Global Warming

A lot of people are solidly convinced not just that global warming is happening, but that human emission of carbon dioxide is one of the main causes, which means that the only way we can reduce or slow global warming is to take significant action to limit carbon emissions.

There is an abundance of scientific evidence for this view, and most of certainly seems convincing. But here's one thing that bothers me. A post at Stubborn Facts points to this graph, from an EPA website:


Figure 1: Fluctuations in temperature (blue) and in the atmospheric concentration of carbon dioxide (red) over the past 400,000 years as inferred from Antarctic ice-core records. The vertical red bar is the increase in atmospheric carbon dioxide levels over the past two centuries and before 2006.

As you can see if you look closely, there are several places where the lines don't exactly match up. So I did a bit more digging. Turns out that there are several scholarly articles finding that CO2 usually lags temperature changes.

One such paper is Manfred Mudelsee, "The phase relations among atmospheric CO2 content, temperature and global ice volume over the past 420 ka," Quaternary Science Reviews 20 (2001): 583-589. His finding is that over the past 420,000 years -- according to the Vostok ice core -- "CO2 variations lag behind atmospheric temperature changes in the Southern Hemisphere by 1.3 [plus or minus] 1.0 ka." ("Ka" means a thousand years.) So, temperature goes up, and CO2 goes up 300 to 2300 years later. Temperature starts to go down, and CO2 goes down 300 to 2300 years later.

Then, in another paper, several scientists found that "high-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 +/- 400 years after the warming of the last three deglaciations. Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere." H. Fischer, M. Wahlen, J. Smith, D. Mastroianni, and B. Deck, "Ice core records of atmospheric CO2 around the last three glacial terminations," Science, Mar. 12, 1999, vol. 283, no. 5408: 1712-4.

So again, CO2 increased "after" the warming, and high CO2 was maintained for thousands of years after "strongly decreasing temperatures" -- which obviously must have been caused by something other than CO2 decline.

And here's a third paper finding that over the past 750,000 years, CO2 lagged behind temperature by up to 2800 years: Siegenthaler et al., "Stable Carbon Cycle–Climate Relationship During the Late Pleistocene," Science 25 November 2005: Vol. 310. no. 5752: 1313-17:

By shifting the time scales of the entire CO2 and deuterium records between 390 and 650 kyr B.P. relative to each other, we obtained the best correlation for a lag of CO2 of 1900 years. This lag is significant considering the uncertainties of age. Over the glacial terminations V to VII, the highest correlation of CO2 and deuterium, with use of a 20-ky window for each termination, yields a lag of CO2 to deuterium of 800, 1600, and 2800 years, respectively.

Maybe these findings are untrue; maybe Antarctic ice core doesn't reflect what's happening in the rest of the planet; maybe there's something else going on here. But if these findings are accurate and relevant, then I can't figure out how a CO2 rise in year 2000 causes a temperature rise that began in year 1. Given the CO2 lag, I have to wonder if we're looking at a basic correlation/causation mistake, much like what happened for decades as to lactic acid (scientists used to be very defensive in their belief that lactic acid in muscles causes fatigue, when the opposite is true -- lactic acid is correlated with fatigue, but only because the body produces it to fuel tiring muscles).

Like the poster at Stubborn Facts, the only attempt at a response that I could find was this blog post by Jeff Severinghaus, Professor of Geosciences, Scripps Institution of Oceanography, University of California, San Diego. (I found lots of other websites that purport to address the lag issue, but they all ended up linking back to the Severinghaus post.)

Here's what he says:

What does the lag of CO2 behind temperature in ice cores tell us about global warming?

This is an issue that is often misunderstood in the public sphere and media, so it is worth spending some time to explain it and clarify it. At least three careful ice core studies have shown that CO2 starts to rise about 800 years (600-1000 years) after Antarctic temperature during glacial terminations. These terminations are pronounced warming periods that mark the ends of the ice ages that happen every 100,000 years or so. Does this prove that CO2 doesn't cause global warming? The answer is no.

The reason has to do with the fact that the warmings take about 5000 years to be complete. The lag is only 800 years. All that the lag shows is that CO2 did not cause the first 800 years of warming, out of the 5000 year trend. The other 4200 years of warming could in fact have been caused by CO2, as far as we can tell from this ice core data.

The 4200 years of warming make up about 5/6 of the total warming. So CO2 could have caused the last 5/6 of the warming, but could not have caused the first 1/6 of the warming.

The lag is "only" 800 years. CO2 "could have caused" some of the warming. But did it actually do so?

It comes as no surprise that other factors besides CO2 affect climate. Changes in the amount of summer sunshine, due to changes in the Earth's orbit around the sun that happen every 21,000 years, have long been known to affect the comings and goings of ice ages. Atlantic ocean circulation slowdowns are thought to warm Antarctica, also.

The fact that there are lots of factors besides CO2 that affect climate doesn't seem to strengthen the case that CO2 "could have caused" most of the past warming.

From studying all the available data (not just ice cores), the probable sequence of events at a termination goes something like this. Some (currently unknown) process causes Antarctica and the surrounding ocean to warm. This process also causes CO2 to start rising, about 800 years later. Then CO2 further warms the whole planet, because of its heat-trapping properties. This leads to even further CO2 release. So CO2 during ice ages should be thought of as a "feedback", much like the feedback that results from putting a microphone too near to a loudspeaker.

In other words, CO2 does not initiate the warmings, but acts as an amplifier once they are underway. From model estimates, CO2 (along with other greenhouse gases CH4 and N2O) causes about half of the full glacial-to-interglacial warming.

So CO2 "acts as an amplifier." But then, what accounts for the fact that the planet may occasionally start to cool even while CO2 is still rising? Why doesn't the CO2 just keep on amplifying, amplifying, amplifying? Severinghaus doesn't even try to explain this.

On the other hand, maybe the big picture looks like this:

1. There are lots of factors that make the earth's temperature go up and down.

2. Once the temperature goes up because of one or more of those factors, CO2 starts to increase several hundred or a thousand years later.

3. An increase in CO2 acts as an amplifier, although it obviously has to be of modest impact (given that it doesn't seem to increase the rate of historical warming, and given that subsequent temperature declines can start even while CO2 is still rising).

4. When the other factors affecting climate start to shift back towards a cooler temperature, they outweigh any effect of CO2, and several hundred years later, CO2 starts to decrease.

5. Nonetheless, in modern industrial times, there is so much CO2 being independently emitted by human activity that the amplifying effect might overwhelm the other factors that otherwise cause the earth's temperature to fluctuate up or down.

Number 5 very well could be true. That's the only logical possibility I can imagine that would account for the current link between CO2 emissions and temperature increases, in the face of past cycles where CO2 has lagged temperature both on the upswing and the downswing.

But is 5 actually true? It seems to me that one would need an account of (a) what those other factors are (whatever is in the category that Severinghaus says is "currently unknown"); (b) exactly how strong those factors are, and under what circumstances; (c) whether those factors are present now, and to what extent; and (d) why those factors are now going to be overpowered by CO2 rather than the other way around. Is there any such explanation out there? What am I missing?