Most are familiar, at least vaguely, with the concept of climate change and the undeniable human drivers thereof. However, oft glazed over is a reflection upon the precise meaning of change in this context.

Change, in the general sense, is the act or instance of making or becoming different (Cambridge Dictionary, 2020) with some sort of a baseline implicit to the definition.

The United Nations body for assessing the science related to climate change (the Intergovernmental Panel on Climate Change (IPCC)), uses the term ‘industrial era’ to refer to the period after AD 1750 when industrial growth and the resulting emissions from the combustion of fossil fuels began to accelerate (as of 23/10/2020, IPCC AR5). This is a somewhat arbitrary cut-off as the Industrial Revolution is historically defined as beginning in 1760. For reference, the steam engine was patented in 1769 by James Watt but, there was probably only a small climate effect of this new technology for at least a few decades.

Ideally, a preindustrial period should reflect an average climate state just before human activities started to demonstrably alter the climate through their activities. This debate also surfaces when trying to precisely define the onset of the geological epoch now referred to as the Anthropocene1,2.

It is clear that there is no perfect choice in selecting a suitable starting point for when humans began to influence the climate. But, there are important details that one must consider:

  1. Which human activities could lead to detectable climate changes? For example, landuse change3,4 such as forest loss or the emissions of fossil fuels.
  2. What scale of human activity changes climate (threshold effects?) this is related to climate buffering. For how long can oceans absorb warming such that it is not detectable prior to a particular threshold CO2 increase?
  3. How long did it take for tipping points to be surpassed?
  4. Given imperfect methods of detecting temperature changes and regional differences in response how representative and certain is the period of change that is selected?
  5. Very few instrumental temperature records exist pre-1850 and therefore we need to rely on proxies.
  6. We want to measure the change in human activities as opposed to other factors. However, anthropogenic change co-occur with: i) internal climate variability (climate oscillations) and; ii) changes in the levels of both solar and volcanic activity.
  7. There may be climate anomalies within a given time period. The period before 1720 (which was referred to as Little Ice Age) is one example.5

To quote Feynman:

“If you thought that science was certain - well, that is just an error on your part.”

This is one of the challenges for policy makers, in particular the United Nations Framework Convention of Climate Change (UNFCCC) whom agreed in Paris to work to limit global surface temperature rise to:

“Well below 2°C above pre-industrial levels.”

Amazingly, a clearly defined rationale for 1750 as the selected pre-industrial level threshold is not provided 5. Ultimately, the 2 °C target is also merely a political consensus that takes into account what policymakers at that time considered to be both realistically achievable and tolerable. There is a lot more work required from the scientific community to determine if this is a meaningful quantity, and if not, what level most appropriately reflects the UNFCCC’s ultimate objective 6. The UNFCCC in addition to being more explicit regarding their rationale, may simply consider including the baseline time period in their nomenclature as suggested by Hawkins et al. (2017)5. For example, “well below 2°C above pre-industrial” (p. 3) might be translated to “well below X°C above time1–time2.” Andy Stirling wrote an excellent article describing why when knowledge is uncertain, experts should avoid pressures to simplify their advice7.

“A move towards plural and conditional expert advice is not a panacea. It cannot promise escape from the deep intractabilities of uncertainty, the perils of group dynamics or the perturbing effects of power. It differs from prevailing approaches in that it makes these influences more rigorously explicit and democratically accountable.” -Stirling, 2010