Theory is all well and good. But if we're
going to invest money in trying to limit C02 emissions to avoid climate change,
really we want something more. We want to have evidence that everything
is working the way it is thought that it should theoretically.
The way to do that is to see if the Earth is in fact warming up.
There are three different major types of temperature measurements.
Those over land, those over sea, and those from satellites.
Land temperature records go back
to the invention of the thermometer by Gabriel Fahrenheit in 1714.
The thermometer is very easy to calibrate,
because you can just stick it in boiling water and then stick
it in ice water and make a scale between those two things.
And since we know today what temperatures ice melts and boils at,
It's very easy to interpret
very old thermometer measurements in terms of what the temperature really was.
This is as opposed to measurements of CO2 concentration or something like that,
that would have used methods that are hard to reconstruct today.
Some kinds of old data are really not worth that much.
But old temperature data is pretty reliable.
One possible
problem with the land temperature measurements is called
the urban heat island effect. This is a very real effect.
The idea is that in a vegetated landscape, the incoming
sunlight energy can be carried back away by the latent heat in water vapor.
Because plants mine water from the ground
and they encourage it to evaporate.
Whereas in a urban landscape The same heat input
can only be balanced by warming up the ground enough...
that infra red energy leaving can
can balance the incoming light.
And so in
maps of temperatures or infra red images from space around cities,
it's very clear that it is warmer in
urban environments than it is in rural environments.
The problem would come if a lot of the places where they measure temperatures...
have transitioned over time from relatively green places like
this, to relatively urban places like that,
and if you then took the warming, it would be due to the urban heat island effect,
and you call that global warming due to rising C02 concentrations.
That would be a wrong attribution. But there have been many
studies that have taken the thermometer data and put them together into the global
average temperature change,
and they take different methods of trying to filter out
the urban heat island effect from their averages,
by taking stations out if they seem like they're likely to be a problem.
And as you can see in this plot, basically everybody is
getting the same answer for the average temperature of the earth.
It turns out that the urban heat Island
effect, while it is a very real effect, Is not
an important one for setting the global average temperature.
It's not confounding the global warming
signal that's being measured by temp, by thermometers on land.