Comment Re:So is it... (Score 2) 49
When they say "pre-industrial levels", when do you think they mean? The 19th century (even though the industrial revolution was well underway), usually 1890 specifically.
What year is used depends entirely on the study. Some start at the advent of satellite measurements, some at the advent of modern ground-based measurements, some with the era of semi-reliable ground-based measurements, some incorporate further back with more fragmentary measurements, and others use proxies - some recent proxies from 200, 300, 400 etc years ago, others thousands, tens of thousands, hundreds, millions of years ago or more. There is no single timeframe that is examined. Numerous studies evaluate each different source, and the different proxies are commonly plotted out relative to each other.