Rising carbon dioxide concentration stops the glacial/interglacial cycle
Posted by Ari Jokimäki on August 24, 2009
I have seen claims from denialists that the past records of temperature from Antarctic ice cores show that we are heading towards an ice age. The reason for this claim can be seen from the temperature reconstruction made from the Vostok ice core deuterium profile. Details are given here which is where I also got the data for the graph presented in Fig. 1.
Figure 1. Reconstructed temperature from Vostok ice core.
The claim suggests that the same is happening now that happened about 130000 years ago, and about 240000 years ago, and about 320000 years ago, that is, the temperature increased quickly and then also decreased quickly and it all happened seemingly in a cyclic process. However, the claim misses few things. First, let us see how high CO2 concentration was during the past 400000 years shown in Vostok temperature reconstruction. There’s also CO2 concentration measured from Vostok ice core. It is described here, and the data is also given there. I have made a graph out of the data, and it is presented in Fig. 2.
Figure 2. CO2 concentration from Vostok ice core.
We find out that CO2 concentration varied between 180 and 300 ppm during the last 400000 years when glacial/intarglacial cycles were going on. The thing that the claim misses is that currently, the CO2 concentration is getting close to 400 ppm which is far more than anything during the past 400000 years. But, does it matter? Here we arrive to the second thing the claim misses.
If we look further back in time than Vostok ice core, we find out that glacial/interglacial cycle has not been always functional. There has been times when global temperature was continuously high for long periods of time (millions of years) without any glacial periods. During those times, also CO2 concentration has been high, much higher than it is today. Based on the knowledge of those past times, Royer (2006) has suggested that glaciation starts only when CO2 concentration decreases below 500 ppm. However, Royer cannot give very accurate limit, but only says that it is below 500 ppm. Above we saw that the maximum value of CO2 concentration were quite consistently 280-300 ppm during interglacial periods of recent 400000 years. Based on this, I would say that it matters that current CO2 concentration is far above the normal interglacial maximum. I cannot say for sure that we have already reached the below 500 ppm threshold Royer suggests, but I also don’t see any reasons to assume that the next glacial would be about to begin. At least the situation is currently heading towards the threshold.
Also, at any case, to change climate, you always need forcings to be such that a change can occur. This is the third thing the claim misses. Currently, there’s no forcing in sight that could turn things around. As Hansen et al. (2008) show, greenhouse gases have always (well, at least the last 65 million years) been the determining factor of where climate goes. Other forcings have only minor roles as initiators. (However, albedo changes can be very strong feedback, but is not usually considered as an initiator of climate changes because there would have to be some pre-existing cause to change the albedo.) Note also that Hansen et al. succesfully explain the glacial/interglacial cycle by greenhouse gas forcing with Earth’s orbital changes acting as the initiator. So, to see when the next ice age is going to start, we only need to see which way the greenhouse gases are pointing.
Currently, greenhouse gases are pointing towards warmer climate.