风险企业：气候与宏观经济

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　 Special Report Risky business: the climate and the macroeconomy  Climate change is a slow-moving process, but it is no less danger- ous for that. It is likely to be one of the key defining features of the coming decades. The longer action is delayed the more costly it will be to deal with the issues. Moreover, a delayed policy response opens us up to potentially catastrophic outcomes, which might be impossible to reverse.  This report examines climate change in three sections: the mechan- ics of climate change; the impact of climate change; and the re- sponse to climate change.  The mechanics of climate change considers the journey from hu- man activity to CO 2 emissions, from CO 2 emissions to atmospheric CO 2 concentrations, from atmospheric CO 2 concentrations to the global temperature and from the global temperature to the global climate. The climate system is complex, non-linear and dynamic. There is considerable inertia in the system so that emissions in the coming decades will continue to affect the climate for centuries to come in a way that is likely to be irreversible . Uncertainty is en- demic, not just about modal effects but also about the shape of the probability distributions, especially how fat the tails are.   The impact of climate change is broad based covering GDP, the capital stock, health, mortality, water stress, famine, displacement, migration, political stress, conflict, biodiversity and species surviv- al. Uncertainty is endemic here as well, trying to evaluate the im- pact of a climate that the earth hasn’t seen for many millions of years. Empirical estimates based on the variability of the climate in recent decades likely massively underestimate the effects.   The response to climate change should be motivated not only by central estimates of outcomes but also by the likelihood of extreme events (from the tails of the probability distribution). We cannot rule out catastrophic outcomes where human life as we know it is threatened.   To contain the change in the climate, global net emissions need to reach zero by the second half of this century. Although much is happening at the micro level, it is hard to envisage enough change taking place at the macro level without a global carbon tax.  But, this is not going to happen anytime soon. Developed econo- mies, who are responsible for most of the cumulative emissions, worry about competitiveness and jobs. Meanwhile, Emerging and Developing economies, who are responsible for much less of the cumulative emissions, still see carbon intensive activity as a way of raising living standards. It is a global problem but no global solu- tion is in sight. Economic Research January 14, 2020

　 David Mackie (44-20) 7134-8325 david.mackie@jpmorgan.com

　Jessica Murray (44-20) 7742 6325 jessica.x.murray@jpmorgan.com

　www.jpmorganmarkets.com 20 Conclusion 19 Geoengineering as an extreme technology 17 Adaptation and mitigation Ecosystems and species survival 15

　Section 3: The response to climate change 16

　CO 2

　emissions as a global externality 16 15 Climate change and conflict 14 Climate change and migration pressure 13 Climate change and health The impact of climate change beyond GDP 13 12 Economic impacts are too small Section 2: The impact of climate change 10 Estimates of climate change on GDP 10 Wealth effects and the discount rate

　12 9

　From temperature to climate 7

　From CO 2 concentrations to temperature 6

　From CO 2 emissions to CO 2 concentrations Section 1: The mechanics of climate change 5 From human activity to CO 2 emissions 5 2

　Introduction

　Contents:

　CO 2 Temperature

　 Introduction In the 800,000 years prior to the industrial revolution, the atmospheric concentration of CO 2 oscillated in a range from 170ppm (parts per million) to 300ppm. This ebb and flow in CO 2 emissions was mainly driven by volcanic activity and ocean fissures. Since the industrial revolution, CO 2 concen- trations have climbed dramatically to the current level of around 410ppm (Figure 1). 1

　This increase in CO 2 concentra- tions reflects the burning of fossil fuels for electricity genera- tion and transportation, industrialization, and changes in ag- riculture and land use (deforestation). Figure 1: Atmospheric concentration of carbon dioxide

　Parts per million (ppm)

　450

　400

　350

　around 1°C (Figure 3). 3 This has been associated with a rise in CO 2 concentrations from 280ppm to around 410ppm.

　However, given the long lags between emissions and temper- ature, the global temperature will keep rising in the coming decades even if CO 2 concentrations are stabilized at current levels. Figure 2: CO 2 concentration and temperature over 800,000 years

　ppm Surface air temperature anomaly mean 1000 years, °C 350 8 6

　300 4

　250 2 0

　200 -2 150 -4 -800,000 -600,000 -400,000 -200,000 0 300

　Source: See footnote 2, J.P. Morgan

　Years before 8000 BC, 0 = 8000 BC 250

　200

　150

　-800,000 -600,000 -400,000 -200,000 0

　Source: See footnote 1, J.P. Morgan Years before 1950, 0 = 1950

　 There has been a relatively close relationship between CO 2 concentrations and temperature over the last 800,000 years (Figure 2). 2

　These long run estimates of CO 2 concentrations and temperature are based on ice core data from Antarctica so they are not estimates of global conditions. But the im- pression is very strong. Over the last 800,000 years, through to the middle of the 19th century, as CO 2 concentrations os- cillated in a 170ppm to 300ppm range, the Antarctic tempera- ture oscillated in a range from -3.5°C to +6.3°C (relative to the average temperature over the last 1000 years).

　More recent data indicate that the increase in the global aver- age surface temperature since pre-industrial times has been

　1 Lüthi et al, High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, Vol. 453, pp. 379- 382, 15 May 2008.; Petit et al, Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Nature 399: 429-436.; C. D. Keeling et al, Exchanges of atmospheric CO2 and 13CO2 with the terrestrial biosphere and oceans from 1978 to

　Figure 3: Global mean temperature anomalies

　°C difference relative to 1961-1990 average 1.0 0.8 0.5 0.3 0.0 -0.3 -0.5 -0.8 1850 1870 1890 1910 1930 1950 1970 1990 2010 Source: Footnote 3, J.P. Morgan

　 Increases in the global average surface temperature affect the earth’s climate system. This system is complex, non-linear and dynamic. It is helpful to think of the climate as the prob- ability distribution of weather outcomes. 4 Each day’s weather comes from this distribution. In fact, the climate system co- vers more than what we normally think of as the weather— temperature, precipitation, wind, cloudiness and storms. It also covers complex features such as snow and ice cover, the sea level, atmospheric and ocean circulation patterns (such as the Gulf Stream and the El Niño Southern Oscillation). All of these interact in complex, non-linear and dynamic ways. Of particular importance are positive feedback mechanisms 2000. I. Global aspects, SIO Reference Series, No. 01-06, Scripps

　Institution of Oceanography, San Diego, 88 pages, 2001. 2 Lüthi et al, High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, Vol. 453, pp. 379- 382, 15 May 2008; Friedrich, T. et al., Nonlinear climate sensitivity and its implications for future greenhouse warming, Science Ad- vances, Vol. 2, 2016 3 Morice, C. P., J. J. Kennedy, N. A. Rayner, and P. D. Jones, Quan- tifying uncertainties in global and regional temperature change us- ing an ensemble of observational estimates: The HadCRUT4 da- taset, 2012 4 Auffhammer, M., Quantifying economic damages from climate change. JEP, Fall 2018

　which create amplification in response to initial shocks. Due to this complexity, climate models, even if they are huge, don’t fully capture everything that is going on.

　If we think of the climate as a probability distribution cover- ing weather and these other aspects, climate change refers to a shift in the moments of this probability distribution. And what matters is not simply the mean and variance, but also the skewness and kurtosis. Skewness and kurtosis determine the fatness of the tails—the likelihood of low-probability, extreme events.

　The Paris agreement on climate change, adopted in Decem- ber 2015, has a central objective of limiting the rise in the global temperature “to well below 2°C above pre-industrial times, and to pursue efforts to limit the temperature increase even further to 1.5°C.” This objective is to be met by the end of the century. Given that the rise in atmospheric CO 2 has already increased the global temperature by around 1°C rela- tive to pre-industrial times, and there is a lagged effect still to come, these Paris objectives look challenging, especially with the US decision to leave the Paris Accord (Table 1, RCP8.5 is a BAU pathway).

　Global greenhouse gas (GHG) 5 emissions in 2017 were around 52GtCO 2 eq (gigatonnes of CO 2 equivalent). If no new policies are enacted relative to what was legislated as of the end of 2017, emissions would rise to 60GtCO 2 eq by 2030 and 70GtCO 2 eq by the end of the century (Figure 4, Busi- ness-as-usual (BAU) scenario). This would likely mean a global temperature increase of around 3.5°C at the end of the century relative to pre-industrial times. To achieve the Paris objective of limiting the temperature increase to below 2°C (with a 67% likelihood), global GHG emissions would have to fall to 42GtCO 2 eq by 2030 and to minus 4GtCO 2 eq by the end of the century. To achieve the Paris objective of limiting the temperature increase to 1.5°C (with a 50% likelihood), global emissions would need to decline to 39GtCO 2 eq by 2030 and minus 10GtCO 2 eq by the end of the century 6 .

　 Table 1: IPCC Representative Concentration Pathways (RCPs)

　 CO 2 concentration Temperature Sea level ppm

　°C

　m RCP 2.6 420 1.0 (0.3-1.7) 0.4 RCP 4.5 650 1.8 (1.1-2.6) 0.5 RCP 6 850 2.2 (1.4-3.1) 0.5 RCP 8.5 1370 3.7 (2.6-4.8) 0.6 Source: IPCC

　Figure 4: Global greenhouse gas emissions

　GtCO2-eq

　 70

　 50

　 30

　 10

　 -10

　1990 2000 2010 2020...

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