Well, I can't pretend to know the answer to that question but after I "stumbled" across the following chart, I realized that conservatives may be deluding themselves with false data. Reproduced below, this chart suggests that there has not been an increase in global temperature changes over the last 4500 or so years. The moment I saw this graph, the hair began to rise on the back of my neck. First, anybody that shows a graph without indicating the units of the y-axis has something to hide. Second, to depict something as variable and complex a global temperature as such a smooth curve suggests dishonesty. Third, the labeling of the chart with the words "Nomanic Time" in bold is bizarre What is Nomanic? Fourth, why does this data go back only 4500 years? Most sources of climate data that goes back several thousand years also include data that go much further back. Fifth where did this data come from? It looks nothing like any climate data that I have seen. Finally, who on earth are Climatologist Cliff Harris & Metereologist Randy Mann? Well keep reading if you are at all interested.
If you look very carefully at the graph, you will find that the baseline of the graph is 57˚F (label on the far right) and there was a point labeled 58˚F for now. They are reporting huge shifts of average global temperature which vary at most a couple tenths of degrees from year to year. The absence of normal variations that one sees in temperature charts indicates that the data must have been made up. Regarding "nomanic times", the Scythians are known as "nomanic invaders" but this is a esoteric word used mostly by historians referring to an obscure Iran-Afghan race. Perhaps it was a mispelling for "nomadic" and a period when the ancient Hebrews were nomadic. This also is consistent with a mostly biblical time line of the earth. The source of the data for the graph is unclear. Finally, if you look up Cliff Harris and Randy Mann, you will find that they are two guys who run a website http://www.longrangeweather.com/About-Us.htm and that neither are trained as a climatologist or a metereologist, unless one considered appearing on television to report weather or studying geology to be training for such a field. Harris apparently is a conservative Christian who believes in looking in the Bible for clues on what the weather will be (Source).
Here are some real data from Nasa and other sources concerning the climate change. You can see that they are very different looking from the graph above.