SUMMARY:
Writing in Nature,
Yan et al. (2019)
report the first direct
measurements of
atmospheric CO2
concentrations
from more than
one million
years ago.
I'd like to report
that Yan et al.
have changed
the world of
climate science.
But real climate
science moves
very slowly,
and has many
unanswered
questions, so
refuting a theory
is considered
to be great
progress.
And that's what
Yan et al. did.
Real science
is completely unlike
government climate
junk science,
where government
bureaucrats with
science degrees,
pretend they
can predict
the future climate,
yet they have made
consistently wrong
climate predictions
since the 1970s !
DETAILS:
In the past 2.6 million years,
Earth’s climate has alternated
between warm interglacial periods
( we're living in the Holocene interglacial now ),
and cold glacials, when ice sheets
spread across North America
and northern Europe:
Before
one million years ago,
interglacials were about
every 40,000 years.
The 40,000-year cycle
is explained by the tilt
of Earth’s spin axis,
relative to its orbit
around the Sun,
varying between
22.1° and 24.5°.
A low tilt angle
leads to cooler
summers,
that promoted
the growth, and
preservation,
of ice sheets.
One million
years ago,
the interglacials
changed to about
100,000 years
apart.
That timing shift,
is known as the
mid-Pleistocene
transition (MPT).
Explaining
the MPT
is one of the
great challenges
of palaeo-climate
science.
The variations
in Earth’s orbit
and tilt did
not change.
So what changed ?
Somehow ,
more energy
became available
to melt ice sheets.
One theory :
A change in
atmospheric
CO2 is
responsible.
Yan et al. show
show the CO2
concentrations
during interglacials
did not change much.
Yan, Y. et al. Nature 574,
663–666 (2019).
Air trapped
in Antarctic ice,
for up to
800,000 years,
has allowed
CO2 levels
to be estimated
for the
800,000 years
before 1958,
when real time
CO2 measurements
began in Hawaii.
Estimates of
earlier CO2 levels
have been made
by measuring
the ratio
of boron isotopes
in sea shells
found in
ancient marine
sediments.
Bereiter, B. et al.
Geophys. Res. Lett. 42,
542–549 (2015).
Chalk, T. B. et al.
Proc. Natl. Acad. Sci. USA 114,
13114–13119 (2017).
But boron isotope
CO2 estimates
are not very precise.
Yan et al.
tried another approach:
Finding very old ice,
nearer to the surface
of Antarctica.
In "blue-ice areas",
an ice flow against
a mountain barrier
gets an upwelling
of old ice towards
the surface.
The authors
studied two
ice cores,
147 and 191
meters deep,
drilled to bedrock
in the blue-ice region
near the Allan Hills
in Antarctica:
Environmental
conditions
draw ancient ice
to the surface.
Yan et al.
analyzed
air trapped
in an ice core
from more than
1 million years ago.
They measured the ratios
of argon isotopes in air
extracted from cores,
to determine the age
of the ice.
The concentration of argon-40
in Earth’s atmosphere is slowly
increasing with time, as it is
produced from the radioactive
decay of potassium-40.
The authors also measured
the ratios of deuterium
(a heavy isotope of hydrogen)
to hydrogen in the ice,
as a proxy of the temperature
at the time the ice was deposited.
Ice in the
lowest 30 meters
of each core is up to
2.7 million years old.
Unfortunately,
the oldest
ice samples
had extra CO2
from the breakdown
of organic material
at the base
of the ice sheet.
Yan et al. obtained samples
from about 1 to 1.5 million
years ago, that they
consider to be undisturbed.
They found interglacial
CO2 concentrations
are similar in the
past 500,000 years,
peaking at 279 ppm
( parts per million )
with a minimum of 180 p.p.m.
at the last glacial maximum,
about 20,000 years ago.
The authors conclude
the relationship between
CO2 levels and Antarctic
temperatures was similar
before, and after, the MPT.
Pre-MPT ice does not contain
very low ratios of deuterium
to hydrogen characteristic
of extremely cold Antarctic
temperatures, nor really low
CO2 levels characteristic of
the most recent
glacial maximum.
Yan and colleagues’ data
add precision to previously
reported estimates of CO2
levels, made using
marine sediments.
They force palaeoclimate
scientists to look elsewhere.