Highway 280 between San Francisco and Palo Alto is an Earth Scientist commuter's dream. The wide stretch of road bends along the San Andreas Fault (the Pacific tectonic plate out one window, the North American plate out the other), and a sunset driver can watch the marine stratus clouds spilling over the Santa Cruz Mountains like melting whipped cream.
This week, I'm out in California, visiting collaborators at the Carnegie Institution at Stanford and enjoying my twice-daily Hwy 280 meditation. My visit happens to overlap with that of Dr. Kerry Emanuel, a brilliant and eloquent climate scientist from MIT, who specializes in interactions between hurricanes and climate (he recently did an interesting Reddit Ask Me Anything).
Dr. Emanuel is a co-author of a new climate science report, What We Know, that's been getting lots of press in the last few days. The report and the broader initiative that it's part of are a push to communicate the scientific realities and risks of climate change, as well as potential societal responses, by the American Association for the Advancement of Science (the organization in charge of Science magazine).
On Tuesday, a New York Times article on the report highlighted an aspect of climate science that I mull over frequently: the uncertainty associated with climate change.
We know lots about the basics of climate science—that more human-produced carbon dioxide is increasing global temperatures and that increased global temperatures have a lot of problematic implications for delicate environmental systems—but there are lots of uncertainty bars on the things that we can predict. For example, we know that the climate warms when there's more carbon dioxide, but we don't know if the climate will get 1°C warmer or 6°C warmer if we double the amount of carbon dioxide in the atmosphere (though we do have an idea of what the relative probabilities of different values in that range are).
But what does that uncertainty mean for taking (or not taking) action to reduce climate change?
Well, that depends on what type of uncertainty you're talking about. Uncertainty in the climate system comes in all shapes and sizes and the different types may mean different things for policy.
Let's say you're trying to buy a house. Before you make the decision of which house to buy, you probably want to take into account what its resale value is likely to be, say, 20 years down the road when you might want to move. When you look at a house, the possible resale value is pretty uncertain. Some parts of that obviously can be reduced: you can get a home inspection, you can research the property values of other houses in the neighborhood, etc. But other parts can't be reduced: you can't predict if the housing market is going to crash somewhere in the next 20 years.
You'd be insane to buy a house without doing things like a home inspection, but it's completely unfeasible to wait to buy a house until you know what the housing market will be like in 20 years.
Uncertainty in climate science comes in those two flavors as well. You have reducible uncertainty (uncertainty that comes from things that we don't yet understand about the climate system), but you also have irreducible uncertainty (uncertainty that comes from natural variability in the inherently chaotic climate system and from the unpredictability of what technology choices society will make).
Forty years ago, one could certainly have made an argument that we needed to know more about the climate system before policy action on climate change would be rational. But the irreducible forms of uncertainty are rapidly overtaking the reducible forms of uncertainty as we make progress in climate science.
(Climate scientists actually think a lot about how to quantify the relative contribution to predictions from these two types of uncertainty—too much for me to attempt to summarize here. For people who want to do more digging, the most broadly cited of these studies is probably this 2009 paper, which is actually very readable, even for people outside of climate science. Or you can read the overview of that paper from the awesome climate science blog, RealClimate.org, run by a whole bunch of highly respected climate scientists.)
The New York Times article I mentioned before makes this point:
"The issue of how much to spend on lowering greenhouse gases is, in essence, a question about how much insurance we want to buy against worst-case outcomes. Scientists cannot decide that for us."
The fact of the matter is that there will always be uncertainty surrounding climate change, how bad it will be, and what its impacts will look like.
Some of that is reducible—it will decrease as we learn more and more about the climate system. But some of it is irreducible—our climate system is an inherently chaotic one and there are limitations on what can be predicted, even with perfect knowledge.
The question then becomes, how much insurance are you willing to buy?
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