I told you so

In 1981 Hansen et al made predictions for the change in global mean temperature expected over the course of the coming century. The figure shows their predictions along side 4 independent estimates of what has actually happened.

In 1981 Hansen et al made predictions for the change in global mean temperature expected over the course of the coming century. The figure shows their predictions along side four independent estimates of what has actually happened.

According to Gore Vidal, the four most beautiful words in the English language are “I told you so”. My hero James Hansen can justifiably speak those words, but I am sure they don’t feel beautiful to him.

In 1981, together with six NASA colleagues, he published a paper in Science magazine entitled ‘Climatic Impact of Increasing Atmospheric Carbon dioxide‘. Science magazine won’t let you read it but it is available online here. The paper is not that difficult to understand and if you are curious about these things, it’s a good read. I particularly liked the inclusion of a simple analogy:

“The surface temperature resulting from the greenhouse effect is analogous to the depth of water in a leaky bucket with constant inflow rate. If the holes in the bucket are reduced slightly in size the water depth and water pressure will increase until the flow rate out of the holes once again equals the inflow rate. Analogously, if the atmospheric infrared opacity  increases, the temperature of the surface and the atmosphere will increase until the emission of radiation from the planet again equals the absorbed solar energy.”

The figure at the top of the page shows Figure 6 from their paper on which I have overlaid four independent estimates of what has actually happened since then. At the time the paper was published,  global mean temperature was declining and the predictions were thus extremely bold. However, looking back the authors predictions now seem conservative. And indeed the authors were careful and conservative, though clear about specific predictions.

In the summary they state

“Potential effects on the climate in the 21st Century include the creation of drought-prone regions in North America and central Asia … erosion of the West Antarctic Ice Sheet … and an opening of the fabled NorthWest passage”

Well, North America has been prone to drought, and the North West passage now regularly opens in summer. Thankfully the West Antarctic Ice Sheet seems relatively stable.

All through the paper the authors consider the uncertainties arising from the simplicity of their model and the many poorly-understood effects – such as cloud cover and solar variability – which affect climate. However, they test their predictions against plausible variations in these factors and find that the predictions of warming are robust against a wide range of plausible feedback effects. They conclude with a wider non-scientific perspective

Political and economic forces affecting energy use and fuel choice make it unlikely that the CO2 issue will have a major impact on energy policies until convincing observations of global warming at in hand. In light of historical evidence that it takes several decades to complete a major change in fuel use this makes large climate change almost inevitable.

However the degree of warming will depend strongly on the energy growth rate and the choice of fuels for the next century. Thus CO2 effects on climate may make full exploitation of coal resources undesirable. An appropriate strategy may be to encourage energy conservation and develop alternative energy sources while using fossil fuels as necessary during the next few decades.

The Climate change induced by anthropogenic release of CO2 is likely to be the most fascinating global geophysical experiment that man will ever conduct. The scientific task is to help determine the nature of future climatic effects as early as possible. The required efforts in global observations and climate analyses are challenging, but the benefits from improved understanding of climate will surely warrant the work involved.

To me these views seem modest, realistic and optimistic. But I bet that although James Hansen and his colleagues predicted the climate 30 years ahead, they never guessed that in the 21st Century the US would have senators such as Paul Brown.

To understand such ignorance we have to turn again to Gore Vidal:

The United States was founded by the brightest people in the country – and we haven’t seen them since.

Acknowledgement: This article is based on a blog story at Real Climate:


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One Response to “I told you so”

  1. The IPCC 5th Assessment Report: The authors speak | Protons for Breakfast Blog Says:

    […] Predicting the future climate of Earth is almost as hard as predicting the weather, and in fact predictions from the 1980s are shaping up pretty well. […]

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