Greenhouse-gas levels near milestone
US: Any day now, the concentration of the greenhouse gas
carbon dioxide sampled from the air wafting above a barren lava field in
Hawaii could be above 400 parts per million (ppm), a level not seen
since the Pliocene, between 3.2 and 5 million years ago.
Carbon dioxide levels were around 280 ppm when the Industrial
Revolution got under way in the 18th century and humans started pumping
greenhouse gases into the atmosphere from burning fossil fuels. Levels
have continued to accelerate higher since then.
Hitting the 400 ppm milestone is symbolic to science and policy
discussions about efforts to control global climate change, according
Ralph Keeling, a geochemist at the Scripps Institution of Oceanography
in La Jolla, Calif.
Human-forced global climate change is primarily driven by rising
concentrations of carbon dioxide. Round numbers such as 350 ppm, 400 ppm
and 450 ppm are discussed as targets for “safe” levels of the greenhouse
gas - a level that will prevent the climate from spinning out of
control, he explained.
“So crossing 400 represents, if you will, a lost opportunity,”
Keeling said.
His late father, Charles David Keeling, a climate science pioneer at
Scripps, began tracking levels of carbon dioxide in 1958 at the Mauna
Loa Observatory. Concentrations then were 316 ppm. Since then, levels
have continued to step higher in a saw-tooth pattern, peaking each May
before drifting lower as plants in the Northern Hemisphere absorb carbon
dioxide to grow, then rising in the fall and winter.
Keeling Curve
The readings plotted on a graph show carbon dioxide concentrations
curving sharply higher over time as humans burn more and more fossil
fuels. The graph is known as the Keeling Curve. Details from the curve,
in fact, show the rate of increase continuing to accelerate, noted
Pieter Tans, the senior scientist at the National Oceanic and
Atmospheric Administration's Climate Monitoring and Diagnostic Lab,
which oversees the Mauna Loa Observatory. When David Keeling first
started keeping records, the five-year average rate of change was 0.7
ppm. Today, it is 2 ppm, more than three times faster, he said.
“That is significant,” Tans told NBC News. “We are just mostly
talking about decreasing emissions. In practice, at least globally
average, emissions are still accelerating and so is the rate of CO2
increase in the atmosphere.” But crossing 400 ppm in and of itself
“doesn't mean anything in particular,” Daniel Sarewitz, a sustainability
scientist at Arizona State University, noted in an email to NBC News.
“It's just another indicator of what we know full well already — that
modern society is enormously dependent on fossil fuels for its
well-being.”
And that dependence on fossil fuels means concentrations of carbon
dioxide will continue to climb for the foreseeable future, he added.
Watching for 400
To mark the crossing of 400 ppm, Scripps Institution of Oceanography
launched a website and Twitter feed to provide daily readings from Mauna
Loa. As of this writing, it stands at 399.29. The hourly number
fluctuates up and down depending on the atmospheric winds and, to a
lesser extent, pockets air that upwell from lower on the island.
Scientists calculate a daily average from hourly readings (when
there's too much noise, a reading isn't produced). The daily numbers are
then averaged to establish weekly, monthly and yearly numbers to be
plotted on the graph.
- ABC NEWS
|
