Is there an upper limit to the greenhouse gas effect?

Editor’s Note: Learn More About Climate recently received an interesting question through our Ask A Scientist feature:

I have recently heard the claim that “the atmospheric absorption bands for CO2 are already saturated” so the addition of more CO2 to the atmosphere is moot. I trust this is not the whole story, and would like to understand why such a claim is not accepted by the (vast) majority of climate scientists.

CU-Boulder second-year graduate student Anondo Mukherjee reached out to his colleagues at the Department of Atmospheric and Oceanic Sciences (ATOC). Here is Anondo’s response:

What is well understood is that greenhouse gasses such as CO2 act as a radiative forcing through the greenhouse gas effect: they keep more energy in the earth system.

The figure below shows the magnitude of the human-caused greenhouse gas effect due to different factors. The units are Watts per meter squared (averaged over all of earths surface). We can think of the energy reaching earths surface from the atmosphere or the sun in those units, Energy per unit time (Power), per unit surface area.

To give some context, the energy reaching earth from the sun is 341.5 watts per meter squared (averaged over the top of earths atmosphere). Not all of this energy reaches earth surface. So for context, the magnitude of the human-caused greenhouse gas effect is about 1% the energy the earth system receives from the sun. Small but very important!

The figure also shows the magnitude changing over time, showing that the greenhouse gas effect has almost doubled between the years 1980 and 2011. That is a huge difference in a very small amount of time for climate (which usually changes of thousands of years). Just that information tells us that the greenhouse gas effect is not slowing down anytime soon.

Radiative Forcing by Emissions and Drivers

Click to enlarge

Now for the question: if we keep raising CO2 concentration, is there an upper limit to the greenhouse gas effect?

According to the radiative transfer, which scientists believe describe the system: the relationship between concentration, and radiative forcing is logarithmic, with no upper bound. So that means whenever we double CO2 concentration, we see the same amount of change of radiative forcing.

The more CO2 we put in to the air, the more energy stays in the earth system (and is not radiated away to space). The reason there is no limit to this, is essentially because the lower layers of the atmosphere can grow warmer and warmer, and also grow higher and higher. So physically the air, earth, and ocean, have no limit to how much energy they can hold.

For example, Venus has a huge greenhouse gas effect, giving it 493 Kelvin of warming compared to earths ~33 Kelvin of warming (mostly due to naturally occurring water vapor). So the surface of Venus is hotter then the surface of Mercury, which is closer to the sun! This is because Venus’s atmosphere is 96 % CO2, almost nothing but greenhouse gas, and at a much higher density then earth’s atmosphere.

Greenhouse Gas Effect on Venus

Greenhouse Gas Effect on Venus

One important caveat – the relationship between warming of earths surface and radiative forcing from the greenhouse gas effect is a bit more complex – it depends on a term that scientists call climate sensitivity. It is not a simple because of nonlinear feedback cycles that are unpredictable. For example the energy from the greenhouse gas effect can go into the ocean, or the earth, or into melting glaciers, or into changing the dynamic patterns of circulation. In the long run there is going to be more warming + other complex effects (see the IPCC 5th assessment)

What we understand really well is this: the more CO2 we put into the earth, the more energy stays in the earth system due to the greenhouse gas effect. The relationship between CO2 concentration and Radiative Forcing is logarithmic without upper bound. the relationship between Radiative Forcing and surface temperature is complex, but certainly will continue to be positive for the near term future.

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