“My girl, my girl, don’t you lie to me,
tell me where did you sleep last night?
In the pines, in the pines, where the Sun don’t ever shine,
I’ll shiver the whole night through.” –Leadbelly, among many other variations
It’s already been a couple of months since I wrote about Global Warming, and was deluged with comments that (to be kind) objected to the scientific consensus that the Earth is getting warmer, and humans are very likely the cause.
So let’s just take a look at the basic physics of how a warm object — like a planet — stays as warm as it does in a cold environment, like interstellar space.
And let’s do this by comparing it with a much more familiar warm object in a cold environment: humans being outside when it’s cold.
Taking a look at the picture below, you can easily imagine that, despite having a normal human body temperature, these people will get cold quickly, and if they don’t do something about it, they run the risk of freezing to death.
And this is simple thermodynamics: a warm object in a cold environment radiates its heat away until it decreases its temperature to match the outside environment.
If you don’t want to freeze to death, and you can’t change the outside temperature, you’ve really only got two options. Either you can add heat to the warm object, so even as you radiate your heat away into the outside, you can maintain a constant body temperature and not freeze to death.
For a human body, you typically do this by burning your stores of chemical energy at a rapid rate. But over longer periods of time, this is a highly dubious method of staying warm. You are much better off taking the simplest route: trying to maintain the heat you already have!
And you know how to do this: you wear clothes and wrap yourself in blankets! Your body is still going to radiate heat away into the outside, cold environment, but the blanket does two very important things to make it harder for that heat to leave you:
- The blanket is going to absorb the heat your body is radiating. By heating up to some intermediate temperature and being in the way of your radiation, it inhibits the rate of heat flow from your body to the outside environment, meaning you lose heat more slowly than you would otherwise. And…
- The blanket is partially reflective, meaning that some of the heat emitted from your body either gets reflected outright from the blanket back towards you or absorbed and re-emitted from the blanket back towards you.
As anyone who’s ever been underneath too many blankets can tell you, this can even make you too hot very quickly!
Now, for systems like blankets and insulated houses, you need to think about more than just radiation; you need to consider conduction and convection to get the details of heat flow right. But for a planet, this simple picture tells most of the story.
Planets generate some of their own heat from radioactive materials in their interior, they receive the rest of their heat from the Sun, and they radiate that heat back into the cold depths of space.
And this simple model works incredibly well for predicting the temperatures on planets like Mercury and our own Moon.
On Mercury, for example, the side facing the Sun gets very hot, as it absorbs the Sun’s radiation and is extremely close to the Sun. The hottest daytime temperatures on the planet Mercury get up to a blistering 800 °F (427 °C), hot enough to boil the element mercury; hot enough to melt tin and lead.
But during the long nights, Mercury radiates its heat away, achieving lows of -270°F (-168°C), giving it the largest natural temperature swing of any known place in the Universe (so far).
But Mercury, the Moon, and other similar bodies in our Solar System behave in this simple, easily predictable manner because they have no atmosphere on them. Not every planet is like this, however.
Because most planets have a blanket in the form of an atmosphere wrapped around them, as exemplified by Venus, above. Venus is perhaps the most extreme example of what happens if you wrap a thick blanket around a planet: the maximum temperature on Venus is 460 °C (860 °F), and despite taking more than 200 days to spin around once, Venus’ temperature is the same whether it’s day or night!
This is despite Venus seeing the Sun as only being one-quarter as bright as someone on Mercury does, and yet Venus is always hotter than Mercury, even comparing Venusian midnight with the hottest part of the Mercurian day! This atmospheric blanket — made out of 96.5% carbon dioxide — is what keeps Venus in its inferno-like state.
Now, knowing what happens on Mercury or Venus is nice, but it’s time to ask the question of what’s happening on the Earth. We definitely have an atmosphere, but it’s very different from the one on Venus. (Thankfully!)
Our atmosphere is nearly all nitrogen and oxygen gases, which are practically transparent to both the light emitted by the Sun and the heat radiated by the Earth. But after nitrogen and oxygen, the next most common elements in our atmosphere are water vapor, argon, and carbon dioxide. While argon is also practically transparent to visible light and the Earth’s thermal heat, both water vapor and carbon dioxide — just as it does on Venus — behave as a blanket on Earth, as you can see from their absorption spectra.
So a fair and accurate picture of our atmosphere is to treat it as a thin blanket, not nearly as effective (and oppressive) as Venus’ atmosphere, but definitely as having a non-negligible impact on our temperature.
Well, what has been the effect of humans on the thickness of this blanket? While water vapor self-regulates its atmospheric density based on factors such as temperature, carbon dioxide does not. In fact, if we look at carbon dioxide density in our atmosphere, recently, and compare it with our carbon dioxide emissions since the start of the industrial revolution, here’s what we find.
Without a doubt, we have increased the thickness of our blanket, albeit just by a little bit. Now, we can argue about, quantitatively, just how much of an impact this thicker blanket has on our temperature. Some argue that it’s quite large, some argue that it’s small enough that it doesn’t matter, and while the models generally agree that a thicker CO2 blanket makes the Earth warmer, there isn’t a consensus as to “how much.” (See here for a roundup of the latest.)
But as far as the thickness of our blanket goes, we can see that it’s increasing, and that more recently, it’s increasing at an accelerating rate.
Wouldn’t it make sense to look at the measured average temperature of the Earth during this time? Our reliable temperature records go back to about 1880, when widespread data from thermometers is first available. Let’s take a look and see what we find.
Now, I am fully aware that showing a correlation between the carbon dioxide levels in our atmosphere — of which the rise is indisputably caused by man — and the global average temperature doesn’t necessarily mean that one caused the other.
But given what we know about carbon dioxide absorbing heat, given what CO2 atmospheres on other planets do, and given the disastrous effects that continued rising temperatures are having (to say nothing of the scientific consensus on the issue), don’t you think — at the very least — it’s time to stop putting on more blankets?
If you believe that blankets keep you warm, then it’s inconsistent of you to believe that emitting carbon dioxide into the atmosphere cannot possibly cause a rise in the Earth’s temperature. And if you still don’t believe it, then I politely invite you to go to Venus.
(My kudos to Alex for his eloquent inspiration for this post.)