Fair point. I can see how a global rise in antacid use could explain it more easily. If antacids are the culprit, I expect the rise in bicarbonate to eventually asymptote.

Still, we know that very high sustained carbon dioxide does risk respiratory acidosis and a rise in serum bicarbonate. The question then is whether a lower sustained rise also has an observed effect.

Fwiw, as per Figure 2A, thus far we see a linear increase in the data, with no sign of it being asymptotic.