John Cook, who writes the Skeptical Science blog, has been criticised – in a blog article called Media Fail: John Cook’s Atom Bombs – for comparing global warming to atomic bombs. This Media Fail post is actually referring to an article in the French Tribune called Climate Change likened to atom bomb by scientists.
In the French Tribune John Cook is quoted as saying
All these heat-trapping greenhouse gases in our atmosphere mean … our planet has been building up heat at the rate of about four Hiroshima bombs every second – consider that going continuously for several decades.
What the Media Fail article claims is that no scientist has claimed that humans are 100% responsible for current temperature trends and hence this comparison is flawed. Indeed, no scientist has claimed that we are 100% responsible for current temperature trends, so that part of the statement is correct. Other factors can and do play a role in changing globel surface temperatures. However, there is a difference between the temperature trends (for global surface temperatures for example) and the top-of-the-atmosphere energy imbalance. What John Cook is referring to is changes in the energy in the climate system, not simply changes in the global surface temperature.
Although humans are indeed not responsible for all changes to global surface temperatures, there is a perfectly good argument for why humans are almost 100% responsible for the top-of-the-atmosphere energy imbalance. The energy imbalance simply refers to the climate system receiving more energy from the Sun than we lose back into space. Normally, we’d expect the system to evolve into some kind of equilibrium in which the surface temperatures are such that – on average – the amount of energy re-radiated back into space matches the amount we’re receiving. Currently, however, global surface temperatures are higher than they’ve been for most of the last few thousand years. If so, the surface of the planet must be radiating at least as much energy – if not more – than it has done for the last few thousand years. The Sun is also not more luminous today than it has been for the last few thousand years, so we aren’t receiving more energy than we have done for the last few thousand years. Yet, we still have an energy imbalance such that we’re receiving more energy than we lose. How is this possible?
Well the current scientific consensus is that the CO2 that we’ve been adding to the atmosphere has been trapping more of the outgoing radiation and reducing the amount of energy reaching the top of the atmosphere. The net effect will be that surface temperatures will need to continue to rise so as to reduce this deficit. Therefore, humans may not be the only contributor to changes in global surface temperatures, but they are most likely the prime contributor to the existing top-of-the-atmosphere energy imbalance.
So, what is the estimate for the energy imbalance? Well, it’s about 0.5 Wm-2. If you multiply this by the surface area of the Earth and by the number of seconds in a year, you get 8 x 1021 J. So, the energy in the climate system is currently increasing at about 8 x 1021J per year. What was the energy of the Hiroshima bomb? Well, it was 6.7 x 1013 J. If you multiply this by 4 and then by the number of seconds in a year you get 8.4 x 1021 J.
There you have it. The energy in the climate system is increasing at the same rate as 4 Hiroshima bombs every second and this energy excess is almost entirely due to the continued increase of CO2 in our atmosphere driven by our use of fossil fuels. So, as far as I can tell, John Cook is entirely correct and this is another example of people assuming that the only indicator of global warming is changes to the global surface temperature and don’t realise that global warming actually refers to energy, not temperature.