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:
The study shows that it is highly likely that all of the observed warming over the past 50 years was caused by human activities and, while natural variability could have contributed up to about a quarter of the amount over any comparable period, the recent slowing in global temperature rise suggests that natural variability has contributed to offset the human-induced component if anything.
Climatologist Michael Mann emails me:
Natural forcing over this period was negative. So the most likely conclusion is that anthropogenic forcing explains more than 100% of the observed warming. There is an internal variability component too, but no reason to believe that it has been positive.
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).
Here’s more background on the study:
Knutti and Huber found that greenhouse gases contributed 0.6–1.1 °C to the warming observed since the mid-twentieth century, with the most statistically likely value being a contribution of about 0.85 °C. Around half of that contribution from greenhouse gases — 0.45 °C — was offset by the cooling effects of aerosols….
The authors calculated a net warming value of around 0.5 °C since the 1950s, which is very close to the actual temperature rise of 0.55 °C observed over that period. Changes in solar radiation — a hypothesis for global warming proffered by many climate sceptics — contributed no more than around 0.07 °C to the recent warming, the study finds.
To test whether recent warming might just be down to a random swing in Earth’s unstable climate — another theory favoured by sceptics — Knutti and Huber conducted a series of control runs of different climate models without including the effects of the energy-budget parameters. But even if climate variability were three times greater than that estimated by state-of-the-art models, it is extremely unlikely to have produced a warming trend as pronounced as that observed in the real world, they found.
“This tightens estimates of past responses,” says Gabriele Hegerl, a climate scientist at the University of Edinburgh, UK, “And it should also lead to predictions of future climate change that are grounded in the kind of changes already being observed.”
A key point of the study is that our actual confidence that humans are warming the planet is stronger than the confidence just from this one study.
Last year, Time magazine reported on a comprehensive new review paper of “100 peer-reviewed post-IPCC studies” in an article titled, “Report: The Case for Global Warming Stronger Than Ever” noting:
By looking at a wide range of observations from all over the world, the Met Office study concludes that the fingerprint of human influence on climate is stronger than ever. “We can say with a very high significance level that the effects we see in the climate cannot be attributed to any other forcings [factors that push the climate in one direction or another],” says study co-author Gabriele Hegerl of the University of Edinburgh.
It is a “settled fact” that the climate system is warming, as the National Academy concluded in 2010. It is now beyond a reasonable doubt that humans are responsible for most of that warming, and highly likely we are responsible for all of it.
- Eight Must-Have Charts Summarize the Evidence for a “Human Fingerprint” on Recent Climate Change
- 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.”