Underlying atmospheric carbon dioxide (CO2) levels will exceed 400 parts per million (ppm) in 2015 and may remain above that level for many thousands of years, according to a leading expert at the US National Oceanic and Atmospheric Administration (NOAA).
Seasonally adjusted global average atmospheric CO2 concentrations have been rising remorselessly since instrument measurements began in the late 1950s and they brushed 398ppm in September (at 397.96ppm) while readings at the Mauna Loa Observatory in Hawaii exceeded 399ppm, according to NOAA data.
“We will have 400 parts per million next year and that is fairly certain,” said Pieter Tans, a senior scientist with NOAA’s Climate Monitoring and Diagnostics Laboratory, in Boulder, Colorado.
The precise figure 400ppm is, in itself, just a number and has no particular physical significance but the level of 400ppm is seen as a milestone by many climate scientists and others. According to Tans, the significance lies in the fact that once the 400ppm threshold is crossed there will no going back and that “on the time scale of human civilisation” it is a one-way trip.
“This is truly irreversible, once the CO2 is in the atmosphere and the oceans then it will remain there for a long, long time, for many thousands and thousands of years until geological processes remove it,” Tans told reportingclimatescience.com. “The last time the Earth was at this level was a couple of million years ago,” he added.
The level 400ppm means, very roughly, that within every cubic meter of dry air, a volume of 400 cubic centimetres – just under a pint – is made up of CO2. More technically, the number describes what scientists call the mole fraction of CO2, expressed as ppm: this is the number of molecules of CO2 in every one million molecules of dried air (that is, with the water vapour removed).
There is a continuing exchange of CO2 between the atmosphere and the oceans. The oceans are always catching up with the rising human-driven atmospheric levels and if human civilisation continues to emit CO2 then atmospheric CO2 levels will continue to rise irrespective of continued CO2 absorption by the oceans.
Tan explained that the sum of the added CO2 in atmosphere and in the oceans will not go away for thousands of years. However, at some point in the future a chemical equilibrium will be established between the oceans and the atmosphere in which there is more CO2 than today both in the atmosphere and in the oceans. This equilibrium will take a few hundred year to get established because it takes time for the deep ocean waters to get to the surface and absorb CO2.
Tans pointed out the ocean lag means that if human CO2 emissions were to stop today then it would be the case that the seasonally adjusted global average CO2 level would sink back down below 400ppm as the oceans caught up with the atmosphere and established an equilibrium or balance at a lower atmospheric concentration than now. “However, it is very unlikely that emissions will stop,” Tans added.
There was considerable press coverage of the fact that global average CO2 levels, that were not adjusted to reflect seasonal variations, exceeded 400ppm earlier this year – in April, May and June – although they have since dipped back below that level.
Tans points out that the unadjusted global average atmospheric CO2 concentration figure would continue to dip below and rise above the underlying level going forward. Indeed, it will dip back down below the 400ppm level during the 2015 northern hemisphere late summer and fall, he predicted.
However, seasonally adjusted figures smooth out the fluctuation which is driven by increased plant activity in the northern hemisphere summer and reduced plant activity in the northern hemisphere winter which superimposes a wave on top of the underlying trend. The significance of the seasonally adjusted data is that it removes this cyclic variation and this means that once the 400ppm threshold is exceeded then the concentration will not dip back below 400ppm, unless there are rapid and significant emission reductions in the very near future.
The global average takes into account the fact that, in general, CO2 levels are higher in the more industrialised northern hemisphere than they are in the southern hemisphere and that CO2 concentrations in the southern hemisphere tend to lag behind. Most industrial emissions happen in the northern hemisphere and as emissions growth has accelerated so the gap between the hemispheres has expanded – from around 1ppm back in the 1950s to around 4ppm now.
Continuous modern measurements of CO2 started at Mauna Loa in 1958 and these are the longest record of continuous measurements of CO2. A programme of global flask sampling from a network of more than 20 sites began in the mid-1970s. This flask data combined with NOAA instrument data is used to construct estimates of the globally averaged CO2 concentration in the marine boundary layer of the atmosphere.
Tans explained that seasonally adjusted CO2 at Mauna Loa closely tracks the seasonally adjusted global average, but is always slightly higher because most of the fossil fuel CO2 is emitted in the northern hemisphere. The seasonally adjusted curve at Mauna Loa reached 399.23 for the October average, but the global average is lagging. The latest global average seasonally adjusted value for September 2014 is 397.96.
NOAA’s Arctic monitoring site in Barrow in Alaska was the first station to report unadjusted local CO2 levels of 400ppm levels in the Spring of 2012. Since then, levels measured on NOAA’s Mauna Loa mountain observatory in Hawaii topped 400ppm for the first time on 9 May 2013. CO2 levels reached 400ppm throughout the northern hemisphere for the first time in April 2014 and the global average is expected to reach 400ppm in late spring of 2015.
Eventually, some time around 2017 on current trends,according to Tans, the 400ppm level will be reached in the Antarctic; and then every monitoring station in the world will consistently be recording levels at or above 400ppm.