Yes, It's Getting Hotter, and, Yes, It's Our Fault
I thought since it's been a while I should take on a weighty subject. So, I decided I would try to prove global warming. If you read this site regularly, you know that this topic is on my mind quite a bit. And, nothing irritates me more than people who dismiss the idea of global warming as nothing more than some kind of conspiracy, a way for Al Gore to get rich and for scientists to get research dollars.
Specifically, I'm talking about "anthropogenic global warming (affectionately referred to as AGW), also called climate change. "Anthropogenic" means caused by humans. Global refers to the fact that the changes span every part of our planet. Some places may get warmer, some colder, some rainier, some drier. Climate is not the same as weather, meaning that one hot spell, cold spell, hurricane, temperature record, or any other anomaly doesn't prove things one way or the other. But on the average things are getting warmer.
Follow along carefully. The science isn't that hard. You can get more details on any of this from Wikipedia or your other favorite reference. The math is very straightforward. I used Excel.
This chart shows the basis of the argument. It shows the rise in carbon dioxide in the atmosphere since just before the start of the twentieth century, along with the average global temperature in the same time period. Note that I average temperatures over an 11 year period, because that is the sun spot cycle and I wanted to remove influence from that. You can see that temperatures rose sharply at the beginning of the century, up until about World War II, fell for several years, and then resumed an even more steep rise, through today (actually, 2007, which is the latest year for which there is complete data). There has been about a 0.8 degree C rise over this period.
Is this significant? Well, since human civilization evolved, the average temperature of this planet has hovered within a range of about a degree, and we are now at the high end of that range with temperatures still rising. The eight highest global temperatures on record were recorded in the last ten years (1998-2007). Not only are temperatures rising, the rate at which they are rising is going up -- the change is accelerating.
So, global warming is real. But how do we know that it is caused by humans?
Well, look at the second line in the graph, the amount of CO2 in the atmosphere each year. This has gone up every year (except for a brief respite around World War II) since the start of the industrial revolution, when humans starting burning fossil fuels. Prior to that, CO2 levels were pretty much constant from year to year.
Ok, so CO2 has gone up. But is there any proof that humans are responsible for this, and how do we know that this is where the temperature rise comes from?
There are actually several ways to show that humans are responsible for the rise in CO2. The most obvious is the time coincidence. The level was constant prior to the mass release of carbon from under the earth into the air. Then it started going up, with the rate of increase going up as people starting burning more fossil fuels. The second piece of evidence is the spatial coincidence. Measurements of CO2 levels have gone up most in parts of the earth where more fossil fuels are burned (e.g., in the northern hemisphere vs. the southern).
But there is a more direct way of measuring the human responsibility for CO2. See, a percentage of carbon in the biosphere is a radioactive form, C-14. But carbon in fossil fuels has been buried deep in the earth so long that the C-14 has decayed. By measuring the amount of C-14 in tree rings, scientists can determine the portion of carbon that came from fossil fuels each year, and that amount has gone up steadily as the CO2 has gone up.
Ok, we're almost there. Temperatures are definitely rising, CO2 has risen along with the temperatures, and humans are responsible for the CO2. Can we prove that the CO2 is responsible for the warming?
To start, let's look at something called the "greenhouse effect". This is caused by a quantum mechanical effect that happens when certain gases (CO2 among them) are struck by infrared (IR) radiation at certain frequencies. It causes energy to be released in all directions. If the IR radiation happens to be heat reflecting off the surface of the earth from solar radiation, some of that energy continues up while some is reflected back to the earth, warming the surface. At a certain temperature increase, the amount of heat going up equals the amount going down, and the temperature reaches a balance.
Currently, that balance point is about 32-33 degrees C above the nominal temperature on this planet. Without the greenhouse effect, the average temperatures would be 32 degrees C (58 degrees F) colder, and life would probably not exist.
There are several gases that contribute to the greenhouse effect. The largest contribution is from water vapor, H2O, which is not significantly affected by human activities. (Except that as the temperature rises the amount of H2O will also rise.) The second is CO2, which as we've seen has risen by about 35% due to human activities in the past 150 years. There are other greenhouse gases that are also affected by human activities, such as methane and chlorofluorocarbons (the latter of which does not occur naturally in the atmosphere).
To determine the relative impact of the various gases, experiments have been performed in which each gas has been removed and the reduction in greenhouse effect measured. Of course, it is not that simple, since the various gases interact and change the greenhouse effect by combination. So, the portion of the greenhouse effect attributable to CO2 is somewhere between 9% and 26%.
So, to do some simple math, CO2 is responsible for between 2.9 degrees C and 8.3 degrees C of the base global temperature. Another 35% on top of that would theoretically bring about a global warming effect of about 1.0-2.9 degrees C. In the real world, of course, it's not quite that simple. There are greenhouse gases other than CO2. There are other climate impact events, such as sunspots and volcanoes. And there is an expected lag time of a couple decades from when the CO2 is put into the atmosphere until when the corresponding temperature rise is seen.
And indeed we've seen a rise of about 0.8 degrees, pretty darn close to what we just predicted. Some of the CO2 warming hasn't been seen yet, and there are other climate-impacting factors that have changed things over shorter spans (a few years). But the fact is, temperatures have gone up almost exactly in line with what is predicted based on the known science behind the greenhouse effect.
So, bottom line, we have shown that global warming is real. We've shown that atmospheric CO2 has gone up in line with the temperature increases, and that those CO2 increases are caused by human activities. And we've shown that the measured warming is exactly what is predicted based on the greenhouse effect.
For someone to dispute AGW, they would not only have to come up with an alternate explanation for why the earth has heated over the past 150 years, but they would also have explain what happened to the heat from the additional CO2. And nobody has done so.
So, global warming is real and caused by people. That still leaves two big questions -- what will the impacts be, and what should we do. There's still lots of room for discussion on both of these topics (and I'll be talking more about them in upcoming posts). But don't let anybody tell you that global warming itself is a hoax.