Why did the researchers base their volcanic study on a model?

In an article at WUWT today, Eric Worrall asks what he claims is an obvious question (archived here), but wasn't - at least not to me (see below). He was writing about an article published in the Journal of Geophysical Research: Atmosphere. The paper was by a team led by Thomas J Aubry from the University of British Columbia. The scientists were exploring the impact of global warming on future volcanic eruptions.

Eruption at Eyjafjallajökull April 17, 2010. Credit: Árni Friðriksson

What the authors found was that under two of three Representative Concentration Pathways (RCP4.5 and RCP8.5, but not so much under RCP2.6), global warming will likely have the effect of reducing the cooling effect of volcanoes.

The reason for this is, as the authors write: "the critical mass eruption rate required to reach the tropopause will increase as a consequence of:

• a decrease in the heights of tropospheric plumes driven by a decrease of the tropospheric temperature lapse rate; and 
• an increase of the tropopause height."