This week brought news that atmospheric carbon dioxide (CO₂) levels at the Mauna Loa atmospheric observatory in Hawaii have risen steeply for the seventh year in a row, reaching a May 2019 average of 414.7 parts per million (ppm). But in truth, the amount of greenhouse gas in our atmosphere is higher still.

The Cape Grim observatory, home of the ‘world’s cleanest air’… and rising greenhouse gases. Author provided

This week brought news that atmospheric carbon dioxide (CO₂) levels at the Mauna Loa atmospheric observatory in Hawaii have risen steeply for the seventh year in a row, reaching a May 2019 average of 414.7 parts per million (ppm).

It was the highest monthly average in 61 years of measurements at that observatory, and comes five years after CO₂ concentrations first breached the 400ppm milestone.

But in truth, the amount of greenhouse gas in our atmosphere is higher still. If we factor in the presence of other greenhouse gases besides carbon dioxide, we find that the world has already ticked past yet another milestone: 500ppm of what we call “CO₂-equivalent”, or CO₂-e.

In July 2018, the combination of long-lived greenhouse gases measured in the “cleanest air in the world” at Cape Grim Baseline Atmospheric Pollution Station surpassed 500ppm CO₂-e.

As the atmosphere of the Southern Hemisphere contains less pollution than the north, this means the global average atmospheric concentration of greenhouse gases is now well above this level.

What is CO₂-e?

Although CO₂ is the most abundant greenhouse gas, dozens of other gases – including methane (CH₄), nitrous oxide (N₂O) and the synthetic greenhouse gases – also trap heat. Many of them are more powerful greenhouse gases than CO₂, and some linger for longer in the atmosphere. That means they have a significant influence on how much the planet is warming.

Southern Hemispheric radiative forcing relative to 1750 due to the long-lived greenhouse gases (carbon dioxide, methane, nitrous oxide and synthetic greenhouse gases), expressed as watts per square metre, from measurements in situ at Cape Grim, from the Cape Grim Air Archive, and Antarctic firn air.

Atmospheric scientists use CO₂-e as a convenient way to aggregate the effect of all the long-lived greenhouse gases.

As all the major greenhouse gases (CO₂, CH₄ and N₂O) are rising in concentration, so too is CO₂-e. It has climbed at an average rate of 3.3ppm per year during this decade – faster than at any time in history. And it is showing no sign of slowing.

Cape Grim/Antarctic carbon dioxide equivalent (CO₂-e) calculated from the long-lived greenhouse gas radiative forcing data shown in the figure above with CO₂ data shown for reference, annual data through to 2018. Inset panel shows the monthly mean CO₂-e data for Cape Grim from 2015 through to March 2019, showing CO₂-e surpassing 500ppm in July 2018.

This milestone, like so many others, is symbolic. The difference between 499 and 500ppm CO₂-e is marginal in terms of the fate of the climate and the life it sustains. But the fact that the cleanest air on the planet has now breached this threshold should elicit deep concern.

Warming on the way

The Paris climate agreement is aimed at limiting global warming to less than 2℃ above pre-industrial levels, to avoid the most dangerous effects of climate change. But the task of predicting how human greenhouse emissions will perturb the climate system on a scale of decades to centuries is complex.

The best estimate of long-term global warming expected from 500ppm CO₂-e is about 2.5℃. But so far, since pre-industrial times, the global climate (including oceans) has warmed by only 0.7℃.

This is partly because industrial smog and other tiny particles (together called aerosols) reflect sunlight out to space, offsetting some of the expected warming. What’s more, the climate system responds slowly to rising atmospheric greenhouse gas concentrations because much of the excess heat is taken up by the oceans.

The amount of heat each greenhouse gas can trap depends on its absorption spectrum – how strongly it can absorb energy at different wavelengths, particularly in the infrared range. Despite its simple molecular structure, there is still much to learn about the heat-absorbing properties of methane, the second-biggest component of CO₂-e.

Studies published in 2016 and 2018 led to the estimate of methane’s warming potential being revised upwards by 15%, meaning methane is now considered to be 32 times more efficient at trapping heat in the atmosphere than CO₂, on a per-molecule basis over a 100-year time span.

Considering this new evidence, we calculate that greenhouse gas concentrations at Cape Grim crossed the 500ppm CO₂-e threshold in July 2018.

This is higher than the official estimate based on the previous formulation for calculating CO₂-e, which remains in widespread use. For instance, the US National Oceanic and Atmospheric Administration is reporting 2018 CO₂-e as 496ppm.

The graph below shows the two curves for the time evolution of CO₂-e in the atmosphere as measured at Cape Grim, using the old and new formulae.

Cape Grim monthly CO2-e from 2015 until Sept 2018 calculated using the old and new formulae.

Some greenhouse gases, such as chlorofluorocarbons (CFCs), also deplete the ozone layer. CFCs are in decline thanks to the Montreal Protocol, which bans the production and use of these chemicals, despite reports that indicate some recent production of CFC-11 in China.

But unfortunately, their ozone-safe replacements, hydrofluorocarbons (HFCs), are very potent greenhouse gases and are on the rise. The recently enacted Kigali Amendment to the protocol means that consumption controls on HFCs are now in place, and this will see the growth rate of HFCs slow significantly and then reverse in the coming decades.

We can change

Australia is at the forefront of initiating measures to curb the impact of HFCs on climate change.

Methane is another low-hanging fruit for climate action, while we undertake the slower and more difficult transition away from CO₂-emitting energy sources.

The significant human methane emissions from leaks in reticulated gas systems, landfills, waste water treatment, and fugitive emissions from coal mining and oil and gas production can be monitored and reduced. We have the science and technology to do this now.

Both in the oil and gas sectors and in urban areas, there are many examples of how methane “hot spots” can be identified and tackled.

It’s a classic win-win that saves money and reduces climate change, and something we should be implementing in Australia in the near future.The Conversation


Zoe Loh, Research Scientist, CSIRO; Blagoj Mitrevski, Research scientist, CSIRO; David Etheridge, Principal Research Scientist, CSIRO; Nada Derek, Research Projects Officer, Oceans and Atmosphere, Climate Science Centre, CSIRO; Paul Fraser, Honorary Fellow, CSIRO; Paul Krummel, Research Group Leader, CSIRO; Paul Steele, Honorary Fellow, CSIRO; Ray Langenfelds, Scientist at CSIRO Atmospheric Research, CSIRO, and Sam Cleland, Officer in Charge, Cape Grim Baseline Air Pollution Station, Australian Bureau of Meteorology

This article is republished from The Conversation under a Creative Commons license. Read the original article.


  1. Does “methane is now considered to be 32 times more efficient at trapping heat in the atmosphere than CO₂, on a per-molecule basis over a 100-year time span” mean that on a mass basis, CH4 is 32×44/16= 88 times the GHG heating potential of CO2? Seems a lot higher than the “1 tonne CH4 = 25 Tonne CO2-e” factor used in a lot of calculations on potential impact of a CO2-e tax.

  2. [The following was “moderated” by The Conversation. That serves as another example that “97%” believe that global warming is caused by Man as non-believers’ opinions are deleted.]

    There definitely is more greenhouse gas than realised. For example, the IR absorbing H2O vapour is 1% or 2% vs 0.04% for CO2
    However, ALL gases are greenhouse gases as, like all substances, they all absorb heat.

    People confuse “heat” with “IR” – which is but one way of transmitting or absorbing heat, but others include conduction and convection (and winds).

    The Greenhouse Effect is a difference in temperature, that T between the S-B temperature, ie the theoretical T without an atmosphere, and the actual T with an atmosphere. (The S-B temperature is the Stefan-Boltzmann temperature which accounts for the distance from the Sun.)
    The Greenhouse Effect results from the extra heat held by a planet’s atmosphere – regardless of how it arrives from the planet’s surface, eg by IR, conduction, convection, whatever.
    The temperature of all molecular species in any small parcel of atmosphere is the same – as one can feel.
    Therefore the heat absorbed by each species is in proportion with their amount (or more accurately by the product amount x specific heat).
    As a minor component at 0.04%, CO2 has only a minor contribution to Temperature, CH4 much less again.
    Therefore CO2, whether or not Man-made, has no perceptible effect upon changing climates – which are real.

  3. My email address is and I had forgotten to mention in my last post a few minutes ago, that 60% of the energy which heats the air is from contact with the ground and that oxygen and nitrogen rely on CO2 and other greenhouse gases to rasdiate this energy.

  4. What I would like to see from CSIRO would be a very detailed scientific analysis of the behavior of carbon dioxide in the atmosphere. It is all very well to talk about the “forcing” used by models etc but we do NOT see any numerical presentation surrounding the absorption characteristics of CO2. For a start, CO2 absorbs only a little more than 10% of the earth’s emitted radiation. Even though there is only a small quantity of CO2 in the atmosphere, the infra red radiation it absorbs is concentrated in about the first hundred metres or less of the lower atmosphere. This warmed parcel of air is lifted rapidly by convection which transports energy upwards far faster than any radiation – radiation of say 1000 W/m^2 at most from the earth – actually about 500 W – while the parcel of air with a thermal capacity of about 1000.0 (1 x 10^3)Jm^-3K^-1 holds heat energy of 10000 J when it is heated by 10 degrees from absorbing radiation. If it rises at 1 m/second (3.6 km/hr, it is effectively taking up awards an energy field of 10,000 W/m^2. In the upper atmosphere, the energy is radiated by green house gases and NOTHING else, since N@ and O@ cannot radiate at that temperature.

    in 1956 Gilbert Plass (mathematical Physicist) showed that increases in CO2 significantly increased the radiation of this energy to space. On the other hand, the lesser energy left at ground level, increased by about the same, the radiation back to ground . More spectroscopically accurate calculations show that in fact the radiation to space increases by about the same amount as calculated by Plass but that the radiation returning to the ground increased by only a miniscule amount.

    Perhaps these calculations could be improved on but there seems to be no recent calculations or presentation of old calculations by your organisation or by any of Australia’s climate “science” units. Nor is there from NASA or UEA. This represents very poor communication of the science.

  5. The trolls are out in force again. But I am sure that the university that granted Charles Young’s PhD in climate science will speak up in his defense and silence them.

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