What 400 ppm C02 Means
On May 2, after nightfall shut down photosynthesis for the day in Hawaii, carbon dioxide levels in
the atmosphere touched 400 parts-per-million there for the first time
in at least 800,000 years. Near the summit of volcanic Mauna Loa—where a
member of the Keeling family has kept watch since 1958—sensors measured this record through
sunrise the following day. Levels have continued to dance near that
benchmark in recent days, registering above 400 ppm for the first time
in eons after midnight on May 7. When the measurements started the daily
average could be as low as 315 ppm, already up from a pre-industrial
average of around 280 ppm.
This measurement is just the hourly average of CO2 levels high in the
Hawaiian sky, but this family’s figures carry more weight than those
made at other stations in the world as they have faithfully kept the
longest record of atmospheric CO2 .
Arctic weather stations also hit the hourly 400 ppm mark last spring
and this one. Regardless, the hourly levels at Mauna Loa will soon drop
as spring kicks in across the northern hemisphere, trees budding forth
an army of leaves hungrily sucking CO2 out of the sky.
It may be next year before the monthly average level reaches 400
ppm—and yet longer still until the annual average reaches that number.
But there is no question that the world continues to inexorably climb
toward higher levels of greenhouse gas concentrations. Barring economic
recessions, the world may be lucky to stop at 450, 500 or even beyond . Last year, humanity spewed some 36 billion metric tons of greenhouse gases, up from 35 billion the year before.
In the coming year, Scientific American will
run an occasional series, “400 ppm,” to examine what this invisible
line in the sky means for the global climate, the planet and all the
living things on it, including human civilization. Some scientists argue
we passed the safe level for greenhouse gas concentrations
long ago, pointing to the accelerating impacts, from extreme weather to
the meltdown of Arctic sea ice. Others argue that we have yet more room
to burn fossil fuels ,
clear forests and the like—but not much—before catastrophic climate
change becomes inescapable. And the international community of nations
has agreed that 450 ppm—linked to a rise of 2 degrees Celsius in global average temperatures—should not be exceeded. We are not on track to avoid that limit, whether you prefer the economic analysis of experts like the International Energy Agency or the steady monitoring of mechanical sensors .
The last time CO2 levels at Mauna Loa were this high, Homo sapiens did
not live there. In fact, the last time CO2 levels are thought to have
been this high was more than 2.5 million years ago, an era known as the Pliocene ,
when the Canadian Arctic boasted forests instead of icy wastes. The
land bridge connecting North America and South America had recently
formed. The globe’s temperature averaged about 3 degrees C warmer, and
sea level lapped coasts 5 meters or more higher.
The world will change again due
to human activity and associated emissions of CO2, perhaps causing
another set of coral reef extinctions like those found during the
Pliocene, among other impacts. When Charles D. Keeling first started his
measurements, CO2 made up some 317 ppm of the air we breathe and climate change was already a concern thanks
to the work of John Tyndall, Svante Arrhenius and Guy Callendar. Every
year since 1958 the sawtoothed line depicting Keeling’s
measurements—readings kept up by his son Ralph—has climbed up, capturing
the rise in greenhouse gas concentrations as well as the world’s
What can be done? In the short term, more potent but shorter-lasting greenhouse gas emissions could be curbed or a concerted effort to develop CO2 capture and storage technology
could be undertaken. Whether we do that or not, given CO2′s long
lifetime in the atmosphere, the world will continue to warm to some
extent; at least as much as the 0.8 degree C of warming to date is
likely thanks to the CO2 already in the atmosphere.
At present pace, the world could reach 450 ppm in a few short
decades. The record notches up another 2 ppm per year at present pace.
Human civilization developed and flourished in a geologic era that never
saw CO2 concentrations above 300 ppm.We are in novel territory again and we show no signs of slowing to get our bearings, let alone stopping.
About the Author: David Biello is the associate editor for environment and energy at Scientific American. Follow on Twitter @dbiello .
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