1. The global mean temperature has already risen by 1 °C (relative to 1850 to 1900) (IPCC 2013, 2018). Half of the rise has occurred during the last 30 years (NASA 2018, IPCC 2014).
2. The years 2015, 2016, 2017, and 2018 were, globally, the warmest years in the modern record (NASA 2019).
3. The temperature rise is almost entirely due to human-made greenhouse gas emissions (U.S. Global Change Research Program 2017, IPCC 2013, 2014).
4. Already the current temperature rise increases the probability of extreme weather conditions in several regions of the globe, such as strong precipitation and heatwaves, leading to elevated rates of regional droughts, floods and forest fires (e.g., IPCC 2012, 2013, 2018, The National Academies of Sciences, Engineering, and Medicine 2016).
5. Global warming is a risk factor for human health (Watts et al. 2015, 2018). Besides the above-mentioned direct consequences, its indirect consequences include the lack of food security and the spread of pathogens and disease carriers.
6. If humanity fails to limit global warming to 1.5 °C, as envisaged by the Paris Agreement, additional severe consequences must be expected for humanity and nature at large in many parts of the world (IPCC 2018).
7. In order to restrict warming to the 1.5 °C limit with high probability, emissions of greenhouse gases must be swiftly reduced and particularly net CO2 emissions must, at the global level, reach zero within the next 20 to 30 years (IPCC 2013, 2018).
Note: The wording above is slightly corrected versus the originally published version. The earlier version was: “In order to restrict warming to the 1.5 °C limit with high probability, net emissions of greenhouse gases (in particular CO2) must be swiftly reduced and must, at the global level, reach zero within the next 20 to 30 years.”
8. Instead, CO2-emissions continue to rise. Given the policy proposals currently on the table, global warming is likely to cross 3 °C by the end of the century and will increase afterwards due to continued emissions and positive feedback dynamics (Climate Action Tracker 2018).
9. Based on current emissions, the remaining CO2-budget left for reaching the 1.5 °C goal will last for about ten years. For the 2 °C goal, the budget is likely to last for about 25 to 30 years (MCC 2018, IPCC 2018).
10. Afterwards, humanity lives on a “CO2-overdraft-loan”: any emitted greenhouse gases have to be removed later from the atmosphere with tremendous efforts (e.g., Rogelj et al. 2018, Gasser et al. 2015). Today’s young people are already supposed to pay off this loan. If this fails, the following generations will suffer from the severe consequences of global warming.
11. Rising temperatures increase the probability of crossing climatic tipping points in the earth system dynamics, i.e., positive feedback loops will become more likely (Schellnhuber et al. 2016, Steffen et al. 2016, 2018). This would result in a situation, where returning to the current temperature regime would become unrealistic for future generations.
12. Oceans are currently absorbing around 90% of the additional heat (IPCC 2013). They have furthermore absorbed about 30% of the CO2 emitted so far. Consequences are rising sea levels, melting of sea ice, acidification and dissolved-oxygen depletion in the oceans. Meeting the goals set by the Paris Agreement is essential to protect humanity and nature, and to mitigate the loss of marine biodiversity and ecosystems, specifically the currently endangered coral communities (IPCC 2018).
13. The human basis of life is threatened in several areas by the crossing of “planetary boundaries”. As of 2015, two boundaries are exceeded with a degree of uncertainty (climate and land use change) and two further boundaries are critically exceeded: the destruction of genetic variability (biodiversity) and the phosphorus and nitrogen biogeochemical cycles (Steffen et al. 2015).
14. We presently face the largest mass-extinction event since the era of the dinosaurs (Barnosky et al. 2011). Global extinction rates are 100 to 1000 times faster as compared to before humanity exerted its influence (Ceballos et al. 2015, Pimm et al. 2014). The past 500 years saw the extinction of more than 300 land-dwelling vertebrate species (Dirzo et al. 2014); the abundance of investigated vertebrate species has dropped on average by around 60% from 1970 to 2014 (WWF 2018).
15. Causes for biodiversity loss are on the one hand habitat destruction by agriculture, deforestation, as well as land consumption by settlements and roads. On the other hand, invasive species play a role, as well as depletion due to over-collection, overfishing and overhunting (Hoffmann et al. 2010).
16. Global warming adds to this: with undiminished CO2 emissions, half of the plant and animal species of the Amazon basin or the Galapagos Islands, for example, can be expected to have vanished by 2100 (Warren et al. 2018). Similarly, global warming is the major threat to the survival of coral reefs (Hughes et al. 2017, 2018, IPCC 2018).
17. The loss of agricultural areas and soil fertility, as well as the irreversible destruction of biodiversity and ecosystems threaten the basis of life and limit the options of current and future generations (IPBES 2018a, 2018b, Secretariat of the CBD 2014, Willett et al. 2019, IAAST 2009a, 2009b).
18. Insufficient protection of soil, ocean, fresh-water resources and biodiversity acts as a risk multiplier in the face of global warming (Johnstone and Mazo 2011). It increases the risk that water shortage and famine in many countries will trigger or aggravate social and military conflicts, and contribute to the migration of larger human populations (Levy et al. 2017, World Bank Group 2018, Solow 2013).
19. A sustainable diet with reduced meat, fish and milk consumption, as well as a reorientation of agricultural methods to resource-saving food production are necessary for the protection of land and marine ecosystems and the stabilisation of climate change (Springmann et al. 2018).
20. Meat production produces less than one fifth of the calories used worldwide on more than four fifths of the agricultural area (Poore and Nemecek 2018), and emits a significant proportion of greenhouse gases (FAO 2013). Since the agricultural area includes permanent pastures and meadows as well as croplands, and most of the former cannot be converted to cropland, another comparison is also illustrative: more than one third of the global cereal harvest is used currently as animal feed (FAO 2017).
21. A transition to increased direct consumption of plant-based foods will reduce both the need for cropland and the level of greenhouse gas emissions while providing additional health benefits (Springmann et al. 2016).
22. Direct government subsidies for fossil-based industries amount to more than 100 billion U.S. dollar per year (Jakob et al. 2015). Taking social and environmental costs (in particular health costs, but also air and water pollution) into account, global post-tax subsidies for fossil fuels are significantly higher. According to experts of the International Monetary Fund (IMF) they amount to about five trillion U.S. dollar per year – that is 6.5% of global gross domestic product (2014) (Coady et al. 2017).
23. According to the polluter pays principle, the cost of climate damages should be attributed to the burning of fossil fuels. One possible approach is the introduction of CO2-prices. As long as a sufficient supply of low-cost renewable energies is not achieved, the resulting financial burden will need to be distributed in a socially responsible way. Examples are direct transfers or tax reductions for particularly affected households or lump-sum payments for citizens (Klenert et al. 2018).
24. Based on already established sustainable energy technologies, a strong reduction in costs and an increase in production capacities is possible. This would, in turn, render a change from burning fossils to an energy system fully based on renewable energy financially feasible and create new economic possibilities (Nykvist and Nilsson 2015, Creutzig et al. 2017, Jacobson et al. 2018, Teske et al. 2018, Breyer et al. 2018, Löffler et al. 2017, Pursiheimo et al. 2019).