Are we still able to terraform Mars using greenhouse effect?

In the last month, a group from University of Colorado, Boulder, published a paper on Nature Astronomy entitled “Inventory of $CO_2$ available for terraforming Mars”, which was highly publicized on popular media. In the work the authors claim that the $CO_2$ reserves on Mars are not enough to support the generation of an artificial greenhouse effect, thus thwarting most of the terraforming plans.

Taking this study in consideration, is there any alternative realistic strategy to perform the martian terraforming process using little or none $CO_2$ to do the greenhouse effect?