Not So Sure About Global Warming? Some Thoughts On Scientific Uncertainty
Thu Jul 26, 2007 at 12:14:39 PM PDT
I've been reading a great book called Uncertain Science... Uncertain World by Henry Pollack - a readable and engaging discussion of decision-making in the face of uncertainty. Pollack argues that decision-makers use uncertainty as an excuse for inaction, when in fact it should be a stimulus for creativity and progress.
This is a huge, and hugely important, issue. What is scientific uncertainty? Why are scientists so cautious? How sure do you have to be that something will happen to act on the possibility? And what does this all mean for climate change?
We don't give it much thought, but we take action in the face of uncertainty all the time. We're not sure we'll get into a car accident, but there's a small chance we will so we wear our seat belts. And though we may never need it, we buy major medical insurance. We act on these possibilities, though they're unlikely, because the consequences, if they occur, are severe.
Yet scientists are held to a different standard. Some people feel that if scientists aren't 100 percent certain about something, there's no need to take action - even when there is a 90 percent chance that inaction will lead to catastrophe. Why?
My colleague Bill Chameides used a great analogy to explain this in a Webcast for teachers. (The section on scientific uncertainty begins about 3 minutes into the video. Side note: the full video is a great intro to climate change science.) He says that some people think of scientific knowledge as a house of cards - if one piece is taken away, the whole house comes down. That is, one uncertainty about climate change means none of the science can be trusted.
In fact, scientific knowledge is more like a jigsaw puzzle than a house of cards - a jigsaw puzzle where you don't have the box top. As you fit the pieces together, a picture begins to emerge. You may not know all the details, but enough pieces are in place to leave no doubt what the big picture is.
In a recent essay [PDF], Henry Pollack made another analogy:
There is a tendency to focus on the weakness of the parts rather than the strength of the whole, supposing that if a single piece of evidence can be discredited, the entire construct will fall like a house of cards. In fact, discrediting a single line of evidence is more like snipping a strand in a net hammock—the hammock continues to be supported by the many strands that remain intact. The scientific evidence for climate change in the natural world is compelling in its totality although individual pieces of the story may indeed be open to some question.
That's where we are now with climate change. The big picture is clear, the hammock is up. The debate is only about individual strands and pieces here and there. Just because scientists don't know everything about a particular topic doesn't mean they don't know anything about it.
In our everyday lives we act based on incomplete information as a matter of course - to the point that we don't even think about it. But for scientists, uncertainty is top of mind. We're trained to notice what's certain and what's not so we can design useful studies. That's why we spell out in such exquisite (or excruciating!) detail what we don't know about any given topic. And that's why the IPCC report (indeed, any scientific report) uses such careful language. For non-specialists, all that careful language can obscure aspects of a topic that are extremely well understood and no longer under debate.
Unlike a car accident, which is serious but unlikely, global warming is both serious and likely. In fact, the clear picture that has emerged from decades of research is that it's already happening. We must act now to reduce greenhouse gas emissions. The challenge facing us can seem overwhelming, but we can do it.
So the next time someone tells you that global warming isn't a certainty so there's no reason to act, ask them what the probability of an accident has to be before they'll wear a seat belt.
- Obviously, Pollack's book and essay [PDF]
- Stephen Schneider's discussions of uncertainty
- Various posts on RealClimate. Most of them are technical (ice sheets, aerosols, etc). A fine example of the philosophical aspect is their post on climate change ethics, particularly the bit about Henry Shue's work