TOA is not at a fixed height in geometric height (z) or geopotential height (Z), or geopotential (Φ); it is approximately at a fixed pressure (p) at p = 0; it is also approximately at a fixed sigma level at σ = 0. (realclimate.org)
Now, a robust 95 % CI is available for almost 60 years, and while that 5 % doesn't sound like much, it's a full additional sigma level in and of itself. (judithcurry.com)
When the drivers might do 1.5 or up to 4.5 it seems hard to say we know to 5 sigma levels what they do unless we retain a broad range. (judithcurry.com)