Real engineers use datum in proper context. Data, as the plural of datum is frequently used in statistics, or in a database field with multiple records, but multiple sources of datum are used in multivariate expressions, such as singular records of multiple fields.
Many designers confuse and interchange the two words.
The word "data" is commonly used as a singular noun to describe any number of records of any number of fields.
Many designers confuse and interchange the two words.
What I describe is the common usage amongst the many hundreds of engineers that I work with and it's been like that for the 30 years that I've been doing it.
And addressing the original point, when the president of the Sierra Club says that the “data are” instead of the “data is”, he is using the uncommon usage of the data a a plural noun.