On the science of climate change
In an interview last week, Computer Science professor David Gelernter told the News: “For human beings to change the climate of the planet is a monstrously enormous undertaking…I haven’t seen convincing evidence of it” (“Gelernter, potential science advisor to Trump, denies man-made climate change,” Jan. 25, 2017). Gelernter is widely rumored to be in consideration for the position of President Donald Trump’s science adviser, and his comments insinuate that human activity is insufficient to cause such change.
While we agree with Gelernter’s premise about the magnitude of human activity needed to alter the planetary climate system, we disagree with his conclusions.
In consulting peer-reviewed scientific literature, we find that the energy expended by billions of people over nearly two centuries is in fact a significant climatological and geological force.
Carbon dioxide (CO2), along with other carbon-bearing molecules in the atmosphere such as methane, is a greenhouse gas that warms the planet — a relationship that has been known since the release of a classic study by Svante Arrhenius in 1896, and which has been confirmed by numerous independent experiments.
CO2 levels have now risen above 400 parts per million by volume, relative to a pre-industrial value of about 280 ppmv. That difference corresponds to about 250 billion tons of carbon added to the atmosphere.
Published in the Proceedings of the National Academy of Sciences, a 2007 global inventory of fossil fuel combustion, cement production and land-use changes such as deforestation indicates that humans have emitted about 500 billion tons of carbon into the atmosphere since 1850.
This is indeed a “monstrously enormous” figure.
It is so enormous that the abrupt atmospheric CO2 rise, reaching levels substantially higher — and at a pace far faster —than those of natural glacial-interglacial cycles, represents only half of the anthropogenic effects on Earth’s carbon cycle. The other half of the emitted carbon has been taken up in roughly equal measures by the land surface and the oceans.
As a result, the oceans have been slowly acidifying.
How sensitive is global climate to CO2 concentrations in the atmosphere?
The connection is quantified using a factor called the “climate sensitivity,” which is the number of degrees Celsius rise caused by each doubling of CO2.
“Sensitivity” is an appropriate term given that even small variations of greenhouse gases can lead to either a completely ice-covered or totally ice-free Earth; as illustrated by many computer simulations of climate, including a pioneering 1992 study by Ken Caldeira and James F. Kasting.
The climate sensitivity factor combines the summed effects of ocean heat uptake, changes in atmospheric humidity, as well as changes in clouds and planetary reflectivity. Climate sensitivity is not constant under all conditions; the estimates summarized by Reto Knutti and Gabriel C. Hegerl in 2008 range within 2-5 degrees Celsius per CO2 doubling.
It should be no surprise, then, that during the same industrial-era time interval when atmospheric CO2 rose by nearly 50%, the averaged global temperature has risen by about 1 degree Celsius, as documented by a team of scientists from Oregon State and Harvard in 2013.
According to a 2004 study by R.J. Klee and T.E. Graedel, carbon emissions are just one example among many cases where human activity has mobilized geological materials at rates far exceeding natural processes. Our species has become uniquely powerful in its environmental potency. Those who deny an anthropogenic cause of global warming have been called “climate skeptics,” a euphemistic term that would appear to give them an elevated ethical standing in critical thought. Based on our consideration of well-documented scientific research, and like the vast majority of Earth scientists — as documented in a 2016 study in the journal Environmental Research Letters — we reject the hypothesis that human carbon emissions have had zero effect on global climate.
We welcome rational discussion on this issue, grounded in reference to peer-reviewed studies by researchers with a long-term and serious engagement in climate science. Skepticism expressed for its own sake — without factual knowledge — does not contribute to scientific advancement and does not belong in the conversation.
David Evans is a professor of Geology & Geophysics. The column is jointly written with 18 other faculty members in the department: Jay Ague, David Bercovici, Ruth Blake, William Boos, Mark Brandon, Alexey Fedorov, Pincelli Hull, Jun Korenaga, Kanani Lee, Maureen Long, Jeffrey Park, Noah Planavsky, Alan Rooney, Brian Skinner, Ronald Smith, Trude Storelvmo, Mary-Louise Timmermans and John Wettlaufer.
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