  We present the first results of three-dimensional (3D) numerical
magnetohydrodynamic (MHD) simulations of the onset of magnetic reconnection via
the tearing instability in dynamically thinning current sheets in the solar
corona. In all our simulations, the onset of the non-linear tearing
instability, which leads to the break-up of the thinning current sheet, does
not occur until after the instability growth time becomes faster than the
dynamic thinning time. Furthermore, as in previous 3D MHD simulations of static
current sheets in the corona, for some parameters, the amount of magnetic shear
is a fundamental switch-on parameter, which has consequences for coronal
heating models. These results open up the possibility of using observable
quantities of coronal current sheets to predict when they will break-up and
release magnetic energy to power various energetic phenomena and/or heat the
atmosphere.
