1. The global annual mean temperature has already risen by 1 °C (relative to annual means between 1850 to 1900) (IPCC 2013, 2018). Half of this temperature increase has occurred during the last 30 years (NASA 2018, IPCC 2014).
2. The years 2015, 2016, 2017, 2018 and 2019 were, globally, the warmest years on record (NASA 2019).
3. The rapid temperature rise has been caused by human-generated greenhouse gas emissions (U.S. Global Change Research Program 2017, IPCC 2013, 2014)
The repercussions if global temperatures continue to rise
4. The current global temperature rise has already increased the probability and magnitude of extreme weather around the globe, such as stronger storms, extreme precipitation events, droughts and heatwaves, which have led 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 major risk for human health on local, regional and global scales (Watts et al. 2015, 2018). In addition to the direct repercussions mentioned above, its indirect consequences include food insecurity and the spread of pathogens and disease carriers.
What are the challenges we are facing
7. To be able to restrict global warming to 1.5 °C, a limit that has been agreed upon by scientists, human-generated emissions of greenhouse gases must be swiftly and drastically reduced, and particularly net CO2 emissions must, at the global level, reach zero within the next 20 to 30 years (IPCC 2013, 2018).
8. Instead, CO2-emissions continue to rise. Given the current global policy proposals, global warming is likely to exceed 3 °C by the end of the century. Temperatures will continue to increase afterwards due to continued human-generated emissions and positive feedback loops that will further amplify greenhouse gas emissions (Climate Action Tracker 2018).
9. Based on current emissions, the remaining CO2-emissions budget that we have left for reaching the 1.5 °C limit will last for about ten years (i.e., until 2030). For reaching the 2 °C limit , the budget is likely to last for about 25 to 30 years (MCC 2018, IPCC 2018).
10. Subsequently humanity will live on a “CO2-overdraft-loan”, because humans have already expended most of the CO2-emissions budget. This means that 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 supposed to pay off this “loan”. If we fail, the following generations will suffer from the severe consequences of global warming.
11. Rising temperatures increase the probability of reaching tipping points in the earth’s climate system, including positive feedback loops that further amplify warming will become more likely (Schellnhuber et al. 2016, Steffen et al. 2016, 2018). Returning to a stable climate would become unrealistic for future generations.
Oceans and biodiversity
12. Oceans are currently absorbing around 90% of the additional heat associated with global warming (IPCC 2013). Furthermore, oceans have already absorbed about 30% of the surplus CO2 emitted by humans. Consequences of heat and CO2 absorption include rising sea levels, melting sea ice, ocean acidification and dissolved-oxygen depletion in the oceans. Meeting the goals set by the Paris Agreement is essential in order to protect humanity and nature, and to mitigate the loss of marine biodiversity and ecosystems, specifically the currently endangered coral communities (IPCC 2018).
13. Human livelihoods are threatened in several distinct areas by hitting the planetary load limits, known as “planetary boundaries”. In 2015, two of nine boundaries were alarmingly passed (climate change and change of land use), while two further boundaries were critically exceeded (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 of Earth’s biodiversity since the era of the dinosaurs (Barnosky et al. 2011). Global extinction rates are 100 to 1000 times faster compared to extinction events that occurred before humanity exerted its influence (Ceballos et al. 2015, Pimm et al. 2014). Over the past 500 years, humans have witnessed the extinction of more than 300 land-dwelling vertebrate species (Dirzo et al. 2014). The abundance of known vertebrate species has dropped on average by around 60% from 1970 to 2014 (WWF 2018).
15. Causes for biodiversity loss are multifold. They can result from habitat destruction due to agriculture, deforestation, intensive land use, and development such as settlements and roads. On the other hand, invasive species play a role in biodiversity loss by outcompeting native species and deteriorating habitat while wildlife populations can become depleted due to over-collection, overfishing and overhunting (Hoffmann et al. 2010).
16. Global warming exacerbates the biodiversity crisis. If CO2 emissions continue to increase, half of the plant and animal species of the Amazon basin or the Galapagos Islandsare will be expected to vanish 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) with massive bleachings being observed globally.
Implications for sustaining human populations during the climate crisis
17. The deterioration and loss of agricultural areas and soil fertility, as well as the irreversible destruction of biodiversity and ecosystems, threaten livelihoods and deplete resources and options required for survival and wealth 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, freshwater resources and biodiversity acts as a risk multiplier in the face of global warming (Johnstone and Mazo 2011). Consequently, the risk of water shortages and famines will increase in many countries which can 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. Meat production creates 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 share of greenhouse gases (FAO 2013). Permanent pastures occupy a large amount of agricultural area to sustain meat production. Also: more than one third of the global grain harvest is used currently as animal feed (FAO 2017).
The green transition
20. A sustainable diet with reduced meat, fish and milk consumption, as well as a reorientation of agricultural methods to regenerative and resource-saving food production, are necessary for the protection of land and marine ecosystems and the stabilisation of climate change (Springmann et al. 2018).
21. A transition to an increased (direct) consumption of plant-based foods will reduce both the need for cropland and the level of greenhouse gas emissions, while providing additional individual health benefits (Springmann et al. 2016).
22. Direct government subsidies for fossil-based industries amount to more than 100 billion U.S. dollars per year (Jakob et al. 2015). Taking social and environmental costs (in particular health costs, but also air and water pollution) into account, subsidies for fossil fuels are significantly higher. 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. One frequently-recommended solution by economists that can be adopted to collectively reduce emissions is the introduction of a CO2-price on emissions. The sufficient supply of low-cost renewable energies is achievable. The financial burden of transitioning will need to be distributed in a socially responsible way. Examples include 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).