The Hiro is an useful energy unit when discussing global warming, and I've certainly used it in the past, but a recent mailing list post I read has convinced me it's unnecessarily alarmist, and probably insensitive too.
It's useful because an energy imbalance of 1 W/m2 over the Earth's entire surface = 8.1 Hiros/second -- the yield of the Hiroshima bomb being 63 terajoules (TJ), or 15 kilotons of TNT.
But a couple of months ago, Alan Robock, who's done a lot of work on nuclear winter scenarios, wrote on a geoengineering mailing list:
I think this is a terrible comparison to make and should not be used. I heard someone this summer who was trained by Al Gore use this and she did not distinguish between all the effects of atomic bombs and the energy equivalent of the atomic explosions.He convinced me: the "Hiro" is inappropriate -- a scare tactic. And a false equivalence that neither captures the essence of global warming (since the energy is spread over the entire planet and not just a city), nor captures or respects the tragedy of the bombs dropped on Japan.
Equating nuclear war and global warming is a false comparison. Nuclear war is a much worse fate for our planet, and is a scare tactic. The number is correct (I did the calculation myself when I first heard of this), but trapping energy has completely different effects from dropping nuclear bombs on people. Using this is distracting, sensational, and easy to refute if the speaker does not carefully explain that it is the energy released by the bombs, with no other effects including radioactivity, fires, and blast.
I told John Cook this at the AGU Chapman Conference on Climate Communication this summer, and he said he would consider this recommendation, but I don't know if he plans to continue to feature this idea on Skeptical Science.
But see http://thebulletin.org/how-many-hiroshimas-does-it-take-describe-climate-change for another point of view.
As someone who in his late twenties often had nightmares about nuclear war and woke up screaming (much to the fright of a few girlfriends), and who when younger thought there would certainly be a global nuclear war in my lifetime, I'm not going to use the "Hiro" any more.
So what is a better unit? I asked Alan via the list, and he responded:
I think an array of incandescent light bulbs around the world would work much better. If you have 100 W bulbs, and want to model an imbalance at the surface of 1 W/m2, then you will need one light bulb every 100 m2, multiplied by 1 over the fraction of energy emitted by each bulb that is heat rather than light. Light bulbs in an array separated by 5-10 m on a side would not perhaps seem dramatic, but if you would zoom out and see them everywhere on Earth, it could be.It isn't as dramatic, which underscores the point: the "Hiro" is used for its incendiary nature, purposely trying to elicit fear. While global warming is, in some sense, scary, it's sloooooooow long-term scary, unfolding over decades, centuries, and millennia, and not wake-up-screaming scary.
Another option is to compare it to sunlight. At the top of its atmosphere, the Earth receives 1,376.6 W/m2 from the Sun, or 2,768 Hiros/second (as the Earth's intercepting area is only πR2). So global warming's 4 Hiros/second is equivalent to 0.1% more sunlight at the TOA.
Or, an energy imbalance of 4 Hiros/s is, over an entire year, an extra 8 billion terajoules, or 13 extra hours of sunlight at the TOA. That sounds impressive, but not in the way something terrifying is impressive.
Or, 4 Hiros/s = 252 terawatts, or about 14 times the power consumption of the entire Earth.