The wide spread of projected temperature changes in climate projections does not predominately originate from uncertainty across climate models; instead it is the broad range of different global socio-economic scenarios and the implied energy production that results in high uncertainty about future climate change. It is therefore important to assess the observational tracking of these scenarios. Here we compare these socio-economic scenarios created in both 1992 and 2000 against the recent observational record to investigate the coupling of economic growth and fossil-fuel CO₂ emissions. We find that global emission intensity (fossil fuel CO₂ emissions per GDP) rose in the first part of the 21st century despite all major climate projections foreseeing a decline. Proposing a method to disaggregate differences between scenarios and observations in global growth rates to country-by-country contributions, we find that the relative discrepancy was driven by unanticipated GDP growth in Asia and Eastern Europe, in particular in Russia and China. The growth of emission intensity over the 2000s highlights the relevance of unforeseen local shifts in projections on a global scale.

Introduction

The wide spread of projected temperature changes in climate projections does not predominately originate from uncertainty across climate models; instead it is the broad range of different global socio-economic scenarios and the implied energy production that results in high uncertainty about future climate change. While the physical-science basis of models used in Intergovernmental Panel on Climate Change (IPCC 1990–2013) [8] reports is very much the focus of the debate in climate research [23], the underlying socio-economic scenarios that determine emissions of greenhouse gases have received comparably less attention. Observations over two decades are now available against which the initial sets of socio-economic scenarios underlying the IPCC reports can be assessed to study the observational tracking. Here we compare these socio-economic scenarios created in both 1992 (IS92 – see Refs. [12,20]) and 2000 (SRES – see Ref. [17]) against the recent observational record to investigate the coupling of economic growth and fossil-fuel CO₂emissions. We find that global emission intensity (fossil fuel CO₂ emissions per GDP) rose in the first part of the 21st century, despite all major climate projections foreseeing a decline. Studying the differences between projections and observations we find that the relative discrepancy was driven by unanticipated GDP growth in wider Asia, particularly in Russia and China. The growth of emission intensity over the 2000s highlights the relevance of unforeseen local shifts in projections on a global scale.

We make three main contributions to the existing literature. First, we provide an assessment of socio-economic scenarios in terms of their growth rates over a long time-span matching the intervals of the IPCC scenarios. The assessment is particularly relevant to investigate any suggested de-coupling of economic growth from fossil-fuel CO₂ emissions. Second, we provide a method to decompose aggregate differences in growth rates into individual contributions when down-scaling is necessary due to a coarser resolution of the projected values relative to observational data. This can be used for future scenarios on a global level to assess whether particular countries (or regions) have led to systematic deviations from the projected paths. Our analysis highlights that unforeseen shifts in single countries can contribute substantially to global differences and our decomposition allows this contribution to be quantified directly. Third, based on our conclusions we provide suggestions for the development of future scenarios.

Four sets of scenarios have been used in the IPCC reports: the first used the SA90 scenarios, the second used the IS92 projections, the third and fourth used the Special Report on Emission Scenarios (SRES), while the fifth relied on the Representative Concentration Pathways (RCPs)². We assess all main socio-economic indicators in the IS92 and SRES: real gross domestic product (GDP), population, and fossil fuel CO₂ emissions, all of which are major components in the Kaya identity [11]. We focus on emission intensity as it is a crucial measure for the environmental impact of energy production and economic growth, and plays a crucial role in projected future warming [1]. Emission intensity combines two projected socio-economic series, real gross domestic product (GDP) and fossil-fuel CO₂emissions³. Additional results for all other socio-economic series are reported in the supplementary material.

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