Human emissions levels were very low in the Little Ice Age, compared to today. The only proto-industrialized part of the world was Europe, and Braudel estimated

Human emissions levels were very low in the Little Ice Age, compared to today. The only proto-industrialized part of the world was Europe, and Braudel estimated that Europe had access to 1.5-2.5 million horsepower total from all machinery (including wind- and water-power). That's roughly 2 gigawatts of power; we might guess that burning fuel for heat produced about the same order of magnitude. (Fuel was very expensive as late as the 1930s; Orwell has some interesting statistics in _The Road to Wiglan Pier_.) Double that is probably enough to account for the rest of the world: not more than 10 gigawatts of human-generated power and heat, some fraction of it from wind and water. 10 gigawatts is probably too large. Today, the United States apparently consumes about 2,000 gigawatts (2 terawatts) of power from power plants, and another 2 from all other sources; Europe, China, and the rest of the world probably consume another 2-4 terawatts apiece, for close to a petawatt total -- about 100,000 times the power output of the world in the Little Ice Age. I don't rule out the possibility that human emissions are having no effect or a limited effect -- or that their effect is positive, by keeping the Little Ice Age from developing into the real thing -- but I'm also not ruling out that the prophets of doom might be on to something. (Jeremiah was simply correct, after all.) I'm hoping that emissions-cap negotiations get somewhere, and that we see more use of alternative energy (especially nuclear and geothermal), more artificial and biological carbon sequestration, more hybrid and electric cars (and is it too much to wish for, to hope that we see more commuter rail?), and general efforts to keep things from getting out of hand. We don't know what happens next, after all, and we only have to get it wrong once. Besides, if these sorts of things start producing hazardous levels of global cooling in 50 years, we can just scale them back, or even remove some filters from remaining coal-fired plants. This sort of effort will be expensive, but I'd rather risk doing more than we need to than not doing enough...