How would you measure the temperature of the earth from mars? Well you could measure the infra red spectrum of the earth and then fit it to a black body spectrum to derive T. If you did that you would get an answer of about 252K, unchanging with time. The greenhouse effect keeps the effective average surface warmer at 288K, but what does this actually mean and how can we measure if it changes? This is not a simple question.
The earth’s temperature at any location is never in equilibrium. It changes daily, seasonally and annually. Incoming solar radiation varies enormously especially near the poles which receive more energy per day in summer than the equator.
The earth cools primarily by moving heat from hot tropical regions towards high latitudes where net IR radiation loss cools the planet, thus maintaining a certain temperature profile.
Rising CO2 levels modify that radiation imbalance profile slightly. Surface temperatures in the tropics are not really warming at all. Any excess heat induces more clouds and more convection while surface temperatures remain constant. What really happens is that the meridional radiation profile changes. Slightly more heat is transported polewards so that hot places are shifting more heat to cold places which are doing the warming. If CO2 levels stop rising then a new temperature and radiation profile would rather quickly be reached. This is then called ‘climate change’ but any such changes are concentrated in colder regions of the world. The global ‘temperature’ itself is not changing, but instead the global distribution of temperature is changing.
Temperatures at the poles during 6 months of darkness would fall well below -150C if there was no atmosphere, similar to the moon. Instead heat is constantly being transported from lower latitudes by the atmosphere and ocean and so that temperatures never fall much below -43C. If more heat is transported northwards than previously, then minimum temperatures must rise, and this is exactly what we observe in individual measurements.
The main problem with all the existing observational datasets is that they don’t actually measure the global temperature at all. Instead they measure the global average temperature ‘anomaly’. This is because measuring any average global temperature is “biased” by the distribution of stations, whereas measuring an average anomaly ( ) is supposedly not. Each monthly station ‘anomaly’ is actually the difference between the measured monthly temperature and so-called “normal” monthly values. In the case of Hadcrut4 these normal values are the 12 monthly averages calculated from 1961 to 1990 at each station.
The basic assumption being made when using anomalies is that global warming is a universal, location independent phenomenon which can be measured by averaging all station anomalies together, wherever they might be distributed. So in principal this then implies that global warming could be measured by just one station alone, which is clearly nonsense.
The use of anomalies introduces a new bias because they are now dominated by the larger ‘anomalies’ occurring at cold places in high latitudes. The reason for this is obvious, because all extreme seasonal variations in temperature occur in northern continents, with the exception of Antarctica. Increases in anomalies are mainly due to an increase in the minimum winter temperatures, especially near the arctic circle. To take an extreme example here is the monthly temperature data and calculated anomalies for Verkoyhansk in Siberia.
Annual temperatures vary from -50C in winter to +20C in summer. That is a seasonal range of 70C each year, and a year to year anomaly variation of ~8C is normal. The only global warming effect evident is a slight increase in the minimum winter temperatures since 1900. That is not due to any localised enhanced greenhouse effect but rather to an enhanced meridional heat transport. Temperatures in equatorial regions meanwhile have only ~4C seasonal variations, and show essentially no warming trend.
Long term changes in temperature anomalies occur mainly in northern continents in winter months. This is not because the earth as a whole is warming up but rather that meridional heat transport from the equator to the poles has increased and the largest effect on ‘anomalies occurs in winter. The average absolute temperature of the earth’s surface is unknown. Basing the evidence for climate change on the 150 year trend in global averaged temperature anomalies still biases the result towards higher latitudes where most of the stations are located. Is it any wonder then that climate scientists are tempted to further amplify the impression of global warming by artificially infilling the Arctic, rather than say Africa or South America. For some reason they are not so keen to infill the much larger Antarctica, because surprisingly it isn’t warming
This is an animation of recent monthly temperature anomalies which demonstrates how most variability in anomalies occur over northern continents.
Note the El Nino’s in 1998 and 2015.