| Since the Paris Climate Agreement was adopted by consensus in December 2015, the global society has made various efforts to respond to this Agreement. The essential element of the Paris Climate Agreement is the so-called Nationally Determined Contributions (NDCs), specifying the long-term greenhouse gas (GHG) reduction targets beyond 2020. Unlike the different legal obligations of developed and developing countries specified in the precedent Kyoto Protocol, the NDCs under the Paris Climate Agreement apply equally to all countries. All the signed countries were asked to submit their Intended Nationally Determined Contributions (INDCs) before the 2015 Paris Climate Summit, which serve, unless otherwise provided, as the initial NDCs after the ratification of the Agreement. However, there is no mechanism to force a country to set a specific target by a specific date, but each target should go beyond previously set targets. Another challenge is that in June 2017, US President Donald Trump announced his intention to withdraw the US from the Agreement. Under the Agreement, the earliest effective date of withdrawal for the US is November 2020, shortly before the end of President Trump’s current term. The US was responsible for 15.5% of global CO2 emissions from fuel combustion in 2015 [1]. The US’s withdrawal has brought a great uncertainty to the implementation of the Paris Climate Agreement and induced a lot of global disputes. Nevertheless, many national governments have initiated their great efforts in reducing GHG emissions. The promoted strategies include but are not limited to deploying energy-efficient technologies, promoting the use of low-carbon fuels, optimizing grid mix, incorporating carbon trade mechanism, developing low-carbon supply chain, advocating low-carbon lifestyles, etc [2]. These efforts are critical in preventing an unconstrained growth of GHG emissions. Taking China as an example, the CO2 intensity in 2014 was decreased by 34% from the 2005 level [3]. In addition, EU achieved an 18.4% net reduction of CO2 emissions from fuel combustion during 2005–2015 [1]. However, there is a lack of integrated and systemic synergy of existing mitigation policies to meet the overall mitigation target. It is unclear whether existing policy instruments are effective enough to achieve the climate target, and if not, how much further efforts are needed; what the co-benefits, risks and spillovers of individual policies are, and how the diversified policies can be combined in an effective way. Besides, there is also a lack in the synergy between climate policies and other socio-economic policies, such as energy security, air pollutions. Most policies are designed with a narrow focus on solving the target issue, while ignoring cobenefits on wider dimensions [4,5]. Such narrowlyfocused policies are inherently inefficient due to the miscounting of social costs and benefits, and lead to the discount of the overall effect. |