It might seem like a fool’s errand to fact check Al Gore. But the man does have a big audience and someone’s got to do it so we rush in. For the next few weeks we are going to entertain ourselves, and we hope you as well, by going through the rant he delivered on stage at the 2023 World Economic Forum, beginning today with this claim, referring to the effect on the climate of the accumulated amount of carbon dioxide released up to now: “And the accumulated amount is now trapping as much extra heat as would be released by six hundred thousand Hiroshima-class atomic bombs exploding every single day on the Earth.” Is 600,000 Little Boy bombs a lot? Sure, if they were actual atomic bombs going off. But as units of energy in the climate system, no.
Of course even a single Hiroshima-class bomb sounds like a lot, because it was concentrated in one place and the combination of the devastation caused by the blast and the radiation that followed killed an estimated 140,000 people. And because it conjures up images of a massive exchange of nuclear weapons. 600,000 such events would certainly wipe out most life on Earth (even if they only involved 1945-style uranium or plutonium fission bombs dropped on Hiroshima and Nagasaki respectively rather than modern hydrogen fusion ones). Which right away tells you that something is wrong with Gore’s statement since, um, notwithstanding all that energy being detonated over our heads life on Earth keeps getting better.
How much energy would be released by 600,000 Hiroshima bombs? According to Wikipedia the bomb dropped on Hiroshima released 63 terajoules (TJ) of energy. (A joule being the work required to generate one watt of power for one second.) So 600,000 such bombs would contain 37.8 million TJ, which is 37.8 Exajoules. That’s the amount of heat Gore claims is trapped every day by the CO2 humans have already released, and it sounds like a lot especially if you know that “exa” is a prefix meaning “times 10 to the 18th power”. But now let’s figure out the sun’s energy because that big hot thing in the sky is in fact very big and hot.
The standard estimate is that the sun sends 1,361 watts per square meter every second to the Earth, which when spread out over a disc with the same diameter as the Earth equals about 173,000 Terawatt-hours or 14,947.2 Exajoules in a day. Which is a lot of Exajoules.
How many? Glad you asked. It turns out that 37.8 divided by 14,947.2 is about 0.25 percent. So that scary-sounding 600,000 Hiroshima bombs works out to the same amount of energy as the sun being just under 0.25 percent brighter. Or about 3.4 watts per square meter. And mentioning Hiroshima in the same breath as climate change turns out to be on the same level of lurid emotional misrepresentation as using “denier” to link climate skeptics to the Holocaust.
Actually Gore’s numbers are skewed a bit high. The latest IPCC report (see here, p. 69) says all the greenhouse gases ever emitted amounted to an increase of energy in the climate system of about 2.7 watts per square meter, not 3.4. Since the solar constant is 1,361 Watts per square meter, 2.7 divided by 1,361 is 0.2 percent. So Al Gore’s estimate in terms of Hiroshima bombs is exaggerated compared to the IPCC’s in terms of energy, as well as utterly over the top in terms of rhetoric.
By Gore’s math or the IPCC’s, it works out to a very small amount of energy, on the order of 2 tenths of one percent. As for expressing it in terms of hundreds of thousands of nuclear bombs, which would annihilate humanity, the fact is that all the greenhouse gases ever emitted by humans have accompanied vast improvements in human well-being, from the taming of fire for cooking to the industrial revolution and the automobile. So even if Al Gore’s numbers weren’t exaggerated, his metaphor was explosive rubbish.