We know that during the industrial revolution, humans developed machinery at a large scale that pumped vast amounts of greenhouse gases—such as carbon dioxide and steam—into the atmosphere, kickstarting the climate change we're all witnessing today. But when exactly did humans start emitting "industrial" levels of emissions into the atmosphere?

In other words, when did our emissions start to affect the way the atmosphere works?

As nations around the world make plans to limit their emissions and to adapt to the effects of climate change, this timeline is actually important. The idea of a "pre-industrial" climate serves as a baseline in determining how global warming impacts the Earth, and how much hotter it can afford to get. During the Paris Climate Accord talks in 2015, which brought together almost 200 countries in order to limit greenhouse gas emissions, negotiators cited "2 ℃" as a tipping-off point at which irreversible damage to the planet happens.

There, 197 parties agreed to curb greenhouse gas emissions to avoid reaching that threshold. Technically, that means 2 ℃ above pre-industrial temperatures, but some climate scientists may have been measuring pre-industrial incorrectly.

At the time of Paris Climate Accord, pre-industrial temperatures were never precisely defined, several scientists argue in a new study, published in Nature. And their findings are troubling. Researchers from Pennsylvania State University, the University of Edinburgh, and the University of Reading found that defining the late 19th century as pre-industrial—as some climate scientists had been doing—is actually too late. People had already emitted a significant amount of carbon by then due to the industrial revolution, which started in the 1750s.

On average, the amount of greenhouse gases in the atmosphere was lower before the 1750s, and global mean temperatures have risen since then. So if we want to maintain a 2 ℃ limit above "pre-industrial" temperatures, the baseline date and temperature chosen is important. But scientists can't decide on a specific date or temperature because natural climate variability, such as the Medieval anomaly or the Little Ice Age, make it difficult to characterize one specific date as "normal."

Practically speaking, an earlier pre-industrial baseline date would mean that parties in the Paris Accord must emit 40 percent less greenhouse gases in order to avoid 2 ℃ warming above these earlier, truly pre-industrial levels. So according to this analysis, Paris negotiators were 40 percent off in their goals.

Worse yet, these researchers argue, the Paris Accord's aspirational warming limit of 1.5 ℃ is impossible. This limit was proposed by The Alliance of Small Island States (AOSIS), a coalition of nations in the tropics and global south including Haiti, Fiji, the Maldives, and the Solomon Islands. They stand to experience worsened typhoons, hurricanes, cyclones, tides, flooding, sea level rise, and ocean acidification when warming exceeds 1.5 ℃, so this group has been pushing nations around the world to place limits at that temperature.

But 1.5 ℃ was only adopted as an "aspirational" limit because it would require countries to cut more emissions than they were willing.

Even with an incorrect late 19th century pre-industrial baseline, which should make it easier to hit our warming targets, studies are finding that we still probably can't stay below 1.5 ℃ or even 2 ℃. One, released Monday, claimed that there's only a five percent chance that we won't exceed 2 ℃ from late-19th century levels by 2100, and another, also published the same day, claimed that 1.5 ℃ warming from late-19th century levels is likely impossible.

Michael Mann, the director of the Earth System Science Center at Penn State University, is one of the researchers behind the study searching for a precise time when humanity's carbon emissions truly were pre-industrial. He told me that redefining pre-industrial levels of carbon emissions isn't just about semantics or scientists arguing over terminology. His team's findings specifically impact the island nations of the AOSIS, he said.

"Based on the result of our analysis, 1.5 ℃ as a limit is probably off the table in the absence of carbon removal strategies," Mann said. These strategies include carbon capture and storage (CCS), or the process of artificially removing carbon dioxide from the atmosphere and storing it, but they're not necessarily realistic, and the technology isn't quite there yet. "[Carbon removal strategies] are really hard to do and really expensive—but what might be more expensive, arguably, is the damage that results if we don't do it."

Mann also told me that we don't know which regions have useless climate adaptation measures based off of an incorrect definition of "pre-industrial."

While Mann thinks that a minority of regions used the wrong definition, he believes that the IPCC Impacts, Adaptation, and Vulnerability Working Group—which makes adaptation recommendations for nations in the UN—will need to conduct a thorough review to determine who will be affected, and when.

But the next IPCC assessment won't come out for another three to four years, so people won't know until 2020 or 2021 if their city, state, or country is unprepared for upcoming decades of climate change damage.