Hiroshimas, light bulbs, and touchstone facts

There’s a rough consensus that the heat gain attributable to man-made climate change is equivalent to about one watt per square meter. How can we visualize that? You could say it’s like we’ve added one always-on 100-watt light bulb to every ten-meter-square piece of the planet’s surface. Or you could say that we’re adding the heat equivalent of 400,000 Hiroshima bombs per day.

The Hirosohima meme is fashionable in certain circles. You can even use a blog widget or Facebook app to dramatize the effect. Is that helpful?

Yes, according to Joe Romm:

In my quarter century communicating on climate change, I’ve found that many people in the media and the public have a visceral belief that “Humans are too insignificant to affect global climate.”

The anti-science CNBC anchor Joe Kernen voiced this conviction when he suggested that “as old as the planet is” there is no way “puny, gnawing little humans” could change the climate in “70 years.”

Certainly humans do seem tiny compared to the oceans or even a superstorm like Sandy. So I don’t see anything wrong with trying to find a quantitatively accurate metaphor that puts things in perspective.

Yes, but not without context. Suppose I told you the effect was an order of magnitude smaller: 40,000 bombs per day. Or an order of magnitude larger: 4 million bombs per day. Do you have any intutions about those numbers? I don’t. And unless you’re a scientist working in this domain you don’t either.

The missing context, in this case, is the amount of solar power reaching Earth’s surface. It’s about 175 watts per square meter (1, 2). That’s a lot of Hiroshimas. But let’s focus on our representative square, ten meters on a side. That’s 100 sqare meters, roughly the footprint of an average house in Spain. How many 100-watt bulbs are we talking about?

175 W/m^2 * 100 m^2 = 17,500W

17,500W / 100W/bulb = 175 bulbs

So the baseline for our representative square is 175 bulbs. If we add one more bulb, we increase the wattage by about half of one percent. Some will intuit that the extra 175th is significant. I do. We’re adding a measurable fraction of Earth’s insolation? Whoa.

Others will intuit that it’s neglible. But will this formulation at least enable us to discuss the effect in a way that everyone can meaningfully visualize? Maybe not. Because it depends on an intuition that varying a global parameter by half a percent is a big deal. Which is like having an intution that varying the planet’s temperature by a degree or two is a big deal. Some will have it, others won’t.

I can’t imagine preventing 400,000 Hiroshimas. I would rather think about turning off every 176th light bulb. But I can’t imagine turning off 5 trillion light bulbs either. So maybe Joe Romm is right. If both visualizations are valid, and if the goal is to communicate the underlying intuition, then I suppose unfathomably many bombs says that more compellingly, to most people, than half a percent of the solar flux at Earth’s surface.

And yet: half a percent of the solar flux? Whoa. That’s a pretty useful touchstone fact.

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