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Roz Pidcock

03.05.2013 | 3:30pm
ScienceScientists’ thoughts on 400 parts per million
SCIENCE | May 3. 2013. 15:30
Scientists’ thoughts on 400 parts per million

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.

Mauna Loa

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 continuous record.

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 is 397.34 ppm.

Mauna _Loa _CO2_2

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 year:

“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 next year.”

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?

Scientific milestone?

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 way off:

“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 while:

“[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 greenhouse gases.”

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 us:

“[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.




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