CO2: The Most Important Controller of Global Average Temperature in Earth’s History

The following is an excerpt from meteorologist Jeff Masters’ Wunderblog, from March 24. Masters is discussing the talk given by Dr. Richard Alley at the American Geophysical (AGU) meeting, the world’s largest scientific conference on climate change, in San Francisco last December. Dr. Alley is the Evan Pugh Professor of Geosciences at the Pennsylvania State University, and one of the most respected and widely published world experts on climate change. Dr. Alley has testified before Congress on climate change issues, served as lead author of “Chapter 4: Observations: Changes in Snow, Ice and Frozen Ground” for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), and is author of more than 170 peer-reviewed scientific articles on Earth’s climate.  Here is what Masters has do say about Alley’s talk to a packed, standing-room only audience of over 2,000 scientists:

Earth’s past climate has been shaped by a number of key “control knobs”–solar energy, greenhouse gas levels, and dust from volcanic eruptions, to name the three main ones. The main thrust of Dr. Alley’s speech is that we have solid evidence now–some of it very new–that CO2 has dominated Earth’s climate over the past 400 million years, making it the climate’s “biggest control knob”. Dr. Alley opens his talk by humorously discussing a letter from an irate Penn State alumnus. The alumnus complains that data of temperatures and CO2 levels from ice cores in Antarctica don’t match:

Co2 lags Earth’s temperature…This one scientific fact which proves that CO2 is not the cause of recent warming, yet…Dr. Alley continues to mislead the scientific community and the general public about ‘global warming’. His crimes against the scientific community, PSU, and the citizens of this great country, and the citizens of the world are significant and must be deal with severely to stop such shameful activities in the future.”

Dr. Alley explains that the irate alumnus is talking about the Antarctic ice core record, which shows that as we emerged from each ice age, the temperature began increasing before the CO2 did, so increased CO2 was not responsible for the warmings that brought us out of these ice ages. Climate change scientists and skeptics alike agree that Earth’s ice ages are caused by periodic variations in Earth’s orbit called Milankovich Cycles. “There’s no doubt that the ice ages are paced by the orbits”, says Dr. Alley. “No way that the orbit knows to dial up CO2, and say ‘change’. So it shouldn’t be terribly surprising if the CO2 lags the temperature change. The temperature never goes very far without the CO2. The CO2 adds to the warming. How do we know that the CO2 adds to the warming? It’s physics!”

Dr. Alley then discusses that the physics that govern how CO2 absorbs and re-emits heat energy, when plugged into state-of-the-art climate models, show that about half of the observed 5 – 6°C natural warming that occurred since the last ice age ended was due to extra CO2 added to the atmosphere. At the peak of the Ice Age, CO2 was about 190 ppm. By the end, it was about 280 ppm (Figure 1). Earth’s orbital variations “forced” a warming, which caused more CO2 to escape from swamps and oceans, with a time lag of several centuries. The increased CO2 reinforced the warming, to double what it would have been otherwise–a positive feedback loop. “Higher CO2 may be forcing or feedback–a CO2 molecule is radiatively active regardless of how it got there”, says Dr. Alley. “A CO2 molecule does not remember why it is there–it only remembers that it is there”. In other words, the fact that higher CO2 levels did not trigger an end to the Ice Age does not mean that the CO2 had no warming effect. Half of the the observed 5 – 6°C natural warming that occurred since the last ice age ended was due to the extra CO2 added to the atmosphere. So, the irate PSU alumnus was half right. The CO2 does lag temperature. However, we can only explain approximately half of the warming since the last ice age ended if we leave out the increase in CO2 that has occurred. “If higher CO2 warms, Earth’s climate history makes sense, with CO2 having caused or amplified the main changes. If CO2 doesn’t warm, we have to explain why the physicists are so stupid, and we also have no way to explain how a lot of really inexplicable climate events happened over Earth’s history. It’s really that simple. We don’t have any plausible alternative to that at this point”…

To read the full post and see the graphs at Masters’ Wunder Blog, click here.

To hear Dr. Alley’s full lecture with slides, click here.

0 thoughts on “CO2: The Most Important Controller of Global Average Temperature in Earth’s History”

  1. Cheers for this piece; I’ll probably link back to it (as I’m often terrible with referencing my blogs) as it is a wonderful quote to express just how important CO2 is for life on Earth and just how much these relatively small concentrations of CO2 have an effect on climate.

  2. Great! Keep up your good work of spreading the word – Australia’s one of the places that is feeling the sting of global warming already, as you know. It’s hard to believe that the denial movement is as active there as it is, considering the glaring evidence staring every Aussie in the face, should they choose to look.


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