Oxygen and hydrogen
isotopes in the ice, and
CO2 in the air bubbles,
are preserved in the
Antarctica and
Greenland
ice cores.
Temperature is inferred
from examining the
isotopic composition
of the water molecules
released by melting
the ice.
For more than
2.5 million years
( the Pleistocene Epoch )
the world was cold,
with long glaciations
( aka “ice ages” ),
separated by brief
warm periods,
that lasted from
10,000 to 15,000 years.
We are in a warm period now,
and have been in the current
Holocene Epoch warm period
for about 11,500 years.
Within the Holocene, there is
strong evidence of mild
harmless warming / cooling
cycles.
We have been living in
a mild warming cycle since
the late 1600's.
Most of the “warm periods”
( aka “climate optimums” )
are believed to have been
at least as warm as today's
climate.
From the Greenland
Ice Sheet Project Two:
( GISP2 ):
The Roman Warm Period
is prominent in the
GISP2 ice core,
about 1,500 to 1,800
years ago.
During that period,
ancient Romans
wrote about grapes
and olives growing
farther north in Italy,
than had been
thought possible,
and there was little
or no snow or ice.
Oxygen isotope data
from GISP2 Greenland
ice cores, show a
Medieval Warm Period
(MWP)
from 900 to 1300 A.D.
During the MWP:
In Europe, grain crops
flourished, the alpine
tree lines rose higher
on the mountains,
and the population
more than doubled.
The MWP was
a global event
with proxy data
also confirming
a warm period in :
Africa
(Lüning et al., 2017),
South America
(Lüning et al., 2018),
North America
(McGann, 2008),
China
(Hong et al., 2009),
and many other areas.
The average level of
CO2 in the atmosphere,
during the evolution and
spread of today's
"C3" plants used for food,
about 300 million years
ago, was approximately
1,000 ppm, much higher
than today's CO2 level
of 415 ppm.
It's no surprise that
greenhouse owners
buy CO2 enrichment
systems, and pay
for the fuel required,
to raise CO2 levels
inside their greenhouses
to at least 1,000 ppm
The atmospheric CO2
concentration today
is “unprecedented”
only because it's so low !
In the geological record,
CO2 concentrations
in the atmosphere
typically rose several
hundred years AFTER
temperatures rose,
indicating the temperature
increase was NOT caused
by the CO2 rise
( Petit et al., 1999;
Monnin et al., 2001;
Mudelsee, 2001;
Caillon et al., 2003 ).
During periods of glaciation,
cooling oceans absorb more CO2.
(think of a cold, sparkling soda pop)
During warmer inter-glacial periods,
oceans absorb less CO2,
or outgas more CO2 into the air.
(think of a warm, flat soda pop)
Plant life absorbs more CO2
from the air during the higher
CO2 warm periods.