Back in 2009, NASA’s Gavin Schmidt was asked, “what percentage of global warming is due to human causes vs. natural causes?” His answer:
Over the last 40 or so years, natural drivers would have caused cooling, and so the warming there has been … 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.
Turns out he was spot on.
A new study in Nature Geoscience, “Anthropogenic and natural warming inferred from changes in Earth’s energy balance” (subs. req’d) finds:
Our results show that it is extremely likely that at least 74% of the observed warming since 1950 was caused by radiative forcings, and less than 26% by unforced internal variability. Of the forced signal during that particular period, 102% (90–116%) is due to anthropogenic and 1% (−10 to 13%) due to natural forcing…. The combination of those results with attribution studies based on optimal fingerprinting, with independent constraints on the magnitude of climate feedbacks, with process understanding, as well as palaeoclimate evidence leads to an even higher confidence about human influence dominating the observed temperature increase since pre-industrial times.
Here’s a figure from the study comparing the magnitude of different “forcing agents” or contributors to warming since the 1950s:
Contributions of different forcing agents to the total observed temperature change. Error bars denote the 5–95% uncertainty range. The grey shading shows the estimated 5–95% range for internal variability. Observations are shown as dashed lines.
Three-Quarters of Climate Change Is Man-Made
That’s not a good headline.
The 74% or “three quarters” probability is where the 95% confidence level is for this one study. As climatologist Kevin Trenberth put it in an email, it is “highly likely” that all of the warming since 1950 is due to human activity: