Radiative Forcings

In my last post, I discussed how scientists have been able to prove that the increase in carbon dioxide in the atmosphere is due to human activity. It’s now time to discuss why carbon dioxide matters. Below is a chart from the IPCC of radiative forcings, which I explain below.

Before I can explain forcings, there is an important concept that we need to remember from high school physics: every system is in an energy equilibrium (the second law of thermodynamics). This applies to the Earth, meaning that the Earth must radiate as much energy as it absorbs. Translated into what actually happens, the Earth must emit as much energy as it absorbs from the sun, meaning that if the amount it absorbs rises, the Earth’s temperature must rise to compensate.

A forcing is anything that perturbs that balance, by either raising or lowering the amount of energy the Earth absorbs from the sun. Positive forcings raise that amount, mostly by trapping sunlight that has already entered the Earth’s atmosphere and has been reflected off the Earth’s surface, preventing it from escaping into space. Negative forcings raise the albedo of the Earth, meaning that they increase the amount of sunlight the Earth reflects.

You’ll notice that human activities have caused positive (red) and negative (blue) forcings, meaning that humans have put pressure in both directions. However, you’ll notice the bar at the bottom showing “total net human activities,” which is clearly positive.

There has been a focus on carbon dioxide for two reasons. First, it is by far the largest forcing. Second, it is much longer lived in the atmosphere than other greenhouse gases. While methane (CH4) is a much stronger greenhouse gas, it only remains in the atmosphere for about ten years. In contrast, carbon dioxide lasts for millennia.

It is important to note the size of the error bar on “cloud albedo effect.” The two aerosol categories are the result of burning fossil fuels, just like greenhouse gases. Many of you will remember the acid rain problem from the late 1980s, which was effectively reduced by the first Bush Administration by implementing a cap and trade policy. These are the same compounds. The reason for the large error bar is that the US Government has so far been unwilling to fund an accurate measure of the aerosol content of the atmosphere.

Aerosols only last about two years in the atmosphere. This means that, were humans to stop burning fossil fuels, the aerosols would all immediately leave the atmosphere, while the carbon dioxide would remain, increasing the pace of warming. If all emissions were to stop today, it’s estimated that temperatures would continue to rise for 20 to 30 more years. They would then stay elevated for over 10,000 years.

The chart measures the net effect of all of these forcings between 1750 and 2005. You’ll note towards the bottom that the researchers included “solar irradiance,” which is the amount of warming that can be attributed to the sun. Many climate change deniers like to assert that the warming is being driven by solar activity, which seems ridiculous when you realize what a small forcing it is. There is, however, a small chance they are correct. Ironically, if they are correct, it means that climate change will be unimaginably more severe than current projections.

Once again we have to return to the error bar on “cloud albedo effect.” If the albedo effect is very large (so at the far left of the error bar), then it would be effectively cutting the warming due to carbon dioxide by a third from current estimates. This would mean that much more of the observed warming has been due to solar activity, as the deniers maintain. However, remember the behavior of aerosols: they last only two years in the atmosphere. Eventually, humanity will have to stop burning fossil fuels; the forcings from greenhouse gases will continue to increase as we burn more. When that happens, the aerosols will be gone, and the net forcing due to human activity will be substantially higher than the current projections.

The chances of this are rather slim; you’ll note that the median of that error bar is still very low. My point in explaining this was simply to show the absurd nature of the arguments of the deniers. There is simply no evidence that the sun has been driving more than a tiny fraction of the warming. And, if it were in fact responsible, then the climate crisis is even larger than the worst scare mongering has predicted.