Ask The Professor

A friend and climate-change skeptic, W., quizzes J., a professor of climatology, about the state of climatology. As I understand it, in the last 25 years, there has been a shift away from chemical explanations for climate change (carbon sequestration in the oceans, as affected by carbonate/bicarbonate sensitivity to temperature), and a shift towards thermohaline circulation sensitivity towards salinity. Presumably the fairly-rapid temperature changes of the Pleistocene can't be explained by deep oceanic processes, which don't change that rapidly. For other, probably more authoritative answers, J. suggests Realclimate.org.

W. writes:

Hey Marc: Got some questions about climate change / global warming. Back in 1990 I taught a night school course for teachers on Climate Change (textbook was Schneider & Londer). I've lost track with the thinking, but now I want to catch up somewhat. So here are the questions:

1.) As of 1990, climatic variation on a scale of 1-10 years was becoming understood in terms of ENSO. Variation on a scale of 10,000-100,000 years was well understood as Milankovitch cycles. But variation on time scales of 10-10,000 years was not understood much at all. There was speculation of CO2 sequestration and release in vertical ocean convections. What is the present interpretation of this time scale? Something in one of your recent emails made me think the vertical ocean circulation has been successful.

J. answers:

It depends on which period you're interested in. In the late Pleistocene, the large temperature fluctuations that occurred were probably caused because of an instability in the Laurentide Ice Sheet (the big one that covered most of Canada). Basically the idea here is that influxes of freshwater into the North Atlantic and the subsequent reduction in the salinity of the surface waters caused a weakening in the North Atlantic thermohaline circulation, thus reducing heat transport into the high latitudes of the Northern Hemisphere. (One example: the Younger Dryas Event, which may have been caused by a sudden re-direction of water from glacial Lake Agassiz.

In the Holocene, the amplitude of temperature changes has been much smaller; various possible mechanisms have been proposed as causes: changes in solar energy flux, increased albedo due to volcanic eruptions, and "internal variability" of the climate system, e.g., things like ENSO, PDO, etc. Nothing definitive, though, as far as I can tell. (Reconstructions of solar-energy flux are very dubious; they are essentially extrapolations based on a rather meager dataset -- 25 years of satellite measurements.)

2.) In 1990, the big unknown in climatic modelling was albedo change due to cloud cover -- will warming result in positive or negative feedback? You seemed to say that this was still an unknown. Is this correct? If so, how much confidence do you have in all the models which predict significant warming over the next century?

J. answers:

There are two main sources of uncertainty: (1) future anthropogenic emissions of greenhouse gases and (2) the effect of cloud feedbacks. For a given emissions scenario, it is still true that the biggest source of uncertainty is the cloud feedback. The latest model intercomparison study that I have seen still shows that the feedback is positive in some and negative in others.

"Confidence"? Hard to say. There is such a range in model sensitivities and of emissions scenarios that it seems likely that the future warming is encompassed by the projections that have been made. The lowest projected temperature rise, made using the least sensitive model (one in which the cloud feedback is strongly negative) and the lowest credible emissions scenario (in which emissions in 2100 are actually lower than 1980 values) is 1.5 degrees C.

3.) In 1990, when talking with believers in anthropogenic warming who pointed to demonstrated warming since 1850 (correlating with the Industrial Revolution), I would counter by saying one could interpret the present warming regime as beginning around 1700, when the Little Ice Age began to let up. In that case, the warming would predate the Industrial Revolution, and would therefore require a different (or at least an additional) cause. Nobody ever satisfactorily rebutted my argument at that time. How would you answer that today?

J. answers:

What you're really talking about here is how the warming of the past century or so compares with natural variability. Climate models that are given constant forcing (e.g., constant solar flux) exhibit a century-scale variability of only a few tenths of a degree C, considerably less than the 0.6 deg. warming that has been observed. In addtion, temperature reconstructions using proxy climate data (e.g., Mann, Bradley, and Hughes) indicate that temperature increase of the 20th century is much greater than the variability over the period 1000-1900. (This study has stood up well to recent attacks, e.g., by McIntyre and McKitrick; I recommend the the discussion at Realclimate.org for more details.)

4.) In 1990 I was not aware of any good theories for the Little Ice Age and the Medieval Maximum. Are there explanations now?

J. answers:

I go back to the reconstructions mentioned in #3, which indicate that the amplitude of global mean temperatures on time scales of centuries was probably only a few tenths of a degree prior to 1900; this is considerably less than some early estimates that were based on much sparser data. So, statements that indicate that the global mean temperature in the middle ages was a degree or more than today's temperature would appear to be unfounded. It seems to me that the "Little Ice Age" is essentially a period in which the Earth was somewhat colder than today. It may be that part of the warming since then (e.g., in the early 20th century) was due to natural causes, but when all of the data and model results are taken together, the preponderance of evidence supports the thesis that most of the warming has been caused by the observed increase in anthropogenic greenhouse gases. This is especially true of the last several decades; there has been a 45 ppm increase in CO2 concentrations since 1975 and an increase in global surface temperature of nearly half a degree Celsius during that time. (These are essentially the only climate-related variables that have shown a large increase over that time.)