PyPSA: Global constraints directly read in as component attributes indexed by year?

Question

I would like to impose a CO2 emissions constraint by year in PyPSA. I found this example on google groups showing how to add it as part of the code. What I would like to do is to have the network read the constraints in by year.

The documentation states that investment_period is an attribute, but when I add more rows to an input global_constraints.csv like below, I get an "ERROR:pypsa.io:Error, new components for GlobalConstraint are not unique". Most global constraints (RPS, CO2 emissions etc. are driven by annual policies. Is there a way to implement this directly from an input csv without having to write explicit constraints?