Scientists are starting to question whether we are using the proper date range for the pre-industrial period when measuring climate change.

An engraving of London in 1865, at the height of the Industrial Revolution.

In today’s climate studies, researchers define the pre-industrial period as being between 1850 and 1900.

The trouble with that date range is, the Industrial Revolution began in Britain in the late 1700s and was in high gear throughout the western world by 1850. In fact, industrial development was at its peak during the years that are currently referred to as “pre-industrial.”

Thus, humans were influencing the global climate as early as the 19th Century, as evidenced by ice cores whose records show increasing levels of carbon dioxide and other greenhouse gases such as methane throughout the 1800s.

Looking at this data, Dr. Ed Hawkins of Reading University in England, and some of his colleagues, tried to deduce which period would be a more appropriate start and end time for “pre-industrial” temperature and greenhouse gas measurements.

In a new paper published in the Bulletin of the American Meteorological Society, they suggest that the period from 1720 through 1800 would serve as a better baseline.

“There is no perfect period,” they wrote in the paper’s abstract, “but we suggest that 1720-1800 is the most suitable choice when discussing global temperature limits. … Our assessment is that this pre-industrial period was likely 0.55-0.8°C cooler than 1986-2005 and that 2015 was likely the first year in which global average temperature was more than 1°C above pre-industrial levels.”

This period also avoids some of the big natural disasters that affected the world’s climate, including gigantic volcanic eruptions in 1809 (location unknown) and Tambora in 1815.

It also excludes the period before 1720, when the sun was relatively inactive, with little sunspot activity, which would have cooled global temperatures.

The trouble with using 1720 through 1800 as a data set is that temperature records from that time are sparse. However, good data does exist for central England, the Netherlands, and Central Europe. Dr. Hawkins’ group used these data to gauge what global temperatures were doing during that period and assess whether they were warmer or cooler.

It turns out that those temperatures were most likely cooler.

“We can assess that there has been a certain amount of temperature change from 1850-1900 up to now, and we think that amount is a lower limit on how much temperature [change] there has been since the true pre-industrial period,” Dr. Hawkins told BBC News.

The researchers also say there are many other temperature observations from the 18th and 19th century that have not yet been collated or digitized. Doing so would give a clearer picture of global temperatures during that era.

Another option, they say, would be to abandon the 1850-1900 reference altogether and use only modern comparisons.

“We obviously have much better observations now and over the past few decades, and so the policy-makers may want to reframe the question. So instead of being, ‘can we keep to two degrees above pre-industrial?’, it becomes ‘can we limit ourselves to an amount below, say, the last 20 years or so?’.”

While using the period of 1720 through 1800 as a pre-industrial benchmark would create a more accurate measure of temperature change, it would take a great deal of work to find more comprehensive records. With that in mind, policymakers may choose to simply abandon the current pre-industrial reference and focus on the last couple of decades instead, said Dr. Hawkins.