Signal detection in global mean temperatures after “Paris”: an uncertainty and sensitivity analysis

Abstract

Abstract. In December 2015, 195 countries agreed in Paris to "hold the increase in global mean surface temperature (GMST) well below 2.0 • C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 • C". Since large financial flows will be needed to keep GMSTs below these targets, it is important to know how GMST has progressed since pre-industrial times. However, the Paris Agreement is not conclusive as regards methods to calculate it. Should trend progression be deduced from GCM simulations or from instrumental records by (statistical) trend methods? Which simulations or GMST datasets should be chosen, and which trend models? What is "pre-industrial" and, finally, are the Paris targets formulated for total warming, originating from both natural and anthropogenic forcing, or do they refer to anthropogenic warming only? To find answers to these questions we performed an uncertainty and sensitivity analysis where datasets and model choices have been varied. For all cases we evaluated trend progression along with uncertainty information. To do so, we analysed four trend approaches and applied these to the five leading observational GMST products. We find GMST progression to be largely independent of various trend model approaches. However, GMST progression is significantly influenced by the choice of GMST datasets. Uncertainties due to natural variability are largest in size. As a parallel path, we calculated GMST progression from an ensemble of 42 GCM simulations. Mean progression derived from GCM-based GMSTs appears to lie in the range of trend-dataset combinations. A difference between both approaches appears to be the width of uncertainty bands: GCM simulations show a much wider spread. Finally, we discuss various choices for pre-industrial baselines and the role of warming definitions. Based on these findings we propose an estimate for signal progression in GMSTs since pre-industrial.

Predicted Chance That Global Warming Will Temporarily Exceed 1.5 °C

“…Observations of global mean near surface temperature are taken as the average of HadCRUT4 (Morice et al, ), NASA‐GISS (Hansen et al, ), and NCDC (Karl et al, ). The Paris Agreement (UNFCCC, ) did not define the period to be used as the preindustrial reference, and different choices will affect whether the 1.5 °C threshold has been exceeded or not (Schurer et al, ; Visser et al, ). In common with other studies (Collins et al, ; Henley & King, ) we take a pragmatic approach (Hawkins et al, ) and use the period 1850 to 1900 to represent preindustrial conditions since earlier periods suffer from a lack of direct observations.…”

Science, Religion, and Public Policy

“…; Visser et al. ). I realize that keeping research portfolios alive in multiple disciplines is not supported everywhere in academia and I am very fortunate to inhabit a niche academic home where this is actually valued.…”

Pathways Toward Sustainable Development

“…As a result of this global emission increase, global temperature is expected to rise from around 1°C above pre-industrial levels in 2016 (Visser et al 2018) to around 4°C by 2100 in SSP2, most likely passing the 2°C target of the Paris Agreement before 2050 (IPCC 2014b; see Figure 21.9). There are substantial differences in temperature increase across different areas of the world.…”