Happy 35th birthday, global warming!

Although surprises may yet be in store for us when larger computers and better knowledge of cloud physics allow the next stage of modeling to be accomplished, the magnitude of the CO2 effect has probably been pinned down to within a factor of 2 to 4.

The AR4 gives the uncertainty range of climate sensitivity as 2-4.5ºC warming for CO2 doubling, so there still is about a factor of 2 uncertainty and Broecker used a value near the very low end of this uncertainty range. Modern estimates are not only based on model calculations but also on paleoclimatic and modern data; the AR4 lists 13 studies that constrain climate sensitivity in its table 9.3.

In Broecker’s paper the warming calculated with the help of climate sensitivity happens instantaneously. Today we know that the climate system responds with a time lag due to ocean thermal inertia. By neglecting this, Broecker overestimated the warming at any given time; accounting for thermal inertia would have reduced his warming estimate by about a third (see AR4 Fig. SPM.5). But again he was lucky: picking ~2ºC rather than the more likely ~3ºC climate sensitivity compensates roughly for this, so his 20th-Century warming of 0.8ºC is almost spot on (the actual estimate being closer to 0.7ºC, see Fig. above). (A modern version of this back-of-envelope warming calculation is found e.g. in our book Our Threatened Oceans, p.82.)

Natural Variability

Broecker was not the first to predict CO2-induced warming. In 1965, an expert report to US President Lyndon B. Johnson had warned: “By the year 2000, the increase in carbon dioxide will be close to 25%. This may be sufficient to produce measurable and perhaps marked changes in climate.” And in 1972, a more specific prediction similar to Broecker’s was published by the eminent atmospheric scientist J.S. Sawyer in Nature (for a history in a nutshell, see my newspaper column here).

The innovation of Broecker’s article – apart from introducing the term “global warming” – was in combining estimates of CO2 warming with natural variability. His main thesis was that a natural climatic cooling

has, over the last three decades, more than compensated for the warming effect produced by the CO2 [....] The present natural cooling will, however, bottom out during the next decade or so. Once this happens, the CO2 effect will tend to become a significant factor and by the first decade of the next century we may experience global temperatures warmer than any in the last 1000 years.

The latter turned out to be correct. The idea that the small cooling from the 1940s to 1970s is due to natural variability still cannot be ruled out, although more likely this is a smaller part of the explanation and the cooling is primarily due to the “dust” neglected by Broecker, i.e. due to the rise of anthropogenic aerosol pollution (Taylor and Penner, 1994). However, the way Broecker estimated and even predicted natural variability has not stood the test of time. He used data from the Camp Century ice core in Greenland, arguing that these “may give a picture of the natural fluctuations in global temperature over the last 1000 years”. Ironically, Broecker’s own later work on Atlantic ocean circulation changes showed that Greenland is likely even less representative of global temperature changes than most other places on Earth, it being strongly affected by variability in ocean heat transport (see our recent post on the Younger Dryas, or Broecker’s latest book The Great Ocean Conveyor). However, Broecker was right to conclude that the buildup of CO2 would sooner or later overwhelm such natural climate variations.

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