Sunspot Cycle Length with North American Land Temperatures and CO2
This graph is based on E. Friis-Christensen and K.Lassen, JSTOR 1991.
Their original plot showed the sunspot cycle length filtered with 1,2,2,2,1 weighting factors (red line) and the North American land temperatures (blue line). It used a sunspot filtering scheme that averaged the cycle lengths over 5 cycles, or about 44 years, including 22 years ahead of the current data point time. The graph used North American land temperatures rather than global temperatures. I have added a sunspot cycle length curve using a 1,2,1 filter (orange line), which means that each plotted value is the previous cycle length plus 2 times the current cycle length plus the next cycle length divided by 4. The reduced filtering shows more curve variability. As the purpose of the graph is to show how Sun activity correlates to temperature, using future cycles in the filtering scheme seems problematic as future cycles can not affect current temperatures. The CO2 concentration is shown as the green curve. The graph suggests that the Sun has a major effect on temperatures; however, the temperature rise since 1980 is greater than can be explained by the cycle length, possibly indicating a CO2 effect. Recent research shows that about half of the temperature rise since 1970 is due to urban development, known as the urban heat island effect.