# What is the “global mean surface temperature change estimate” an estimate of?

NASA publishes what it calls an "estimate" of the Earth's global mean surface temperature. Digging through papers I've been able to find some documentation of how these numbers are computed, but I have had a frustratingly hard time answering what seems to me a much more basic question: what is this "estimate" an estimate of? That is, what is the abstract conceptual quantity that we'd really like to know, if we could? Without that, I don't understand how we can evaluate how well, for instance, the methods they use to blend together data sources of different resolutions and reliabilities, since the definition wouldn't have any stronger "correctness" criterion than just "this is how we put these numbers together".

It seems to me that the conceptual quantity they're trying to measure is, for a given year:

• Imagine gridding up the earth into equal-area grids and putting a thermometer in each one.
• For each thermometer, imagine taking the limit of the average reading as number of readings increases for a one-year timespan from midnight on Jan 1 to just before midnight on Dec 31 of the given year
• Now take the limit of the average of all those single-thermometer limits for every thermometer in your grid, as the size of the grid squares decreases.

But this is really just a guess as to what they're after -- I have skimmed over several papers on this subject and I haven't been able to find any discussion of this at all. Is this just something basic that climate researchers know that I'm missing?

• Are you talking about GISS Surface Temperature Analysis (GISTEMP) (data.giss.nasa.gov/gistemp)? This map shows the change since a 30-year baseline period of 1951-1980: data.giss.nasa.gov/gistemp/maps. The idea is to gauge how a current regional (or a global average) temperature varies from past trends. The map shows, for example, a current anomaly of 7.5 degrees C in portions of Alaska and northern Asia above temperatures during the baseline period. – jeffronicus Jun 10 '17 at 22:54
• This is a more fundamental question than that. What does "average surface temperature" even mean, independent of how a particular research team estimates it? By way of contrast, "global CO2 emissions per year" is unambiguous, even if we can't know its exact value: There is some specific amount of CO2 we emitted in any given year, so the game is just to estimate that quantity given that we aren't omniscient. It's not obvious what "average surface temperature" even means, though -- what exactly are we averaging? – jacobm Jun 11 '17 at 15:38
• One problem is that climatologists often talk about temperature anomalies, not average temperature, but question 8 on ncdc.noaa.gov/monitoring-references/faq/anomalies.php essentially validates what you're saying (area-weighted average temperature). – Barry Carter Jun 12 '17 at 2:49