The consensus of the scientific community is that anthropogenic release of carbon dioxide by burning fossil fuels is the leading cause of climate change in the past couple hundred years. The other theories you mentioned (particularly the Milankovitch cycles and sunspots) are theories related to global climate (like ice age cycles), however, you will find that according to many of those models, the Earth should be cooling rather than warming.

You are looking at a long time span here, 2.5m years. So I wouldn't spend too much time talking about the last 50 years... especially in reference to co2, which, over almost the entire period, is a product of temperature. A single sentence is all it warrants.

Have a look over this, particularly section IV, page 16 on.

ftp://meteor.geol.iastate.edu/data/2005/...

Make your own decisions based on the data and support it that way. Avoid using a voting argument that your professor would consider anti-science, at least for a university level assignment.

Yes, the things you mention are leading theories, and your paper would not be complete unless you include something about human influence even if it is to show that you do not think it is significant.

I note that your assignment is not about global warming, [3] but rather about climate change. In that case, it is very difficult to argue that human influence is not important. Radioactive fallout http://www.sciencedirect.com/science/art...

was rare before World War 2. http://en.wikipedia.org/wiki/Low-backgro...

As old nuclear plants age, http://edition.cnn.com/video/?/video/us/...

and become more numerous, http://maptd.com/worldwide-map-of-nuclea...

this is becoming a common occurrence. http://www.simplyinfo.org/?p=10254

As for temperature, and other climate aspects, there is a lot of natural variation. There are proposed irregular cycles that seem evident, http://en.wikipedia.org/wiki/Bond_event

but the mechanisms are not really understood. One proposal for the Bond event is the heat/cold is stored in ocean water that is sequestered http://answers.yahoo.com/question/index;...

in the Thermohaline circulation to emerge at semi regular intervals every 15 centuries or so. http://en.wikipedia.org/wiki/Thermohalin...

Regardless of the cause of temperature change, they all ultimately result in an imbalance in the energy balance of Earth. http://en.wikipedia.org/wiki/File:Breakd...

Every theory, including the greenhouse theory, http://en.wikipedia.org/wiki/File:Greenh...

must address this.

Important theories:

1. Ocean Currents [1] have been affected by continental drift during the 2.6 thousand millennia of the Quaternary. [1a]

2. CO2, which is supposed to be the main driver according to Warmists, increases and decreases with ocean temperature, but lags behind a bit, [2] indicating that ocean warming is more likely to be causal to atmospheric CO2 increases rather than the other way around.

You may want to watch this video first.