It is "extremely likely" that human activities are "by far the dominant cause of warming" in Earth's climate since 1950, according to a study published this weekend in the journal Nature Geoscience. The amount of warming contributed by natural forces — for example, changes in solar radiation — during that time was, say the study's authors, "near zero."
Most predictions of temperature increase as a result of greenhouse gas emissions employ a technique called 'optimal fingerprinting', which involves statistical analysis of complex climate models. "Optimal fingerprinting is a powerful technique, but to most people it's a black box," said Reto Knutti of the Institute for Atmospheric and Climate Science in Switzerland, one of the co-authors of the new study, by way of explaining its complexity.
Knutti and co-author Markus Huber instead focused on measurements of factors that influence the planet's total energy budget – for example, incoming radiation from the sun, solar energy leaving Earth, heat absorbed by the oceans and reflected back by snow and ice cover, as well as the effects of greenhouse gases — and ran them thousands of times over, in different combination of varying parameters, in a much simpler model.
Despite the difference in technique, the results were remarkably similar to what existing models had found and what has been observed to be occurring.
They calculated that, with all those factors considered, Earth should have warmed by about 0.51 degrees C over the last 60 years, very close to the observed increase of 0.55 C. Changes in solar radiation were responsible for no more than 0.07C of that increase, they found.
Even factoring in the possibility of a natural random "swing" in Earth's climate, the authors found it was "extremely unlikely" that such a swing could account for more than 26 percent of the observed temperature increase; even if the planet's climate were three times more variable than generally considered, it remained "extremely unlikely that internal variability could produce a trend as large as observed."
That latter observation prompted some reports of the study to carry headlines along of lines of "75 percent of warming is caused by humans." But, as climatologist Kevin Trenberth pointed out in an email that Joe Romm excerpted on his Climate Progress blog, such headlines are somewhat misleading:
"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."
Indeed, the Nature Geoscience paper points out that, by themselves, greenhouse gases actually should have warmed the planet by about 0.85 C since the 1950s. The primary reason global temperatures have not increased that much is because the warming effect of carbon dioxide and other greenhouse gases has been partly negated by the cooling mechanisms of sulfate aerosols, from for example industrial activities and volcanic eruptions, which scatter light from the sun and reflect its energy back into space.
Piers Forster, Professor of Climate Change at the University of Leeds in England, told the Carbon Brief blog that the new study is "pretty convincing stuff":
Observations and the physical law of energy conversation have been used to show greenhouse gases are responsible for global warming and that alternative scenarios violate this law of nature. Previous proofs have relied on complex climate models, but this proof doesn't need such models — just careful observations of the land, ocean and atmospheric gases
Huber and Knutti conclude that their study, combined with existing evidence, "leads to an even higher confidence about human influence dominating the observed temperature increase since pre-industrial times."
Chinese factory worker Geng Peili hangs laundry to dry in a courtyard beside a power plant in Beijing, China, Nov. 28, 2011. Reports state that China, the world's largest emitter of greenhouse gases, is rallying key allies to push developed nations to agree to binding targets for reducing carbon emissions ahead of climate change talks in Durban even as it maintains that developing countries continue to be exempt as these would hamper efforts to alleviate poverty. EPA/How Hwee Young/Corbis