Special Report on Global Warming of 1.5 °C

The Special Report on Global Warming of 1.5°C (SR15) was published by the Intergovernmental Panel on Climate Change (IPCC) on 8 October 2018. The report, approved in Incheon, South Korea, includes over 6,000 scientific references, and was prepared by 91 authors from 40 countries. In December 2015, the 2015 United Nations Climate Change Conference called for the report. The report was delivered at the United Nations' 48th session of the IPCC to "deliver the authoritative, scientific guide for governments" to deal with climate change. Its key finding is that meeting a target is possible but would require "deep emissions reductions" and "rapid, far-reaching and unprecedented changes in all aspects of society". Furthermore, the report finds that "limiting global warming to 1.5°C compared with 2°C would reduce challenging impacts on ecosystems, human health and well-being" and that a 2 °C temperature increase would exacerbate extreme weather, rising sea levels and diminishing Arctic sea ice, coral bleaching, and loss of ecosystems, among other impacts.

SR15 also has modelling that shows that, for global warming to be limited to 1.5 °C, "Global net human-caused emissions of carbon dioxide () would need to fall by about 45 percent from 2010 levels by 2030, reaching 'net zero' around 2050." The reduction of emissions by 2030 and its associated changes and challenges, including rapid decarbonisation, was a key focus on much of the reporting which was repeated through the world.

When the Paris Agreement was adopted, the UNFCCC invited the Intergovernmental Panel on Climate Change to write a special report on "How can humanity prevent the global temperature rise more than 1.5 degrees above pre-industrial level". Its full title is "Global Warming of 1.5°C, an IPCC special report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty".

The finished report summarizes the findings of scientists, showing that maintaining a temperature rise to below 1.5 °C remains possible, but only through "rapid and far-reaching transitions in energy, land, urban and infrastructure..., and industrial systems". Meeting the Paris target of is possible but would require "deep emissions reductions", "rapid", "far-reaching and unprecedented changes in all aspects of society". In order to achieve the 1.5 °C target, emissions must decline by 45% (relative to 2010 levels) by 2030, reaching net zero by around 2050. Deep reductions in non- emissions (such as nitrous oxide and methane) will also be required to limit warming to 1.5 °C. Under the pledges of the countries entering the Paris Accord, a sharp rise of 3.1 to 3.7 °C is still expected to occur by 2100. Holding this rise to 1.5 °C avoids the worst effects of a rise by even 2 °C. However, a warming of even 1.5 degrees will still result in large-scale drought, famine, heat stress, species die-off, loss of entire ecosystems, and loss of habitable land, throwing more than 100 million into poverty. Effects will be most drastic in arid regions including the Middle East and the Sahel in Africa, where fresh water will remain in some areas following a 1.5 °C rise in temperatures but are expected to dry up completely if the rise reaches 2 °C.

Main statements

The report states that global warming would likely rise to 1.5°C above pre-industrial levels (defined as being the average during 1850–1900) between 2030 and 2052 if warming continues at the 2018 rate. SR15 provides a summary of, on one hand, existing research on the impact that a warming of 1.5°C (equivalent to 2.7°F) would have on the planet, and on the other hand, the steps needed to limit global warming to 1.5°C.

Even assuming full implementation of conditional and unconditional Nationally Determined Contributions submitted by nations in the Paris Agreement, net emissions would increase compared to 2010, leading to a warming of about 3°C by 2100, and more afterwards. In contrast, limiting warming below or close to 1.5°C would require to decrease net emissions by around 45% by 2030 and reach net zero emissions by 2050 (i.e. keeping total cumulative emissions within a carbon budget). Even just for limiting global warming to below 2°C, emissions should decline by 25% by 2030 and by 100% by 2075.

Pathways (i.e. scenarios and portfolios of mitigation options) that would allow such reduction by 2050 describe a rapid transition towards producing electricity through lower-emission methods, and increasing use of electricity instead of other fuels in sectors such as transportation. On average, the pathways describing the proportion of primary energy produced by renewables as increasing to 60%, while the proportion produced by coal drops to 5% and oil to 13%. Most pathways describe a larger role for nuclear energy and carbon capture and storage, and less usage of natural gas. They also assume that other measures are simultaneously undertaken: e.g. non- emissions (such as methane, black carbon, nitrous oxide) are to be similarly reduced, energy demand is unchanged, reduced by even 30% or offsetted by an unprecedented scale of carbon dioxide removal methods yet to be developed, while new policies and research allows to improve efficiency in agriculture and industry.

Impact of 1.5°C or 2°C warming

According to the report, with global warming of 1.5°C there would be increased risks to "health, livelihoods, food security, water supply, human security, and economic growth". Impact vectors include reduction in crop yields and nutritional quality. Livestock are also affected with rising temperatures through "changes in feed quality, spread of diseases, and water resource availability". "Risks from some vector-borne diseases, such as malaria and dengue fever, are projected to increase."

"Limiting global warming to 1.5°C, compared with 2°C, could reduce the number of people both exposed to climate-related risks and susceptible to poverty by up to several hundred million by 2050." Climate-related risks associated with increasing global warming depend on geographic location, "levels of development and vulnerability", and the speed and reach of climate mitigation and climate adaptation practices. For example, "urban heat islands amplify the impacts of heatwaves in cities." In general, "countries in the tropics and Southern Hemisphere subtropics are projected to experience the largest impacts on economic growth."

Weather, sea level and ice Many regions and seasons experience warming greater than the global annual average, e.g. "2–3 times higher in the Arctic. Warming is generally higher over land than over the ocean," and it correlates with temperature extremes (which are projected to warm up to twice more on land than the global mean surface temperature) as well as precipitation extremes (both heavy rain and droughts). The assessed levels of risk generally increased compared to the previous IPCC report.

The "global mean sea level is projected rise (relative to 1986–2005) by 0.26 to 0.77m by 2100 for 1.5°C global warming" and about 0.1m more for 2°C. A difference of 0.1m may correspond to 10 million more or fewer people exposed to related risks. "Sea level rise will continue beyond 2100 even if global warming is limited to 1.5°C. Around 1.5°C to 2°C of global warming," irreversible instabilities could be triggered in Antarctica and "Greenland ice sheet, resulting in multi-metre rise in sea level." "An ice-free Arctic summer is projected once per century" (per decade) for 1.5°C (respectively 2°C). "Limiting global warming to 1.5°C rather than 2°C is projected to prevent the thawing over centuries of a permafrost area in the range of 1.5 to 2.5 million km2."

Ecosystems "A decrease in global annual catch for marine fisheries of about 1.5 or 3 million tonnes for 1.5°C or 2°C of global warming" is projected by one global fishery model cited in the report. Coral reefs are projected to decline by a further 70–90% at 1.5°C, and even more than 99% at 2°C. "Of 105,000 species studied, 18% of insects, 16% of plants and 8% of vertebrates fare projected to lose over half of their climatically determined geographic range for global warming of 2°C."

Approximately "4% or 13% of the global terrestrial land area is projected to undergo a transformation of ecosystems from one type to another" at 1°C or 2°C, respectively. "High-latitude tundra and boreal forests are particularly at risk of climate change-induced degradation and loss, with woody shrubs already encroaching into the tundra and will proceed with further warming."

Limiting the temperature increase Human activities (anthropogenic greenhouse gas emissions) have already contributed of warming. Nevertheless, the gases which have been emitted so far are unlikely to cause global temperature to rise to 1.5°C alone, meaning a global temperature rise to 1.5°C above pre-industrial levels is avoidable, assuming net zero emissions are reached soon.

Carbon budget

Limiting global warming to 1.5°C requires staying within a total carbon budget, i.e. limiting total cumulative emissions of CO2. In other words, if net anthropogenic CO2 emissions are kept above zero, a global warming of 1.5°C and more will eventually be reached.

The value of the total net anthropogenic CO2 budget since the pre-industrial era is not assessed in the report. Estimates of 400–800 GtCO2 (gigatonnes of CO2) for the remaining budget are given (580 GtCO2 and 420 GtCO2 for a 50% and 66% probability of limiting warming to 1.5°C, using global mean surface air temperature (GSAT); or 770 and 570 GtCO2, for 50% and 66% probabilities, using global mean surface temperature (GMST)). This is about 300 GtCO2 more compared to a previous IPCC report, due to updated understanding and further advances in methods.

Emissions around the time of the report were depleting this budget at 42±3 GtCO2 per year. Anthropogenic emissions from the pre-industrial period to the end of 2017 are estimated to have reduced the budget for 1.5°C by approximately 2200±320 GtCO2.

The estimates for the budget come with significant uncertainties, associated with: climate response to CO2 and non-CO2 emissions (these contribute about ±400 GtCO2 in uncertainty), the level of historic warming (±250 GtCO2), potential additional carbon release from future permafrost thawing and methane release from wetlands (reducing the budget by up to 100 GtCO2 over the century), and the level of future non-CO2 mitigation (±400 GtCO2).

Necessary emission reductions Current nationally stated mitigation ambitions, as submitted under the Paris Agreement, would lead to global greenhouse gas emissions of 52–58 GtCO2eq per year, by 2030. "Pathways reflecting these ambitions would not limit global warming to 1.5°C, even if supplemented by very challenging increases in the scale and ambition of emissions reductions after 2030." Instead, they are "broadly consistent" with a warming of about 3°C by 2100, and more afterwards.

Limit global warming to 1.5°C with no or limited overshoot would require reducing emissions to below 35 GtCO2eq per year in 2030, regardless of the modelling pathway chosen. Most fall within 25–30 GtCO2eq per year, a 40–50% reduction from 2010 levels.

The report says that for limiting warming to below 1.5C "global net human-caused emissions of CO2 would need to fall by about 45% from 2010 levels by 2030, reaching net zero around 2050." Even just for limiting global warming to below 2°C, CO2 emissions should decline by 25% by 2030 and by 100% by 2070.

Non-CO2 emissions should decline in more or less similar ways. This involves deep reductions in emissions of methane and black carbon: at least 35% of both by 2050, relative to 2010, to limit warming near 1.5°C.