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Received Sep 1, 2017; Revised Mar 12, 2018; Accepted Mar 29, 2018
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1. Introduction
To limit global mean warming to well below 2°C, in accordance with the Paris Agreement [1], 190 countries submitted their Intended Nationally Determined Contributions (INDCs), which outline the intended post-2020 emission plans of each country [2]. INDCs became the first target of greenhouse gas (GHG) mitigation reached through a bottom-up approach by nationally intended efforts, so it is easier to monitor a level of commitment than before through a top-down system. However, the impacts of these emission-reduction efforts on climate warming are poorly understood and their adequacy to meet the long-term goal of stabilizing the global mean temperature to 1.5°C or 2°C above the preindustrial level is still unknown.
To simulate climate response under INDC scenarios, running a full suite of comprehensive Earth system models (ESMs), such as the CMIP5 models (Coupled Model Intercomparison Project), is unrealistic due to the high computational cost, while running only one certain model is not representative of climate response of the Earth system due to potential model biases. Recent studies have shown a near linear relationship between cumulative carbon emissions and temperature change [3–10]; thereby providing a way to evaluate climate response under INDC scenarios without the need of running additional simulations by comprehensive Earth system models.
In the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) [11], future climate change was projected under a set of Representative Concentration Pathway (RCP) scenarios, using a model ensemble of comprehensive Earth system models (CMIP5) [12] and a reduced-complexity carbon-cycle and climate model (Model for the Assessment of Greenhouse Gas Induced Climate Change (MAGICC)) [13, 14]. The ratio of temperature increase to cumulative carbon emissions, also referred to as the transient climate response to cumulative carbon emissions (TCRE), is relatively constant over time and is independent of the CO2 emissions pathway [4, 15, 16]. Expert judgements [4, 5, 7, 10] based on multiple lines of evidence estimate TCRE to be likely between 0.8°C and 2.5°C per 1000 GtC (5 to 95%), of...