Is it the beginning of the end?

There is a recent post on Watts Up With That (WUWT) suggesting that it is the beginning of the end : warmists in retreat on sea-level rise and climate sensitivity. It links to a blog post by someone called Pierre Gosselin who claims that the New York Times is conceding a low sensitivity.

In his post, Pierre Gosselin makes a number of claims and statements that I thought I would discuss here. He starts by saying

1. The New York Times first have to learn that science is not about insisting you are correct, or being obsessed with a popular pet hypothesis. In the article Justin Gillis and scientist James Annan come across as having a very hard time departing from the hypothesis that CO2 climate sensitivity is high.

Yes, I agree that science is not about insisting that you are correct, or being obsessed with a pet hypothesis. I do, however, find it a little rich coming from someone who seems to fall within the camp that typically dismisses any climate science that suggests that we should be concerned about anthropogenic climate change (AGW). Admittedly, this camp doesn’t really have a pet theory. Anything that attempts to reduce or dismiss the significance of AGW is acceptable.

2. CO2 lags temperature, history has proven. Moreover, although CO2 has risen tremendously (in relative terms) over the last 15 years, the global temperature has not. For the last 15 years, sensitivity has been zero. Worse, there’s strong, mounting evidence that there isn’t going to be any warming for another 10 or 20 years at least. (Hint: oceans).

I found this comment interesting for a number of reasons. His comment about the last 15 years is presumably based on temperature anomaly data which I addressed in an earlier post. His comment about the next 10 or 20 years lacks evidence but his hint implies that heat will be going into the oceans. This is the point. That means global warming is happening. Just because the energy isn’t currently heating the surface doesn’t mean that global warming has stopped. It is, however, the start of his comment that I find most interesting.

Has history shown that CO₂ lags temperature? Yes, in some sense it has. Is it relevant? No, noone is claiming that all rises in global surface temperatures have to be due to increases in atmospheric CO₂ concentrations. If anything it helps to make the case for AGW. The figure below shows the Vostok ice core record that goes back more than 400000 years. It shows both temperature and CO₂ concentrations. There are a series of cycles, each lasting about 100000 years, in which the temperature rises by about 10^o C and then falls back down again. This is associated with a similar rise and fall in the CO₂ concentrations.

Vostok ice core record showing temperature and carbon dioxide concentrations for the last 420000 years.

It turns out that the rise is CO₂ concentration in the Vostok ice core record lags the temperature rise by between 600 – 800 years. People have used this to claim that this proves that the current rise in CO₂ concentrations cannot be driving up global temperatures. The interpretation is incorrect. What we see in the Vostok record is a consequence of what is known as Milankovitch cycles.

The Earth’s orbit around the Sun is not fixed. It varies with a period of about 100000 years. These variations result in changes to the amount of energy the Earth receives from the Sun and where this energy is deposited. At the beginning of a cycle the Earth’s temperature will start to rise. The ability of water to retain CO₂ depends on temperature and so this rise in temperature results in a rise in the atmospheric CO₂ concentration. This increased CO₂ concentration further amplifies the warming which then further increases the atmospheric CO₂ concentration. The net result is that the surface temperature increases by about 10^oC and the CO₂ concentration rises by about 100 ppm. An important point, however, is that the orbital variations alone cannot explain this temperature rise. It requires amplification due to enhanced levels of atmospheric CO₂.

Later in the cycle, however, the orbital variation changes the amount of energy the Earth is receiving so that temperatures start to decrease. The cooler oceans are able to absorb more CO₂ and atmospheric CO₂ levels also start to decrease. This reduces the warming influence of CO₂ and the system returns back to where it was at the beginning of the cycle. This process can also repeat. There is essentially a feedback relationship between temperature and CO₂ levels.

The problem today, is that the rise in CO₂ is not because of a rise in surface temperatures. It’s because of us releasing CO₂ into the atmosphere through our use of fossil fuels. Pierre Gosselin goes on to say

3. There’s no correlation between CO2 and temperature. Of the last 130 years, CO2 has risen 100% of the time, but temperature have risen only during 45 of those years (1920-45 and 1978-98). That’s a very lousy correlation.

This, again, is essentially illustrating why people are concerned. Anthropogenic global warming does not imply that temperatures should increase at all times. There are other processes that can lead to changes in surface temperature. Just because there have been periods of cooling, does not mean that global warming is not happening. The important point is that the cooling periods do not result in a corresponding reduction in CO₂ concentrations (as they have in the past). Humans are causing CO₂ levels to continue to rise even when the surface temperatures are dropping. HotWhopper has an interesting post with diagrams that illustrate exactly this point. At the end of some period of cooling, the surface temperatures do not return to where they were at the beginning of the warming phase. They return to the level determined by the underlying warming trend.

This is why it’s concerning. In the past, what we might call natural variations could control the process and, in some sense, make it self-limiting. Warming would lead to rising CO₂ levels that would amplify the warming, but the start of a cooling phase would reduce CO₂ levels and consequently reduce surface temperatures. Now that we’ve removed the dependence of CO₂ concentrations on surface temperatures we run the risk of a runaway process. It may not happen. Maybe we don’t have enough fossil fuels for this to be a real issue, but there is currently no feedback in the system that can act to prevent runaway global warming (unless we choose to do so ourselves).

Pierre Gosselin finishes by saying

4. It is clear that scientists are now abandoning the high CO2 sensitivity values in droves. Naturally we don’t expect them to abandon the high values and to run right over to the low CO2 climate sensitivity values. The next logical step is that they adopt the middle sensitivity values, and act like those are going to be catastrophic as well.

Really? I appreciate that there have been recent studies suggesting that climate sensitivity might be between 1.5 and 2^oC but most predict that it is between 2 and 3^oC and there’s no evidence that any significant number of climate scientists are suddenly suggesting that it is probably much lower than previously thought.

This post has got rather long. I’m meant to be preparing a talk for a meeting next week and am clearly in procrastination mode. To summarise though

1. There is no evidence to suggest that global warming stopped 15 years ago. If anything, there is evidence suggesting it has accelerated. It’s time people stopped using temperature anomaly data alone to determine whether or not global warming is happening.

2. Yes, in the past increases in CO₂ levels have lagged increases in temperature. This does not prove that what we are currently experiencing cannot by due to the currently observed increases in CO₂ concentrations. If anything, this past data illustrates the importance of CO₂ in determining surface temperatures.

3. Scientists are not abandoning higher climate sensitivity values. Yes, there are some recent papers suggesting that it might be lower than previously thought, but these are actually consistent with most earlier studies and most studies still support a higher climate sensitivity than these newer studies.

So, I don’t think there is any evidence to suggest those who are called “warmists” are in retreat. I just think there are some who would like this to be true. As many would agree, wishing it to be true does not make it so.