Tree ring widths are one of the best a ways of piecing
together climate changes of the last thousand years or so.
A tree has a seasonal cycle of growth, If it's
growing in a place that has a seasonal cycle in the weather.
This method doesn't work in the tropics,
where it's warm all the time.
It works in the higher latitudes, where you have a distinct summer and winter.
Most of the growth
happens in the summertime.
And wintertime, you get a layer that has a different color.
You can count the rings in a stump,
and each one of those rings corresponds to a year.
How wide the ring is,
in a particular year, depends on a whole bunch of things.
In addition to temperature, it can depend on the availability of water.
It can depend on sunlight, the age of the tree, things like that.
There's a lot of noise in this method that has to be averaged
out, or gotten rid of in one way or another.
You get the primary calibration of tree ring width
to temperature by comparing it with the instrumental temperature record,
the time span over which there are measurements from thermometers.
That can give you a calibration
you can project further back into the past
to say If the width was here, then using
that same calibration the temperature must have been there.
And the way noise is further reduced,
is by looking at spatial patterns of these records from all
around the world.
This is a terribly drawn map, but you can use your imagination
and figure this is South America and North America.
If you have a distribution of tree ring records that
cover the whole surface of the earth, or a large fraction
of it, you can compare the variations that you see in
the tree rings with patterns of variation like the El Nino cycle.
From the instrumental record, we have some idea that when the Earth's
climate changes, it changes with particular spatial patterns.
The way that noise is eliminated from these tree ring records ..
is by accepting the temperature changes from records that are coherent, in
some way that's recognizable from the way the climate works today.
And calling that real information.
Then if there's some weird wacko record here that does something totally
different than everybody else it will get excluded, by this method.
It's not a secret, but there is a problem with the tree ring data.
And that is that, in the most recent few
decades of the instrumental records, starting from about 1970s, to today.
The temperature, as measured by thermometers, has gone up, but the
tree rings don't show that, and it's not clear why that is.
It could be because rising CO2 concentrations
actually fertilize the trees in some way.
It could be because one of the components of acid rain is nitric acid.
Which is nitrate, a fertilizer.
Maybe fertilizer deposition has impacted the tree
ring, the fidelity of the tree ring method.
Does this destroy our confidence in the
tree ring method to put together ancient climates?
No. it doesn't destroy it. It undermines it a little bit.
Like I said before, these proxy measurements are what they are.
They're as good as they are and we have to use them with some caution.
If we could actually have thermometers from a thousand years
ago, that would be better, but this is what we have.
It's possible that there's a cap on the the sensitivity of the method.
We have to live with that possibility.