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There are many possible causes of the warming observed at the surface of the earth in the 20th century. Possibilities include:
1) The sun.
2) Changes in earth's albedo (either natural or caused by humans).
3) Greenhouses gases (e.g. carbon dioxide, methane, chlorofluorocarbons) emitted by humans.
There is substantial evidence that the sun's output increased during the 20th century. One way that the sun's output can be evaluated is by evaluating sunspots. More sunspots mean a brighter sun (i.e., more solar output). This may be surprising, since sunspots are darkened areas on the sun, but it's true. Also, sunspots change over a cycle that has a nominal length of approximately 11 years. A shorter sunspot cycle also means a brighter sun (more output), while a longer sunspot cycle means a dimmer sun (less output).
In a 1991 article in Science magazine, two researchers (E. Friss-Christiansen and K. Lassen) presented a figure that related sunspot cycle length to temperture. The figure appeared to show a very striking correlation. A hyperlink to the figure is below. The figure is not in English, but the x-axis contains dates, from 1860 to 2000. There are two y-axes. The left y-axis is sunspot cycle length in years, with longer periods (11.5 years) at the bottom, and shorter years (10 years) at the top. The right y-axis is global temperature variation, in degrees Celsius. Graph from 1991 Science article by E. Friss-Christiansen and K. Lassen
Solar activity (output, in Watts/square meter) has been has been related to Northern hemispheric temperature going all the way back to the 1600's (when sunspots were first documented by Europeans). The correlation for this longer temperature record seems even more striking. Solar variation versus temperature (and CO2) back to 1600
From looking at these two graphs, you might say, "Well, that settles it! The warming in the 20th century was simply caused by the sun! Case closed! Let's grab a beer, and talk about something more interesting!" But it's not quite that simple. The energy flux (power per unit of area) from the sun can be measured in space. When this flux is measured, the flux doesn't seem to be enough to directly cause the measured amount of warming. In fact, the IPCC calculates that the flux increase is approximately X watts/square meter, which they calculate to be only about 1/10th of the flux increase caused by greenhouse gases (except water). Therefore, if the sun is causing all of the observed warming, some sort of indirect effect would have to be involved. One such possible indirect effect is that changes in the sun's strength may change cloud cover over the earth.
There is substantial disagreement between the IPCC documentation on the effects of the sun on the global warming of the 20th century, and the opinions of many experts who study the sun. As mentioned above, the IPCC estimates that the sun was responsible for only approximately 10% of the warming of the 20th century. Experts on the sun's effect on climate generally estimate a much higher effect for the sun. Their estimates range as high as 30%, 50%, 70%, or even more. Let's hold off on making a decision, until we've looked at some other possible causes of global warming.
As mentioned above, changes in albedo can be both natural, or caused by humans. I'm going to focus on changes caused by humans. In particular, I'm going to focus on the changes in albedo that cause the "urban heat island" effect. An "urban heat island" is caused by the asphalt and concrete in cities (e.g., asphalt roads and asphalt shingles on roofs). Many urban areas also lack trees, which cool by providing both shade and evaporation during the day. The urban heat island effect is well documented, and definitely worthy of research for mitigation.
But from a global warming standpoint, perhaps the most important aspect of urban heat islands is that they can falsely make it appear that more warming is occurring than really is. If a temperature measurement station is originally in a rural area, and then the area around the station becomes more urbanized, the station's temperature measurements show warming that may not be occurring in the whole region.
It's possible to adjust surface temperature measurements to "cancel out" the effects of urban heat islands. This can be done by removing stations that are known to be influenced by the heat island effect from the temperature measurement database. But it may not be possible to completely cancel out the effects of urban heat islands, especially for more recent years. This is because an urban heat island can only be detected by comparing records of nearby sites. If one site experiences a recent rise in temperature, it's not always possible to tell whether that rise is duplicated at other sites, or is an anomoly.
Most of the reasoning behind the amount of warming that is produced by CO2, methane, and CFCs comes from computer modeling of the earth's atmosphere. This is not my field of expertise, but as far as I'm aware, in order for global warming to be caused by CO2, methane, and CFCs, the models all agree that two things must be true: 1) the temperature of the lower troposphere must increase faster than the temperature at the earth's surface, and 2) the temperature at the poles must increase faster than other areas of the globe. If either of these two phenonena is not occurring, it's blow to the entire theory of global warming caused by human emissions of CO2, methane, and CFCs. At present, the predonderance of evidence is that neither of these phenomena is occurring, which is a huge blow to the entire theory of gloabl warming caused by human emissions of CO2, methane, and CFCs.
The most reliable method we have of measuring temperatures in the atmoshper
It's a tough call to say how much of the surface warming observed in the 20th century was due to human emissions of CO2, methane, and CFCs. Probably at least some of the warming was due to emissions of these gases. But, without question, the sun was responsible for some of the observed surface warming. The fact that atmospheric temperatures (as measured by satellites) haven't increased even faster than surface temperatures, is a huge blow to the idea that much of the warming was caused by emissions of CO2, methane, and CFCs. Likewise, the recent evidence that the temperatures at the South Pole haven't increased is another huge blow. However, in order to make predictions about the effects of future changes in temperature caused by emissions of CO2, methane, and CFCs, it's necessary to make some sort of "engineering judgement" about the effects of CO2, methane, and CFCs on the temperatures of the 20th century. The choice would of course vary between 100% of the temperature change caused by CO2, methane, and CFCs, and 0% (no effect) caused by CO2, methane, and CFCs. I'm going to use my engineering judgement to estimate (guess) that 50% of the surface warming observed in the surface temperature measurements of the 20th century was caused by human emissions of CO2, methane, and CFCs. Taking a value of 0.6 degrees Celsius total warming, 1/2 of that would be 0.3 degrees Celsius, or approximately 0.5 degrees Fahrenheit caused by human emissions of greenhouse gases.
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