When planes were grounded for 9/11 it was calculated that the absence of the vapor trails in the high altitude atmosphere, caused the temperature to drop 2 degrees. That has to do with the fact that high altitude clouds trap heat in.
The volcanic ash is causing low altitude clouds. So it is blocking sun light.
Large scale volcanic blasts have cooled the Earth thousands of times in its history. But temperature is one thing and C02 is another. If a global cooling persisted long enough to cause an expansion of polar ice then more light would reflect and a new cycle could form. And the cooler oceans can absorb more gas including C02 and methane. But this volcano does not have the power to block out enough sun to cause a 2 or 3 degree cooling for a 24 month period.