George, My name is Charlie :)
It's hard to isolate the effects of BC in nature because of all the other processes that simultaneously affect regional climate. Snow is disapppearing more quickly in springtime in Asia than North America. Flanner et al. ACP 2009 show this is consistent with absorbing aerosol effects. Painter et al. GRL 2007 show that snow downwind of dust (which is less absorbing than BC) melts earlier. I doubt one can find any serious claims that BC, in "normal amounts", cools climate. The caveat on normal amounts, BTW, is to recognize that extreme amounts (e.g., nuclear winter) could, theoretically, cool the surface by shadowing it while stabilizing the atmosphere above, i.e., net heating in the column but a colder surface (weird, huh?). You're on the right track that snow only melts when T > 0. What happens when BC changes T from -10 to -9 is that snow crystals, on average, grow larger (this is called thermal metamorphism). Larger crystals are darker, darker crystals absorb more sunlight, so the BC-induced warming triggers a positive thermodynamic feedback which, absent other processes, warms the snow further. If the BC causes any snow to melt, then the aging is compounded by "traditional" snow-albedo feedback due to the darker ground uncovered by the snow. Either way it's game over, snow-man. And you are exactly right that BC effects on snow are most pronounced in regions on the cusp of freezing (mid-latitudes in early spring, higher latitudes a month or two later, or higher altitudes appropriately staggered). "Chris" -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineering@googlegroups.com. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
