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Well below 2 °C: mitigation strategies for avoiding dangerous to catastrophic climate changes

Yangyang Xu, Veerabhadran Ramanathan

The historic Paris Agreement calls for limiting global temperature rise to “well below 2 °C.” Because of uncertainties in emission scenarios, climate, and carbon cycle feedback, we interpret the Paris Agreement in terms of three climate risk categories and bring in considerations of low-probability (5%) high-impact (LPHI) warming in addition to the central (∼50% probability) value. The current risk category of dangerous warming is extended to more categories, which are defined by us here as follows: >1.5 °C as dangerous; >3 °C as catastrophic; and >5 °C as unknown, implying beyond catastrophic, including existential threats. With unchecked emissions, the central warming can reach the dangerous level within three decades, with the LPHI warming becoming catastrophic by 2050. We outline a three-lever strategy to limit the central warming below the dangerous level and the LPHI below the catastrophic level, both in the near term (<2050) and in the long term (2100): the carbon neutral (CN) lever to achieve zero net emissions of CO2, the super pollutant (SP) lever to mitigate short-lived climate pollutants, and the carbon extraction and sequestration (CES) lever to thin the atmospheric CO2 blanket. Pulling on both CN and SP levers and bending the emissions curve by 2020 can keep the central warming below dangerous levels. To limit the LPHI warming below dangerous levels, the CES lever must be pulled as well to extract as much as 1 trillion tons of CO2 before 2100 to both limit the preindustrial to 2100 cumulative net CO2 emissions to 2.2 trillion tons and bend the warming curve to a cooling trend.

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Summary. Basically, for a safe climate, all three levers (CN, SP, and CES) must be deployed as soon as possible. The CN and SP levers must be deployed by 2030 and 2020, respectively; the cumulative CO2 emissions from preindustrial must be limited to 2.2 trillion tons of CO2 (or 0.6 trillion tons of carbon); and the CES lever should extract and sequester as much as 1 trillion tons of CO2 (CES1t), depending on when the CN lever is deployed. If the CN lever is deployed as early as 2020, the required CES is much less than 1 trillion tons.
We propose that mitigation goals be set in terms of climate risk category instead of a temperature threshold. In this paper, we offer three broad risk categories, but it is likely that a more granular set of categories is required. The temperature threshold has served policy very well; however, given the imminence of dangerous warming within decades, the focus must broaden to include extreme climate changes. Precipitation, flooding, fire, and drought will all become serious sources of concern. The temperature will still occupy our attention because of the heat stress phenomenon and the likelihood of approximately half of the population exposed to deadly heat by 2050 (Box 2).
We conclude with a commentary on the feasibility of the mitigation options considered thus far. Over 24 technological measures to reduce SLCPs have been detailed previously (39) (details are provided in SI Appendix). These measures include providing clean cook stoves to the poorest three billion of the world’s total population and installing particulate filters in all diesel vehicles to reduce global BC emissions by nearly 80% and also reduce air pollution-related mortalities by ∼2 million; routine maintenance of gas pipes and banning gas flaring to reduce methane leaks; recovering methane from landfills, water sewage treatment plants, and farm manure; replacing HFCs with other available refrigerants that have negligible greenhouse effects; and installing catalytic converters in vehicles to reduce emissions of ozone precursors.
CN levers require switching from fossil fuels to renewables such as wind, solar, geothermal and nuclear sources, among others. Also, CO2 emissions from industrial processes should be eliminated. This requires electrification of all end uses and production of electricity from renewables (40). Since many renewables (solar and wind) are intermittent, storage is a crucial issue. Batteries, hydrogen production by renewables, and pumped hydropower are all possible options for storage. While about 50% of reductions are possible with scaling up of existing technologies, innovations are required for achieving carbon neutrality in a cost-effective manner (40). Achievement of carbon neutrality also requires societal transformation, governance, and market mechanisms such as cap and trade and carbon pricing (40). The encouraging sign is that 52 cities, 65 businesses, and numerous universities have already embarked on the CN pathway (41). Some of these living laboratories, like California and Stockholm, have shown that the gross domestic product (GDP) can be decoupled from carbon emissions. Their carbon emission per GDP has decreased by 20% while bending the carbon emissions curve. The technology development and innovations from these living laboratories should be scaled to the world to greatly accelerate efforts to achieve CN within decades.
Of the three levers recommended here, the third lever dealing with CES is the most challenging and formidable due to lack of scalable technologies. However, many technologies are being explored, including capturing CO2 in bioenergy power plants (42), biochar production by pyrolysis and storage in soils (43), restoration of soil organic pools (44), chemical weathering of rocks, mineral sequestration, reforestation, and urban forestry, among others. The availability of land and conflict with food production is another important constraint in some of the CES solutions. Major breakthroughs are needed urgently, and in the meantime, the best option is to start on the CN goal by 2020 and mitigate the SPs as soon as possible, since cost-effective technologies are already present to immediately start bending the emission curves.

Proceedings of the National Academy of Sciences, Vol. 114, No. 39. (26 September 2017), pp. 10315-10323, 
Key: INRMM:14438775



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