International climate change agreements typically specify global warming thresholds as policy targets, but the relative economic benefits of achieving these temperature targets remain poorly understood. Uncertainties include the spatial pattern of temperature change, how global and regional economic output will respond to these changes in temperature, and the willingness of societies to trade present for future consumption. Here we combine historical evidence with national-level climate and socioeconomic projections to quantify the economic damages associated with the United Nations (UN) targets of 1.5 °C and 2 °C global warming, and those associated with current UN national-level mitigation commitments (which together approach 3 °C warming). We find that by the end of this century, there is a more than 75% chance that limiting warming to 1.5 °C would reduce economic damages relative to 2 °C, and a more than 60% chance that the accumulated global benefits will exceed US$20 trillion under a 3% discount rate (2010 US dollars). We also estimate that 71% of countries—representing 90% of the global population—have a more than 75% chance of experiencing reduced economic damages at 1.5 °C, with poorer countries benefiting most. Our results could understate the benefits of limiting warming to 1.5 °C if unprecedented extreme outcomes, such as large-scale sea level rise, occur for warming of 2 °C but not for warming of 1.5 °C. Inclusion of other unquantified sources of uncertainty, such as uncertainty in secular growth rates beyond that contained in existing socioeconomic scenarios, could also result in less precise impact estimates. We find considerably greater reductions in global economic output beyond 2 °C. Relative to a world that did not warm beyond 2000–2010 levels, we project 15%–25% reductions in per capita output by 2100 for the 2.5–3 °C of global warming implied by current national commitments, and reductions of more than 30% for 4 °C warming. Our results therefore suggest that achieving the 1.5 °C target is likely to reduce aggregate damages and lessen global inequality, and that failing to meet the 2 °C target is likely to increase economic damages substantially.

This report was produced for the Abe Fellows Global Forum 2017 symposiums on climate change, held in partnership with Stanford University's Walter H. Shorenstein Asia Pacific Research Center (Abe Global | Stanford, October 20, 2017) and the Asia Society Texas Center (Abe Global | Houston, October 18, 2017), respectively. 

Energy-intensive production has been both a leading contributor to climate change as well as one of the keys to modern economic growth over the last several centuries. In the post-WWII era, the “economic miracles” of Asian growth—starting with Japan, and followed by South Korea, Taiwan, China, and now increasingly India—have lifted hundreds of millions of people out of poverty. At the same time, these “economic miracles” have created huge pollution problems which have adversely affected the health of millions of people while speeding up the effects of climate change.

Some early developers from this group—including Japan—have made efforts to clean up their air and water, created more energy efficient economies, lowered their carbon footprints and contributed to initiatives to slow global warming. The Fukushima nuclear power plant disaster forced Japan to take even more aggressive action to reduce energy consumption and lessen its impact on the global environment. In contrast, the United States, as a sizeable nation-state both in its geographic area and economy, is one of the world’s largest polluters and recently made recent headlines when it withdrew from the Paris Agreement negotiated at the 2015 United Nations Climate Change Conference (COP21).

Putting into place effective measures to curtail climate change while creating sustainable societies requires international cooperation. The series of extreme weather events in the US in 2017 are only some the most recent disasters to remind us of climate change’s threat to our economy, our society, and our individual daily lives.

Integrated assessment models generate climate change mitigation scenarios consistent with global temperature targets. To limit warming to 2 °C, cost-effective mitigation pathways rely on extensive deployments of CO2 removal (CDR) technologies, including multi-gigatonne yearly CDR from the atmosphere through bioenergy with carbon capture and storage (BECCS) and afforestation/reforestation. While these assumed CDR deployments keep ambitious temperature targets in reach, the associated rates of land-use transformation have not been evaluated. Here, we view implied integrated-assessment-model land-use conversion rates within a historical context. In scenarios with a likely chance of limiting warming to 2 °C in 2100, the rate of energy cropland expansion supporting BECCS proceeds at a median rate of 8.8 Mha yr−1 and 8.4% yr−1. This rate exceeds—by more than threefold—the observed expansion of soybean, the most rapidly expanding commodity crop. In some cases, mitigation scenarios include abrupt reversal of deforestation, paired with massive afforestation/reforestation. Historical land-use transformation rates do not represent an upper bound for future transformation rates. However, their stark contrast with modelled BECCS deployment rates implies challenges to explore in harnessing—or presuming the ready availability of—large-scale biomass-based CDR in the decades ahead. Reducing BECCS deployment to remain within these historical expansion rates would mean either the 2 °C target is missed or additional mitigation would need to occur elsewhere.

The availability of climate model experiments under three alternative scenarios stabilizing at warming targets inspired by the COP21 agreements (a 1.5 ºC not exceed, a 1.5 ºC with overshoot and a 2.0ºC) makes it possible to assess future expected changes in global yields for two staple crops, wheat and maize. In this study an empirical model of the relation between crop yield anomalies and temperature and precipitation changes, with or without the inclusion of CO2 fertilization effects, is used to produce ensembles of time series of yield outcomes on a yearly basis over the course of the 21st century, for each scenario. The 21st century is divided into 10 year windows starting from 2020, within which the statistical significance and the magnitude of the differences in yield changes between pairs of scenarios are assessed, thus evaluating if, and when, benefits of mitigations appear, and how substantial they are. Additionally, a metric of extreme heat tailored to the individual crops (number of days during the growing season above a crop-specific threshold) is used to measure exposure to harmful temperatures under the different scenarios. If CO2 effects are not included, statistically significant differences in yields of both crops appear as early as the 2030s but the magnitude of the differences remains below 3% of the historical baseline in all cases until the second part of the century. In the later decades of the 21st century, differences remain small and eventually stop being statistically significant between the two scenarios stabilizing at 1.5 ºC, while differences between these two lower scenarios and the 2.0ºC scenario grow to about 4%. The inclusion of CO2 effects erases all significant benefits of mitigation for wheat, while the significance of differences is maintained for maize yields between the higher and the two lower scenarios, albeit with smaller benefits in magnitude. Changes in extremes are significant within each of the scenarios but the differences between any pair of them, even by the end of the century are only on the order of a few days per growing season, and these small changes appear limited to a few localized areas of the growing regions. These results seem to suggest that for globally averaged yields of these two grains the lower targets put forward by the Paris agreement does not change substantially the expected impacts on yields that are caused by warming temperatures under the pre-existing 2.0ºC target.

Worldwide, humans are facing high risks from natural hazards, especially in coastal regions with high population densities. Rising sea levels due to global warming are making coastal communities’ infrastructure vulnerable to natural disasters. The present study aims to provide a coupling approach of vulnerability and resilience through restoration and conservation of lost or degraded coastal natural habitats to reclamation under different climate change scenarios. The Integrated Valuation of Ecosystems and Tradeoffs (InVEST) model is used to assess the current and future vulnerability of coastal communities. The model employed is based on seven different bio-geophysical variables to calculate a Natural Hazard Index (NHI) and to highlight the criticality of the restoration of natural habitats. The results show that roughly 25 percent of the coastline and more than 5 million residents are in highly vulnerable coastal areas in China, and these numbers are expected to double by 2100. Our study suggests that restoration and conservation in recently reclaimed areas have the potential to reduce this vulnerability by 45 percent. Hence, natural habitats have proved to be a great defense against coastal hazards and should be prioritized in coastal planning and development. The findings confirm that natural habitats are critical for coastal resilience and can act as a recovery force of coastal functionality loss. Therefore, we recommend that the Chinese government prioritize restoration where possible and conservation of the remaining habitats for the sake of coastal resilience to prevent natural hazards from escalating into disasters.

Elevated atmospheric CO2 concentrations ([CO2]) are expected to increase C3 crop yield through the CO2 fertilization effect (CFE) by stimulating photosynthesis and by reducing stomatal conductance and transpiration. The latter effect is widely believed to lead to greater benefits in dry rather than wet conditions, although some recent experimental evidence challenges this view. Here we used a process-based crop model, the Agricultural Production Systems sIMulator (APSIM), to quantify the contemporary and future CFE on soybean in one of its primary production area of the US Midwest. APSIM accurately reproduced experimental data from the Soybean Free-Air CO2 Enrichment site showing that the CFE declined with increasing drought stress. This resulted from greater radiation use efficiency (RUE) and above-ground biomass production at elevated [CO2] that outpaced gains in transpiration efficiency (TE). Using an ensemble of eight climate model projections, we found that drought frequency in the US Midwest is projected to increase from once every 5 years currently to once every other year by 2050. In addition to directly driving yield loss, greater drought also significantly limited the benefit from rising [CO2]. This study provides a link between localized experiments and regional-scale modeling to highlight that increased drought frequency and severity pose a formidable challenge to maintaining soybean yield progress that is not offset by rising [CO2] as previously anticipated. Evaluating the relative sensitivity of RUE and TE to elevated [CO2] will be an important target for future modeling and experimental studies of climate change impacts and adaptation in C3 crops.