Moves to implement the Paris Agreement on climate change should consider a new analysis of historical temperature changes, research suggests.
Previous attempts to gauge global warming over the years have defined the pre-industrial age as the period 1850-1900.
However, a new study shows that by this time, when the industrial revolution was under way, warming may already have occurred.
Research estimating global temperatures from 1400-1800 could improve definitions of pre-industrial temperatures.
It could also help decide a baseline temperature for the Paris Agreement on climate change.
The Paris Agreement aims to keep average global temperature increases below 2C, and preferably not more than 1.5C, above pre-industrial levels.
Researchers have used their estimates of historical global temperatures to help explore how varying definitions of pre-industrial temperatures could impact upon actions needed to meet the Paris targets.
Setting out a pre-industrial baseline temperature would also affect the volume of greenhouse gases that may be emitted by nations around the world within the terms of the deal.
Researchers at the Universities of Edinburgh, Reading and Pennsylvania State used computer model simulations to estimate global temperatures for various periods between 1400 and 1800.
They found that this timescale included periods of similar warmth to the late 19th century and periods when temperatures were 0.2C cooler. Their findings indicate that during the period 1850-1900, temperatures may already have begun warming.
The latest research suggests that using the late 19th century as a pre-industrial starting point may be optimistic in terms of meeting the Paris goals.
The study, published in Nature Climate Change, was supported by the European Research Council, Natural Environmental Research Council, National Centre for Atmospheric Science and Royal Society, UK, and the National Science Foundation, US.
It’s important that a consensus is reached on the definition of pre-industrial temperatures, to allow scientists and policymakers to properly assess progress to reduce emissions. There is no simple answer to this – for example, temperatures at the start of the last millennium were very different to those in the 17th century. These differences in definition could have a large impact on further emissions under the Paris agreement.