It is commonly quoted that the world needs to keep the global temperature below 2°C (3.6°F) above pre-industrial levels. If temperatures rise above this, many think it will cause widespread ecological disaster, as ice caps melt, sea levels rise, currents shift, and coral reefs die. But there is some debate as to what exactly the “pre-industrial” levels should be.

The Paris climate agreement, in which all of the world’s governments came together to agree to keep global temperatures well within the 2°C (3.6°F) limit and to endeavor below 1.5°C (2.7°F) above pre-industrial temperatures, doesn’t actually define what this means.

Most researchers take the pre-industrial baseline as the average global temperature between 1850 and 1900, but this throws a few problems into the mix. By this point in time, the Industrial Revolution was already in full swing, with fossil fuels being burned and carbon emitted as the standard. This time period also contains some large volcanic eruptions, which would skew the data.

This makes using the 1850-1900 baseline to represent “pre-industrial” temperatures a little tricky. The authors of a new study, published in the Bulletin of the American Meteorological Society, suggest we should really be comparing modern temperatures to those from between 1720 and 1800, as during this time the natural conditions were similar to how they are now, meaning that any observed differences would be down to humans and not variation.

Yet this brings up its own clear problem. While we can accurately gauge the atmospheric conditions – such as carbon dioxide and methane concentrations – dating back hundreds of thousands of years by analyzing the tiny bubbles of air trapped in ice, how do we measure the temperature?

Well, it turns out that temperature records for some places do exist that stretch back this far, primarily from central England, the Netherlands, and Europe. Coupling this with modeling, it can give some indication as to how temperatures have shifted from the 1700s to now. The team found that during this pre-industrial period up to 1986-2005, global temperatures have likely increased by more than 0.6°C (1.08°F).

It also means that 2015 will have been the first time in recorded history that global temperatures have been higher than 1°C (1.8°F) above pre-industrial levels. These new calculations do have a wider range due to the increased uncertainties with modeling the temperature from so long ago, but they still show that we are well on our way to smashing that 1.5°C (2.7°F) limit.

Inevitably, many will chastise this as an attempt to “shift the goal posts”, but in reality it simply means that policy and targets can be more accurately set and our impact on the environment more accurately judged.