may suffer more.” The water cycle can not increase in areas like India.. depending so much on summer melting of glaciers. & rebuilding in winter. I live in very arid Baja Calif. We had very little rain this past winter. Should be green now in May.. very brown. Of course, we all revel in warm summer days.. many plants will not grow in dry heat conditions.. I really see global situation growing into DISASTER.
Is Tav really calculated as T(min+Tmax) / 2 even at modern weather stations where you could calculate a real daily average?
Yes I think is still simply that because otherwise it would not be consistent with the past.
C02, UHI/changes in land use etc. obviously get mentioned most often but I would be interested to see a calculation by the physicists of the “entropy” part i.e. the direct warming of the atmosphere by humanity’s total energy consumption, e.g. 100% of all heating energy + 80% of all locomotive fossil fuel energy + 80% (or whatever) of lighting energy + X% of air con, +, +, etc. or maybe it would be simpler to just take the world’s total energy consumption and strike a view on the proportion directly heating the atmosphere? Is it significant?
From my rough calculations the thermal effect of Fossil Fuel on the atmosphere is 0.5 deg per decade.
Thermal Effect of Fossil Fuel burning
Atmosphere weight 5.4E+18 kg
Cp of Atmosphere 1.005 kJ/kg/K
Fossil Fuel emissions per year – CO2 3.40E+13 kg
Enthalpy of C to CO2 393.5 kJ/Mol
Atomic Mass of CO2 45 g/mol
Enthalpy of C to CO2 kJ/kg 8744 kJ/kg
Enthaply of FF emissions per year 3.0E+17 kJ
Delta K from FF Emissions/yr 0.055 deg K/yr
Thanks! That’s a big number relative to the published trends eg UHA 0.11degC per decade. Where does it go?
As I see it (not a scientist!) CO2 has caused a little warming, and is not responsible for 100% of the observed warming at the surface. The lower troposphere appears to warming at a steady rate(ish) of 0.14 deg c per decade (or more if you look at RSS). The release of CO2 to raise the ppm from 280 to 400 thus accounts for the 120ppm…which has caused the surface to rise by 1 degree c.
The ‘crisis’ or ’emergency’ often quoted by the BBC, Channel 4, Guardian, and Independent is because it is believed (by some) that releasing another 120ppm will cause another 1 degree c rise. However, it appears that CO2 doesn’t work that way (based on the past two decades of CO2 release and warming).
We are about to enter the cooling phase of the Atlantic Multidecadal Oscillation (AMO). This will drop global temps a little, I presume, though no one appears sure.
The only thing I, as a layman, am sure about is that something happened around 1980. The lines of temp graphs aren’t steadily progressive, and that makes me suspicious of a single cause. Also, as it’s a basic tenet of the theory that the stratosphere should cool (and it hasn’t continued to cool), then I’m deeply suspicious of the whole thing. Certainly, I am thankful for the above explanation, as I seem to remember glorious summer days and weeks in the 1960s in southern England (daytime temps), but I don’t remember many warm nights!
An increasing urban heat effect obviously shows nights warming faster than days. So what? 5% of earth’s land surface has 95% of the population. The remaing 95% of the land surface (where there aren’t so many thermometers) only has 5% of the population.
” So what? 5% of earth’s land surface has 95% of the population. ”
Then how do you explain that of the 40,000 GHCN daily weather stations worldwide, about 2,200 2.5 degree grid cells encompass the 17,000 that have valid data for 1981-2010?
Since the Earth’s land surface is about 30% of the total area, and a 2.5 degree grid has about 10,000 cells, this means that about 70% of the Earth’s land surface hosts at least one station somewhere.
Your 5% represent only cities.
In fact, 90% of the Earth’s land surface is used by people and their infrastructures, not just cities, towns, farmlands and industrial areas, but also all the networks that connect them.
Only 10% are true wilderness.