Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 °C and 2 °C

Abstract

Abstract. Robust appraisals of climate impacts at different levels of global-mean temperature increase are vital to guide assessments of dangerous anthropogenic interference with the climate system. The 2015 Paris Agreement includes a two-headed temperature goal: "holding the increase in the global average temperature to well below 2 • C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 • C". Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing. Here we provide an assessment of key impacts of climate change at warming levels of 1.5 • C and 2 • C, including extreme weather events, water availability, agricultural yields, sea-level rise and risk of coral reef loss. Our results reveal substantial differences in impacts between a 1.5 • C and 2 • C warming that are highly relevant for the assessment of dangerous anthropogenic interference with the climate system. For heat-related extremes, the additional 0.5 • C increase in global-mean temperature marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions. Similarly, this warming difference is likely to be decisive for the future of tropical coral reefs. In a scenario with an end-of-century warming of 2 • C, virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards. This fraction is reduced to about 90 % in 2050 and projected to decline to 70 % by 2100 for a 1.5 • C scenario. Analyses of precipitation-related impacts reveal distinct regional differences and hot-spots of change emerge. Regional reduction in median water availability for the Mediterranean is found to nearly double from 9 % to 17 % between 1.5 • C and 2 • C, and the projected lengthening of regional dry spells increases from 7 to 11 %. Projections for agricultural yields differ between crop types as well as world regions. While some (in particular high-latitude) regions may benefit, tropical regions like West Africa, South-East Asia, as well as Central and northern South America are projected to face substantial local yield reductions, particularly for wheat and maize. Best estimate sea-level rise projections based on two illustrative scenarios indicate a 50 cm rise by 2100 relative to year 2000-levels for a 2 • C scenario, and about 10 cm lower levels for a 1.5 • C scenario. In a 1.5 • C scenario, the rate of sea-level rise in 2100 would be reduced by about 30 % compared to a 2 • C scenario. Our findings highlight the importance of regional differentiation to assess both future climate risks and different vulnerabilities to incremental increases in globalmean temperature. The article provides a consistent and comprehensive assessment of existing projections andPublished by Copernicus Publications on behalf of the European Geosciences Union. 328C.-F. Schleussner et al.: Climate impacts at 1.5 • C a...

Global drought and severe drought-affected populations in 1.5 and 2 °C warmer worlds

“…Regardless of the socioeconomic and political achievability of these goals (Sanderson et al, 2017), there is a lack of scientific knowledge about the relative risks (e.g., drought risks and their potential impacts) associated with the implications of 1.5 and 2 • C warming. This has naturally attracted contributions from the scientific community (Hulme, 2016;Schleussner et al, 2016;Peters, 2016;King et al, 2017).…”

Beyond 1.5 °C: vulnerabilities and adaptation strategies for Caribbean Small Island Developing States

“…It is projected that by 2030, climate-related threats will be high in the eastern Caribbean and will push the proportion of reefs at risk to 90% in 2030, and up to 100% by 2050, with about 85% at high, very high or critical levels. Virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching Schleussner et al (2016). This forecast has serious implications for the Caribbean's economic survival.…”

“…A study by Schleussner et al (2016) revealed that as a result of a 1.5°C temperature increase, SIDS hotspots will suffer a substantial and rapid increase in climate change risks which include risks for unique and threatened systems such as coral reefs and risks by extreme events. This change will be significant because of limited adaptive capacity and high exposure.…”

Assessments of the north hemisphere snow cover response to 1.5 °C and 2.0 °C warming

“…interest on this initiative ( Boucher et al, 2016;Hulme, 2016;Peters, 2016;Rogelj and Knutti, 2016;Schleussner et al, 2016;Mitchell et al, 2017). The Intergovernmental Panel on Climate Change (IPCC) has also scheduled to propose a special report on the impacts of 1.5 C in 2018 (http://www.ipcc.ch/report/sr15/pdf/information_note_expert_review.pdf).…”

“…The Intergovernmental Panel on Climate Change (IPCC) has also scheduled to propose a special report on the impacts of 1.5 C in 2018 (http://www.ipcc.ch/report/sr15/pdf/information_note_expert_review.pdf). However, present comparison studies regarding to the differences between 2 °C and 1.5 °C are all through analyzing CMIP5 outputs under the Representative Concentration Pathway (RCP) scenarios ( Vuuren et al, 2011;Schaeffer and Hare, 2012; 5 Schleussner et al, 2016). For example, based on the CMIP5 model outputs, Schleussner et al (2016) assessed the impacts of 1.5 °C and 2 °C warming levels on the extreme weather events, water availability, agricultural yields, sea-level rise and risk of coral reef loss, and concluded that substantial risk reductions with 1.5 °C compared to 2 °C warming, and further showing the regional differentiation in both climate risks and vulnerabilities.…”