(t - l) / l * 100%, where t = value this year and l = value last year
2009
(final mass - initial mass) / initial mass
The differences in mass don't deal with the proportional aspect of the solutions, making the real results less accurate. The percent was calculated to give the exact difference, along with...
(Final-Initial)/(Initial) is the formula for percent change (multiply by 100 to make it a percentage) I think this is what you are looking for.