Does global warming trap as much heat as 600,000 Hiroshima-class bombs every day?

Question

In January 2023, in an interview at the World Economic Forum, former US Vice President Al Gore claimed that we're adding the equivalent of 600,000 Hiroshima bombs every day in heat to the atmosphere:

We're still putting 162 million tons into it [the atmosphere] every single day, and the accumulated amount is now trapping as much extra heat as would be released by 600,000 Hiroshima-class atomic bombs exploding every single day on the earth. That's what's boiling the oceans, creating these atmospheric rivers and rain bombs, and sucking the moisture out of the land and creating the droughts and melting the ice and raising the sea level and causing these waves of climate refugees, predicted to reach one billion in this century.

He claims that the amount of thermal energy trapped each day is equivalent to 600,000 Hiroshima bombs, which appears to be an exaggeration for the purpose of making a point. Is that true?

Answer 1

This article from 2016 attributes Al Gore's claim (with a figure of 400,000 Hiroshima bombs per day which Gore was citing at the time) to a 2012 TED talk by James Hansen, a climate researcher who was at the time the director of the NASA Goddard Institute for Space Studies in New York City.

Here is a relevant quote from Hansen's talk:

The total energy imbalance now is about six-tenths of a watt per square meter. That may not sound like much, but when added up over the whole world, it's enormous. It's about 20 times greater than the rate of energy use by all of humanity. It's equivalent to exploding 400,000 Hiroshima atomic bombs per day 365 days per year. That's how much extra energy Earth is gaining each day.

The figure of six-tenths of a watt per square meter comes from the journal article "Earth’s energy imbalance and implications", by Hansen, Sato, Kharecha and von Schuckmann, which appeared in the peer-reviewed journal Atmospheric Chemistry and Physics in December 2011. The article gives an estimated figure of 0.58 (with an error margin of 0.15) watts per square meter for the Earth's overall energy imbalance during the period 2005-2010. This is taken from the abstract on the first page. In the body of the paper there is a more in-depth discussion.

I haven't converted the 0.58 watts per square meter figure into a "Hiroshima bomb" unit of energy, but that should be easy to do (and since Hansen cites that figure based on his own paper I expect his calculation would check out). Maybe someone else can supply that detail.

If the figure was 400,000 bombs in 2016 and is now being cited as 600,000 bombs, maybe there are updated estimated that have come out in the interim period and Al Gore is referring to. I don't know the source for those updated figures.

Bottom line: Gore's claim is based on peer-reviewed research. That doesn't guarantee the results he is relying on are correct, but they have at least been subjected to scrutiny by scientists applying the standards of scientific research for truthfulness and accuracy.

P.S. The 2011 paper by Hansen and his colleagues has 645 citations on Google Scholar, so seems to have been pretty influential and to have attracted even more attention and scrutiny than a typical peer-reviewed article. If the methodology of the research were flawed that would likely be pointed out by some of the later articles citing the original study, which might also provide more accurate estimates.

Answer 2

In my opinion, while the analogy is technically correct it's still somewhat misleading for the following reasons:

Use of an obscure comparison metric

Heat output from a Hiroshima-class bomb is a highly unusual metric. I.e. imagine asking "How long is the United States from coast to coast?" and someone answering with "Around 6.4m Hiroshima bomb widths". This is technically correct but since the vast majority of people have no idea how long that bomb was, it's of dubious value.

It would be better to use something like "total energy use of the United States per year", in which case we can calculate that the US uses the equivalent of ~1.6m Hiroshima bombs per year. Therefore human-generated CO2 heats up the planet every day by ~3.5 months worth of US energy use. This metric is still somewhat obscure, but at least it's much closer to something used by consumers on a daily basis.

Unnecessary association with radiation and deaths

In the minds of most people the Hiroshima bomb is associated with:

1. The deaths in Japan
2. The radiation fallout
3. The heat generated from the explosion

By invoking the bomb as the reference metric, Al Gore is creating a mental connection between global warming and radiation/death, rather than heat alone. He never explicitly says so, but many listeners will nonetheless start thinking about CO2 output as equivalent to the use of radioactive weapons.

To better understand why this is misleading, imagine someone asking "How much does the median Fortune 500 CEO make in a year?". We could then answer with "Around 45 times more than Hitler, adjusting for inflation". This is technically correct, but the answer is indirectly creating an unnecessary association between Fortune 500 CEOs and the Nazi leader. A fair answer would use a metric like "salary of median US household" or another neutral term to provide a fair analogy.

Answer 3

According to Heat stored in the Earth system: where does the energy go? Earth Syst. Sci. Data, volume 12, pages 2013–2041, (2020):

The study obtains a consistent long-term Earth system heat gain over the period 1971–2018, with a total heat gain of 358±37 ZJ,

358 Zettajoules per 48 year is 20 exajoules per day, meaning 20 x 10^18 joules per day.

The Hiroshima bomb released 6.3 x 10^13 joules

So the equivalent energy of about 300,000 such bombs per day.

Gore's claim seems reasonably correct, especially considering heat energy of the Earth is increasing faster now than in 1971, as seen in the above figure 6 of the reference.

OP is stating "600,000 Hiroshima bombs every day in heat to the atmosphere", but Gore isn't limiting this to the atmosphere, instead stating "boiling the oceans", which of course is hyperbole, but the heat is going into the oceans and land, not just the atmosphere. As seen in the above figure, most of the energy goes into the oceans.