Guest Post By Bill DiPuccio
Many of the comments made on your Three Hypotheses reinforce my belief that the word “significant”, when applied to human influence on climate, may require further qualification in order to remove any confusion.
Anthropogenic forcing is a continuum: It can dominate climate change, modulate natural variability, or have little to no impact at all. I believe what you mean is that human influence is “significant (but not necessarily dominant)”.
The problem, in part, is one of parameter, scale and location. Though spatially small, the UHI effect in the center of a large city may produce a 5F-10F difference in max summer temps when compared to outlying rural locations. In this case, the anthropogenic effect is the dominant factor in meso-scale climate change as regards the increase in average summer temp. Deposition of black carbon aerosols may have a large measurable impact on regional climates in polar and glacial areas by accelerating melting, but little or no measurable influence in the tropics. In both of these examples, locations downstream will naturally experience some effect along the impact continuum.
When parameter, scale and location are taken into consideration, Hypothesis 2A becomes a composite that more accurately reflects the breadth and complexity of climate change as well as the current state of climate science which is becoming increasingly nuanced. It takes into consideration the potential dominance of natural causes or CO2 on specific parameters, scales and locations, without exaggerating (as do Hypothesis 1 and 2B) their influence.