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GARY WITT: Hello, I'm Dr. Gary Witt. I want to talk to you about James Hansen and his climate predictions from 1988. Learning objectives-- to explain the circumstances around Hansen's 1988 model, the reaction to it at the time and later, and more deeply, to appreciate the difficulty of a prediction like this but look at a detailed example of a successful prediction.

So who is James Hansen? Hansen started out as a physicist, an astronomer. He studied under Dr. Van Allen, who discovered the Van Allen radiation belts around the Earth in the 1960s at Iowa. And then he was hired by NASA to look at the data coming in from probes being sent to Venus in the late '60s and '70s. And his specialty was planetary atmospheres. So he was studying the data coming in from Venus. And what he and his colleagues realized is that Venus is a lot hotter than it should be given its distance from the sun because it has a very thick atmosphere of carbon dioxide.

Well, during this time period, CO2 was being measured more carefully and being discussed a lot more here on planet Earth. And Hansen decided, probably against the advice of a lot of his colleagues, that he wanted to switch from studying Venus to studying the Earth because CO2 was increasing on the Earth-- still very, very low level, of course, nothing like Venus. But he was concerned about what is this going to do to the atmosphere of the Earth as CO2 increases.

So he began focusing on the Earth's atmosphere itself. And in 1981, he published his first paper on how increasing CO2 would affect the temperature of the Earth. And this was in the nature pretty much of a prediction in '81. And it turns out his predictions subsequently weren't that different from the one in '81. And the one in '81, as a prediction, was pretty close.

But by the end of the 1980s, as temperature records are coming in, the '80s turned out to be quite warm. He was asked to testify before the US Senate. And he made some bold predictions. So first of all, Hansen said that the Earth is warmer in 1988 than any other time in history of measured temperature but also that this warming is caused by human actions, by the accumulating greenhouse gases in the atmosphere.

But he also said that the greenhouse effect is significant enough to be causing some of the extreme temperature variations that we've seen on the Earth already. So this was not a prediction. This is about he's looking at the data that they have about the Earth's climate and saying that it's already being affected by the accumulating greenhouse gases.

So to summarize what Hansen said, he was claiming that the signal from climate change had risen above the noise of natural climate variation. The United Nations formed the Intergovernmental Panel on Climate Change to study and monitor climate change. At the time of Hanson's prediction, they offered their own assessment of the climate.

So this is Andrew Dressler, a climate scientist at Texas A&M, looking back at Hanson's prediction. And he said that at the time, Hanson was going out on a limb. And you can see this from this is the text here is from the IPCC. And they were saying just a year or two after Hansen's testimony, that the warming that you see is of the same magnitude as natural climate variability.

So basically, as a statistics student, you can look at this, it's like a hypothesis test, right? The null hypothesis is always going to be the status quo. The null hypothesis will always be that the climate hasn't changed, that natural variability is just occurring. The alternative, the research hypothesis, the bold prediction that Hansen's making is that CO2 is causing the temperature to increase.

So the data at that time, it was still the signal wasn't strong enough that everybody could agree that the evidence is overwhelming enough to reject the null hypothesis in favor of the alternative and to say that we have a statistically significant increase in temperature that couldn't just be due to natural variability.

But Hanson stuck his neck out and he felt sure that because he had studied the data more carefully and more deeply that this was evidence of climate change. And, of course, the rubber meets the road. And you know what happened since then. Who was right? Recently, in the last five years, this was 30 years after the fact of Hansen's predictions, other climate scientists opined on how he did.

Here on the left is Michael Mann, well-known climate scientist at the University of Pennsylvania, saying that Hansen was remarkably prescient back then. He said that Hansen basically predicted the warming that we've seen three decades ahead of schedule. And Rahmstorf, whose real specialty is oceanography, but he's generally a climate scientist at the Potsdam Institute in Berlin. He's just saying that the signal of climate change has risen way above the noise, and that what we're seeing is climate change, a global warming due to increased CO2.

It's important to understand that Hansen also, like I said, in his '81 paper, and then he reiterated in '88 testimony, that he actually made specific predictions about how temperature was going to evolve in the future. And he did it with three scenarios-- model A, B, and C.

So model B is what is usually referred to as business as usual, so carbon dioxide output increasing from the '80s but not radically increasing. Model A was a large increase in CO2 emissions. And model C would have been lower CO2 emissions had the people of Earth decided that oh, this is a huge problem and we've got to drastically reduce our burning of fossil fuels. Obviously, that didn't happen. What happened was more like model B.

So these three models reflect choices that could have been made sort of economically or socially by people. Hansen's model, given the amount of CO2, says that the trend in temperature is clear. There will be natural variation around the trend, but it's the CO2 that defines the trend. And if you watch our later videos, you'll see that.

So model B reflects business as usual, which is close to what we've had today. And if you take a look at his model B prediction here in red, this is going forward up to today from when he issued it. So Hansen's report is his prediction is in red. The other squiggly lines here from NASA and NOAA and Berkeley, those are the actual temperature records. So these are three different institutions that have kept up with what happened to the temperature over this time period.

So the red line is Hansen's prediction. The other squiggly lines are the actual temperature record. And then this pink simple regression linear model that's fitted through it is just the trend in the actual temperature. And it's going upward because the carbon dioxide output has gone upward.

So what we did in this little video was try to explain to you, James Hansen's model and especially appreciate how hard it is to step out on a limb. And we could see that with the reaction of the other climate scientists. They were really bowled over, that he did it so well because successful predictions about the future are extremely difficult. Thanks for watching this. I hope you'll have a chance to watch the other two-part videos that go into this in more detail with a multiple regression example.

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