“Being told about the effects of climate change is an appeal to our reason and to our desire to bring about change. But to see that Africans are the hardest hit by climate change, even though they generate almost no greenhouse gas, is a glaring injustice, which also triggers anger and outrage over those who seek to ignore it.” –Sigmar Gabriel
With all of the scientific issues subject to politicization in this world, there’s arguably none that raises such strong emotions as the issue of global warming and climate change. This is the final installment of a three-part series on how one could figure out whether global warming was real, and if it was real, if it was caused by human activity. You can read part 1, which talked about the factors that determine the temperatures of the inner planets, and part 2, which talked about the mechanisms by which an atmosphere (and the gases in it) raise the temperature on any world that has one. Including ours.
Today, with that in mind, I’d like to focus on what’s been going on in the relatively recent past here on Earth.
This is the Earth’s atmosphere, with which we’re all familiar. What you might not be as familiar with is what that atmosphere is made out of, in terms of its chemical composition. Yes, you know that it’s got Oxygen in it and Carbon Dioxide, but the breakdown is incredibly important. You see, as we learned in part 2, the Earth’s temperature is 33 °C (59 °F) warmer than it would be if it didn’t have an atmosphere.
So what’s that atmosphere made out of?
Mostly Nitrogen, which is about 78% of our dry atmosphere, followed by Oxygen, at about 21%. There’s also about 1% argon, an inert gas, followed by small amounts of carbon dioxide, neon (another inert gas), methane, and other trace elements and molecules.
It’s important that I say “dry atmosphere” here, because, well, our atmosphere isn’t ever really dry. We’ve got this pesky little thing on our planet that prevents that from ever really happening.
And by “little”, of course, I mean our oceans, which contain about 300 times the mass of the entire atmosphere combined. Because of how chemistry (evaporation, vapor pressure, etc.) works, that adds around an additional 1% of our atmosphere, on average, in the form of water vapor. That number is highly variable, but that’s one component we really have no control over.
There are others; we don’t control the water vapor, the clouds, the oxygen or the ozone. (At least, not yet.) But the amount of carbon dioxide in our atmosphere has changed substantially over the past few centuries, and that is, without a doubt, due to human activity.
Up until the end of the 18th century, Carbon Dioxide levels were pretty stable at about 270-280 parts-per-million (ppm) in our atmosphere, changing by small amounts due to things like volcanic eruptions, forest fires, and other natural activity. But with the advent of the industrial revolution, all that started to change.
For the first time in natural history, hundreds of millions of years worth of carbon — carbon that had been stored under the surface of the Earth — the remnants of carbon-based organisms that had been buried underground and turned by time into oil, coal, and other resources, was being burned and returned to the atmosphere, all at once.
You can do the math for yourself, and you’ll find that since the dawn of the industrial revolution, we have burned-and-added about 1.5 trillion metric tonnes of Carbon Dioxide to the atmosphere.
This should be a little surprising, because if you do the math about how much Carbon Dioxide is in our atmosphere right now, it’s “only” about 2.1 trillion metric tonnes (or about 400 ppm), which is an increase of only around 0.7 trillion tonnes from pre-industrial revolution levels (270 ppm). So where did the other 0.8 trillion tonnes go?
Into the ocean. Any idea what you get when you mix carbon dioxide (CO2) with water (H2O)? You get H2CO3, also known as carbonic acid. (And yes, it was our old buddy Arrhenius who figured that out, too.) If you’ve ever heard of ocean acidification, this is where it comes from, and this is undoubtedly what’s causing it.
But that’s not what all this is about; back to the issue at hand: global warming. Based on what we went over in the first two parts, we know that planets absorb light in mostly the ultraviolet, visible, and near infrared, and then radiate that energy back into space in the mid-and-far infrared. At least, they try to, unless something in the atmosphere absorbs some of that infrared energy, and re-radiate it back to the planet’s surface. How good are Earth’s gases at doing that?
They’re only okay, but they’re important enough that they’ve warmed the planet by 33 °C (59 °F) over what it would be without an atmosphere at all. In fact, of that 33 °C (59 °F) that Earth is warmed by due to the atmosphere, atmospheric science has been able to quantify how much is due to the different components:
50% of the 33 K greenhouse effect is due to water vapor, about 25% to clouds, 20% to CO2, and the remaining 5% to the other non-condensable greenhouse gases such as ozone, methane, nitrous oxide, and so forth.
So if 20% is due to Carbon Dioxide, and we’ve increased the Carbon Dioxide level by 50%, does that mean we’re in for another 3.3 °C (5.9 °F) of warming?
Not necessarily. There are other factors that come into play, and when you do something to heat the Earth up, it has many natural mechanisms to attempt to regulate itself.
There’s latent heat stored in glaciers and icecaps, and if you start to melt them, that releases cooler water into the oceans, lakes and rivers. For small increases in Carbon Dioxide, plant activity will increase, removing some of that greenhouse gas from the atmosphere.
The danger is in what happens if we add too much Carbon Dioxide to the atmosphere too quickly, which could mean the Earth’s temperature would start to rise in response to an increased greenhouse effect.
And that’s exactly what’s happened. We had what appeared to be normal temperature fluctuations — consistent with what was historically observed — up until the late 1970s. But after that, coincident with an exponentially rising increase in Carbon Dioxide concentrations, the average temperature of the Earth began rising, too.
Other methods of showing global average temperature vs. time — such as taking the average global temperature over each decade — show the same, steady increase over time since the end of the 1970s.
The vast majority of the heat, by the way, isn’t going into the Earth’s surface or the Earth’s atmosphere; that’s just the places where it’s easiest for humans to measure the temperature on Earth.
As you’d expect, given that the Earth’s oceans have a low albedo, cover the majority of the surface, convect easily, and run around 2-3 miles deep on average, the vast majority of the heat increase has wound up in the oceans.
So, undoubtedly, the Earth has warmed, and — to the best of our measurements — it appears to be warming still.
There could have been other, natural explanations for this warming, such as increased solar output, which has been correlated with temperature increases in the past. But, in fact, the opposite has been happening, and the current solar cycle is showing substantially decreased solar activity, which should’ve resulted in a cooling effect, had all other things been equal.
It cannot be proven that human activity is the cause of global warming, but based on what we know about planetary science, Earth’s atmosphere, human activity and the warming we’re observing, it seems very, very unlikely that anything else could be the cause. Not the Sun, not volcanoes, not any natural phenomenon that we know of.
Next year, a broad scientific report (the IPCC’s AR5) will come out, and they will be taking a full, in-depth look at this and other global warming issues. Now that you know that global warming is real, and now that you understand why it’s really likely that it’s caused by human activity, I hope you’ll start asking what they right way is to start addressing this problem. I’d like for humans to live happily and successfully on this world for thousands of generations to come, and that starts with taking care of this world today. I hope you’ve enjoyed this series, and I hope you’ll continue to join me on our journey through the Universe, near and far.