Chemical Connections to Climate Change

Chemical Connections to Climate Change

Did you know that climate change can be used as a backdrop to discuss several ideas in chemistry? In this post I’ll describe how several chemical concepts including Beer’s Law, the chemical composition of the atmosphere, combustion chemistry, and solutions can all be used in a discussion of how CO₂ keeps our planet warm – and how excess CO₂ warms it even more.

Composition of the atmosphere

We’ll start by treating our atmosphere as a solution. Several different gases make up our atmosphere (Table 1). Because N₂(g) is the compound present in greatest amount in the atmosphere, N₂ is the “solvent” in our atmospheric “solution”. The solute present in largest amount is O₂ (g), but certainly there are many others.

                                                Table 1: Gases present in the atmosphere

The greenhouse effect

Some of these solute gases warm our planet by absorbing infrared (IR) light radiation that would otherwise escape into space. Without this warming effect (called the greenhouse effect), the average temperature on Earth would be a chilly 255 K (that’s -18 ^oC, or the temperature on an extremely cold winter day). Because solute gases in our atmosphere absorb IR light, Earth’s average temperature is 288 K (that’s 15 ^oC, about the temperature on a slightly cool spring day). Thus Earth is 33 K warmer than what would be expected if there was no atmosphere.¹

While both N_2(g) and O_2(g) are present in very large amounts in the atmosphere, these gases do not absorb IR light and therefore do not contribute to the greenhouse effect. On the other hand, CO_2(g) and H₂O_(g) do absorb IR light and upon doing so gain energy which is transferred to the rest of the Earth. Even though these gases are present in small amounts, they are very good at absorbing infrared light. Thus, these atmospheric gases are the main contributors to the greenhouse effect. Of the 33 K increase in temperature due to the greenhouse effect, H₂O_(g) contributes half (16.5 K) and CO₂ contributes 20% (6.6 K). Clouds (25%) and other trace gases (5%) make up the remainder.¹

Connection between climate change and Beer’s Law

Large amounts of CO_2(g) are regularly pumped into the atmosphere though various combustion reactions, such as that found during the burning of isooctane (the main component in gasoline):

2 C₈H_18 (g) + 25 O_2 (g) à 16 CO_2 (g) + 18 H₂O _(g)                  Equation 1

Climatologists tell us that the resulting increase in atmospheric CO_2(g) concentration has the effect of increasing the average temperature on Earth. So let’s ask the question: How much hotter would we expect the average temperature of Earth to rise upon addition of more CO_2(g) into the atmosphere?²

We’ll gain some insight into this question using Beer’s Law, which describes the amount of light absorbed by a compound in a solution:

A = ebc                       Equation 2

Where A is the amount of light absorbed, e is the molar absorptivity of the compound (a measure of how well the compound absorbs the light), b is the path length through which the light travels, and c is the concentration of the compound in the solution. In our case, A is the amount of IR light absorbed by CO_2(g) in the atmosphere, e is how well CO_2(g) absorbs IR light, b would be the thickness of Earth’s atmosphere, and c is the concentration of CO₂(g) in the atmosphere.

Back in 1850, the concentration of CO₂(g) in the atmosphere was 285 ppm.³ Today, the concentration of CO₂(g) is over 400 ppm.^3,4 Let’s use these values and Beer’s Law to estimate the ratio of IR light absorbed by CO_2(g) in 1850 (A₁₈₅₀) vs. today (A_today):

calculation

Based on Beer’s Law, we’d expect CO_2(g) to be absorbing about 40% more IR light than it did back in 1850, due to this increase in concentration.

Let’s find the expected global rise in temperature due to this 40% increase in light absorbed by additional CO_2(g). The baseline contribution of CO_2(g) to the greenhouse effect is 6.6 K (see above). Let’s set this value equal to the contribution of CO_2(g) to the greenhouse effect in 1850. Multiplying this value by 1.4 gives: 6.6 K x 1.4 = 9.2 K. Thus we would predict that the additional CO₂ in the atmosphere would increase the contribution of CO_2(g) to the greenhouse effect from 6.6 K to 9.2 K. That’s a change of +2.6 K (9.2 K – 6.6 K). Indeed, average global temperatures have risen 1.0 K since 1850.⁵ Our estimate is in right the ballpark, but well over two times too high. Surely this is because there are many more factors on the global scale that our simple approach has not taken into account. Nevertheless, we can at least gain some insight into how increased CO_2(g) concentrations in our atmosphere have caused the average global temperatures to rise…and we can do so using chemistry!

Do you ever bring up climate change or global warming in your classes? If so, how do you approach the issue? What kinds of chemical concepts do you use to discuss these issues? We would love to hear from you, so be sure to let us know what you think.

Notes and references (accessed 1/25/2017):

1. Schmidt, G.A.; Reto, R.A.; Miller, R. L.; Lacist, A.A. J. Geophys. Res. 2010, 115, D20106.
2. Noticing that combustion reactions release water (Equation 1), it is natural to ask why no one is concerned about increases in global temperature due to increases in atmospheric water vapor. The answer is that water does not “build up” in the atmosphere the way CO₂ does (and some other gases do). When “too much” water vapor builds up in the atmosphere, it rains, removing the excess water. 
3. https://data.giss.nasa.gov/modelforce/ghgases/Fig1A.ext.txt
4. https://www.co2.earth/daily-co2
5. http://berkeleyearth.org/land-and-ocean-data/#section-0-0