Climate Forcings

In my previous post, I discussed the relationship between the orbit of the Earth around the Sun and fluctuations between periods of climate warming and cooling. In this post I’d like to explain how that works. I already pointed out that the Earth is currently defying all previous behavior by experiencing warming when it should be entering a period of cooling. The data I present at the end of this post will show why.

Climate change is caused by climate forcings. James Hansen, of Columbia and NASA, has provided this definition of a climate forcing: it is “an imposed perturbation (disturbance) of the planet’s energy balance.” So what does this mean?

The concept is actually fairly simple. The Earth is in energy equilibrium, with that equilibrium defined by how much energy the Earth absorbs versus how much it radiates. In other words, the amount of energy the Earth absorbs from the Sun must equal the amount of heat it radiates into space. If one of these values changes, then the other must adjust. For example, if the amount of solar radiation reaching the Earth decreases (as explained in the global cooling scenario in my prior post), the temperature of the Earth must also decrease. This is what causes ice ages. Conversely, if the amount of solar energy that reaches the Earth increases, the Earth’s temperature must rise.

Now, we’ve been recording temperature data for 130 years or so and, over that time period, temperatures have been rising. As I noted in my previous post, the opposite should be true: based on the Earth’s current orbit around the sun, temperatures should be falling. Climate forcings explain why this is so. The amount of sunlight that is absorbed by the Earth is not simply reliant on the amount of radiation that reaches the Earth; the composition of the Earth’s atmosphere also has an effect.

Since 1750, the Sun has accounted for a climate forcing of about 0.2 to 0.3 watts per square meter. A watt is a unit of power, meaning that it measures how much energy changes per second. For a climate forcing, this is averaged out for each square meter of the Earth’s surface. The other “natural” climate forcing comes from volcanoes, which account for a forcing of about 0.15 watts per square meter of the same time period. These calculations come with a bit of uncertainty, but even at the highest approximations, natural forcings account for only about 0.5 watts per square meter.

Climate forcings can also be negative; these, by definition, would drive down temperature, though bear in mind that all of these forcings must be added together. Thus, if there is a positive forcing of 0.3 watts from one source and a negative forcing of 0.2 watts from another, the net forcing is 0.3-0.2=0.1 watts. The smallest negative forcing comes from land cover change. This accounts for a forcing of about -0.15 watts.

After this, things get complicated. No government has been willing to fund measurements of the level of aerosols in the air. These are small particles formed by burning fossil fuels that scatter and absorb sunlight, which leads to a negative climate forcing. The net forcing of aerosols falls between -1.5 and -2 watts per square meter, though this has a high level of uncertainty. (I will return to this topic in a later post).

This brings us to positive climate forcings caused by human activity. Black carbon aerosols account for a climate forcing of about 0.5 watts per square meter. Methane, nitrous oxide, chlorofluorocarbons, and ozone account for about 1.5 watts per square meter. This leaves carbon dioxide, which accounts for another 1.5 watts per square meter. This means that human-caused positive climate forcings account for about 2.5 watts per square meter.

One claim made by climate skeptics is that it is the sun that is driving climate change. As I noted in my previous post, this makes no sense based on the Earth’s orbit. But as the information above indicates, it can be empirically shown to be false. The net forcing from the sun is only 0.2 to 0.3 watts; the net positive forcing from greenhouse gases is about 2.5 watts. This is an order of magnitude difference, meaning that there is really no question that human activity is driving global warming.

When you add all of these values together, you get a range of a minimum of about 0.85 watts per square meter and a maximum of about 1.45 watts per square meter. In other words, there is a substantial positive net climate forcing. This means that more energy is being absorbed by the Earth than is being radiated into space. To achieve equilibrium, the Earth’s temperature must rise relative to where it was in 1750. It has begun to do so, but the climate shifts slowly; warming would continue for quite some time even if we were to stop all emissions today.