Another proxy we need for the past, another thing that we'd really like
to know, is how bright the sun was.
That way, when trying to piece together past changes in climate, we can
say how much of that could be driven by changes in the intensity of the sun.
There's a proxy for the solar
intensity that you never would have believed,
until you'd seen it for yourself, and seen
how the proof of it is always
to compare what the proxy is trying to measure
with the real data for when you have measurements.
And if it works reliably in that time span then you
can project it into the past and make estimates of the past.
This proxy for the solar intensity relies on the magnetic
field from the sunc which shields the earth, to some extent, from cosmic rays.
Cosmic rays are coming in from space all the time and when they come in and hit
the atmosphere they can do some nuclear chemistry and
create these radioactive elements like carbon 14 and beryllium 10.
And those are then deposited in ice cores.
We can measure the concentration, in particular beryllium 10, in ice cores
in order to back out what the solar intensity was in the past.
And so here you see a plot of that correlation going
through the instrumental record and it seems to work remarkably well.
Temperature reconstructions of the last thousand years or
so are colloquially and affectionately called the hockey stick because
the very first one of these showed very flat
temperature and then a strong increase just at the end.
This has been repeated now a dozen times,
with different methods for illuminating the the noise in the tree
ring records, for example.
And they're all basically coming up with the same picture.
You have period of general warmth during medieval times.
This was the time when the Vikings were sailing the world and
people were building cathedrals in gratitude to benevolent god in Europe.
After that came a time from about 1300 to about 1750 called
the Little Ice Age. This was a time with nasty
weather in Europe. Not just cold, but periods of
drought, periods of intense rain, periods of just strange kind of nasty weather.
Right in the middle of this time is a period called
the Maunder Minimum which was a time when there were no sun spots.
Right after the telescope was invented,
just a few decades after that, and sun spots were discovered,
for some reason the sunspots went away on the sun.
The intensity of the sun today is found
to be correlated with the number of sun spots.
We have this roughly 11 year sun spot cycle and when
there's lots of sun spots the sun is brighter than when there are fewer.
We know that this period is
a time when when the sun was relatively cool,
and that can explain why the Little Ice Age was colder.
You can see that the reconstructed temperatures from
the tree rings have a lot more variability in them.
compared to that from the bore hole temperatures which are very smooth.
They can only tell us about the larger changes
in temperature rather than all the little wiggles.
And you see also
where the tree rings don't match the instrumental record in the last few
decades of the hockey stick period that we mentioned before,
the decline in sensitivity of the tree rings during that time.
In general, the temperature changes we see in the last 1,000 years
are roughly comparable to the temperature changes we've seen in the last 30 years.
The Little Ice Age was maybe one degree C
colder than 1950 temperature on Earth.
The Medieval warm time was maybe 1/2 a degree warmer than the 1950 temperature.
And where we are today is about 0.7 or 0.8
degrees warmer than that temperature.
The temperature changes we've seen so far are comparable to those
that have affected history just in the last 1000 years or so.
And these were significant changes.
The Little Ice Age was significantly different than the Medieval time.
But it wasn't apocalyptic.
There are some false conclusions that can be drawn from
this record too that I want to warn you of.
One is since we see some natural climate change,
does that mean that climate change is harmless?
No.
Not really. There's no reason to draw that.
And it also doesn't mean that humans can't also change the climate.
If the climate changed in the past it
might have been because the sun changed its intensity
or something like that.
It doesn't mean that we are excluded from ever effecting earth's temperature.
And any other conclusion that I've heard drawn
is that, since the Medieval time was actually
a good time to be in Europe, relative to the Little Ice Age, when it was worse,
does that mean that any amount of warming is good?
Not really.
Because the forecast
for the end of the century totally blows all of this off the scale.
The forecast for the year 2100 is like 3 or 4 degrees centigrade.
A little bit of warming might be good.
A lot of warming is a different story altogether.