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• Wednesday, July 2, 2008 03:10 PM

  Greenhouse warming

  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

  —manacker Read manacker's other letters
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