Plotting it this way is just a shift and rescaling. As the other commenter already showed this will lead to a plot that looks pretty much the same in terms of difference relative to the past data. I think standard deviation makes sense here since we don’t really have an innate sense of what is a meaningful change in ocean surface temperature and the point is to show that we are already far beyond normal here.
And expressing a number as some percentage more or less than another number is a great way to make it meaningless.