Picture (above): Macro of snowflake. Credit: Aaron Burden (Unsplash).

This study looks at the average climate in the northern hemisphere with a simplified climate model. It considers the atmospheric effects of mountain ranges (Tibet, Rockies), contrasts between land and ocean surface, and ocean currents at the surface (such as the Gulf Stream) and their impact on winter climate.

The researchers found the individual effect of each forcing was substantially different when compared to the result when all three forcings were combined. This is because the forces individually affect the background temperature, so together they magnify the individual effect even further. In short, the total impact of all three forcings is more than just the sum of its parts. This effect was felt from the land surface up to the stratosphere.

The researchers also found that while individual forcings were more important in some regions of the Northern Hemisphere, overall it was not possible to discern if any single forcing played a greater role than the other.