How 8,000,000,000 people warmed 5,500,000,000,000,000 tons of air
And why we'll be able to stop
Prior to human intervention, the Earth’s climate was relatively stable from century to century. On longer time scales, you’d find the occasional ice age and whatnot. But things didn’t change at the speed we see today. The world’s population has grown remarkably, as have our technological capabilities. But even so, it’s a big planet, and we are still fleas on the back of an elephant. How have we managed to actually heat up an object as large as the world? And can we reverse course?
We spend a lot of time in the details of climate change, its myriad causes and effects. Occasionally, for perspective, it’s worth reviewing the big picture.
When something is stable, like the pre-industrial climate, we tend to think of it as static: “lacking in movement, action, or change”. However, nothing about the atmosphere is ever lacking in movement. Instead, stability was due to the system having achieved balance. Over the eons, critical parameters such as temperature and gas mix had wandered until they reached an equilibrium, where motion in one direction was matched by motion in the opposite direction.
Why do we burn fossil fuels? For heat, which gives us both warmth and motion. But the heat of those fires is not what’s warming the Earth. It’s the side effects, such as carbon dioxide, that are tipping the balance of forces that control our climate. That’s good news, because it means that halting climate change doesn’t require giving up the things we want. There are many ways of obtaining warmth and motion without cooking the planet; solar, wind, nuclear, geothermal, hydro, and wave energy are all climate-safe. Digging stuff up and burning it is the only energy source that triggers warming. In the long run, preserving the climate will be easier than destroying it.
I have a big project underway: a concise, comprehensive guide to climate change mitigation. Today’s post, adapted from that project, is a review of the basic science of climate change and the greenhouse effect. I’m sure this is not new to you, but give it a go anyway. Rather than recapping the dry facts, I’ll try to put them in perspective, to give you an intuitive feel for why human activities are having such a large impact, and why we’ll be able to stop.
Exposed to Extremes
Every time you go outside, you’re standing under a frigid, yawning abyss that extends to the edge of the universe. The temperature in outer space is -270°C, or -455°F. It’s cold out there.
Nestled in the cozy embrace of Earth’s atmosphere, it’s easy to forget that. But without the atmosphere, temperatures would plunge to lunatic, science-fictional levels every night. Then the sun comes up, and a colossal nuclear furnace, almost a million miles across, blasts us with 173 quadrillion watts – as if the sky were filled with one hundred trillion space heaters, spaced just a few feet apart.
On the moon, right next door and exposed to the same conditions, temperatures swing from insane cold (-130°C / -208°F) to literally boiling hot (120°C / 250°F) with every sunrise and sunset. On Earth, the tenuous layer of atmosphere insulates and buffers us from these wild swings1.
In other words, the temperature we experience at the Earth’s surface is a delicate balance of energy flooding in from the sun, and energy leaking out to space.
Energy Flows
When you have a mix of temperatures, they tend to even out: heat flows from hot to cold. Our experience of this is based on heat conduction (between objects that are touching one another, as when you pick up a hot dish straight out of the microwave) or convection (riding along with a moving fluid, as when you open the door on a cold winter day and freezing air pours in). In space, nothing is touching and there’s no fluid, so these mechanisms don’t apply.
Instead, the energy flows between the Sun, Earth, and outer space consist of radiation – the same effect that makes you feel toasty when standing in front of a fire. This radiation is a ray that shoots instantly from one place to another, like a beam of light. In fact, exactly like a beam of light, because visible light is simply a form of radiation. The warmth that you feel when stepping into direct sunlight, and the warmth that you feel under a heat lamp, are both caused by radiation landing on your skin. In the case of the sun, it’s white light; from the heat lamp, it’s “infrared” light – basically, a color that’s even redder than what we call red. Our eyes don’t pick it up, but it’s the same sort of stuff as visible light.
The sun is glowing white-hot; that’s why it emits light. A heat lamp glows infrared-hot (edging into actual red-hot, which is why the the heating coils look red). The Earth also glows infrared-hot, though it’s farther down the spectrum than a heat lamp. The infrared glow from the Earth radiates into space, taking heat with it.
Because the Earth isn’t nearly as hot as a heat lamp, its infrared glow is weak. However, that radiation emanates from every inch of land and ocean, 24 hours a day, 365 days a year. This is enough to balance the blazing white solar radiation, which is much more intense at its source, but less so when spread across the Earth’s full surface. The sunlight absorbed by the Earth carries about the same energy as the infrared glow shining from the Earth into outer space. That’s not a coincidence: over time, temperatures adjusted until these flows were in equilibrium.
The Greenhouse Effect
One fact about radiation – at least, the “electromagnetic” kind we’re talking about – is that it’s easily blocked. That’s why a simple parasol, or a canopy of leaves, can shade you from the hot sun.
Glass, of course, does not block visible light. (Duh; that’s like the whole point of glass windows.) But it does block some infrared light. So if you build a big glass box and set it out in the sun – in other words, if you build a greenhouse – then the sun’s rays can shine in, but infrared from the warm interior can’t shine out. With energy flowing in but not out, the inside will get hot. This is also why cars get so hot when left out in the sun.
Some gases, such as carbon dioxide, have this same property of blocking infrared light. And so just as glass walls trap heat inside a greenhouse, “greenhouse gases” trap heat in the atmosphere; hence, the “greenhouse effect”.
Clouds block both visible and infrared light. The net effect might be to warm the Earth, or to cool it, depending on the cloud’s altitude. Chemicals which affect cloud formation, therefore, can also affect the balance of energy flow.
Over Time, A Small Imbalance Has a Big Effect
The atmosphere itself is not static. Natural processes such as photosynthesis, plant respiration, and exchanges of gas between the air and ocean move far more carbon than the best efforts of the fossil fuel industry. However, fossil fuels are a new factor, not part of the equilibrium, and they push in just one direction: up.
In a day, or a year, the amount we burn is not really significant. But we’ve kept burning, and burning, and over decades and centuries it’s added up. Even now, the amount of excess CO₂ in the atmosphere isn’t much on a planetary scale. Suppose that, for visualization purposes, we were to freeze that excess; the resulting layer of dry ice would be just 1.35 millimeters thick. That’s 1/20th of an inch. This gossamer shell, along with even more tenuous wisps of other greenhouse gases, blocks only a tiny fraction of the Earth’s infrared glow; tipping the energy balance only slightly.
On average, allowing for day/night and equator/pole variations, the amount of sunlight reaching the top of the atmosphere is 340 watts per square meter. The infrared radiation escaping from the Earth back into space is just slightly less, about 339.2. In other words, the flows of energy in and energy out are almost identical; if we compare the size of the flow to the land area of the United States, the imbalance is proportional to Hawaii.
This slight imbalance in energy flow, caused by the historical accumulation of a slight imbalance in flows of greenhouse gases, is entirely responsible for global climate change. Think of it as the ultimate heat pump: our total energy consumption is “only” about 20 terawatts, but the resulting imbalance in energy flow between the Earth and outer space is 500 terawatts2. Inadvertently, we stumbled onto the worst possible way of generating power. Fortunately, to stop, all we need to do is to adopt literally any other approach.
The moon’s slower rotation also contributes to the wide temperature swings; it takes about 29 days from sunrise to sunrise, as compared to just 24 hours on Earth. But at the Earth’s poles, night lasts six months, and even there temperatures don’t get nearly as cold as the moon.
To be clear, burning fossil fuels isn’t the only human activity that drives climate change. For instance, deforestation and agriculture play a significant role. But fossil fuels are the lion’s share of the problem.