As we begin a new year — on a day that is hot and windy 40 degrees in Melbourne, I might add — these continuous questions about climate science and the issues surrounding climate change seem even more poignant. Another year, another international climate change conference, another year with little climate change policy being advocated by our government.

So as part of your summer reading, delve into these questions on CO2, global warming and what levels of CO2 are “safe” or “traditional”. And just to note the calibre of the scientists answering these questions, today we have Spencer Weart — Director Emeritus at the American Institute of Physics, historian and author of seven books — answering one of them.

Got a question about climate science? We’re continuing to run the Ask a climate scientist series on Rooted. Keep the questions coming by emailing me directly.

These answers are coming from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. The AGU only comments on science, not climate policy.

Crikey reader Steven asks:

I have heard that the increasing CO2 levels will only heat the atmosphere slightly (.07 degrees is the amount that was mentioned) and that most of the warming predicted will come from the increase in water vapor that will have be released into the atmosphere due to this slight increase in temperature and not directly from the increase in temperature due to the CO2. Is this true? If this is true will the water vapor cause the warming by the greenhouse effect or by causing an increase in cloud cover?

Spencer Weart, Director Emeritus at the American Institute of Physics, responds:

That’s an important question. Yes, an increase in carbon dioxide would not warm up the Earth very much. It would have an effect — not as small as .07 degrees (wherever that figure came from it is too small), but even doubling the CO2 level in the atmosphere would not bring more than a degree of warming all by itself.

In terms of direct effects on radiation, water vapor accounts for about half the greenhouse effect, clouds for about a quarter, CO2 for 20% and other greenhouse gases 5%. What makes CO2 important is that it regulates the rest of the system. After all, the Earth is a wet planet. Water cycles in and out of the air, oceans, and soils in a matter of days, exquisitely sensitive to fluctuations in temperature.

By contrast CO2 (and other, less important greenhouse gases like methane) linger in the atmosphere for much longer timesclaes (decades or centuries). Thus it is these gases that act as the “control knob” that sets the level of water vapor. If all the CO2 were somehow removed, the temperature at first would fall only a little. But then less water would evaporate into the air, and some would fall as rain.

With less water vapor (and also less clouds retaining heat at night) the air would cool further, bringing more rain… and then snow. Within weeks, the air would be entirely dry and the Earth would settle into a frozen state — just as people calculated already in the nineteenth century for a planet without an atmosphere.

On the other hand, raising the CO2 level as humans are now doing raises the temperature enough so that more water evaporates and stays in the air, and this brings its own important warming effect. That’s why evidence from both computer models and climate changes in the distant past show that a doubling of CO2 must mean a rise of roughly 2-5 degrees C.

As for clouds, the best recent studies show that changes in cloudiness with increasing temperature might have a slight additional warming effect, but not a very important one.

Answer reviewed by Clare Murphy (Paton-Walsh) from the University of Wollongong.

Crikey reader George asks:

“Traditional, or Safe” CO2 concentrations have been asserted to be 280 – 300 parts per million (ppm) in the atmosphere. While 450 ppm seems like a large increase over 300 ppm, expressed as a percentage it isn’t. We are talking about a change from 0.030% to 0.045%. I cannot find experimental results which show that such a change, in a closed system, with constant radiation heat inflow makes any difference at all. Has it been done? Where is it?

Dr. Lin Chambers, physical scientist at the NASA Langley Research Center, responds:

A better way to think about this is as an increase to 450 from 280-300 is ~450/290 or ~155%. That is a pretty significant increase. It is really not appropriate to think about it in terms of absolute concentration in the atmosphere (i.e., 0.03%), because most of the gases in the atmosphere (mainly nitrogen and oxygen) are essentially transparent to radiation. So their presence does not matter in this regard.

I like to make the analogy to poisons. There may be only a very small amount in your body, but depending on the properties of the poison that can be enough.

Also, it may be worth referencing the work of Arrhenius, who did this calculation over 100 years ago and pointed out the impact of increased CO2 on surface temperature.

Answer reviewed by Dr Peter C. Griffith  from the NASA Goddard Space Flight Center.

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Peter Fray
Peter Fray
Editor-in-chief of Crikey
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