Many researchers in 1999 published a paper examining ice core data from the Vostok lake, which allows us to see 400,000 years into the past (see the previous post about how ice core stratigraphy works).
Here is one graph from their paper (http://www.nature.com/nature/journal/v399/n6735/full/399429a0.html) representing their results:
The graph is a little tough to read, but if you can see on the sides there are 5 lines, representing CO2, temperature, CH4 (methane), oxygen-18 isotope (for explanation see the previous post about ice core stratigraphy) and insolation, respectively.
One of the important parts of this graph is how the greenhouse gases and temperature are related; the graphs seem to be very, very similar, further suggesting a relationship between them.
A very noticeable part of this graph, though, is its cyclical nature. Temperature and greenhouse gas concentrations fluctuate on a cycle, which could fuel the argument from climate change skeptics that climate change is just a natural cycle.
But what cause these cycles? Well, for the most part they are caused by different tilting and spinning of the earth. First, the actual tilt of the Earth’s axis changes in a cyclic nature and this influences the temperature of the Earth. Second, the radius of the orbit around the Sun also changes, affecting temperature. Third, precession, which is the “wobbling” of the Earth around its axis, also affects temperature.
Antarctic Glaciers (via Wikimedia Commons) have an extension of this graph showing new data, and it is clear that the current levels of carbon dioxide far exceed that of the natural cycles in the past.
This cycle is known as the Milankovitch cycle, and it lasts about 100,000 years. However, we should be currently in the cooling part of the cycle, but the levels of carbon dioxide put in by humans have caused us to deviate from this cycle.
This graph is from the NOAA and shows where we should be in the cycle and how different the last cycle was from our current position. (For more graphs and visuals, see http://ossfoundation.us/projects/environment/global-warming/natural-cycle).
So far, current data suggests that climate change is anthropogenic and is a problem that we are currently facing.