A: Probably almost all of it
There still seems to be some confusion on this basic question.
A year ago, NASA scientist Gavin Schmidt was asked on RealClimate: “What percentage of global warming is due to human causes vs. natural causes?” His answer is straightforward:
Over the last 40 or so years, natural drivers would have caused cooling, and so the warming there has been “¦ is caused by a combination of human drivers and some degree of internal variability. I would judge the maximum amplitude of the internal variability to be roughly 0.1 deg C over that time period, and so given the warming of ~0.5 deg C, I’d say somewhere between 80 to 120% of the warming. Slightly larger range if you want a large range for the internal stuff.
Absent the increasing GHGs, we probably would have cooled, since
- We’ve had a couple of big volcanoes.
- We’re just coming off “the deepest solar minimum in nearly a century.”.
- The underlying long-term trend had been cooling (see Human-caused Arctic warming overtakes 2,000 years of natural cooling, “seminal” study finds, see figure below).
The more important point is that the rapid increase in the human-driven component of the forcing are increasingly dwarfing the small, slow natural forcings, rendering them increasingly irrelevant (see “Humans boosting CO2 14,000 times faster than nature, overwhelming slow negative feedbacks“). In the Anthropocene Epoch, humankind’s destiny is in its hands.
- How carbon dioxide controls earth’s temperature; NASA’s Lacis: “There is no viable alternative to counteract global warming except through direct human effort to reduce the atmospheric CO2 level.”
- How do we really know humans are causing global warming?*
- NASA: “We conclude that global temperature continued to rise rapidly in the past decade” and “there has been no reduction in the global warming trend of 0.15-0.20°C/decade that began in the late 1970s.”