While most climate scientists, including the Intergovernmental Panel on Climate Change, implicitly define "pre-industrial" to be in the late 1800's, a true non-industrially influenced baseline is probably further in the past, according to an international team of researchers who are concerned because it affects the available carbon budget for meeting the 2 degrees Celsius (3.6 degrees Fahrenheit) warming limit agreed to in the Paris Conference of 2015.
"The IPCC research community uses a definition of preindustrial that is likely underestimating the warming that has already taken place," said Michael Mann, distinguished professor of atmospheric science and director, Earth System Science Center, Penn State. "That means we have less carbon to burn than we previously thought, if we are to avert the most dangerous changes in climate."
The researchers explored a variety of date ranges for defining a "pre-industrial" baseline and the likelihood that, compared to those baselines, the global temperature averages could be held to 2 degrees C (3.6 degrees F) or to the preferred 1.5 degrees C (1.7 degrees F). They report their results today (July 24) in Nature Climate Change.
"When the IPCC says that we've warmed 1 degree C (1.8 degrees F) relative to pre-industrial, that's probably incorrect," said Mann. "It's likely as much as 1.2 degrees C (2.16 degrees F)."
Because greenhouse gas concentrations have been increasing since 1750 it would be preferable to define a baseline prior to then, but actual instrumental measurements of temperature did not exist before the 1800s. There are also natural phenomena that preclude defining a single unique value for pre-industrial average global temperature.
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