Global warming: how much heat, exactly?

It is often difficult to visualize what we are doing to our planet. But a simple calculation shows that the greenhouse effect generated by fossil fuels can be seen as the equivalent of turning on more than a hundred 1 kW electric heaters for each human being on the earth. And we can't turn them off!

If you look at the way climatologists describe global warming, you'll see that they use a lot the term "forcing"; that is, the additional effect of human activities to the natural heating from sunlight. Not all forcings increase temperatures, some tend to reduce it; for instance, atmospheric particulate. The overall result is called "imbalance" or "net forcing." You can think of a forcing in terms of someone trying to budge a person who doesn't want to move. If the person pushing is stronger, the net resulting force will cause the person being pushed to move. In the case of climate, the warming forcings are stronger than the cooling forcings, and the net result is a rise in temperature.

As we keep emitting CO2 and other gases in the atmosphere, the greenhouse forcing increases, as you see in the figure below (Hansen 2011).

In this figure, forcings are measured in terms of W/m2 (watts per square meter), as it is generally done in climate science. Unfortunately, it is a kind of unit that doesn't convey a feeling of the magnitude of what we are doing to our planet. A few watts per square meter are approximately equivalent to a single Christmas light, and that doesn't look worrisome. But, if you take into account the effect on the whole planet (510 million km2), then the overall forcing is gigantic. From Hansen's figure, you can calculate something like 1500 TW (terawatts, or trillions of watts) for the greenhouse gas forcing and around 500 TW for the net forcing. These numbers vary depending on which factors are considered; for instance, Zhang and Caldeira, (2015) consider the effect of CO2, alone, and calculate a forcing of 1.57 W/m2, that is about 800 TW. (For more data, see also Steve Easterbrook and Dana Nuccitelli et al.).

In any case, we are talking about huge numbers, at least hundreds of TW. For comparison, think that the total primary energy generated by burning fossil fuels is "just" about 15 TW, and practically all of it is eventually turned into heat. So, the indirect greenhouse warming effect is 1-2 orders of magnitude larger. We may also compare with the total solar irradiation that arrives to the earth's surface, around 90,000 TW (Szargut 2003). The effect of the sun is much larger than the human forcing, but not so much larger that the latter can be considered negligible. So, it is not surprising that human activities are causing a detectable warming of the whole planet.

Note also that we have been discussing just the effect of the current forcing. But, whereas the heat from burning fossil fuels is rapidly dissipated, CO2 remains in the atmosphere for a long time, tens of thousands of years and even more (Archer 2005). And this CO2 will keep warming the earth for a total effect that Zhang and Caldeira (2015) estimate as about 100,000 times larger than the direct thermal effect of the combustion that created it.

At this point, we can try to visualize these values by comparing them to something familiar. As an order of magnitude, let's take the number calculated from Zhang and Caldeira's paper, that is a forcing of 800 TW resulting from CO2 alone. That would correspond to 800 billion electric heaters of 1 kW each, all turned on together.

Considering that there are more than seven billion people on the earth, we may think that the CO2 forcing, alone, is equivalent to each one of us turning on a hundred electric heaters of one kW each. The net forcing - the actual heat being added to the atmosphere - is smaller, but the overall picture doesn't change: we can visualize it as corresponding to 250-400 billion heaters, about 50 per person. And, if we want to make the number of heaters proportional to energy consumption, people living in the rich West would be associated with many more. Imagine each home in your neighborhood stacking hundreds of electric heaters in their front yard, all turned on at full power, and you have some idea of what we are doing to our planet.

So, we have turned the heat on, and now we discover that there is no way to turn it off. (At least not as easily as you can turn off an electric heater). The best we can do, for the time being, is to avoid adding too many heaters to the ones already on. Even that seems to be extremely difficult, but we can at least try.




h/t Steve Easterbrook, Dana Nuccitelli, John Cook, Ben, Michael Tobis, John Abraham, and others. Note also that there are other attempts to visualize the total amount of heat generated by human forcing, for instance, in terms of "four Hiroshima bombs detonated per second" Spectacular, but probably less intuitive than electric heaters.

 

References

Archer, David (2005) Fate of fossil fuel CO 2 in geologic time  Journal of Geophysical Research vol. 110 (C9) p. C09S05

Hansen, J., M. Sato, P. Kharecha, and K. von Schuckmann, 2011: Earth's energy imbalance and implications. Atmos. Chem. Phys., 11, 13421-13449, doi:10.5194/acp-11-13421-2011.

Szargut, Jan T. 2003. “Anthropogenic and Natural Exergy Losses (exergy Balance of the Earth’s Surface and Atmosphere).” Energy 28 (11): 1047–54. doi:10.1016/S0360-5442(03)00089-6.

Zhang, Xiaochun,  Caldeira, Ken, (2015) Time scales and ratios of climate forcing due to thermal versus carbon dioxide emissions from fossil fuels,  Geophys. Res. Lett, 42, 11,  1944-8007