If what that paper means by "worst-case" is the same as the standard definition in this literature (hard to tell because of the paywall), then it essentially assumes continual acceleration of human contributions to warming, which is extremely implausible. (There are even some papers which argue that there are not enough fossil fuels to enable those rates of warming in the first place.) The rate of warming which is actually projected nowadays (see the last link in that comment) is well below that which is considered in the paper, and was found by the other papers I linked to have limited effects on at least the Atlantic circulation.