This post is written to describe the basic effects of greenhouse gases, and CO2 in particular, for those who are unfamiliar with how the greenhouse mechanism works. Apologies to any physicists who might be reading this, I’m going for a very gross generalization here, not a paper for Nature.
For starters, let’s define the “greenhouse effect”. In this context, it is:
The ability for something to allow energy into a system, but also to prevent or retard its exit.
This sounds paradoxical, doesn’t it? How can something let energy in freely, but then prevent it from getting out? I’ll give you an example that probably everyone reading this will have encountered.
It’s the middle of summer and you walk out into a parking lot to get into your car – and inside the car it is insanely hot. So hot that it could kill you if you had to stay in there. Yet out in the parking lot, it might be uncomfortably warm, but not lethally so.
What has happened here is that the inside of your car is subject to a greenhouse effect as a result of the nature of the glass in the windows. Visible and other wavelengths of light can pass through the glass – they enter the car freely through the windows (which is why you can see through them), and some are absorbed by the interior of the car. Some bounce back out, giving us a view of the interior.
(Image from Wikipedia)
Light, of all sorts, is made of photons. Different photons have different wavelengths, and those wavelengths represent how much energy is contained in that photon. Wavelength changes inversely to energy, so a short-wavelength photon will be higher energy than a long-wavelength one. X-Rays – very high energy, very short wavelength. Radio – very low energy, very long wavelength. Imagine it this way: if the photon is crammed up into a really small wavelength, it’s going to be very compact and energetic. If it’s stretched out a great deal, it’ll be slow and lethargic. Wavelengths also govern how photons interact with things – if a thing (like an atom in your steering wheel) is a size that matches the wavelength of the photon, that thing can absorb the photon. If it isn’t the right size, the photon passes it by. So visible light photons pass through glass windows because the glass molecules aren’t the right size to stop them – but the metal and plastic molecules of the car body are the right size, and absorb or bounce them back.
Back to your car: it’s the photons that get absorbed that we need to understand – they end up in the materials inside the car: the seats, the steering wheel, the dashboard, etc. They impart energy into these materials, and then those materials get “energized.” That’s not a technical term or anything, they just have absorbed the energy of the photon – the photon is gone, and the atom or molecule is now a little bit jazzed up. After a period of time, the atom or molecule of steering wheel calms down again, by emitting the extra energy it built up when it absorbed the photon. It does this by emitting photons itself – in this case, on the infrared wavelength, which while we can’t see it, we sense infrared as heat.
“Hot” in our body’s language means we are absorbing infrared photons – the warmth of your coffee cup, the heat from a fire, all that is infrared.
Now here’s the catch: glass molecules are the right size to catch infrared. So while your car is sitting in the parking lot, all kinds of visible and other light is getting in through the windows, some bounces back out, but the majority of it gets absorbed inside the car, and then re-released as infrared. That infrared can’t get out directly because the glass won’t let it pass – so more and more builds up inside, and the interior of the car gets hotter and hotter.
So how does this relate to carbon dioxide in the atmosphere?
CO2 in the atmosphere operates similarly to the glass in your car…ultraviolet, X-Rays, radio, and visible light all pass through it without issue. Infrared gets blocked, however – much like the glass of your car, CO2 is the right size to catch infrared. It’s this one property that makes things interesting in this context.
CO2, along with other gases, acts in a similar way to a blanket – it keeps heat in the atmosphere. It does this similarly to how the glass in your car keeps heat in your car. What happens is that light passes down to earth from space, is absorbed when it comes down here, and is re-emitted as infrared. For example, sunlight comes down, sinks into the pavement of your parking lot, and makes it hot (it is emitting infrared). If there was no atmosphere, the infrared would radiate away into space freely. However, we do have an atmosphere – and the infrared has to pass through this. CO2 is part of that atmosphere, and catches some of that infrared, gets excited, and then emits infrared again when it calms down. Those emissions from the CO2 molecule will be sent out randomly, half towards space and half back towards the earth. This results in the “blanket” of CO2 keeping some heat in the atmosphere. When you add more CO2, there’s more to catch infrared, and a heavier blanket effect.
In the 1800s, CO2 existed in the atmosphere to the tune of about 280ppm (parts per million – for every million molecules of anything in the air, 280 of those were CO2). That amount came from mostly natural sources – biological respiration, volcanoes, forest fires, etc. Over the last half-million years, that concentration has fluctuated between 180 to 300 ppm, and those fluctuations take place over thousands and thousands of years. As with any concentration of anything, there are ways things enter the system, and things exit it. The system being earth’s atmosphere, and the something being CO2. Respiration by living things being mostly responsible, volcanoes being a dramatic example (their amount of CO2 entering the system is measured to be about 300 million tons per year). Plants remove CO2 from the air, and CO2 also dissolves into the oceans, where plants and animals can take it up and eventually turn it into sodium bicarbonate (a common ingredient in sea shells), and other body parts of plants and animals – which, when they die, fall to the floor of the oceans and are eventually subducted through tectonic shifts.
Nature as we experience it today evolved to deal with a pretty steady amount of CO2 in the atmosphere, and had millions of years to adapt to changes. There’s a lot of input and output – see that chart I put up there, which was current in 2007. As you can see, the total ins and outs are pretty big – about 750 billion tons or so in, and in 2007, about 735 billion came out. That produced a net gain of 15 billion tons per year. Some of that gain got absorbed by the oceans, but the lion’s share went into the atmosphere.
Over the last 10,000 years or so, we’ve had a fairly steady concentration of CO2 – the blanket has stayed about the same. In 1880, the concentration was as I mentioned above, 280ppm. In 2007, it reached 375ppm.
Just a few months ago in 2013, that concentration reached 400ppm.
That means the blanket of CO2 in the atmosphere is 43% heavier than it was a little more than a century ago.
(Image from the New Scientist)
So where did all that extra CO2 come from? Well, to put it simply, it came from us. The industrial revolution is marked most distinctly by the fact that we discovered vast stores of volatile chemicals in the earth – hydrocarbons. These became the fuels we know today: gasoline, coal, diesel, jet fuel, etc. We found that these burn very handily, and if we harness the power of burning them, we could drive machines, keep ourselves warm, and so on.
So we burned them. We still do. Do you drive a car? You probably burn them now. The Volkswagen Golf 1.6 liter TDI, for example, burns fuel for every km you drive. It’s one of the most efficient vehicles there is. And it still releases 99 grams of carbon for every kilometer you drive. (That’s 5.6 ounces per mile, for my American and British folks.) I would take a guess that most readers are driving vehicles that pump out a lot more. Now multiply that weight times the number of miles you drive to work or school. Then x2 because you have to drive home. Now x5 for a work-week. Now x50 for weeks in a year (I’m assuming you take two weeks holiday).
If you drive the TDI above on my commute (9.5km), that means you would be pumping 470.25 kilograms of carbon into the air every year. And that only takes into consideration the driving habits. It does not reflect how much gets burned to keep your lights on, to keep your house heated in the winter and cooled in the summer, how much is used to pump the water that flushes your toilet or runs through your shower. There are hundreds of millions of cars in the United States alone.
Remember how I pointed out that volcanoes, the primary source of carbon dioxide in the atmosphere, that they put out about 300 million tons of CO2 every year? And how this is a naturally balanced value, where geological and biological systems generally absorb this stuff to keep the atmosphere relatively stable?
Humans were not a big contributor to CO2 levels in the 1800s. However, since the beginning of the 1900s, humans have increased their burning of carbon fuels and have exceeded volcanism. By a lot.
We pushed out 31.6 billion tons of CO2 last year (2012). That’s about one hundred times the amount of all the volcanoes on the planet combined. We humans are a force of change exceeding geological scale.
In addition to the greenhouse effect that CO2 is having on the atmosphere, the CO2 is also being absorbed into the oceans – and this is having a very serious effect through acidification of the waters. I may post something about that as well. But for now, this post is just on greenhouse effects…something for you to consider. These things I’ve posted up here aren’t opinions. They’re observations. So when you see someone arguing about whether humans are causing climate change, point them to this stuff above – these are facts, not subject to debate or opinion. And sadly, they lead inexorably to the conclusion:
We are causing this. The only option to avoid the overheating of our one and only home, is for us to change how we behave.
Think about it, every time you climb into the car, every time you use energy. Every time you vote in an election.
Update: to try to add a little scale to the effects this has, I put together a post about annual ice loss and how it equates to the amount of energy our globe absorbs.
Update 2: corrected biggest source of CO2
Update 3 16.10.2018 – fixed broken link on New Scientist graphic