The UN Paris Agreement on climate change aims to ensure increases in global temperature are less than 2°C above ‘pre-industrial’ levels, with an aspirational 1.5°C limit. However, the ‘starting line’ of the pre-industrial era is not defined by the UN agreements, or by the Intergovernmental Panel on Climate Change (IPCC).

A new analysis by an international team of researchers aims to better define the pre-industrial baseline, informing the world’s decision makers on the required limits to greenhouse gas emissions needed to meet the terms of the Paris agreement. The study concludes that 2015 was likely the first time in recorded history that global temperatures were more than 1°C above pre-industrial levels.

Which period is ‘pre-industrial’?

The invention of an efficient steam engine by James Watt in 1784 meant humans were able to effectively convert fossil fuels into energy, a process that releases carbon dioxide (CO 2 ) into the atmosphere. [Though coal was being burnt for industry before then too].

As the industrial revolution gathered pace in the 19th century, the resulting increase in CO 2 and other greenhouse gases from land-use change and fossil fuel burning started to warm the Earth.

To better define the Paris Agreement’s commitments, we need to select a suitable starting point for when humans began to influence the climate. However, there is no perfect choice.

Previously, the period 1850-1900 has been used as the historical baseline, but this period includes some large volcanic eruptions and is after greenhouse gas concentrations had already started to rise. We suggest that the earlier period of 1720-1800 is a better choice for this baseline. This is because the major natural factors that also affect Earth’s climate – the levels of solar and volcanic activity – were both at similar levels to today (see Figure 1).

This is important because we want to assess the changes due to human activity, rather than changes because of other factors. For example, the early 1800s were cooled by several large volcanic eruptions and so are unsuitable for a baseline. And before 1720 there was less solar activity and also several large volcanic eruptions.

So, by how much have global temperatures changed since pre-industrial? A problem with our chosen pre-industrial period is the limited availability of observed temperature records. We can make an assessment of changes in global temperature since the 1720-1800 period, but the uncertainties are quite large. In our study, we considered the long temperature records that do exist for central England, the Netherlands and Europe, as well as our understanding of historical changes in factors such as greenhouse gases, the sun and volcanic eruptions.

We assessed that global temperatures likely increased by more than 0.6°C from the pre-industrial period up to 1986-2005. This means that the year 2015 was at least 1°C warmer than the pre-industrial era and 2016 was likely to be more than 1.1°C warmer (see Figure 2).

What does this mean for the Paris agreement?

Now we are able to define the starting line for the global temperature limits stated in the Paris agreement, we can also better define the finishing line. The IPCC 5th Assessment Report used 1850-1900 as a historical baseline (but did not formally define this as ‘pre-industrial’), and estimated the warming from then to 1986-2005 at 0.61°C. The new analysis finds that this historical warming assumed by the IPCC is at the lower limit of what we assess is the true change since pre-industrial times.

There is also more to learn. We need volunteers to help rescue additional observations of climate from various undigitised sources to better understand the climate of the 19th century. The suggested 1720-1800 period also offers a target to paleoclimate reconstructions to inform this topic.

The study also suggests that policymakers might avoid the inevitable uncertainty in defining pre-industrial by reframing the temperature limits in terms of a more modern baseline. For example, 2°C above the pre-industrial era might be translated into X°C above 1986-2005 (or another modern baseline).

This would allow use of better observations and atmospheric reanalyses to define the starting point, but would require more thought into what X should be. It would also better inform the difficult decisions about how much greenhouse gas emissions would need to be reduced to avoid breaching agreed global temperature thresholds.