While stratospheric ozone depletion has already been shown to be the primary driver of the expansion of the tropics in the Southern Hemisphere, the researchers are the first to report that black carbon and tropospheric ozone are the most likely primary drivers of the tropical expansion observed in the Northern Hemisphere.

Again, based on MODELS not data.

I do, however, find this somewhat more likely a driver for any slight temperature increase than CO2, part of the remediation if that is shown by actual data not models to be true would be to raise the standard of living worldwide, reduce the amount of cooking done over wood/coal/charcoal/dried dung fires, replaced by electricity availability to the no longer starving masses. This would also have great benefits on the health of those who rely on primitive means of food preparation, on infant and elderly mortality and on overall improvement in the ecology (which again does include humanity)

Study results appear in the May 17 issue of Nature.