Thread by @DekeArndt

Occasional* Reminder that the choice of base period for global temperatures does not change trends or other findings. And a bit of an explainer on the choices we make (1/n)

* - "occasional" typically means "I spent some time on the phone on this today"

The pretty graphic upthread is the annual global temperature anomaly (or departure) from the 20th century average. Each red dot is an individual year. Dots above the black line were warmer than the 20th century average; dots below were cooler than the 20th century average. 2/n

Pink bars are decadal average anomalies, very simply the average of the ten red dots in a decade. The most recent decade is in a different shade because it's the current one, and partly it only has 9 member years. It's getting warmer, but that's not why we're here right now. 3/n

We're here to look at base periods, and how they [DON'T] affect things. Here's the very same data, but with pretty red circles plotted as less pretty blue and red bars. Same logic applies. You can click this here very graph in CAG:
ncdc.noaa.gov/cag/global/tim…
4/n

The trend since 1880? 1.26 degrees Fahrenheit per century (0.70 deg Celsius). 5/n

Sometimes, this data is plotted versus the 1981-2010 average (often called "normal" - a problematic but historically entrenched term). The red circles are lower; because they're being compared to the warmer 1981-2010 period. But the trend is identical: 1.26 deg F / century. 6/n

Sometimes, people want to compare to "preindustrial" times. We often use 1880-1900 as an imperfect but representative proxy. The circles in this case are higher, because the comparison period is cooler, but the trend through the years is the same: 1.26 deg F / century. 7/n

Changing the "base period" moves the dots up and down in unison (probably better to say the y-axis moves, but let's not complicate things). Each frame tells us something relevant to a specific period of reference. The rate of change is not altered. 8/n

Similarly, in life, perspective helps us see details relevant to our point of view, but perspective does not change the truth. Your perspective is unique and beautiful. So is the truth. They work together to make your life rich and worthwhile.. 9/9