"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 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)."
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"A widely used metric for climate change mitigation is how much carbon we can still burn and remain below 2 degrees C," said Mann. "It's what we call the 'carbon budget.'"
A pre-industrial baseline that truly contained no human-caused warming would alter the amount of carbon that could be put into the atmosphere. Measured in gigatons of carbon, to account for the 0.2 degrees C likely unaccounted for in previous estimates of human-caused warming, we would need to burn 40 percent less carbon to remain below the 2 degree C threshold, according to Mann.
"Either the Paris targets have to be revised," said Mann. "Or, alternatively, we decide that the existing targets really were meant to describe only the warming since the late 19th century."
If nothing else, Mann says that the community needs to be far more precise in defining what baselines are being used in setting targets.
Read more at Allowable 'Carbon Budget' Most Likely Overestimated
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