Washington Post - Chris Mooney
For the first time since humans have been monitoring, atmospheric concentrations of carbon dioxide have
exceeded 410 parts per million averaged
across an entire month, a threshold that pushes the planet ever closer
to warming beyond levels that scientists and the international community
have deemed “safe.”
The reading from the Mauna
Loa Observatory in Hawaii finds that concentrations of the
climate-warming gas averaged above 410 parts per million throughout
April. The first time readings crossed 410 at all occurred on April 18,
2017, or just about a year ago.
 |
| Recent CO2 measurements at Mauna Loa Observatory in Hawaii. (Scripps Institution of Oceanography) |
Carbon dioxide
concentrations — whose “greenhouse gas effect” traps heat and drives
climate change — were around 280 parts per million circa 1880, at the
dawn of the industrial revolution. They’re now 46 percent higher.
As
you can see in the famed “saw-toothed curve” graph above, more formally
known as the Keeling Curve, concentrations have ticked upward in an
unbroken progression for many decades. But they also go up and down on
an annual cycle that’s controlled by the patterns and seasonality of
plant growth around the planet.
The
rate of growth is about 2.5 parts per million per year, said Ralph
Keeling, who directs the CO2 program at the Scripps Institution of
Oceanography, which monitors the readings. The rate has been increasing,
with the decade of the 2010s rising faster than the 2000s.
“It’s
another milestone in the upward increase in CO2 over time,” Keeling
said of the newest measurements. “It puts us closer to some targets we
don’t really want to get to, like getting over 450 or 500 ppm. That’s
pretty much dangerous territory.”
“As a
scientist, what concerns me the most is not that we have passed yet
another round-number threshold but what this continued rise actually
means: that we are continuing full speed ahead with an unprecedented
experiment with our planet, the only home we have,” Katharine Hayhoe, a
climate scientist at Texas Tech University, said in a statement on the
milestone.
Planetary carbon dioxide levels have
been this high or even higher in the planet’s history — but it has been a
long time. And scientists are concerned that the rate of change now is
far faster than what Earth has previously been used to.
In the
mid-Pliocene warm period more
than 3 million years ago, they were also around 400 parts per million —
but Earth’s sea level is known to have been 66 feet or more higher, and
the planet was still warmer than now.
As a
recent federal climate science report (co-authored by Hahyoe) noted, the
400 parts per million carbon dioxide level in the Pliocene “was
sustained over long periods of time, whereas today the global CO
2 concentration
is increasing rapidly.” In other words, Earth’s movement toward
Pliocene-like conditions may play out in the decades and centuries ahead
of us.
Even farther back,
in the Miocene era
between 14 million and 23 million years ago, carbon dioxide
concentrations in the atmosphere are believed to have reached 500 parts
per million. Antarctica lost tens of
meters of ice then, probably corresponding to a sea level rise once again on the scale of that seen in the Pliocene.
Farther
back still, at the Eocene-Oligocene boundary around 34 million years
ago, Antarctica is believed to have had no ice at all, with atmospheric
carbon dioxide concentrations of 750 parts per million.
These
data points help show why it is that scientists believe that planetary
temperatures, sea levels and carbon dioxide levels all tend to rise and
fall together — and thus, why Earth is now headed back toward a period
like the mid-Pliocene or even, perhaps, the Miocene, if current trends
continue.
Keeling said that the planet,
currently at 1 degree Celsius (1.8 degrees Fahrenheit) above
preindustrial levels, is probably not yet committed to a warming of 1.5
or 2 degrees Celsius, but it’s getting closer all the time —
particularly for 1.5 C. “We don’t have a lot of headroom,” he said.
“It’s
not going to be a sudden breakthrough, either,” Keeling continued.
“We’re just moving further and further into dangerous territory.
Links
