The (economic) impacts of climate change: Some implications for Asian economies

Auffhammer, Maximilian Working Paper The (economic) impacts of climate change: Some implications for Asian economies ADBI Working Paper Series, No. 1051 Provided in Cooperation with: Asian Development Bank Institute (ADBI), Tokyo Suggested Citation: Auffhammer, Maximilian (2019) : The (economic) impacts of climate change: Some implications for Asian economies, ADBI Working Paper Series, No. 1051, Asian Development Bank Institute (ADBI), Tokyo This Version is available at: https://hdl.handle.net/10419/222818 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/3.0/igo/ ADBI Working Paper Series THE (ECONOMIC) IMPACTS OF CLIMATE CHANGE: SOME IMPLICATIONS FOR ASIAN ECONOMIES Maximilian Auffhammer No. 1051 December 2019 Asian Development Bank Institute The Working Paper series is a continuation of the formerly named Discussion Paper series; the numbering of the papers continued without interruption or change. ADBI’s working papers reflect initial ideas on a topic and are posted online for discussion. Some working papers may develop into other forms of publication. Suggested citation: Auffhammer, M. 2019. The (Economic) Impacts of Climate Change: Some Implications for Asian Economies. ADBI Working Paper 1051. Tokyo: Asian Development Bank Institute. Available: https://www.adb.org/publications/economic-impacts-climate-change-implications- asian-economies Please contact the author for information about this paper. Email: [email protected] Maximilian Auffhammer is a George Pardee Jr. professor in sustainable development and associate dean of social sciences at the University of California, Berkeley. The views expressed in this paper are the views of the author and do not necessarily reflect the views or policies of ADBI, ADB, its Board of Directors, or the governments they represent. ADBI does not guarantee the accuracy of the data included in this paper and accepts no responsibility for any consequences of their use. Terminology used may not necessarily be consistent with ADB official terms. Working papers are subject to formal revision and correction before they are finalized and considered published. Asian Development Bank Institute Kasumigaseki Building, 8th Floor 3-2-5 Kasumigaseki, Chiyoda-ku Tokyo 100-6008, Japan Tel: +81-3-3593-5500 Fax: +81-3-3593-5571 URL: www.adbi.org E-mail: [email protected] © 2019 Asian Development Bank Institute ADBI Working Paper 1051 M. Auffhammer Abstract This paper provides an overview of how economists think about climate change impacts with a focus on Asia. It is designed to discuss the steps along the causal chain from physical impacts to impacts on human and natural systems. It starts with a summary of the projected physical impacts of climate change on Asia based on the IPCC’s 5th assessment report. It then discusses the concept of the social cost of carbon and how integrated assessment models are used to obtain estimates of this number. This includes a discussion of how a mapping from physical climate impacts to economic damages – called the damage function – is obtained. A recent literature using a mapping of weather/climate into GDP growth rates is discussed, including impact estimates from the literature. The second part of the paper contains an extensive discussion of the mitigation challenge, both physical and economic and concludes with a discussion of the policy challenges going forward. Keywords: climate change, economic damages, carbon mitigation JEL Classification: Q54 ADBI Working Paper 1051 M. Auffhammer Contents 1. INTRODUCTION ......................................................................................................... 1 2. PHYSICAL IMPACTS OF CLIMATE CHANGE ........................................................... 1 3. ECONOMIC IMPACTS OF CLIMATE CHANGE ......................................................... 5 3.1 Integrated Assessment Models and Social Cost of Carbon ............................ 6 3.2 Damage Function Estimation ........................................................................ 10 3.3 Overview of Estimates at the Country Level for Asian Economies ................ 14 4. MITIGATION ............................................................................................................. 17 5. GLOBAL POLICY ...................................................................................................... 22 6. A PATH FORWARD .................................................................................................. 23 REFERENCES ..................................................................................................................... 26 ADBI Working Paper 1051 M. Auffhammer 1 1. INTRODUCTION Climate change has been widely touted as the biggest environmental challenge humankind will encounter over the next centuries. The Intergovernmental Panel on Climate Change (IPCC) projects that in the absence of significant policy intervention, unmitigated climate change may lead to unprecedented changes in the climate system— including, but not limited to, changes in temperature, precipitation, and sea levels (IPCC 2013). These changes in the climate system will have significant impacts on human society. If we are serious about engaging in mitigation actions, costs will be incurred in the near future to achieve emissions reductions. These will bring about hard to measure benefits in terms of avoiding climate change further in the future. As climate change is a global phenomenon, it is not a single economy or group of economies that can solve this problem. It will take efforts by economies large and small to tackle this problem. Furthermore, impacts are not evenly distributed. The IPCC (2014) points out that poorer populations, especially in agrarian societies, are likely to suffer the most. In terms of sectors, agriculture and energy use are two that are highly exposed. Direct and indirect effects of climate change have also been shown to affect labor productivity, crime, violent conflicts, happiness, migration, mortality, and morbidity (Carleton and Hsiang 2016). However, our understanding of the impacts for most of these sectors is limited to specific locations and time periods. Also, solid methods to quantify impacts are still being developed. This conference paper proceeds as follows. Section 2 provides a summary of climate change impacts on Asia based on the fifth assessment report by the IPCC. Section 3 discusses the methodologies used by economists to quantify damages from climate change and provides an overview of two recent papers which provide some empirical estimates of the impact of climate change on individual countries’ GDP, as well country level estimates of the implied social cost of carbon. Section 4 discusses historical emissions and future emission scenarios more broadly and provides some insights into the contribution to emissions by different countries. Section 5 briefly discusses global efforts to arrive at a global climate policy regime. Section 6 discusses country level policy options going forward, and concludes. 2. PHYSICAL IMPACTS OF CLIMATE CHANGE Joseph Fourier hypothesized the greenhouse effect in 1924. In 1896, Svante Arrhenius was the first scientist to make a quantitative prediction of global warming if one doubled the atmospheric concentration of carbon dioxide. Over the past ~125 years, scientists have studied the impact of higher greenhouse gas emissions on the global climate. The IPCC issues assessment reports roughly every seven years, which are meant to synthesize the current state of climate science. Working Group I (WG1) “aims at assessing the physical scientific basis of the climate system and climate change. Its main topics include: changes in greenhouse gases and aerosols in the atmosphere; observed changes in air, land and ocean temperatures, rainfall, glaciers and ice sheets, oceans and sea level; historical and paleoclimatic perspective on climate change; biogeochemistry, carbon cycle, gases and aerosols; satellite data and other data; climate models; climate projections, causes and attribution of climate change” (IPCC 2013). The report includes summary predictions for the entire globe, but also provides regional projections of the impacts of climate change. For a more comprehensive and recent review, I suggest consulting Hsiang and Kopp (2018) or the WG1 report (IPCC 2013). Working Group II (WG2) “assesses the vulnerability of ADBI Working Paper 1051 M. Auffhammer 2 socio-economic and natural systems to climate change, negative and positive consequences of climate change and options for adapting to it.” In short, therefore, WG1 deals with the physical climate system and modeling, while WG2 summarizes the literature on impacts on human and natural systems (IPCC 2014). Figure 1 is taken from the fifth assessment report (AR5), which contains a chapter on impacts for Asia. Figure 1 displays a map for what the IPCC considers to be Asia. Figure 1: IPCC AR5 WG2 Asia Map Source: IPCC (2014). The main conclusions drawn by the IPCC for their AR5 are paraphrased as follows (IPCC 2014). • Warming trends and increasing temperature extremes are observed across most of the Asian region over the twentieth century. Observations show a rising number of warm days and a decreasing number of cold days. This trend is not slowing down. Precipitation trends have changing degrees of variability across Asia. • Due to increasing water demand and suboptimal management practices, water scarcity is thought to be a major challenge going forward. Water availability is of massive importance due to the large population in the region. While future projections at the subregional level do not provide a clear forecast, rapid population growth and rising incomes can put additional pressures on water resources in the region. Management of water resources is paramount. • Climate change will affect food production, yet the impacts will be variable depending on the location. However, it looks like many regions will experience a climate-driven decline in production, the clearest predictions being for reduced rice production due to the shortening of growing periods. It is noted that CO2 ADBI Working Paper 1051 M. Auffhammer 3 fertilization may offset some of these drops in yield. Overall there may be some winners—Kazakhstan’s cereal production, for example—and some losers (Western Turkmenistan and Uzbekistan). It is further noted that the area dedicated to high-yielding wheat could decrease significantly in the Indo-Gangetic Plain. Rising seas will cause problems for low-lying areas. • There has been an observed shift in phenology, along with the growth and distribution of plant species and permafrost degradation. It is expected that climate change will amplify these impacts going forward. • There is evidence that marine and coastal systems are experiencing increased stress from climate and other factors. Rising sea levels are expected to lead to increased coastal erosion and high water levels. There may be damage to mangroves, salt marshes, and coral reefs. • It is expected that climate change will compound impacts caused by urbanization and industrial and broader economic development. “Climate change is expected to adversely affect the sustainable development capabilities of most Asian developing countries by aggravating pressures on natural resources and the environment. Development of sustainable cities in Asia with fewer fossil fueldriven vehicles and with more trees and greenery would have a number of cobenefits, including improved public health” (IPCC 2014a). • Human health, security, livelihoods, and poverty will be increasingly affected by extreme climate events. Mortality and morbidity, especially for vulnerable groups, is expected to increase due to heatwaves. The risk of diarrheal diseases, dengue fever, and malaria is expected via the increased risk of floods and droughts. • AR5 concludes that “studies of observed climate changes and their impacts are still inadequate for many areas, particularly in North, Central, and West Asia” (IPCC 2014a). The call is for better projections of all significant precipitation, which affects water supplies. The assessment report further highlights a greatly limited understanding of the impacts of climate change on a large number of sectors, but singles out the urban environment. Table 1 identifies the IPCC’s summary of the state of scientific evidence in physical impact estimation. The crosses in the table suggest that for the Asian region there is insufficient evidence and that there are critical knowledge gaps. This table can serve as a map for a future research agenda, as understanding the historical and future impacts of climate change on these sectors is key for optimal policymaking. The IPCC synthesizes physical and socio-economic impacts published in the literature. The literature attempting to estimate damage from climate change has exploded, even since publication of the AR5. In order to fill the gap for some of the sectors pointed out by the IPCC as lacking evidence, economists and statisticians have developed a number of methods to empirically estimate the projected costs of climate change. The next section discusses the evolution of these impact estimation techniques and how they are used in practice. ADBI Working Paper 1051 M. Auffhammer 4 Table 1: State of Evidence Regarding Observed and Projected Impacts of Climate Change Topics/Issues North Asia East Asia Southeast Asia Sector O = Observed Impacts, P = Projected Impacts O P O P O P Freshwater resources Major river runoff / x / / / / Water supply x x x x x x Terrestrial and inland water systems Phenology and growth rates / / / / x x Distributions of species and biomes / / / / x x Permafrost / / / / / x Inland waters x x / x x x Coastal systems and low-lying areas Coral reefs NR NR / / / / Other coastal ecosystems x x / / x x Arctic coast erosion / / NR NR NR NR Food production systems and food security Rice yield x x / / x / Wheat yield x x x x x x Corn field x x x / x x Other crops (e.g., barley, potato) x x / / x x Vegetables x x / x x x Fruits x x / x x x Livestock x x / x x x Fisheries and aquaculture production x / x / x / Farming area x / x / x x Water demand for irrigation x / x / x x Pest and disease occurrence x x x x x x Human settlements, industry, and infrastructure Floodplains x x / / / / Coastal areas x x / / / / Population and assets x x / / / / Industry and infrastructure x x / / / / Human health, security, livelihoods, and poverty Health effects of floods x x x x x x Health effects of heat x x / x x x Health effects of drought x x x x x x Water-borne diseases x x x x / x Vector-borne diseases x x x x / x Livelihoods and poverty x x / x x x Economic valuation x x x x / / continue on next page ADBI Working Paper 1051 M. Auffhammer 11 Source: Auffhammer (2018). When you leave your house in the morning, you have to decide what to wear. If you look outside your window and it is cold and rainy, you will wear warmer clothes and bring an umbrella, if you have one. The next day it may be sunny and warm, and you will leave your home in a short-sleeved shirt. What you encounter on a day to day basis is weather. In most places, weather is highly variable across and even within seasons. Summer months are usually warmer and drier, while winter months are usually colder and wetter. Climate can coarsely be characterized as the full statistical distribution of weather. If that distribution is stationary—meaning the moments of the distribution are constant over time—one would still expect day to day variation in weather, but on average expect a similar number of hotter and cooler days during the summer, etc. Let’s use a sector which is highly sensitive to weather: electricity consumption. Assume a location with a cool pleasant climate, like San Francisco. Fashionable San Francisco old and young alike live in houses and apartments that usually do not have air conditioning equipment. What this means is that on the occasional very hot day, San Franciscans will complain loudly and head to the park to eat ice cream, hoping for cooler days. Since these hot days are a rare occurrence in a world without climate change, the cost of installing and operating air conditioning may be greater for most residents compared to the benefits they would derive from the few days they would use it. However, if San Francisco inherits the climate of Beijing, which is much warmer and more humid, especially in the summer, most San Franciscans would probably find it optimal to install air conditioning. Hence electricity use would go up. We call this adaptation. The top left panel of Figure 4 displays the weather from a pre-climate change world in light gray. The temperature series is mapped via the damage function into electricity consumption. This pre-climate change damage function is shown in the top right panel by the solid line. This translates into the pre-climate change electricity consumption shown by the dark solid line in the bottom right panel. Now, if climate changes and the weather is drawn from a distribution with a higher mean and variance, as indicated by the dark solid time series in the top left panel, the question arises as to what the right damage function is. If individuals understand that climate has changed, they will adapt (in our case, install more air conditioning) and the damage function changes. The new damage function is steeper, especially at higher temperatures. This means that the new normal weather is mapped into electricity consumption through the “with adaptation” damage function in the top right panel. The correct post-climate change consumption time series is the light gray dotted time series in the bottom right panel. The difference between the dotted line and solid line in the bottom panel is the damage from climate change. Why is this so important? A damage function one would want to use in an IAM should account for adaptation, carry a causal interpretation, and have global coverage. This sounds straightforward, but turns out to be quite difficult in practice. The literature on damage function estimation goes back about five decades and can be split into roughly five strands. The first and most widely adopted approach for estimating climate change damage stems from a seminal paper by Mendelsohn, Nordhaus, and Shaw (1994). They study the agricultural sector for the United States, but the method has been applied in a number of other sectors (e.g., energy consumption). The insight behind this method can best be demonstrated by Figure 5 below. ADBI Working Paper 1051 M. Auffhammer 12 Figure 5: Crop Choice and Profits in the Long Run Note: Imagine a single farmer, who is currently growing crop 1 and earning profits corresponding to the y value at point A. If faced with a significantly hotter climate, the farmer becomes indifferent between growing crop 1 and crop 2 at point B. If climate warms further still, the farmer would be much better off at point C (switching to crop 2) rather than at point D (continuing to grow crop 1). Source: Auffhammer (2018). Assume that a farmer is currently planting crop 1 as (s)he is operating in an area with a cool climate. If the climate gets warmer, (s)he could continue to grow the same crop and make profits as given by point D. However, the farmer could make profits as indicated by point C, simply by switching crops. If one assumes rational, profit-maximizing farmers, the impact of climate change on profits is given by the thick black line in the figure, the envelope of the individual crop/use profit functions, which are functions of, for example, temperature. Econometrically, this is relatively straightforward to implement. If one has data on profits and climate for farmers across different climate zones, one could assume that, conditional on other confounders, the cross-section would give one the envelope of the profit functions exactly. How does one implement this in practice? One runs a cross-sectional regression of outcomes of interest (e.g., agricultural profits) on complex functional forms of long run averages of weather (=climate) and other observable confounders such as soil quality and proximity to roads, to name some examples. The advantage of this method is that it actually identifies the effect of climate on the outcome of interest. The disadvantage of this method is that if one omits confounders that explain variation in the outcome of interest and that are correlated with climate, the estimation suffers from omitted variables bias. The consequences of this can be grave. Another method, which is less frequently used, is simply using time series regressions of outcomes of interest on functions of weather. One example is Franco and Sanstad (2008), who regress electricity load for the state of California on population weighted averages of weather. Other more recent examples of this work are Auffhammer, Baylis, and Hausman (2017) and Wenz, Auffhammer, and Levermann (2017), who estimate the impacts of weather on electricity for the entire US and European countries individually. The advantage of this approach is that it allows researchers to estimate dose response/damage functions for variables that are only available at very coarse levels of aggregation. However, these regressions only identify the effect of weather on the ADBI Working Paper 1051 M. Auffhammer 13 outcomes of interest and do not incorporate the impacts of adaptation, which is of course a significant, if not detrimental drawback. A third method, which has been more frequently applied in the recent damages literature, has employed the use of longitudinal data sets, often referred to as panel data sets. Early examples of these papers are Auffhammer, Ramanathan, and Vincent (2006) and Deschênes and Greenstone (2007). These papers build on the time series regression approach discussed above, but have the advantage that one has observations on an outcome of interest for multiple units (e.g., states or counties) over a number of years. The existing literature has used variation across time and cross-sectional units to estimate the dose response/damage function. The advantage of this approach is that one can control for confounders through a fixed effects strategy that controls for unobservable differences that are time invariant across units and shocks common to all or a subset of units for a given year. Hence the risk of omitted variables bias that the Ricardian approach suffers from is much lower here. The drawback is the same issue that time series regressions suffer from. The response functions estimate a relationship between an outcome of interest and weather—not climate, which lacks an adaptation response. A recent paper by Burke and Emerick (2016) provides a fourth and very clever way to address the confoundables and the adaptation issue at the same time. The authors demonstrate a new method using data on US agricultural output across counties, benefiting greatly from the availability of highly disaggregated annual data over a long period. They calculate a long difference in agricultural output, where they determine the difference between a five-year moving average at the end and the beginning of their sample period. This long difference is the outcome of interest, telling us how much agricultural yields or profits have changed over, say, 40 years. On the right-hand side they use trends in temperature over the same time period. The differencing is equivalent to including unit fixed effects, and using trends gives the estimated coefficients a long run interpretation, which contains adaptation. Burke and Emerick (2016) show for their data that there is little evidence of an adaptation response. This approach has not been used widely outside agriculture. The fifth and most recent approach has incorporated panel data estimation techniques, but augmented the specifications by interacting the weather response coefficients with cross-sectional variables measuring both climate (long run averages of weather) and income. A recent example of this work is Auffhammer (2018). The idea is a simple one. The weather response of, for example, energy consumption is different in areas with a hot versus a cool climate. Interacting the weather variables with a cross-sectional climate variable allows for empirical estimation of this response heterogeneity. An interaction of the weather variables with cross-sectional income allows for heterogeneity in the weather response as a function of income. Richer economies are thought to be able to adapt more easily, and this interaction captures this difference in response. The beauty of this approach is that if one has projections of income and climate, one can simulate how the damage function changes as climate and incomes change going forward. Carleton et al. (2019) make some strong assumptions, but using a dataset on mortality for the majority of humankind, provide extrapolations of climate change-induced mortality for all countries in the world to the end of the twenty-first century. This approach is possible with shorter time series than the long differences approach. It also has the benefit of shifting the response function, which the long differences approach cannot do, in a forward looking way. If one is interested in the detection and attribution of historical climate change, I would argue that the long differences approach is the best tool available, data permitting. ADBI Working Paper 1051 M. Auffhammer 14 The papers discussed above mostly focus on single economies for individual sectors. In order to construct a damage function for all sectors across the world, one would need to write a few thousand empirical papers to get good coverage. I would encourage young scholars to start writing these papers. Another approach is to estimate a single damage function using GDP data, measuring the value of all goods and services produced for a country in a given year. Burke, Hsiang, and Miguel (2015) have written a paper providing a correlation between the growth rate of GDP and a nonlinear measure of changes in temperature to estimate how damaging climate change will be to the major world economies. While there is discussion about the empirical model adopted in the paper, they show a highly nonlinear relationship between GDP growth rate and temperature which resembles an environmental Kuznets curve—an inverse U. Growth rates peak at about 13° Celsius and are increasing at lower temperatures and decreasing at higher temperatures. The issue with approaches like this is that GDP does not measure everything that has value. It excludes the value of non-market resources, which is likely to be significant. Furthermore, this paper again uses short run variation. For technical reasons discussed in McIntosh and Schlenker (2006), the approach here includes some adaptation response but not complete adaptation. In the next section, I will discuss some of these results and provide some context for the Asian economies. 3.3 Overview of Estimates at the Country Level for Asian Economies While the previous section discussed the different approaches to damage estimation, there is far from complete coverage for the different sectors of each economy. There are a number of studies using the hedonic approach for agriculture; a few panel data studies for agriculture, mortality, and energy consumption (Carleton and Hsiang 2016); and one study that uses the panel data approach accounting for adaptation (Carleton et al. 2019). There is, of course, a larger literature in field journals and white papers which use a variety of methods for single sectors. In this section, I provide an overview of the types of result targeted at providing estimates of the impact of climate change. The first set of estimates stems from the paper by Burke, Hsiang, and Miguel (2015) discussed in the previous section, using a simple regression framework accounting for partial adaptation. The projected climate change impacts shown in Table 3 for selected Asian Development Bank (ADB) member countries are presented for the worst-case emissions scenario and are calculated relative to per capita GDP in the year 2010. The estimates from this paper are the only country-level impacts of climate change currently available for most countries. The average impact on per capita GDP across countries by the mid-century is –13.6%, and the average impact by the end of the century is –12.35%. These numbers do not sound unreasonable, and are slightly higher than the number predicted by some integrated assessment models. Yet what stands out here is the massive spread in impacts. For mid-century, the range of impacts just from climate change on per capita GDP is from –40 (Pakistan) to +88% (Mongolia). If we go to the end of the century, the spread of projections becomes even larger. Projected impacts range from –87% (Pakistan) to +881% (Mongolia). These projections have received a significant amount of media attention, and it has been widely reported that global per capita GDP is projected to decrease by roughly 23% by the end of the century. Before using these numbers in decision-making, it is important to remember that this paper has been criticized for making some strong functional form assumptions, where temperature affects the growth rate instead of the level of per capita GDP, which means that the shocks propagate through time. A recent working paper by Newell, Prest, and Sexton (2018) has pointed out that functional form assumptions have significant consequences ADBI Working Paper 1051 M. Auffhammer 15 for the point estimates. Furthermore, GDP only measures market impacts and ignores non-market impacts, and is hence incomplete. Table 3: Climate Change Impacts on Selected ADB Member Countries Country Change in GDP p.c. (2040–2059) in % Change in GDP p.c. (2080–2099) in % Afghanistan –4.78 –28.43 Armenia 19.28 73.50 Australia –12.60 –44.77 Azerbaijan –2.53 –18.17 Bangladesh –36.49 –83.65 Brunei Darussalam –34.16 –81.47 Bhutan –1.17 –11.76 People’s Republic of China –7.51 –34.07 Fiji –23.63 –65.40 Georgia 5.52 8.94 Indonesia –31.44 –77.93 India –38.78 –86.16 Japan –5.97 –28.23 Kyrgyz Republic 29.53 132.64 Cambodia –38.94 –81.57 Korea, Republic Of 3.09 1.89 Kazakhstan 32.17 158.87 Lao People's Democratic Republic –32.21 –79.17 Sri Lanka –32.14 –78.71 Mongolia 87.81 881.11 Malaysia –33.53 –80.70 Nepal –31.08 –78.09 New Zealand –0.41 –6.41 Papua New Guinea –24.30 –66.96 Philippines –30.61 –76.38 Pakistan –39.54 –87.07 Solomon Islands –31.35 –77.07 Thailand –37.81 –84.70 Tajikistan 1.36 –9.47 Turkmenistan –14.10 –51.11 Uzbekistan –8.45 –37.20 Viet Nam –33.60 –80.82 Vanuatu –26.14 –69.40 Samoa –27.87 –72.30 Source: Burke, Hsiang, and Miguel (2015). ADBI Working Paper 1051 M. Auffhammer 16 Another recent paper (Ricke et al. 2018) has used the GDP damage function used in Burke, Hsiang, and Miguel (2015) and another paper by Dell, Jones, and Olken (2012) to calculate a country-specific SCC. As discussed above, in terms of global policymaking, the number that matters from a global perspective is the global SCC. There is, of course, the issue that mitigation happens at the country level. There is an argument for a domestic SCC, which suggests that countries will only find it optimal from a domestic perspective to take action if the benefits of action outweigh the costs. Hence one would want to calculate a country-specific SCC, which ignores damage caused elsewhere. The only framework currently which has country-level resolution is the Burke, Hsiang, and Miguel (2015) framework. Ricke et al. (2018) used this framework to calculate a social cost of carbon at the country level and characterize the uncertainty around it. To give us an idea of the magnitude, I have used their publicly available estimates and generated the median, 16.7th and 83.3rd percentiles at the country level for ADB member countries with available estimates. I only included runs for SSP3 and 5, using the emissions scenario RCP 8.5 and a constant discount rate of 3%, yet using all available damage functions in the paper. Hence these estimates differ slightly from the main estimates in the paper. The aggregate SCC across all countries for this study is estimated to be $1,060, which is, of course, significantly higher than the IWG estimate of $42 per ton. This is largely due to the damage function estimated by Burke, Hsiang, and Miguel (2015). If one takes that approach at face value, Table 4 reports the estimated SCC at the country level. What emerges from this table is that the SCC for the smaller countries is, well, smaller. Population and economy size matter in this calculation. Hence bigger economies are more likely to have larger estimates. The People’s Republic of China (PRC) and India have CSCCs north of $100. Very few countries have CSCCs that are negative. The most noteworthy of these is Mongolia which, according to Burke, Hsiang, and Miguel (2015), is projected to experience significant increases in per capita GDP from climate change. While, for the purposes of this paper, the country-level estimates are interesting, I would like to caution the reader from taking these point estimates at face value, since they depend on two papers (Burke, Hsiang, and Miguel 2015; Dell, Jones, and Olken 2012) which adopt a very specific functional form to arrive at a damage function. The overall damages predicted by these models are much larger than those predicted by more recent incarnations of the classic IAMs. The DICE model by Nobel Laureate Bill Nordhaus in a recent publication (Nordhaus 2017) estimates a SCC of $31 dollars. This is, of course, significantly lower than what Ricke et al. (2018) report. An important route forward, in my view, is to build empirically validated sectoral damage functions that cover as much of the globe as possible and aggregate across space. The Climate Impact Lab is currently working on developing a credible method to do so, which should be applied to more sectors than mortality, agriculture, and energy consumption. The Climate Impact Lab comprising experts from the University of Chicago, UC Berkeley, Rutgers University, and the Rhodium Group, is aggressively pursuing this approach. I would encourage readers to follow their advances and start expanding this literature.4 4 The Climate Impact Lab can be found at https://www.impactlab.org. ADBI Working Paper 1051 M. Auffhammer 17 Table 4: Country Level Social Cost of Carbon (CSCC) Country CSCC (US$, 17th Percentile) CSCC (US$, Median) CSCC (US$, 83rd Percentile) Afghanistan –2.94 0.88 2.22 Armenia –0.29 –0.08 0.04 Australia 1.20 5.89 7.53 Azerbaijan –0.08 0.33 0.56 Bangladesh 7.57 10.33 11.68 Brunei Darussalam –0.02 0.12 0.14 Bhutan 0.08 0.16 0.20 People’s Republic of China 45.68 115.23 155.16 Fiji 0.03 0.04 0.05 Georgia –0.10 0.08 0.19 Indonesia 15.24 25.64 29.92 India 76.31 116.98 131.53 Japan 0.61 10.41 14.35 Kazakhstan –6.27 –2.03 0.01 Kyrgyz Republic –1.53 –0.61 –0.13 Cambodia 0.47 0.88 1.04 Korea, Republic Of –3.53 2.97 5.92 Lao People's Democratic Republic 0.37 0.52 0.60 Sri Lanka 1.12 1.90 2.23 Mongolia –16.34 –6.03 –2.49 Malaysia 2.06 5.91 6.96 Nepal 1.32 1.68 1.92 New Zealand –0.17 0.40 0.68 Pakistan 8.42 11.38 13.24 Philippines 5.45 8.34 9.59 Papua New Guinea 0.56 0.75 0.86 Solomon Islands 0.03 0.05 0.06 Thailand 3.95 8.41 9.96 Tajikistan –0.25 0.10 0.28 Turkmenistan 0.35 0.71 0.88 Uzbekistan 0.27 1.29 1.82 Viet Nam 4.45 6.34 7.42 Vanuatu 0.02 0.04 0.04 Samoa 0.00 0.01 0.01 Source: Ricke et al. (2019). 4. MITIGATION While the previous sections discussed projected physical and economic impacts of climate change, at the very heart of the problem is the fact that emissions of greenhouse gases have grown at a steady pace since the dawn of the industrial revolution. Figure 6 below shows the trajectory of emissions since 1751. ADBI Working Paper 1051 M. Auffhammer 18 ADBI Working Paper 1051 M. Auffhammer 19 Figure 6: Global Emissions of CO2 t = ton. Source: Global Carbon Project (GCP); CDIAC. The figure makes two points. First, the growth of emissions in the post-war period was massive. Annual emissions went from roughly 5 billion tons in 1950 to 35 billion tons in 2017, a sevenfold increase. The second aspect to notice is that global fuel mix has changed tremendously. While in the early parts of the 20th century the vast majority of emissions came from coal, today these account for less than half. Liquid fuels, largely driven by increases in transportation demand, are now the second biggest source of emissions, followed by natural gas. Since CO2 is a stock pollutant when using a human time horizon, one should ask who the biggest contributors to the stock of greenhouse gases in the atmosphere are by region. Figure 7 below breaks down the cumulative emissions for several countries and aggregates. This figure provides two interesting insights. In the early days of industrial carbon emissions, the lion’s share of emissions came from the home of the industrial revolution, the United Kingdom, with almost 100% of cumulative emissions. In the late nineteenth century, emissions in the United States started growing rapidly, and the US share in cumulative emissions rose to a peak of 40% by the end of World War 2. What one observes starting in the late twentieth century is the emergence of the PRC and India as major sources of annual emissions. The growth in emissions was so rapid that even though aggregate annual emissions were growing quickly during this period, the Chinese share in cumulative emissions broke 10% of total emissions, while India’s is hovering around 5%. Maybe the most significant insight behind this figure is that the slope of the emissions trajectories for the US and the EU is declining and the share for the rapidly developing economies of India and the PRC is increasing, with the rate of change increasing. ADBI Working Paper 1051 M. Auffhammer 20 Figure 7: Cumulative CO2 Emissions for a Subset of Countries and Aggregates Source: Our World in Data based on Global Carbon Project (GCP). The drivers of this growth are not surprising. In the climate literature, there is a simple decomposition of emissions called the KAYA identity. It hypothesizes that CO2 emissions can be decomposed into a product of population, GDP per capita, energy intensity per unit of GDP, and carbon intensity (carbon per unit of energy consumed). If one takes this equation at face value, it suggests that more populous, richer, energy-intensive countries with a more carbon-intensive energy sector will have larger emissions. This is not surprising and has been confirmed in largely cross-sectional decomposition analyses. Taking a closer look at one part of this equation, the income emissions relationship is instructive. One common way to look at this is to plot the relationship between per capita emissions and per capita income, which compares measures per person across countries A snapshot of how average income and emissions correlate is provided in Figure 8 below for the year 2016. It is important to note that this figure is plotted on a log-log scale. This suggests a highly nonlinear relationship between per capita emissions and income, higher incomes being consistent with significantly higher emissions. Sub-Saharan economies are shown to have a per capita GDP below $1,000 per person and emissions below 0.1 tons per year. The United States, whose GDP is approaching $60,000 per capita, has per capita emissions exceeding 10 tons per person year. That is a difference of two orders of magnitude. The sign of this effect is intuitive, as wealthier countries consume (and produce) more goods and services, and therefore have higher emissions. ADBI Working Paper 1051 M. Auffhammer 27 National Academies of Sciences, Engineering, and Medicine. 2017. Valuing Climate Damages: Updating Estimation of the Social Cost of Carbon Dioxide. Washington, DC: The National Academies Press. https://doi.org/10.17226/24651. Newell, Richard G., Brian C. Prest, and S. Sexton. 2018. The GDP-Temperature Relationship: Implications for Climate Change Damages. RFF Working Paper. http://www.rff.org/research/ publications/gdp-temperature-relationship- implicationsclimate-change-damages. Nordhaus, William. 2017. Revisiting the Social Cost of Carbon. PNAS, 114(7): 1518–1523. Ricke, K., L. Drouet, K. Caldeira, and M. Tavoni. 2018. Country-Level Social Cost of Carbon. Nature Climate Change 8(10): 895. Rose, Steven K., Delavane B. Diaz, and Geoffrey J. Blanford. 2017. Understanding the Social Cost of Carbon: A Model Diagnostic and Inter-Comparison Study. Climate Change Economics 8(02): 1750009. Wenz, Leonie, Anders Levermann, and Maximilian Auffhammer. 2017. North–South Polarization of European Electricity Consumption under Future Warming. Proceedings of the National Academy of Sciences 114(38): E7910–E7918.