There is some variability up and down, but the trend is clearly up, and quickly accelerating. It doesn’t seem like much, but we can clearly see the effects of the warmer temperatures, including shrinking ice sheets, declining Arctic ice, and retreating glaciers all over the world. As a result of shrinking ice sheets, retreating glaciers and warming oceans, we are seeing rising sea levels. Even people who were skeptical of global warming are coming to realize that the temperature of the earth is rising. Now the question is: What is causing these rising temperatures?
Scientists have investigated several possible causes of warming, but their research has determined that only one cause explains the accelerating rise in global temperatures. The reason that the planet is getting warmer is due to the rise in greenhouse gasses, primarily carbon dioxide (CO2). Popular discussion in the media might lead you to think that the idea that these gasses could lead to rising global temperatures is a recent claim, but this is not the case. This relationship was proposed in the nineteenth century. Scientists were investigating the relationship between CO2 concentrations in the atmosphere and temperature. They theorized that increasing CO2 would cause the temperature to rise. The test of a scientific theory is whether or not its predictions turn out to be true. Laboratory experiments verified that CO2 and other gasses do act as a greenhouse gas. More importantly, we have been conducting a 100 year experiment that tests this theory on the earth itself. Sadly, it has turned out to be true. As atmospheric CO2 levels have increased, so have world temperatures.
A good scientific theory doesn’t just make predictions. We expect that the theory should explain a phenomenon. In this case, we want to know why temperatures increase when CO2 levels increase. Broadly speaking, the greenhouse effect works like this:
- The sun warms the surface of the earth.
- Some of this heat is radiated back toward space.
- Some of the heat that is radiated toward space is absorbed by molecules of greenhouse gasses.
- Greenhouse gas molecules then radiate some of this heat back to the earth.
Now you might wonder, why doesn’t the radiated heat from the earth just pass through the greenhouse gasses like the initial heat radiation from the sun? The answer is where this theory really gets interesting. The radiation from the sun is mostly visible light and ultraviolet radiation. The shorter wavelength of this light passes through the greenhouse gasses. The radiation radiated back toward space is infrared radiation. The wavelength of infrared radiation is longer and more likely to hit the large molecules of greenhouse gases.
Scientists have also determined the individual wavelengths absorbed by different greenhouse gasses. This enables them to measure how much each gas contributes to the greenhouse effect. You can read about that here.
Finally, research has shown that the reason that greenhouse gasses, and especially CO2, have increased significantly over the last half-century is due to the burning of fossil fuels. If we want to prevent runaway warming in the next century, we need to quickly develop alternative sources of energy.
The NASA and NOAA websites have enormous amounts of great information on climate change. At least it is still available as of February 6, 2017.