Scientists' thoughts on 400 parts per million
- 03 May 2013, 15:30
- Roz Pidcock
The concentration of carbon dioxide in earth's atmosphere is
poised to pass 400 parts per million. That might not sound like a
lot, but it's nearly half as much again as pre-industrial levels,
and higher than it's been for hundreds of thousands of years.
We asked scientists what the number means, and what we should
take from the fact that this once distant prospect is now just
around the corner.
On the side of a Hawaiian volcano sits the Mauna Loa
Observatory, where scientists have been measuring levels of
carbon dioxide in the atmosphere since the 1950's.
Other observatories around the world monitor carbon dioxide too.
But the emblematic Keeling curve - named after C. David Keeling who
started the measurements in 1958 - is the longest standing
On May 1st, the instruments at Mauna Loa were recording 399.39
parts per million of carbon dioxide in the atmosphere -
and media reports have suggested that we're
about to pass the 400 ppm mark. Averaged over a month, the
values are a bit lower. The latest monthly average for March
Monthly averaged atmospheric carbon dioxide concentration at
Mauna Loa. Source NOAA
Will it be this year?
Plants and trees absorb carbon dioxide as they grow. This means
concentrations of the gas rise and fall with the seasons, reaching
a peak in May just before summer in the northern hemisphere.
Professor Ralph Keeling, who took over the Mauna Loa record from
his late father, explains
why this means it's not yet certain we'll reach 400 ppm this
"Each year, the concentration of carbon
dioxide at Mauna Loa rises and falls in a sawtooth fashion, with
the next year higher than the year before ... If carbon dioxide
levels don't top 400 ppm in May 2013, they almost certainly will
In fact, globally averaged carbon dioxide concentrations are a
little lower than at Mauna Loa. The most recent estimate for
February is just under 396
ppm, which means global concentration may not pass the 400 ppm
mark for another year or two.
Nevertheless, the prospect of crossing the 400 ppm mark has got
people talking. So what's so special about 400 ppm?
In terms of the global impacts of climate change, scientists
don't attach much specific importance to the 400 ppm threshold. The
consequences of climate change are not expected to change
dramatically once we reach 400 ppm.
But scientists tell us approaching 400 ppm is a symbolic
reminder of the long term trend in greenhouse gas emissions.
Professor Joanna Haigh from Imperial College tells us the prospect
of atmospheric carbon dioxide reaching 400 ppm once seemed a long
"I wrote my doctoral thesis in 1980
about how changes in atmospheric carbon dioxide would affect the
stratosphere. At that time the carbon dioxide concentration was 320
ppmv and, while we knew it was increasing, the prospect of it
reaching 400 ppmv seemed distant."
While 400 ppm "has no special significance scientifically",
Haigh says that it's a "salutary reminder of the unremitting
anthropogenic emissions of carbon dioxide into the atmosphere and
of the global warming that will ensue."
Of course, in a way it's not surprising that we're here. As
professor Richard Allan from the University of Reading tells us,
rising emissions mean reaching 400 ppm has been on the cards for a
"[Approaching 400 ppm] is not
significant in itself since a rise in global carbon dioxide
concentrations above 400 ppm is inevitable and expected to occur
within the next two years."
More significant than the 400 ppm value itself is the comparison
between atmospheric carbon dioxide levels now and in the context of
earth's history. Professor Sir Brian Hoskins from the Grantham
Institute for Climate Change tells us:
"To me the striking fact is that human
activity has already driven the greenhouse gas carbon dioxide to a
level more than 40 per cent above the maximum levels it had during
the previous million years, and it is going to stay at least this
high for thousands of years into the future."
"Wake up call"
Surpassing the 400 ppm mark could well have more political
significance than scientific. Dr Tim Leuker from Scripps
Institution of Oceanography, which monitors the carbon dioxide
measurements at Mauna Loa, says:
"The 400 ppm threshold is a sobering
milestone, and [passing it] should serve as a wake up call for all
of us to support clean energy technology and reduce emissions of
To get a rough idea of what this might mean in temperature
terms, the UN's official climate body, the Intergovernmental Panel
on Climate Change, estimated in 2007 that in order to limit global
warming to a best estimate of between
two and 2.4 degrees above pre-industrial levels, global carbon
dioxide concentration should not exceed 400 ppm.
Allan expresses the concern that passing 400 ppm raises, telling
"[T]he inexorable rise in carbon dioxide
concentrations over the last 100 years and anticipated continued
increases, primarily due to the burning of fossil fuels, is highly
significant because of its influence in warming climate and the
resulting increased damage to societies and the ecosystems upon
which they depend."
Passing the 400 ppm mark - whether it happens this year or next
- may be a milestone of sorts, even if it's now an inevitable one
on the path to higher atmospheric concentrations. With global
emissions showing no sign of slowing, it may well be that within
our lifetimes we look back on 400 ppm as a fond memory.