The National Oceanic Atmospheric Administration announced on May 10 that
atmospheric carbon dioxide, or CO2, concentrations surpassed
400 parts per million, or ppm, in the clean air above Mauna Loa, Hawaii, where
researchers have been testing the air for CO2 for more than 50
years.
Ordinarily, a gas in the atmosphere passing a certain level would garner
little attention, even from scientists. CO2 is a different story,
but why? Why should society care that CO2 is now as high as 400 ppm?
The reasons are multiple, but all trace back to the relationship between CO2
and temperature. In this Part 1, I will discuss why scientists place so much emphasis on
the amount of CO2, and how today’s CO2 levels compared to
the past. In Part 2, I will explore the impacts of a 400 ppm CO2 world,
particularly how it might affect sea-level rise.
Why is CO2
important for temperature?
First and foremost, CO2 is no ordinary gas. Though a minor
constituent of Earth’s atmosphere, CO2 plays an outsized role in
influencing climate because of its “greenhouse” properties. The structure of a
carbon dioxide molecule allows it to reflect radiation emitted by the Earth’s
surface that would otherwise return to space, trapping heat just like a greenhouse
does. This can get very complicated very quickly, and Skeptical
Science has an in-depth examination of the subject for those wanting more
detail. But here’s the bottom line: When
atmospheric CO2 concentrations increase, more outgoing radiation is
absorbed, leading to warming of the Earth surface.
The relationship between CO2 and climate is especially
evident when looking back through Earth’s history. A particular field of
environmental science, paleoclimatology, studies past climates using clues from
archives of Earth’s history, including rocks, sediments and ice. With
information gathered from these archives, scientists seek to understand how and
why Earth’s climate has changed in the past and what these mechanisms mean for
current and future climate change.
Bärbel Hönisch |
“Paleoclimate studies provide very strong evidence that there is a tight
connection between CO2 and climate,” notes Bärbel Hönisch, an assistant
professor in the Department of Earth and Environmental Sciences at Columbia
University. Gas bubbles found in ice cores from Antarctica over the last
800,000 years demonstrate the close linkage between past atmospheric CO2
levels and past temperatures, estimated from the ice’s composition. Taken in
the context of that long-term record, atmospheric CO2 today is
nearly 50 percent higher than the highest level measured in the ice cores.
400 in Context: The
Pliocene
Peter deMenocal |
For paleoclimatologists, the 400 ppm CO2 level is significant
not so much for the absolute number, but rather because it suggests a return to
a type of climate not seen on Earth for millions of years. “We’re passing
through a number easily divisible by 100, but a year from now it will be 402
[ppm]… it’s an inexorable climb to higher values,” says Peter deMenocal, professor
and chair of the Department of Earth and Environmental Sciences at Columbia
University.
As a consequence of this climb, “We are starting to head back into deep
geologic time,” notes Rob DeConto, professor of climatology at University of
Massachusetts-Amherst.
Rob DeConto |
When was the last time CO2 levels were 400 ppm? Scientists
know from ice cores that this cannot be any point in the past million years or
so. To look at time periods before the ice cores, CO2 cannot be
measured directly; rather, it has to be inferred from the geologic record. By
measuring “proxies” of atmospheric CO2, such as physical or chemical
variables that relate to atmospheric CO2, one can reconstruct CO2
levels in the past with a reasonable degree of precision.
But such measurements are not smooth sailing. An expert in CO2 proxies, Hönisch
is candid about the difficulties. “There are only a handful of studies to
reconstruct past CO2. It’s difficult—a lot of these proxies are in
their infancy, and they are challenging and expensive measurements to make,”
she says.
Maureen Raymo |
That the Pliocene is an analog for today’s CO2 levels is
little comfort to paleoclimatologists. “If you go back to the Earth 3 million
years ago, it was a very, very different world than today,” says deMenocal.
For instance, DeConto, Hönisch, and Raymo all note that Greenland,
covered in nearly a two mile-thick ice sheet today, was largely ice-free in the
Pliocene. Scientists estimate that global sea level during the Pliocene was
anywhere between 30 and 90 feet higher than it is today, according to a 2012
study. Temperatures along the East Coast of the United States were likely 5
degrees warmer on average than today. That may not sound like much, but imagine
every 95-degree summer day being 100 degrees instead.
At higher latitudes, the temperature changes were even greater. Ellesmere
Island, high within the Arctic Circle, is a striking example of the difference
between the Pliocene and the present. Today, Ellesmere Island is covered with
snow and glaciers, only reaching temperatures above freezing during the height
of summer. But back in the Pliocene, Ellesmere Island was covered with forests,
and was even home to an ancestor to modern camels. Evidence from these flora
and fauna suggest Ellesmere Island was, on average, a whopping 32 degrees
warmer than today during the Pliocene.
Ellesmere Island, then and now: Left: Artist’s
rendition of High Arctic camels on Ellesmere Island during the Pliocene (Julius
Csotonyi/Canadian Museum of Nature). Right: The Osborn Range on Ellesmere
Island at present day (Ansgar Walk/Wikimedia Commons).
Out of Equilibrium
Now that CO2 has hit 400 ppm, why haven’t camels returned to
Ellesmere Island? And why didn’t the temperature on Ellesmere increase by 32
degress on May 11?
The reason is that climatic response to CO2 is not
instantaneous. “Our climate is vastly out of equilibrium with CO2
levels today, largely because of the oceans,” says deMenocal. Water has a large
capacity to accumulate excess heat.Add up all the water in the ocean and you
get a massive heat sink that takes a long time to adjust to the impact from CO2
changes.
Because paleoclimate records encompass broader time intervals, they give
an image of what conditions look like at equilibrium. “The Pliocene is a good
analog for how future conditions will be when they come to equilibrium with the
current concentration of CO2, but that equilibrium can take two to
three thousand years,” says Raymo. In short, we haven’t reached the point yet
where the world’s climate behaves as would be expected in a 400 ppm world.
But we are well on our way. Even though it takes millennia to achieve
equilibrium between CO2 and temperature, some effects of higher CO2
are already being realized. Parts of the climate system that are highly
sensitive to changing CO2, such as Arctic sea ice and polar
temperatures, are already responding. And the world is warming. “If you were born
in the 1990s, almost every year of your life has been a record warm year. It’s
remarkable,” says deMenocal.
“We are living in a world where the rate of change is accelerating,”
adds deMenocal. “It’s like steering a supertanker through a narrow channel, but
we have no idea how to steer the ship. And the ship is gaining speed.”
Spectacular, clear, informative piece. Can't wait for part II.
ReplyDelete-James Inhofe
Nice job Jesse!
ReplyDelete-Jeff Salacup
Nice work, and nice blog, Jesse. But I think the most urgent about CO2 is ocean acidification, maybe in part 3?
ReplyDelete(As ecologist I must say the main problem is biodiversity loss, because it is our security net to the global changes we are producing or also natural changes... but this is another story)
See you in ICP