 Let's look at the pattern of surface temperature changes over the past century. We are looking at surface temperatures relative to a base period from 1951 to 1980, so we're looking at whether the temperatures are warmer or colder than the average temperature over that late 20th century baseline. And we're looking at five-year chunks. We can see that in the 1930s, for example, there was some warming at high latitudes, but not global in nature. We can see that in later decades, the 1960s, the 1970s, there was some cooling over large parts of the northern hemisphere. We'll talk about that later on in the course that might have had an impart component due to aerosol production by human activity. And of course, as we get into the late 20th century, we see large-scale warming that is unprecedented over at least the period covered by the instrumental record.