  Meidt et al. 2015 derived short lifetimes of 20-30Myr for giant molecular
clouds (GMCs) in M51. Their novel approach utilizes a decline of the GMC
population during their inter-arm passage from one spiral arm to the next.
Using the inter-arm travel time "t_travel" as a fiducial clock, they converted
the decline rate to GMC lifetimes. They implicitly adopted zero as the pattern
speed of the spiral arms, resulting in a very short t_travel. However, it is
well established that the spiral arm pattern in M51 is rotating (Meidt et al.
2008, 2013), and that t_travel should be measured with respect to the rotating
spiral pattern. Here we use the same method and data of Meidt et al. 2015 and
re-evaluate GMC lifetimes by accounting for the pattern speed given by Meidt et
al. 2013. This correction gives a longer t_travel and consequently longer GMC
lifetimes of 60-500Myr.
