The chart is relative to the 20th century average, so even though values before 1980 are less than 0, it doesn’t mean warming hadn’t started. It is just that ar

The chart is relative to the 20th century average, so even though values before 1980 are less than 0, it doesn’t mean warming hadn’t started. It is just that around 1980 we got clearly above the average of the 20th century. Imagine this wasn’t deviation from average, but absolute values and ignore the blue/red colors. Then notice the trend — it is getting warmer through all the 20th century. I think, I can easily see a trend line already from 1910.