The third way of measuring temperatures is by satellite.
And at first blush, this would seem like it is the ideal way of making measurements
because it's out in space and nobody's touching
it and it's covering the whole world.
It's actually not the best way to get a time series measurement of temperature
because orbits of the satellite can decay, and over the time span
that we're talking about there were several satellites that flew,
so they all had to be calibrated against each other
in order to come up with a time series.
The satellite is really good at
making a map, an instantaneous snapshot of surface temperatures.
But for a time series it's a bit more work.
And for a long time the satellites didn't show
the same warming as the land and the sea surface temperature records were showing.
This seemed like it could be a major problem.
It turns out that the way that satellites work
is they measure microwave radiation that comes from molecular oxygen.
And just like greenhouse black body
radiation, this wavelength of light
is more intense when the oxygen is warmer.
The difficulty is that the satellite is
sensitive not just to the temperature right at
the ground, but it's sensitive to temperatures
throughout a range of altitudes in the atmosphere.
And so if there's cooling higher up, that can pull
the temperature the satellite sees down.
You don't get the full signal
from the ground, which is what we're interested in.
After the latest round of corrections,
you can see in this plot that the satellite temperatures, although very
spiky, are also consistent with the land and sea surface temperature records.
And that they show this warming in the last few decades that will
turn out to be the smoking gun for a human impact on earth's climate.