A different way of measuring the temperature is over the ocean.
And this is good because it doesn't have any possibility of
an urban heat island effect and so it's a separate check.
But there has been a change in the way that temperatures have been measured at
sea over time. In in old times they would cast
a bucket over the side on a rope and then pull a
bucket full of sea water on deck and stick a thermometer in it.
And this had the possibility of bias, because water
on the outside of the bucket could evaporate and could cool that bucket down
just a bit relative to what it was in the actual water that the boat is floating in.
In the mid '40s, there was a transition in the sea surface
temperature data toward measurements that were made
on the cooling water intake to the engine.
They have to bring water in to cool down the engine as it's running,
you measure the temperature as the water
comes in, before you've dumped heat into it.
And that can tell you a sea surface temperature.
Of course this cooling intake might be coming in from a little
bit below the surface; it may not be exactly the same temperature as here.
To put together the land and the sea surface temperature records.
you can't just compare them directly
because they're different quantities.
The land tends to heat up more than the ocean would
the same reason as this urban heat island effect.
The ocean would be like a super
vegetative place with lots and lots of
capability for water vapor to carry away heat.
And also, the ocean is a big heat sink itself, and so the land surface
is actually expected to warm more quickly than the ocean surface for those reasons.
The way to compare these
different temperatures is within a climate model,
where you impose the temperature
at the sea surface from the measured ship data,
and then you use the model
to calculate, to predict the temperatures
in the atmosphere, both in the air just above the
water, but also, more importantly, over the land.
And then you can compare these model predicted land
temperatures with the actual temperatures measured at those weather stations.
And it turns out that the correction, there was
a significant correction that had to be made between this
technique of measuring sea surface temperature in buckets versus
measuring the sea surface temperatures from from the cooling intakes.
But aside from that, the temperature signals that come
out from this are consistent with those from the after that
correction is made, both the land and the sea surface temperatures
are telling us the same thing about the temperatures of the Earth.