In my previous post I discussed some of the evidence for climate change and why there is some scepticism. The main point of contention is the hockey stick graph which shows a sudden rise in temperatures starting around 1900 and becoming particularly fast after 1950. The criticism is that there were problems with the analysis that first produced this graph. However, it has been reproduced many times using other methods and so the scientific consensus is that it correctly represents temperature variations over the last 1000 years. Possibly the most surprising thing I found was the possibility of a medieval warm period (around 1000 AD) that may have been as warm as it is today. It’s not clear that it was (not all analyses suggest this) and it’s not clear that it’s relevant.
Here’s why. The figure below shows temperature variations and variations in CO2 over the last 400000 years. There is clearly a relationship between temperature and CO2. This is thought to be a consequence of Milankovich cycles which cause the eccentricity (ellipticity) of the Earth’s orbit and tilt of the Earth’s axis to vary with time. This results in a variation in the total amount of energy that the Earth receives from the Sun and, more importantly, causes where it is deposited to vary with time.
Over the course of a Milankovich cycle the Earth will at times receive more energy from the Sun than at other times. Furthermore, where the energy is deposited on the Earth’s surface also varies during the cycle. This will, of course, cause the Earth’s temperature to vary. The variation shown in the above figure is, however, much bigger than can be explained by variations due to the Milankovich cycle. What is thought to happen is that when the polar ice caps melt they release CO2 into the atmosphere. This produces a feedback cycle. The increased CO2 increases the temperature through greenhouse warming which then releases more CO2. Later in the Milankovich cycle, the Earth will start to cool, the ice caps start to reform, the CO2 levels drop, greenhouse warming reduces and the Earth’s temperatures reduces. As can be seen from the above figure, each cycle lasts about 100000 years, the CO2 varies from about 180 parts per million (ppm) to about 280 ppm, and the temperature varies by about 10o C.
There are a number of interesting things about the above graph. It’s fairly easy to show that in the absence of an atmosphere, the Earth would have an average temperature of about -15o C. Instead, today, it has an average temperature about 30o higher than this. Today the CO2 levels are about 300 ppm, which might suggest that each 100 ppm of CO2 produces 10o C of warming. The above figure would seem to be consistent with this since the CO2 level varies by 100 ppm and the temperature varies by about 10o C. I suspect that this is a little simplistic as there are many other greenhouse gases (methane, water vapour) but I suspect that variations of 100 ppm in CO2 must produce variations in temperature of at least a few degrees. That CO2 produces greenhouse warming is irrefutable, as is the fact that the Earth is 30o C degrees warmer than it would be in the absence of greenhouse warming.
Here’s where things gets even more interesting. In the past 400000 years the CO2 levels varied from between 180 ppm to 280 ppm. If you look at the extreme right of the above figure, you see the red line suddenly shoot up to about 350 ppm. This is shown in more detail in the figure below which shows the CO2 levels since 900 AD till today. It’s clear that it was at about 280 ppm until just after 1800 and increases rapidly after 1950, reaching almost 350 ppm today. This rise in CO2 levels is almost certainly associated with our increasing use of fossil fuels. What’s more, the CO2 levels started rising before the temperature started rising. One can’t then argue that the rising CO2 levels has lead to the rise in temperature, and not the other way around (i.e., the increased CO2 is not because of a natural increase in the temperature).
This increase of 70 ppm over what the levels would have been had we not been using fossil fuels could – using my simple estimate – lead to 7o C of warming. Interestingly, a recent report suggests that the world is on track for 6o C of warming if we don’t cut carbon emissions. Maybe my crude estimate is not that wrong.
Here’s where it gets scarier. Since about 1950 we’ve increased the CO2 levels by 1 ppm per year. This means we could easily get to CO2 levels significantly higher than those seen over the last 500000 years. If so, we could reach a tipping point where the process runs away and doesn’t stop until the atmosphere is almost exclusively CO2, and the temperature is well above what would be suitable for life of any kind. This may seem extreme but such a process (known as the Runaway Greenhouse effect) has indeed happened on the planet Venus, which today has an atmosphere almost exclusively consisting of CO2 and a surface temperature of 500o C, almost 500o C warmer than it should be based on the energy it receives from the Sun.
This post has got slightly long, but here is my basic summary of the situation.
- Despite criticism, the hockey stick graph – first determined by Mann, Bradley and Hughes (1999) – is essentially correct. It has been replicated by others using different techniques.
- There is definitely a relationship between temperature and CO2 levels and CO2 definitely acts as a greenhouse gas with variations of 100 ppm producing variations in temperature of as much as 10o C.
- The CO2 levels today are 80 ppm (~ 30 %) higher than they have been for the last 500000 years. This is certainly a consequence of our extensive use of fossil fuels. This increasing CO2 level started prior to the increase in temperature.
- It is therefore clear that the increased temperatures (climate change/global warming) is causes by the CO2 being released by our excessive use of fossil fuels.
- If we don’t do something soon we could reach a point where the planet undergoes a runaway greenhouse process which won’t stop until the temperature is too high for life to exist.
I think that pretty much says it all. This is extremely serious and I think it is probably time we all started acting to encourage our governments to start taking this seriously and to start doing something about it. The consequences of doing nothing are quite likely be catastrophic.