The Atlantic posted a similar article recently. Full disclosure mk, I only skimmed that one and I haven't read your article yet. I promise to do it tonight.
That said, you said . . .
I wonder what contributes to the other 1.8mm.
I'm no scientist or expert, but there is A LOT of water out there. We have ice and snow capped mountains, for example, that are quickly losing said ice and snow and replacing it with less each year, both in various balances of quantity and duration. This leads to problems like droughts and such, because rivers, lakes, and resevoirs depend on snow melt to replenish themselves.
As far as quaintity is also concerned, the ice in the ocean in the arctic circle has been receding and thinning for quite some time. That's why countries such as the U.S. and Canada and Russia are looking at the area for new navigational and resource opportunities. What was once unpassable water is now passable.
I'm sure there are other things I'm not thinking of but those big two easily come to mind. Like I said, I don't know what both articles do and don't cover, but hopefully they discuss the impact that this ice loss has in concerns of the Albedo Effect. Our ice cover does a great job of deflecting a lot of the Sun's energy. Without it, the oceans and land masses that used to be covered in ice but are no longer, or are covered for a shorter amount of time, are now absorbing more heat than they used to. In terms of compounding an already dire problem, that increase in absorbed heat is probably a big factor.