This is an essay I wrote for school about a year ago.
Global Warming - Cyclical or Our Fault?
Izzy [Last Name]
Global warming is in no way a new thing. Throughout history, there have been periods of time when it has been a lot colder than it is now (the series of ice ages definitely being some of our foremost evidence, as we didn't have much documentation way back when), but also a lot warmer. During the Cenozoic era 65 million years ago (began when the dinosaurs died out, also the beginning of mammals), Earth was about 18° - 27° Fahrenheit hotter than it is now. Clearly humans weren't alive back then, so I can say without a doubt that this was not our fault. Earth's temperature shot up even further after this period, leveled off, went down for a while, we had a few ice ages, and now the temperature is rising again. (Details can be found here: http://math.ucr.edu/home/baez/temperature/)
There are many theories on what caused all this climate change, the Greenhouse Gas Effect most common among these. Greenhouse gases in the atmosphere make the atmosphere retain more of the heat from the sun, as it traps and re-directs the heat as it reflects back off the Earth. Contrary to common belief, green house gases are actually good for us. If we had no greenhouse gases in the air then we'd all be dead, because it'd be very, very, very cold. We are certainly putting more of these, CO2, SO2 and so on into the air, but the effect of that is dwarfed by the two main greenhouse gases, which are water vapor and methane. Though we have significantly added to the amount of methane through cattle ranches, flooded rice fields, and other things, all these gas emissions combined are still behind termites when it comes to the biggest producers of the stuff worldwide.
Human production of CO2 and SO2 also doesn't amount to much. A day, the average size volcano can emit anywhere from 20 tons to 10 million tons of S02, depending on volcanic activity, type, and the volume of magma involved. For example, the large explosive eruption of Mount Pinatubo on 15 June 1991 expelled 3-5 km3 of dacite magma and injected about 17 million tons of SO2 into the stratosphere. Along with this, volcanoes release more than 130 millions of tons of C02 into the atmosphere every year. Don't forget that there are thousands of volcanoes! C02 typically does not pose a direct hazard to life because it becomes diluted to low concentrations very quickly. But in certain circumstances, C02 may become concentrated at levels lethal to people, especially in volcanic areas where C02 emissions occur.
The point is, volcanoes and termites combined, in one year, produce way more SO2 and CO2 than the human race has in its entire history. Volcanoes came before us, volcanoes will stay pretty much long after we're gone. The planet is going to be fine pretty much fine whatever we do, but we stand some chance of wiping out a fair amount of life. So to conclude, it's mostly a natural process (heh, actually it's kind of arrogant to actually think we have so much affect on our planet), but we are doing something to make it worse, and being natural doesn't mean it'll be good for us.
Personally, I'm waiting until 12:01 AM, 1 January, 2013 before any "I told you so.." messages go out.