Carbon-dioxide concentrations hit their highest level in 4m years
The measure of global warming
AT NOON on May 4th the carbon-dioxide concentration in the atmosphere
around the Mauna Loa Observatory in Hawaii hit 400 parts per million
(ppm). The average for the day was 399.73 and researchers at the
observatory expect this figure, too, to exceed 400 in the next few days.
The last time such values prevailed on Earth was in the Pliocene epoch,
4m years ago, when jungles covered northern Canada.
There have already been a few readings above 400ppm elsewhere—those
taken over the Arctic Ocean in May 2012, for example—but they were
exceptional. Mauna Loa is the benchmark for CO2 measurement
(and has been since 1958, see chart) because Hawaii is so far from large
concentrations of humanity. The Arctic, by contrast, gets a lot of
polluted air from Europe and North America.
The concentration of CO2 peaks in May, falls until October
as plant growth in the northern hemisphere’s summer absorbs the gas,
and then goes up again during winter and spring. This year the average
reading for the whole month will probably also reach 400ppm, according
to Pieter Tans, who is in charge of monitoring at Mauna Loa, and the
seasonally adjusted annual figure will reach 400ppm in the spring of
2014 or 2015.
Mauna Loa’s readings are one of the world’s longest-running
measurement series. The first, made in March 1958, was 315ppm. That
means they have risen by a quarter in 55 years. In the early 1960s they
were going up by 0.7ppm a year. The rate of increase is now 2.1ppm—three
times as fast—reflecting the relentless rise in greenhouse-gas
emissions.
