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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

Basic rebuttal 43 - There's no correlation between CO2 and temperature. There is long-term correlation between CO2 and global temperature; other effects are short-term.

Okay, here goes. This is my first attempt at a rebuttal, be honest, but be nice too :)

Scientists have lots of ways to check how much CO2 is in the atmosphere. One of the best is sampling ice from really old glaciers, because when the ice freezes it contains air bubbles. These hold information about the atmosphere at the time when the water froze. Using this method we can sample the Earth's atmosphere going back hundreds of thousands of years.

What we've discovered about CO2 in the atmosphere is that it follows a cycle. Every hundred thousand years or so it reaches a peak and then drops back down to a trough. This cycle is repeated again and again, always the same.

For the past million years or so the peak has stayed constant. Every cycle the CO2 levels reach a high point of 280 parts per million (PPM). That is, for every million cubic centimeters of air, 280 cubic centimeters are CO2.

The graph below (Figure 1) shows the temperature in blue and the CO2 levels in red for the last 400,000 years. The two lines follow a similar pattern. When the CO2 goes up the temperature follows. When the CO2 comes down, so does the temperature. They don't exactly match but there is a definite correlation.

Every day the sun shines down visible light onto the planet giving us energy and heat. The atmosphere acts as a thin blanket keeping some of this heat in and allowing some to be reflected back into space. The Earth soaks up a large percentage of this visible light energy, then radiates it back out again as infrared radiation. Some of this stays in the atmosphere and a lot disappears into space. That is the main system that keeps the temperature on Earth reasonably constant. So what is all the fuss about CO2?

CO2 is a gas that has always been in our atmosphere. It allows visible light to pass through from the sun, but it reflects infrared radiation. What this means is that visible light travels from the sun to the Earth where it is absorbed. When it is radiated back out from the Earth as infrared much of it passes up through the atmosphere and disappears off into space. With higher levels of CO2 in the atmosphere, more infrared is reflected back to Earth, in effect trapping the heat. Over time this causes the Earth's temperature to rise.

There are other systems that affect Earth's temperature. The sun goes through periods of high activity and low activity – sometimes it shines hotter than others, sometimes cooler. For the last few years the sun has been in one of it's low activity or cooler phases. Oceans play a part too, sometimes soaking up heat, sometimes releasing it. In the Pacific, La Niña and El Niño show just how extreme these kinds of oceanic interactions can be and how they can affect the world, rather than just the local area. These and many other interactions can affect the temperature readings on a daily or yearly basis.

Because there are so many interactions going on that affect the temperature in the short term, in order to get the real picture we need to look at long term data. Doing this averages out all the small short term effects to give a much more realistic picture. Looking at the graph above you can see that the two lines don't match up exactly. Sometimes when one line goes up the other comes down. If we just look at a short period we could get the idea that there is no correlation. However, looking at the whole graph shows that the two lines over long periods follow a very similar pattern.

That is why many of us haven't noticed huge changes in the temperature, even though we keep hearing about global warming. We're only experiencing it in the short term. Studies show that over the last 100 years global temperatures have risen around one degree Celsius. That may not sound like much, but bear in mind that the difference between an ice age and an ice free period on Earth is around 5 degrees Celsius.

Updated on 2010-08-27 by gpwayne.

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