Thursday, July 13, 2006

Polling climate change: how do we know that climate change is of human origin?

Pew just released a large survey on global warming, and the results are mixed. A full 70% of respondents think the Earth is warming (it is, no one is even arguing against that any more), and even Republicans generally agree (though only 58%). When asked about causes, less than 2/3 of people who agree that there is a warming attribute it to human activities. Compared to the overwhelming scientific consensus that humans are causing most of the warming, that's unfortunate.

And that ambiguity has consequences. People who said that the warming was caused by humans were more likely to think it was a serious problem and that something could and should be done about it. Of those who think the warming is caused by human activities, 81% think something can be done. Among those who don't think it's anthropogenic, only 48% think anything can be done.
Of those who think something can be done, twice as many think it will take major sacrifices to solve the problem as think technology can solve the problem. Myself, I think technology combined with small sacrifices and neutral changes in behavior can solve the problem.

It's encouraging that less than a quarter of the population doesn't think the warming is a problem (meaning that some people who don't think it's happening still think it's either somewhat or very serious). It ranks low in people's list of priorities, ranking above only gay marriage on average. Democrats and Independents agreed that it was more important than that, but not by much.

Because it's cropping up here and there, it's worth explaining how we know the recent warming is of human origin. The first reason that I, a non-climatologist, think humans are behind the warming is that various climatologists agree it is. All the major scientific societies say so, the (UN based) Intergovernmental Panel on Climate Change pulled the experts together and they said so, and they know the literature. But that smacks of an appeal to authority, so let's move forward.

Naomi Oreskes decided to evade the filter of official bodies and their statements by surveying the literature on climate change. She conducted a search for scientific papers published between 1993 and 2003 that used the term "global climate change." She took that sample of 928 papers from the literature, and classified each according to how it stood relative to the IPCC's statement that "Human activities ... are modifying the concentration of atmospheric constituents ... that absorb or scatter radiant energy. ... [M]ost of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations." Of the 928 papers, 75% explicitly or implicitly backed that consensus. A quarter dealt with methods or with paleoclimate, and took no position relative to climate change in the last 50 years. And exactly 0 papers in her sample dissented from the consensus that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations." Of course, as she explained: "The scientific consensus might, of course, be wrong. If the history of science teaches anything, it is humility, and no one can be faulted for failing to act on what is not known." In a letter, she also pointed out specifically that she only sampled the literature, and dissenting views may have been published, but "the existing scientific dissent has been greatly exaggerated."

This is not an appeal to authority, it's an appeal to expertise. While selective bodies like the IPCC might have a selection bias in the process by which someone becomes a contributor, no such bias exists for the scientific literature beyond the knowledge and competence necessary to do original research. As a non-expert advising non-experts, I'd say that surveys like this are the best way to assess the status of a scientific theory. If there's a raging debate in the literature, consider it uncertain, if no dissenting views are showing up, assume that there is no scientifically credible dissent.

Why is this consensus so strong?

The first reason gets to very simple principles. We know that carbon dioxide traps heat. It absorbs infrared light that should radiate into space, trapping heat which in turn warms the atmosphere. We know this happens on Earth, on Mars, on Venus. No one disputes it, and we've known it for centuries. We also know that carbon dioxide concentrations are rising. A measuring center at Mauna Loa, high above the ground and far from industry that might have modified the local atmosphere, has measured concentrations since 1958, and shows clearly that the concentration in the well-mixed atmosphere has been rising sharply. We can measure air pockets trapped by falling snow in glaciers and extend that trend back in time for centuries, and we find that the modern increase began around the time the steam engine came into regular use, and has proceeded at an unprecedented rate and to unprecedented concentrations.

So we know that there's a lot more of a gas that we know causes warming. It would be surprising if that didn't increase temperatures, and we do indeed know that temperatures have been rising. Where did all that carbon dioxide come from? Since we know we've been burning fuels that produce the gas for 200 years, it would again be surprising if the increase didn't result from human activities. Where did all that excess smoke go, if not into the atmosphere?

We can test that, too. The gas is trapped in various forms, and changing concentrations could result from various natural processes. The trick is that there are two kinds of carbon: carbon-12 and carbon-13 (carbon-14 also exists, but it's too rare for us to worry about here). Carbon-13 is heavier because it has an extra neutron. Plants tend to use the lighter carbon in their growth, so plant matter has more carbon-12 than the atmosphere. Fossil fuels are remains of ancient plants, so they also have more of the lighter carbon, and when you burn them, you add carbon-12 based carbon dioxide to the atmosphere.

That lighter carbon then gets incorporated into trees, and by looking at the ratio of heavy and light carbon in tree rings, we can trace the rise of anthropogenic carbon dioxide beginning about 150 years ago. And by looking at the layers of the ocean we can see that the carbon dioxide absorbed more recently (at the surface) is lighter than carbon dioxide absorbed long ago. We can also measure carbon ratios of air trapped in glacial ice, and all three sources show that light carbon started getting more common about 1850, around when the Industrial Revolution started kicking in. There are other handy ways of figuring this out, too.

Any explanation for the recent increase in temperature that doesn't address the 30% bump in carbon dioxide from human sources simply isn't credible. One must either explain how carbon dioxide doesn't actually trap heat, undermining the basic physics of the thing, or invent a massive climate driver as well has huge feedbacks that would erase the effects of rising carbon dioxide. Plus you'd have to explain why every prediction you'd make if carbon dioxide really were the culprit comes out right.

That last part didn't always seem to be quite right. There were satellite measurements of the lowest parts of the atmosphere which didn't seem to match predictions of models of atmospheric warming. For years, people denied the validity of climate models and the predictions they make by claiming that the models failed to predict what the satellites were measuring. Scientists scrutinized the models and the data from the satellites, and realized that there were errors in the way they were analyzing the satellite data. The models were giving the right results.

Some people think that solar variability could be driving climate change. That's a fine hypothesis, and one that's falsified when you see that solar activity has no trend over the last 50 years. Some of the change may well be driven by solar changes, but not the recent warming. Nor could this solar effect explain what all that extra carbon dioxide does in the atmosphere. Again, any hypothesis that doesn't account for the heat that the added carbon dioxide must be trapping cannot be credible.

That most of the recent warming is of human origin is not in serious debate by the people who study it. That could well change with new discoveries, but we should base our decisions and our views on the phenomenon on data, not hope. And the data show that we are driving this warming.