This is interesting:
First is a graph of solar irradiance since the early 1600s. Notice the sharp rise in the early 19th century, the fall from 1840-1890 and then the marked rise from 1910 or so until 1960 when the sun stabilized at a high level or irradiance.
Now, look at the historical temperature record for Fort Snelling near Minneapolis, Minnesota, in the center of the North American continent, which has fewer oceanic influences than other places and thus responds faster to changes in solar radiation.
What we see is that the temperature at Fort Snelling reflects changes in solar irradiance pretty closely. Notice the marked cooling between 1840 and 1890. Notice the warming between 1890 and 1960. This coincides with the substantial increase in solar irradiance.
If you look at this as a basis for climate change, then you can start to build on top of that with other variables: The PDO and AMO have moderate influences on our climate. So do the Pacific-North American Oscillation, Arctic Oscillation, and the North Atlantic Oscillation. You also have aerosols and greenhouse gases.
What shocked me most about these two graphs is that I dug and dug and couldn't find a good reason as to why the climate cooled so much during the mid-late 19th century. I think I may have found the culprit staring me in the face (well, not anymore now that the sun has set).
What is significant is the stability of solar irradiance since 1960. Notable warming has taken place during this time. But one must also keep in mind that other drivers have influenced this: All of the associated oscillations were in a position from the late '70s to the late '90s that supported a warmer/wetter regime. Since the mid-90s, it seems the AO and NAO have stabilized back to normal and the PDO and AMO have switched regimes. And yet 1998-2006 was by far the warmest period of any in the record.
If you look at the Minneapolis climate record from 1820-1997, it doesn't appear that there is all that much warming going on.. certainly not outside the realm of normal.. but 1998, for whatever reason, changed that.
Any thoughts as to why things have been so warm since 1998?
Edit: One idea I do have is that a very strong El-Nino can have some sort of longer term affect on temperatures. If you look at the period of 1877-1883 or so, you notice a period of warmer years sticking out like a sore thumb. That period was preceded by a very very strong El-Nino in 1877/78 that led to an unprecedentedly warm winter. Like 1997/98 which was quite warm here, the following winters, despite being "La-Nina winters" as 1878/79 and 1879/80 were, were still warmer than normal.
Just some thoughts on that.