How is atmospheric CO2 measured in the Southern Hemisphere?

Why does the atmosphere over Tasmania record lower levels of CO2 than that over Hawaii? Ula Majewski

Last week the greenhouse gas monitoring site at Mauna Loa in Hawaii recorded daily levels of atmospheric carbon dioxide that approached the 400 parts per million molar (ppm) benchmark.

Annual mean Southern Hemispheric levels of carbon dioxide, as recorded at the Cape Grim Air Baseline Air Pollution Monitoring Station in north-west Tasmania, are expected to reach the 400ppm milestone during 2016.

Where do we measure atmospheric carbon dioxide?

Cape Grim is a key international monitoring facility, operated by the Bureau of Meteorology, and is where much of CSIRO’s international global atmospheric research is centred. Measurements have been made here since 1976.

Cape Grim is one of three key sites identified by the World Meteorological Association for long-term carbon dioxide measurements. The others are Mauna Loa in Hawaii (since 1956), which last week measured daily recordings exceeding the 400ppm benchmark, and Barrow in Alaska (since 1973).

CSIRO has also measured Southern Hemispheric carbon dioxide over the past 2000 years in air trapped in Antarctic surface ice - called firn - and deeper ice cores.

Why do we measure atmospheric carbon dioxide?

Carbon dioxide is one of the primary greenhouse gases. Others include methane, nitrous oxide and synthetic gases such as refrigerants and fire retardants.

Carbon dioxide levels in the atmosphere are rising mainly because of the burning of fossil fuels and deforestation.

Increasing greenhouse gases in the atmosphere leads to climate change. The amount of warming produced by a given rise in greenhouse gas concentrations depends on feedback processes in the climate system, such as the water vapour response. This both amplifies, by water vapour, and dampens, by cloud formation, the temperature increase due to these long-lived greenhouse gases.

Over half of the carbon dioxide input to the atmosphere is absorbed by natural sinks in the land plants and oceans.

Land and ocean carbon dioxide sinks respectively removed 30% and 24% of all anthropogenic carbon dioxide emissions over the period 2000-2008. This constitutes a massive natural ecosystem service helping to mitigate humanity’s emissions.

Why are southern hemisphere carbon dioxide levels lower?

Carbon dioxide is currently rising at close to, perhaps a little above, 2 parts per million molar (ppm) per year.

The annual mean carbon dioxide level at Mauna Loa is not expected to exceed 400ppm until 2015, although both Cape Grim and Mauna Loa may reach 400ppm a year earlier if the current growth rate of 2ppm per year accelerates.

Cape Grim baseline CO2 measurements in April 2013 averaged 392ppm for the month.

There is a clear difference between levels of carbon dioxide measured in the Southern and Northern hemispheres, because industrial and other population-based sources of carbon dioxide emissions are concentrated in the Northern Hemisphere.

What we are seeing at present in the Mauna Loa May measurements are observations fluctuating around 400ppm. These will return to sub-400ppm levels later this year (September) when absorption by vegetation - what’s known as the “annual carbon dioxide draw down” - will affect Northern Hemisphere atmospheric CO2 levels.

From measurement to action

Air and ice measurements allow us to trace the dramatic rise in carbon dioxide levels from about 280ppm before the start of the industrial era around the year 1800, to 392ppm in 2012. That’s an increase of 40%, largely due to human activities.

To have a 50:50 chance of keeping human-induced average global warming below 2°C, it will be necessary to stop almost all carbon dioxide emissions before cumulative emissions reach one trillion tonnes of carbon.

The world has already emitted more than half of this quota since the industrial revolution. At current accelerating growth rates for the combustion of fossil fuels, the rest will be emitted by the middle of this century.

Cape Grim measurements of carbon dioxide are publicly available at -