CO2 in Earth's atmosphere nearing levels of 15m
years ago
Last time CO2 was at similar level temperatures were
3C to 4C hotter and sea levels were 20 metres higher
The level of CO2 in the atmosphere is nearing a level
possibly never experienced by a hominoid.
Jonathan
Watts Global environment editor
@jonathanwatts
Published
onThu 9 Jul 2020 10.50 BST
The amount
of carbon dioxide in the Earth’s atmosphere is approaching a level not seen in
15m years and perhaps never previously experienced by a hominoid, according to
the authors of a study.
At
pre-lockdown rates of increase, within five years atmospheric CO2 will pass 427
parts per million, which was the probable peak of the mid-Pliocene warming
period 3.3m years ago, when temperatures were 3C to 4C hotter and sea levels
were 20 metres higher than today.
But it
seems we must now go much further back to see what’s ahead.
Some time
around 2025, the Earth is likely to have CO2 conditions not experienced since
the Middle Miocene Climatic Optimum 15m years ago, around the time our
ancestors are thought to have diverged from orangutans and become recognisably
hominoid.
For the
paper published in the journal Nature Scientific Reports, a team of researchers
from the University of Southampton constructed a new high-resolution record of
atmospheric CO2 during the Pliocene using data derived from the boron levels in
tiny fossils about the size of a pin head collected from deep ocean sediments
of the Caribbean Sea.
This
confirmed trends previously observed in ice cores, but also allowed a more
precise estimate of the CO2 range in that geological epoch, when levels of
solar radiation were the same as today.
“A striking
result we’ve found is that the warmest part of the Pliocene had between 380 and
420 parts per million CO2 in the atmosphere,” one of the co-authors Thomas
Chalk, said. “This is similar to today’s value of around 415 parts per million,
showing that we are already at levels that in the past were associated with
temperature and sea-level significantly higher than today.”
“Currently,
our CO2 levels are rising at about 2.5 ppm per year, meaning that by 2025 we
will have exceeded anything seen in the last 3.3 million years.”
The authors
said the study of the past provided a guide to what is likely to happen in the
future as the Earth responds to the buildup of greenhouse gas from the past two
centuries of industrial emissions.
“Ice sheets
today haven’t had a chance to catch up with CO2 forcing. We are burning through
the Pliocene and heading towards a Miocene-like future,” said another of the
authors, Gavin Foster, a professor of isotope geochemistry at the University of
Southampton. “We now have to go further back in time to find situations that
are relevant.”
During the
Middle Miocene, ice sheets shrank further and sea levels were much higher than
the Pliocene. Foster said this was long before anything recognisably human had
evolved on Earth.
The steady
increase in temperatures in the current era were highlighted on Thursday by a
new international collaboration coordinated by the World Meteorological
Organization and led by the UK’s Met Office. In the first of what will be an
annually annually-updated five-year climate prediction, the scientists noted
that there is a 20% chance of the world temporarily reaching 1.5C above
pre-industrial levels before 2025.
“This study
shows – with a high level of scientific skill – the enormous challenge ahead in
meeting the Paris agreement on climate change target of keeping a global
temperature rise this century well below 2C above pre-industrial levels and to
pursue efforts to limit the temperature increase even further to 1.5C,” said
the WMO secretary-general, Petteri Taalas.
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