Climate policy plays an important role in keeping the global temperature blow 1.5-2, and the

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1 The climate impact of the American policy choice based on the earth system model BNU-ESM Liu Changxin ABSTRACT Climate policy plays an important role in keeping the global temperature blow 1.5-2, and the technology innovation is the key to determining the effectiveness of climate policy. In this study, we investigated the climate impact of American s policy choice using the economic model (EMIRC) and earth system model BNU-ESM. Three emission scenario is designed based on the assumption of whether the American following its INDC and has technological innovation. The results show that it the American did not implement the INDC and has no technology progress, there would be an extra 176.7Gt cumulative carbon emission in the end of 21 st compared to that if all countries following its INDC. This extra cumulative carbon emission lead to an additional 62 ppm CO 2 concentration and 0.4 global warming in Besides, the earth system model results also show that even all the countries following the IDNC, it is hard to keep the temperature warming below 1.5. This study implies that American withdrawal from the Paris agreement is not conducive to achieving the 1.5 goal, and more stringent emission reduction targets are required for the world to control the global warming below 1.5. Key words climate policy, INDC, Paris agreement,earth system model, economic model, climate impact

2 1. Introduction Climate change is an interdisciplinary issue involves energy, health, policy et al., and has been of great interest to scientist and public for a long time. It is well recognized that the temperature rise should be limited below1.5-2 above pre-industrial levels to avoiding the dangerous climate change [1-3].Thus climate negotiations which provides public platform to discussion how to allocate or distribute the remaining limited carbon emission fairly between governments are particularly important. This has made great progresses in recent decades and has significant contribution on global and regional climate mitigation and adaptation. The recent Paris agreement adopted by 195 member countries established a global system of international cooperation addressing climate issue after 2020, which is considered to be a milestone and new starting in the climate negotiations. Almost all the parties submitted its intended nationally determined contributions (INDC) to the Paris agreement. For example, the INDC target submitted by China is the carbon emission intensity will drop 60-65% in 2030 compared with that in 2005; while the INDC submitted by the USA is the carbon emission should drop 26% compared with the amount of year All the plans seem accordance with the results of the negotiations and lead to better future. However, President Donald Trump s decide to withdraw from the Paris agreement on June 1, which is out of expectation for most scientist and government. This decision is generally believed can affect the global confidence and enthusiasm in addressing the climate issues, and will have significant impact on the effectiveness of emission reductions. Lots of researchers criticized Trump s option [4-10], for example, some studies pointed out that Trump s decision may influence the American energy structure and the energy innovation [9,11,12] The innovation rate may decide the decline speed of the carbon intensity. For example, Seo [13] argues the important role of technology progress in protecting the climate change. Xing, Hanaoka et al.[14] have studied the energy transition s effect on greenhouse emission and economic growth. Pan, Chen et al. [15] find that China's non-fossil energy will account for 50-70% and 85% of primary energy consumption in 2050 and 2100 to achieve the IPCC s 2degrees C goal. If the American s energy innovation declined, its carbon emission would increase proportional to the GDP growth, suggesting that the global climate may be greatly influenced by the energy technology innovation. However, what is the consequence and quantitative impact of Trump s decision on the global carbon emission and climate system is still unknown so far. In this paper, we investigated this issue by combining an economic model (EMIRCE) and earth system

3 model (BNU-ESM). The EMICE has the complete energy and economy process, and can be used to design the emission pathway include or exclude the American s INDC and technology innovation. While the earth system model BNU-ESM has the fully ocean and land carbon cycles, and which is closer to the real earth system change compared the simple climate model and can give the detail earth system changes under the prescribed emission pathway. This paper is organized as followings. The MRICE model description and emission scenario development method is given in sections 2. The section 3 gives the earth system and experiment description. Section 4 gives the results of the experiment. A summary and discussion are provided in the last section. 2. The MRICE model and emission scenario design 2.1 The MRICE model The EMRICE is a General equilibrium model, which divides the world into 10 regions: China, American, Japan, the Europea Union, India, Russian, high income countries, upper middle income countries, lower middle income countries and low income countries. These countries economic system are connected by the regional economic linkages. In this model, economic system in each country (or each region) is based on macro-dynamic economic model, while China, America, Japan, India, Russia is used the CGE model. 2.2 Energy carbon emissions calculation In the EMRICES model, the global carbon emission is the sum of all countries or regions carbon emission. (1) Where C(t) represents the global total carbon emission at the time t, C i (t) represents the carbon emission of country i at time t. Each country s carbon emission is aggregated from all the economic sectors. (2) Where represents the carbon emission of economic sector j in country i at the time t. Each economic sector s carbon emission is calculated by different energy and its carbon emission parameter. The energy structure includes coal, oil, nature gas and non-fossil fuel. Χ (3)

4 Χ...(4) Where is the energy intensity of the economic sector j in country i at the time t with the respect of energy k. It represents the energy technology level. is the output level of the economic sector j in counry i at the time t. Each country or region have its own technology innovation rate, is the carbon emission parameter for energy k. 2.3 INDC targets According to the Paris Agreement, the current Intended Nationally Determined Contribution (INDC) carbon emission goal is difficult to limit the surface temperature warming under 2 above pre-industrial levels (UNFCC, 2015). So the carbon emission scenario in this paper is calculated using the most stringent emission reduction targets put forward by each country in INDC. For example, China s INDC target is that its carbon emission intensity will drop 60-65% in 2030 compared with that in 2005, so we use the decline target of 65% in the paper. Most of the countries INDC can be directly or indirectly divided into two Classes. For example, the America, EU, et al give its INDC target based on the total emission level; while China, Japan, India, et al provided the clear reduction target mainly based on its carbon emission intensity. For these two different accounting methods, the following two equations are used to calculate the emission in the target year, respectively. (5) (6) Where E 2030 represents the carbon emission in the target year (2030 year), En 2005 is the emission intensity in the base year, and which is provided by the word bank, is the emission intensity decline rate provided by INDC, and is the Gross Domestic Product (GDP) under the baseline scenario, represents the total carbon emission reduce rate. For the high-income countries, upper middle income countries, lower middle income countries and low income countries, some countries in those regions have not yet submitted independent contributions (such as North Korea) and some countries which have submitted contributions have no clear carbon emission reduction targets (such as Egypt, Bolivia, etc.), meanwhile there are a lot of different emission reduction benchmark years and targets, so it s difficult to give an accurate value to global carbon emission reduction targets as a whole. We adopt different approach to solve this problem. For the high income countries (e g. the Canada,

5 New Zealand, Australia, Norway, Singapore, et al. ), the carbon emission reduction rate of each country is about 30% compared with that in 2005, so the total emission in the target year is about MtC. The INDC targets of upper middle income countries, lower middle income countries and low income countries are mainly based on carbon emission intensity and the emission reduction rate in the baseline scenario. The specific INDC goals, years and carbon emissions targets in each countries or regions are shown in table 1. Table 1: INDC targets and targets years converted carbon emission Country INDC targets Benchmark Target Emission in (decline of the year year target year carbon emission (MtC) intensity (%)) China 65% America 26% Japan 25.4% The EU 40% India 35% Russia 25% High countries income 30% Upper middle income countries 35% Benchmark scenario Lower middle income countries 35% Benchmark scenario Low countries income 35% Benchmark scenario Scenario design We set three scenarios to analyze the situation after the US withdrawing the Paris agreement. a) The America and other countries following their INDC target, noted INDC; b) America does not execute its

6 Gt C INDC target but with technology progress, INDC-NA; c) America does not execute its IDNC and without technology progress, INDC-NA-NT. Table 1 gives the detail description of these three scenarios. The time evolution of these three scenario can be found in Figure.1. Though scenarios have similar temporal evolution characteristics, carbon emission under the IDNC-NA-NT is significantly higher than that of the INDC and INDC-NA after 2030; meanwhile, there is litter difference between the INDC-NA and INDC, which indicate that the American technology progress plays important role in the future carbon emission changes. Besides, the global carbon peak at around 2020, and flowed by a rapid decline by 2030, and then slow down till to In the end of the 21 st, the cumulative carbon emissions gap is about 176.7Gt C between the INDC-NA-NT and the INDC scenario. Table 2 Scenarios design Name Carbon emission targets Technology progress in America(energy intensity) INDC Follow INDC (America implemented) Yes INDC-NA INDC-NA-NT Follow INDC (America doesn t implemented) Follow INDC (America doesn t implemented) Yes No INDC INDC-NA INDC-NA-NT Year Fig.1 Changes of carbon emission under the INDC, INDC-NA and INDC-NA-NT 3. Earth system model description and experiment design 3.1 The BNU-ESM model

7 The BNU-ESM is a fully coupled model developed by Beijing Normal University, and is one of the models on behalf of China to participate the phase 5 of CMIP5. The BNU-ESM consists of four separate components to simulate the atmosphere (CAM3.5), land (CoLM), ocean (MOM) and ice (CICE4.1). These four component is coupled by the coupler of CPL6.0, which is used to coordinate and responsible for the energy and information flux exchange between different components. Ji et al gave the fully description of BNU-ESM and showed that the BNU-ESM can reproduce many basic characters of the earth system, such as the temperature, ESNO, sea ice extent, et al [16,17]. Besides, the BNU-ESM is also widely used in climate attribution and projection [18-20]. For example, Wei et al quantified the responsibilities for CO 2 emissions of developed and developing world on the historical and future climate change [18]. 3.2 Data and experiment design According to the emission scenario, we designed two experiments to quantify the American climate policy impact on the future climate. One is the INDC experiment forced by the IDNC scenario, and the other is the INDC-NA-NT experiment forced by the INDC-NA-NT scenario. Except for the CO 2 condition, other forces (e.g the Ozone and aerosol et al) are provided by the RCP4.5. It show be mentioned that these two experiments include the fully land and ocean carbon cycle. So firstly, we have to covert the annual global carbon emission amount (Gt C) to the monthly gridded CO 2 flux (units: kg/m2/s). In this study, we used the method provided by Yang et al to realize this goal [21, 22], and the gridded INDC and INDC-NA-NT scenario has the same spatial and temporal distribution with the Representative concentration pathways. Figure.2 gives the CO 2 flux of the gridded INDC and INDC-NA-NT, it is clear that the emission is mainly distributed over the east of Asian, the south east of North American and the west of European.

8 Fig.2 The gridded carbon flux and the differences for the INDC and INDC-NA-NT experiment in Results 4.1 The climate change The CO 2 concentration keep increasing with the rate of 2.26 ppm/yr and 2.95 ppm/yr under the INDC and INDC-NA-NT experiment, respectively. Both of the increase rates is in the range of RCP4.5 (1.83ppm/yr) and RCP8.5 (5.96ppm/yr), and much closer to that of the RCP4.5. In the end of the 21 st, the CO 2 concentration of INDC and INDC-NA-NT reaches to 582 ppm and 644 ppm, respectively (Fig.2). Fig.3 The CO 2 concentration of RCPs and simulated by the INDC and INDC-NA-NT experiment

9 Fig.4 The surface air temperature simulated by CMIP5 (Coupled Model Intercomparison Project Phase 5) multiple models and the INDC and INDC-NA-NT experiments (the anomly is related to the average of 1986 to 2005 year) Figure 4 shows the time evolution of the global surface temperature of INDC, INDC-NA-NT and CMIP5 multiple model simulations under RCPs (Representative Concentration Pathways), and it is clear that temperature also show significant increasing trend under the INDC and INDC-NA-NT scenario, and the warming trend is between the multiple model ensemble of RCP8.5 and RCP4.5. In the end of the 21 st ( ), the temperature is projected to increase 2.7 and 3.1 compared to the average of 1986 to 2005, which is in the range of RCP4.5 (2.6 C±0.8 C) and RCP8.5[23]. (5.2 C±1.2 C). The warming of INDC is about 0.4 higher than that of the INDC-NA-NT, and both these two scenario is hard to control the global warming below 1.5 in the end of the 21 st. 4.2 The economic loss of climate change The economic method of assessing the impact of climate change is brought in to interpreting the difference impact of the two scenarios. The method is the damage function of climate change, which is part of IAM. The damage function from DICE model is adopted here and the result is showed in figure 5. As it can be seen, the climate change would result in a big economic loss in the future, which can be about 14% of China s GDP and 10% of USA s. It reveals that the USA may suffer low climate change economic loss than China because of the better adaptive ability. It is also obvious that the economic loss would be lower by 2% in scenario 2 comparing with scenario 1. That means the technology progress can save 2% of the GDP in the future every year.

10 14.00% 12.00% 10.00% 8.00% 6.00% 4.00% 2.00% 0.00% % INDC INDC-NA-NT (a) USA 18.00% 16.00% 14.00% 12.00% 10.00% 8.00% 6.00% 4.00% 2.00% 0.00% % INDC INDC-NA-NT (b) China Fig.5 The economic loss of climate change under the two scenarios. 5. Conclusion and discussion This paper investigated the impact of American climate choice on the global carbon emission and climate change by combining the MIRCE model and earth system mode BNU-ESM. The American climate choice is assumed whether it following the INDC or has technology innovation. The MIRCE projection results show that both the IDNC target and technology innovation can effectively reduce the American s carbon emissions. There is about extra 3.2Gt C if the American dosn t execute the INDC

11 and has no technology progress compared to either following its INDC or has technology innovation in the end of the 21 st. These carbon emission gap lead to an extra 62 ppm CO 2 concentration and about 0.6 warming. Besides, our results also show that even all the countries following its most stringent INDC target, it is difficult to keep the temperature below 1.5 in the end of the 21 st. It is suggested that both China and USA would bare huge economic loss from the climate change, especially for China. And technology progress would cut down the loss by 2% of GDP if technology would not be affected by the USA s climate policy. Our results implies that, more stringent emission reduction targets are required to control the global warming below 1.5, and technology innovation is the key to controlling the carbon emissions. So the American withdrawal from the Paris agreement is not conducive to achieving the 1.5 target, unless it promotes more positive technology innovations. In addition, the inspiration for other countries is the technological change is imperative, especially for developing countries. This study try to quantitatively answer the climate change impact of the U.S. withdrawal from the Paris agreement from a natural science and social science perspective. However, the above results are based on a single economic model and earth system mode, and the results are calculated from the offline mothed rather than the online bidirectional coupling. So there are still uncertainty and areas for improvement in the further study. For example, we should use the multi-economic model and the Earth system model ensemble method or the fully coupled economic-earth system model to solve similar issue. Acknowledgements This work was funded by the Major Research Development Program (No. 2016YFA ), the National Natural Science Foundation of China (No ), the national-level major cultivation project of Guangdong Province entitled: The construction and application of coupled earth system and social economic models (No. 2014GKXM058).

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