Wednesday, 14 April 2010
Humans, and other animals, release CO2 as a byproduct of their respiratory system. While an individual human only releases about 1 kg of CO2/day1; a population of approximately 6.7 billion humans means a total of about 6.7 million metric tons (at ~2204 lbs/metric ton) are released per day as people go about their normal lives. That’s a big number; adds up to around 2.7 billion tons of CO2 a year. But it’s *not* what most climate scientists are worried about, and here’s why. There are 2 sources, or reservoirs, of anthropogenic CO2 in the environment.
The first is the plant/animal carbon cycle (this is simplified, there are also ocean/volcano/etc interactions, but they are a slowly changing natural source that we don’t affect that much); this is the cycle where plants ‘breathe’ in CO2 as a carbon source for their growth. Humans then eat part of the plant for food, and end up respiring CO2. Because humans don’t consume all plant material’ this carbon-cycle (not looking at fuel used to produce/transport the food) has a net-neutral/net-negative carbon contribution to global CO2 levels. This is where human society stood for thousands of years. Local farmers grew plants and raised livestock. They even burned wood for light and heat. But, where did that carbon for that come from? The atmosphere, what they breathed out was recycled into the food they produced which let them keep breathing.
The other (important to climate scientists) source of carbon for the carbon-cycle is carbon which is ‘new’ to the cycle. This is primarily carbon which has been naturally sequestered as coal, oil, and gas (all fossilized plant material). Here is a source of carbon which hasn’t played a role in the atmosphere (and thus the climate) for hundreds of thousands of years (depending upon source, time to fossilize, etc). Humans are extracting this carbon and consuming it at prestigious rates.2 This last source is well worth reading as it describes how we know that the ‘recent’ increase in CO2 levels are due to these extracted carbon sources. It was published in 2004, so some of the information is a little dated, I saw an article recently, and now cannot find, which seemed to indicate that we could see the CO2 levels drop faster than the 700-1000 year span of time suggest in 2. However, since I cannot find it, take that with a grain of salt until the source ‘can’ be found. (Cool thing with science, till you find new information you work off the last best data you had J ).
So why should we care at all? Well, CO2 *is* an important atmospheric gas. This transcribed lecture from Columbia University3 would be a good read to understand what the greenhouse effect is to start with. Here is the key point. “According to this calculation, the effective temperature of Earth is about 255 K (or -18 °C)”. In other words, without an atmosphere to absorb and reflect back to the surface some of the reflected sunlight, our planet would have a high-temp of about -18oC (-0.4oF); which is a bit too cold for plant life. So, where does the rest of the heat come from? The greenhouse effect.
I’m gonna dip into chemistry for a second here so bear with me. When a particle absorbs a photon of light (electromagnetic radiation) it can do so in several ways. How it does so is dependent upon the energy of the incoming photon. Here’s the relationship between a photon’s energy and its wavelength. The shorter the wavelength, the higher the frequency, and the higher the energy of the photon; in the visible region this is from 400 nanometers (nm) which is the ultra-violet/blue region up to about 720 nm which is red/infra-red region. Some people can see slightly above/below this band of wavelengths, but that’s the human ‘average’ for the most part. But, the photons of blue light at 400 nm have much more energy than the photons of red light.
An example is in order here. The equation for the energy of a photon is E = hc/λ. The E is energy measured in Joules. The c term is the constant for the speed of light, 3.0*108 m/sec, h is Plank’s Constant, with a value of 6.626*10-34 m2kg/s (or meters squared *kilograms per second). The lambda, λ, term is the wavelength of light (converted to meters). When you determine the units of c, h, and λ you get m2kg/s2. Or, 1 Newton*meter (a Newton is a measure of force with units of kg*m/s2). So. . . Back to our photons; at 400 nm (blue) (4.00*10-7 m) a photon would have an energy of 5.0*10-19 Joules (J). A photon at 720 nm (red) (7.20*10-7 m) would have an energy of 2.8*10-19J. Almost half as much energy.
So, back to the absorbing photons, what does it matter? Well, when an atom absorbs a photon that causes it to glow (fluorescence, or phosphorescence) the energy of the photon exceeds the energy required to excite an electron, when the electron decays back to it’s unexcited state it releases a new photon (and glows). There is energy loss in the excitation so the new photon will have a longer wavelength (lower energy) than the original. But, what if the photon doesn’t have the energy to excite an electron? Well, it causes the atom or molecule to vibrate in 3 dimensions. This is work energy which warms the atom or molecule up. At a high enough energy the atom or molecule can release a photon of energy in the infra-red (or longer) region of light. This is how your infra-red heaters work for instance, or how your spa lamps work. If you go back to the Columbia University source and it’s calculations, it shows what region of light the Earth emits light at. It is in the infra-red (~1100 nm).
But here’s the thing, different gases and liquids absorb and transmit light at different wavelengths. So, since we know the light from the sun is coming in, in the visible region (why we can see), we know that most of the atmospheric gases are able to transmit its light with minimal absorbance (we’ll get more into this in a later post). But what about in the infra-red when the Earth is emitting those ‘reflected’ photons? Ah, there’s a different story. Water, CO2, CH4, CFC’s, they all have absorption regions within the infra-red region of the electromagnetic spectrum. So, our sun (Sol) emits crap-tons of energy, some of which reaches Earth. As it passes through the atmosphere some of it is scattered (causes changes in atmospheric color), some is absorbed (clouds) and the rest reaches the ground. Now, assuming you have a low-reflectance surface (dirt, asphalt, ocean water) the light gets absorbed, loses some of its energy warming up the ground (motion of the molecule) and gets re-emitted as a photon of infra-red light. (If you have a high-reflectance surface, such as ice, most of it gets reflected more like a mirror and less energy is lost/transferred to the ground, this is important for later).
So now our photon has come from Sol, passed through Earth’s atmosphere, been absorbed by the ground and warmed it up. Now, the ground emits this photon as infra-red light which heads back out into space. But along the way something new has happened to the photon. It can get absorbed by all sorts of stuff in the atmosphere, water vapor, CO2, methane (CH4) among other things. So, now it gets absorbed, again, heats up the molecule it collided with, gets re-emitted as an even ‘lower’ energy photon, and keeps heading out into space, or might get reflected back to earth, to be absorbed/re-emitted ‘again’. So, if Earth had an atmosphere with no greenhouse gases, the reflected infra-red light would just head out into space and the surface of the planet would be chilly. But the greenhouse gases trap these photons and cause additional warming. Not a ‘bad’ thing, it’s what lets us live. But what happens when the amount of photons trapped increases? Warming. So there’s the basic mechanism behind ‘how’ greenhouse gases contribute to global warming in general (keeping it from being below the freezing point of water *all* the time), and how we know that humans are dumping a lot of additional greenhouse gases into the atmosphere.
In the future I’ll talk about feedback effects which can cause ‘forcing’ issues, why sea level rise isn’t due solely to melting glaciers and ice-caps, solar energy, and a few alternative energy sources that are being looked at, some of the limitations, and ideas for the future.