Carbon dioxide & 800-year lag behind temperature

How alarmists think

As we have explained in 2006, Vostok ice core records show that the carbon dioxide concentration averaged over a few centuries has been correlated with temperature at least for half a million of years. However, we know for sure that the temperature was the cause and the CO2 concentration was its consequence, not the other way around. It follows that the greenhouse effect hasn't been important in the last half a million of years.

There are many ways to see it. The 800-year lag is the most popular one, it has been featured in the Global Warming Swindle (especially in this two-minute-long segment), and we will discuss it below. However, there are other ways to see that the influence of temperature on the concentration of gases has been more important than any influence in the opposite direction. For example, the ice core records show that the concentration of methane was correlated with temperature, too. If the CO2 concentration were the primary cause, we would have no explanation why the CH4 concentration was also correlated. In fact, CO2 and CH4 play the very same role in the ice core records. If some combination of them determined the temperature, we would still have no explanation why these two concentrations were correlated with one another.

Moreover, easy reasoning can be used to show that the ability of oceans to store gases decreases with increasing temperature and this effect is clearly much stronger than the greenhouse effect.

The 800-year lag

However, the most popular - and the most straightforward - explanation of the direction of the causal relationship is the fact that in all cases, the CO2 concentration only changed its trend roughly 800 years after temperature had done the same thing. There have been many papers that showed this fact and incidentally, no one seems to disagree with it. For example, a recent paper by Lowell Stott et al. in Science (2007) showed that 19,000 ago, when the last ice age started to go away, CO2 lagged by about 1,000 years, too.

Every sane person knows that this detailed insight implies that the greenhouse effect couldn't have been among the most important effects. Not only the ice core data fails to provide us with evidence supporting the greenhouse theory of the climate; it provides us with strong evidence against it.

The greenhouse effect has been much less important than outgassing. Although we add more CO2 than what Earth has seen for millions of years, the small characteristic importance of the greenhouse effect probably wins.

For whatever reason, some people are not willing to accept this obvious conclusion. That's why they invent various bizarre verbal constructs to circumvent the otherwise inevitable conclusion. The whole "group" at RealClimate.ORG has agreed that there was a lag. But they say that in the first 800 years when the influence of temperature on CO2 is manifest, it was indeed temperature that drove the gases. But in the remaining 4200 years of the trend, it was surely the other way around: CO2 escalated the warming, they say.

Everyone who has basic understanding of feedback theory knows that what they talk about is a textbook example of a positive-feedback system, and if the climate were such a system, the mutually supportive interactions would lead to exponentially escalating temperatures in one of the possible directions. That's clearly not observed in the data and the positive-feedback hypothesis is thus falsified.

Hi stet,

You wrote, “It [greenhouse warming] was a hypothesis at the time Arrhenius, a Nobel prize winner, postulated it more than a century ago.”

Correct, stet. It was a hypothesis then. It remains a hypothesis today, despite the many studies that have been made by many people since then.

It will become more than a hypothesis when causation (not simple correlation over a relatively short time period) becomes validated with physically observed data (not GCM outputs).

I am not saying that this hypothesis is incorrect. I am not saying that it is not possible that CO2 (plus other natural and manmade GHGs) have caused some warming of the climate, along with other factors that we do not yet fully understand.

I am just pointing out that the only period where an obvious correlation exists is the period of rapid warming and rapid increase in atmospheric CO2 starting in 1976.

To quote from IPCC AR4 Chapter 3 (p.240): "The 1976 divide is the date of a widely acknowledged 'climate shift' (e.g., Trenberth, 1990) and seems to mark a time (see Chapter 9) when global mean temperatures began a discernable upward trend that has been at least partly attributed to increases in greenhouse gas concentrations in the atmosphere."

Other periods do not fit the hypothesis that well. IPCC does not even spend much time discussing other warming periods, preferring instead to concentrate on the “late 20th century warming” starting around 1976. In fact, IPCC goes on to say, "The picture prior to 1976 has essentially not changed and is therefore not repeated in detail here." Makes sense for them to concentrate on post 1976 for obvious reasons.

In their discussion (Chapter 9, p.681) IPCC states that: "The [model] simulations also show that it is not possible to reporoduce the large 20th-century warming without anthropogenic forcing regardless of which solar or volcanic forcing reconstruction is used." And (pp.685,686): "Climate simulations are consistent in showing that the global mean warming observed since 1970 can only ne reproduced when models are forced with combinations of external forcings that include anthropogenic forcings." "No climate model using natural forcings alone has reproduced the observed global warming trend in the second half of the 20th century."

IPCC makes no mention at all of the late 19th-century warming period but they do mention the early 20th century warming period briefly in Chapter 9 (p.691): "Detection and attribution as well as modelling studies indicate more uncertainty regarding the causes of early 20th-century warming."

So the logic goes something like this:

1. Our models cannot explain what caused the early 20th century warming

2. We know that CO2 caused the late 20th century warming.

3. How do we know this?

4. Because our models cannot explain it any other way.

Regards,

Max