History is
usually limited
to what
has survived
of written records,
and in prehistory,
to random fragments
unearthed by the
archaeologists and
paleontologists.
Climate history
depends on proxies
such as glacial ice
cores, annual accretion
of stalagmites, the growth
of corals and the
incremental layers
of bone in the ears of fish.
Tree rings are different.
Each growing season,
a tree adds an outer layer
of cambium to its core
of dead wood.
The exact width
of the ring, and its
microscopic structure,
give a clear picture
of growing conditions
during one growing
season.
A German oak-pine
tree-ring series
gives an annual
record of more
than 12,500 years !
Unlike carbon14-dating,
which offers a range,
tree rings pinpoint
the growing conditions
for a particular year.
But tree rings
tell us about
rainfall and
temperature,
such as
detailing
a long period
of drought during
the decline of the
Roman Empire
in the 4th century.
Tree growth ring data
can tell a lot about
temperatures and
moisture for each
year’s growth.
Unfortunately, It is
impossible to separate
temperature changes
from water changes.
No one knows
if tree rings are
better thermometers,
or better rain gauges.
I would also expect
growth rings to be
bigger during periods
of higher CO2 levels,
which started after
the trough of the
Great Recession
in the 1930s, and
ramped up in the
1950s.
Sunlight, CO2,
water, and soil
minerals are
all significant
components
required for
tree growth.
So tree rings
can not be
reliable
historical
thermometers.