I sometimes hear my fellow climate realists say that a globally-averaged surface temperature has little or no meaning in the global warming debate. They claim it is too ill-defined, not accurately known, or little more than just an average of a bunch of unrelated numbers from different regions of the Earth.
I must disagree.
The globally averaged surface temperature is directly connected to the globally averaged tropospheric temperature through convective overturning of the atmosphere. This is about 80% of the mass of the atmosphere. You cannot warm or cool the surface temperature without most of the atmosphere following suit.
The combined surface-deep layer atmospheric temperature distribution is then the thermal source of most of the infrared (IR) radiation that cools the Earth in response to solar heating by the sun. Admittedly, things like water vapor, clouds, and CO2 end up also modulating the rate of loss of IR to space, but it is the temperature which is the ultimate source of this radiation. And unless the rate of IR loss to space equals the rate of solar absorption in the global average, the global average temperature will change.
The surface temperature also governs important physical processes, for instance the rate at which the surface “tries” to lose water through evaporation.
If the globally averaged temperature is unimportant, then so are the global average cloudiness, or water vapor content. Just because any one of these globally-averaged variables is insufficient in and of itself to completely define a specific physical process does not mean that it is not a useful number to monitor.
Finally, the globally averaged temperature is not just a meaningless average of a bunch of unrelated numbers. This is because the temperature of any specific location on the Earth does not exist in isolation of the rest of the climate system. If you warm the temperature locally, you then will change the horizontal air pressure gradient, and therefore the wind which transports heat from that location to other locations. Those locations are in turn connected to others.
In fact, the entire global atmosphere is continually overturning, primarily in response to the temperature of the surface as it is heated by the sun. Sinking air in some regions is warmed in response to rising air in other regions, and that rising air is the result of latent heat release in cloud and precipitation systems as water vapor is converted to liquid water. The latent heat was originally picked up by the air at the surface, where the temperature helped govern the rate of evaporation.
In this way, clouds and precipitation in rising regions can transport heat thousands of kilometers away by causing warming of the sinking air in other regions. Surprisingly, atmospheric heat is continually transported into the Sahara Desert in this way, in order to compensate for the fact that the Sahara would actually be a COOL place since it loses more IR energy to space than it gains solar energy from the sun. (This is because the bright sand reflects much of the sunlight back to space).
Similarly, the frigid surface temperature of the Arctic or Antarctic in wintertime is prevented from getting even colder by heat transport from lower latitudes.
In this way, the temperature of one location on the Earth is ultimately connected to all other locations on the Earth. As such, the globally averaged surface temperature — and its intimate connection to most of the atmosphere through convective overturning — is probably the single most important index of the state of the climate system we have the ability to measure.
Granted, it is insufficient to diagnose other things we need to know, but I believe it is the single most important component of any “big picture” snapshot of climate system at any point in time.