The last time the Earth had a 15-year cooling trend of any kind...
I was asked by a friend to identify the last time the Earth experienced a 15-year cooling trend. The way I answered this was to use a rolling regression on GISS surface data (R code at the bottom). Turns out that the last time was before I was born. The period from February 1958-January 1973 (cooling of -0.00188ºC per decade) was the last 15-year cooling trend in GISS surface data. Every 15-year period since has shown a warming trend of some magnitude—and yes, that even includes trends starting in 1998. Figure 1. Graph of temperature trends over each 15-year period. Each point represents the trend over the preceding 15-year period. Now as to the last time the Earth showed a statistically significant 15-year cooling trend, well, that's a bit tougher to answer. The code I'm using cannot account for autocorrelation, which means that it is biased toward showing significant time series trends when in reality the trends are not signific...