From Scientific American, quoting Nature Climate Change:
The temperature baseline used in the Paris climate agreement may have discounted an entire century’s worth of human-caused global warming, a new study has found.
Countries in the Paris climate agreement set a target of keeping warming below 2 degrees Celsius by curbing carbon emissions compared to their preindustrial levels. But a new study shows that the preindustrial level used in the agreement, based on temperature records from the late 19th century, doesn’t account for a potential century of rising temperatures caused by carbon dioxide emissions. Accounting for those gases, released from about 1750 to 1875, would add another one-fifth of a degree to the baseline temperature, the study found.
Published yesterday in Nature Climate Change, the research suggests there’s less time than previously believed to address global warming, said Michael Mann, a climatologist at Pennsylvania State University.
The study estimates that there may have already been 0.2 degree Celsius of warming, or 0.36 degree Fahrenheit, built into Earth, he said. That means the Paris Agreement would have to be more aggressive, according to the study, which was also written by researchers from the universities of Edinburgh and Reading in the United Kingdom.