Compared to 1900, how much of California has been surfaced with asphalt, concrete and heat sink construction materials and heat creating populations. Yes there was a Medieval Climate Optimum. And warm ups greater than today's. As a history lover I like to correlate this image with history. As I look at this chart it does appear 1900 was a tad cooler than today. But, still in the warm up after the Little Ice Age. Be thankful you live in an interglacial period. If you are really worried about it, the solution is as easy as setting off some volcanoes.
