Image Credit: NASA / Robert Simmon
Image Credit: National Geophysical Data Center
As I promised yesterday, in honor of Earth Day, today I'll debunk some claims that global warming is being caused by something outside of the Earth. In fact, scientists are in agreement that global warming is caused by human activity. But, since I can speak with authority on astronomical issues, I'll stick to that for today.
Above are three graphs. The top two come from NASA's Earth Observatory website. The very top graph is the amount of carbon dioxide in Earth's atmosphere over the past century. Notice how fast it has been climbing, especially in the last 50 years or so. The second graph is a plot of average global temperatures over the same time period, set so that zero is the average temperature from 1951-1980. It also is going up dramatically, especially over the last 50 years or so. In fact, if we use records of global temperatures and carbon dioxide recorded in the ice of Antarctica, we find that temperatures and carbon dioxide mirror each other over the past 400,000 years. That's pretty strong evidence that the two are closely linked; there are many other experiments and measurements that prove that the link exists.
Despite this, some have claimed that global warming is due to an increase in radiation from the sun. The brightness of the sun is hard to measure, partly because the sun is so freakin' bright that it blows away all but the most carefully crafted instruments. We also have to measure the brightness of the sun from space, because Earth's atmosphere allows variable amounts of sunlight through, even on seemingly clear days. So, we only have records of the sun's brightness going back to about 1980, when satellites started measuring the sun's brightness.
The bottom graph above shows the brightness of the sun measured by different satellites (different colors of points). Notice that different satellites get different measurements! That just shows how hard it is to get an absolute number. But, since most of the satellites overlapped with one another, we can assume that measurements taken by two satellites at the same time should give the same answer, so we can shift those points to the same value. We astronomers would call this "relative photometry," and it is well-accepted practice. The black points in the graph of the sun's brightness show the relative photometry of the sun.
What you can see is that the relative brightness of the sun varies every 11 years; this is the normal solar sunspot cycle. The sun is a tiny bit brighter when it is more active, and a tiny bit fainter when it is less active. So, just compare the high points (from about 1980, 1990, and 2000) -- they are all at the same level! Look at the low points (about 1986 and 1997) -- those are also the same level. So, although the sun does vary in brightness, it does so in a regular fashion, and has not gotten systematically brighter over the last 30 years.
Now compare the sun's brightness to the graph of Earth's temperature, which shows an overall increase of about 0.5 degrees Celsius (about 1 degree Fahrenheit) since 1980. The Earth is getting steadily warmer, the sun is no brighter than it used to be.
This, folks, is pretty strong evidence falsifying the hypothesis that global warming is caused by a brighter sun. The sun is no brighter than it was 30 years ago, but the Earth is warmer than it was 30 years ago.
Now this doesn't mean that the sun doesn't affect Earth's climate. In the late 1600s, the sun went through an extended period of low activity called the Maunder Minimum, and the Earth did cool off by about 1 degree Celsius during this time. Also, over hundred million year time scales, the sun is getting brighter; we estimate that in just 1 or 2 billion years or so, the sun will become bright enough to evaporate Earth's oceans. But the sun is not more active than normal, and its gradual increase in brightness is far too slow to explain the Earth's current warming.
Other people claim that other bodies in the Solar System appear to be warming up, specifically, Mars, Jupiter, Triton (a moon of Neptune), and Pluto. But half of Jupiter's heat does not to come from the sun at all, and more rigorous studies show that the total amount of heat energy over Jupiter has not changed appreciably. Triton's south pole has just reached summer, which lasts a good 40 years, and so would be expected to warm up. Pluto gets much closer to the sun at certain parts of its 250-year long orbit; it reached its closest point in 1989. The extra warming from the sun melted some of Pluto's outer layers of methane ice into methane gas; methane gas is a known and very effective greenhouse gas, and so we'd expect Pluto to warm up even more once it has some methane. Over the next few decades, though, this gas should cool off as Pluto moves away from the sun, and freeze back onto Pluto's surface.
That leaves Mars. The evidence for global warming on Mars is that the southern polar ice cap has shrunk a little more each of the past three Martian summers than in the previous summer. Does this mean global warming is happening on Mars? The majority of Mars climate modellers say no, and that Mars may be cooler now than it was in the 1970s. The truth is, it is only in the last 15 Earth years or so that Mars has been under constant, close scrutiny by a flotilla of satellites; that adds up to about 7 or 8 Martian years. We've had better data on Earth for decades, and the Earth often shows short-term (few-year-long) climate swings (like El Nino). Long term monitoring of Mars's climate is needed to determine what it's long term trend is. We know the long term trend on Earth, it's up (see the graphs at the top again).
In short, there's no convincing evidence that global warming has causes related to astronomical phenomena, and, in fact, there is strong evidence to the contrary.
I wish I could say that global warming is not happening or is not caused by humans, and that we all can go on living our lives and not have to worry about it. Unfortunately, it is real, and we are the cause. I gain nothing from stating that fact.
But facts are stubborn things; and whatever may be our wishes, our inclinations, or the dictates of our passion, they cannot alter the state of facts and evidence.
The fact of global warming means that we all are going to have to make some tough and unpopular lifestyle changes. The sooner we start acting these changes, the less painful the costs will have to be. The longer we wait, the more expensive global warming becomes, the more drastically we have to act, and the more painful the choices we will have to make.