Estimating Changes in Global Temperature since the Preindustrial Period

TLDR: The United Nations Framework Convention on Climate Change (UNFCCC) process agreed in Paris to limit global surface temperature rise to “well below 2°C above pre-industrial levels. But what period is preindustrial? Somewhat remarkably, this is not defined within the UNFCCCs many agreements and protocols as mentioned in this paper.

Abstract: The United Nations Framework Convention on Climate Change (UNFCCC) process agreed in Paris to limit global surface temperature rise to “well below 2°C above pre-industrial levels.” But what period is preindustrial? Somewhat remarkably, this is not defined within the UNFCCC’s many agreements and protocols. Nor is it defined in the IPCC’s Fifth Assessment Report (AR5) in the evaluation of when particular temperature levels might be reached because no robust definition of the period exists. Here we discuss the important factors to consider when defining a preindustrial period, based on estimates of historical radiative forcings and the availability of climate observations. There is no perfect period, but we suggest that 1720–1800 is the most suitable choice when discussing global temperature limits. We then estimate the change in global average temperature since preindustrial using a range of approaches based on observations, radiative forcings, global climate model simulations, and proxy evidence. Our assessment is that this preindustrial period was likely 0.55°–0.80°C cooler than 1986–2005 and that 2015 was likely the first year in which global average temperature was more than 1°C above preindustrial levels. We provide some recommendations for how this assessment might be improved in the future and suggest that reframing temperature limits with a modern baseline would be inherently less uncertain and more policy relevant.

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Estimating changes in global temperature since the pre-
industrial period
Citation for published version:
Hawkins, E, Ortega, P, Suckling, E, Schurer, A, Hegerl, G, Jones, P, Joshi, M, Osborn, TJ, Masson-
delmotte, V, Mignot, J, Thorne, P & Van Oldenborgh, GJ 2017, 'Estimating changes in global temperature
since the pre-industrial period', Bulletin of the American Meteorological Society.
https://doi.org/10.1175/BAMS-D-16-0007.1
Digital Object Identifier (DOI):
10.1175/BAMS-D-16-0007.1
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Hawkins, E., P. Ortega, E. Suckling, A. Schurer, G. Hegerl, P. Jones, M. Joshi, T.
Osborn, V. Masson-Delmotte, J. Mignot, P. Thorne, and G. van Oldenborgh,
2017: Estimating changes in global temperature since the pre-industrial period.
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Estimating changes in global temperature since the pre-industrial period1
Ed Hawkins
∗
, Pablo Ortega, Emma Suckling2
NCAS-Climate, Department of Meteorology, University of Reading, Reading, UK3
Andrew Schurer, Gabi Hegerl4
School of GeoSciences, Grant Institute, University of Edinburgh, Edinburgh, UK5
Phil Jones6
School of Environmental Sciences, University of East Anglia, Norwich, UK and Center of
Excellence for Climate Change Research, Department of Meteorology, King Abdulaziz
University, Jeddah, Saudi Arabia
7
8
9
Manoj Joshi, Timothy J. Osborn10
School of Environmental Sciences and Climatic Research Unit, University of East Anglia,
Norwich, UK
11
12
Val
´
erie Masson-Delmotte13
Institut Pierre Simon Laplace, Laboratoire des Sciences du Climat et de l’Environnement
(CEA-CNRS-UVSQ), Gif-sur-Yvette, France
14
15
Juliette Mignot16
Climate and Environmental Physics, Physics Institute & Oeschger Center for Climate Change
Research, University of Bern, Switzerland and LOCEAN/IPSL (Sorbonne Universit
´
es,
UPMC-CNRS-IRD-MNHN), France
17
18
19
Peter Thorne20
Department of Geography, Maynooth University, Maynooth, County Kildare, Ireland21
Geert Jan van Oldenborgh22
Koninklijk Nederlands Meteorologisch Instituut, De Bilt, Netherlands23
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∗
Corresponding author address: Ed Hawkins, Department of Meteorology, University of Reading,
Reading, UK. RG6 6BB.
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E-mail: e.hawkins@reading.ac.uk26
2

ABSTRACT
The United Nations Framework Convention on Climate Change (UNFCCC)
process agreed in Paris to limit global surface temperature rise to ‘well below
2
◦
C above pre-industrial levels’. But what period is ‘pre-industrial’? Some-
what remarkably, this is not defined within the UNFCCC’s many agreements
and protocols. Nor is it defined in the IPCC’s Fifth Assessment Report (AR5)
in the evaluation of when particular temperature levels might be reached be-
cause no robust definition of the period exists. Here we discuss the important
factors to consider when defining a pre-industrial period, based on estimates
of historical radiative forcings and the availability of climate observations.
There is no perfect period, but we suggest that 1720-1800 is the most suit-
able choice when discussing global temperature limits. We then estimate the
change in global average temperature since pre-industrial using a range of
approaches based on observations, radiative forcings, global climate model
simulations and proxy evidence. Our assessment is that this pre-industrial
period was likely 0.55 − 0.80
◦
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. We provide some recommendations for how this
assessment might be improved in future and suggest that reframing temper-
ature limits with a modern baseline would be inherently less uncertain and
more policy-relevant.
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