(1) Go to the Berkeley Earth Data Overview site
here.
(2) Clicking on the text "+Global Time Series Data" reveals a link labeled
"Monthly Global Average Temperature (annual summary)".
If you are using Chrome and if you click on that link,
the text file of data will appear in your browser window.
If instead you right-click on the link,
you will be offered an opportunity to download the file to your computer.
To download just the monthly data in csv format, click here.
Alternatively, here is the URL you can enter into a Google Sheet:
https://www-users.cse.umn.edu/~mcgehee/Course/Math1901/material/BerkeleyEarthMonthly.csv