Apparently there's evidence for increases and decreases in speed of propagation. The difference is really small, on the order of 0.1 degrees per year, which is

Apparently there's evidence for increases and decreases in speed of propagation. The difference is really small, on the order of 0.1 degrees per year, which is something like 7 miles at the surface and much less at the core. Some models claims it goes from 0.1 degree faster to slower, every 35 years or so.