As a child I was under the impression that the atom bombs used against Japan in WWII were the only nuclear weapons to have ever been detonated. Growing up, I was shocked to learn that from the end of the war up until the 1990s, major world powers (the USA, USSR, UK, France, China, India, and Pakistan) have conducted over 2000 nuclear weapons tests, with 520 of these being detonated above ground. North Korea still continues to conduct tests today. Furthermore, many of these weapons are far more powerful than the weapons used in Japan, giving far more potential for environmental harm.
Given the amount of energy released, and the sheer destructive force of the explosions, could they have contributed to the rising levels of greenhouse gases? CO2 levels sharply increased around the time these tests were becoming prevalent and this hasn't slowed down since. Is this linked or just mere coincidence?
No, in terms of anthropogenic climate change, the nuclear weapons tests were negligible. Nuclear explosions are not a significant source of CO2 - the rise of CO2 is due to increasing use of fossil fuels during this time, it coincides with the atom bomb testing because both are the result of a increasingly industrialized world.
As to the heat produced by the explosions, it also is tiny compared to the excess heat the earth is absorbing due to anthropogenic greenhouse gases. Researchers have cast the rate of global warming in terms of atom bombs - the increase in ocean temperatures alone (where most of the excess heat is being stored) is roughly equivalent to 1.5 Hiroshima-size atomic bombs per second!
One incidental impact the atmospheric testing did have was to release large quantities of carbon-14 into the atmosphere. Over the decades since atmospheric testing stopped, that carbon-14 has been seeping into the ocean, and indirectly provided a good measure of ocean ventilation and overturning ... key factors in gauging the ocean's role in climate. So the radioactive tracer produced by bomb tests actually helped to improve understanding of important processes of the global climate system.
More about bomb radiocarbon entering the ocean available in this PDF at NOAA.
