18
Jul
09

How do we measure the temperature of the earth?

Three questions really:

  • How do we measure the earth’s temperature today, with all our observations and measurements?
  • How do we measure the earth’s temperature a hundred years ago, when we may have measured some things and not others?
  • How do we measure the earth’s temperature 5,000 or 10,000 or 100million years ago, when clearly we were not taking notes?

Here is a simple graph, reported in New Scientist:

It looks good and clear, but then I look to the left: what on earth is “Temperature anomaly”?? It doesn’t seem to help me much. In fact it simply says that an “anomaly” of zero was reached in about 1940. Negative before that, positive after. Not much wisdom there.

Same here, from Canada:

Again, what is that measurement column at the left? And clearly we need more scale than since 1860.

So here’s an interesting one from India:

Ignore the green line for now – we’re not up to CO2 yet!

The 0 line on the left hand scale here is clear – it is the average temperature for the 1961 to 1990 period. I’m OK with that – it’s as good a reference point as any. So what is this graph suggesting? Half a million years seems to be a good sample period for global temperature. During that time has mostly been cooler than it is now (now, for now, being that ’61-’90 period). Never has it been more than a few of degrees warmer. As recently as around 25,000 years ago it was almost 10 degrees cooler. If I had to pick an earth average by eye, I’d say it is about 5 degrees cooler than now.

Here is a graph that focuses that picutre in on the last 1000 years. I understand that it is a famous graphic – the ‘hockey stick’ graph. (It is reproduced here from a warming-sceptical site).

A few comments about this one. First, the 1000 year period vanishes in the scale of the graph above it. It makes this millennium seem like like a relatively stable period! Second, notice the blue and the red lines. Red is temperature directly measured by thermometers. Blue is temperature implied by other means. I don’t know why the blue ends around 1950 – I would have liked to see an assurance that the derived temperature readings continued to match the thermometer readings. Third, the measures are for the “Northern Hemisphere”. I’m not sure what that means (I know what the Northern Hemisphere is(!) but I’m not sure if that makes it a good indication of global temperature, or whether it is all of the Northern Hemisphere.

And here is one of the graphs that Senator Fielding may have been given – I assume that because the commentary surrounding it is very similar to the turn of phrase that Fielding was using.

For me, the time scale is just far too small. Temperatures bounce up and down a lot within a general trend.

Time to pack it in for now. I’ll try to draw together what the diagrams tell… I’d love to get hold of the datasets from which such graphs are made. Any clues?

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