Special Report on Global Warming of 1.5 °C

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 (CO₂) 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 1.5 °C (2.7 °F) 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, CO₂ emissions must decline by 45% (relative to 2010 levels) by 2030, reaching net zero by around 2050. Deep reductions in non-CO₂ 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, CO₂ 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-CO₂ 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.

Pathways limiting global warming to 1.5 °C with no or limited overshoot would require rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems. These systems transitions are unprecedented in terms of scale, but not necessarily in terms of speed, and imply deep emissions reductions in all sectors, a wide portfolio of mitigation options and a significant upscaling of investments in those options. The rates of system changes [...] have occurred in the past within specific sectors, technologies and spatial contexts, but there is no documented historic precedent for their scale. — IPCC, SR15 Summary for policymakers, p. 17

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 0.77 m by 2100 for 1.5 °C global warming" and about 0.1 m more for 2 °C. A difference of 0.1 m 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 0.8–1.2 °C (1.4–2.2 °F) 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 1.5 C "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. Such measures could be undertaken in the energy sector and by reducing nitrous oxide and methane from agriculture, methane from the waste sector, and some other sources of black carbon and hydrofluorocarbons.

On timescales longer than tens of years, it may still be necessary to sustain net negative CO₂ emissions and/or further reduce non-CO₂ radiative forcing (*), in order to prevent further warming (due to Earth system feedbacks), reverse ocean acidification, and minimise sea level rise.

(*) Non-CO₂ emissions included in this Report are all anthropogenic emissions other than CO₂ that result in radiative forcing. These include short-lived climate forcers, such as methane, some fluorinated gases, ozone precursors, aerosols or aerosol precursors, such as black carbon and sulphur dioxide, respectively, as well as long-lived greenhouse gases, such as nitrous oxide or some fluorinated gases. The radiative forcing associated with non-CO₂ emissions and changes in surface albedo is referred to as non-CO₂ radiative forcing.

Pathways to 1.5 °C

Various pathways are considered, describing scenarios for mitigation of global warming, including portfolios for energy supply and negative emission technologies (like afforestation or carbon dioxide removal).

Examples of actions consistent with the 1.5 °C pathway include "shifting to low- or zero-emission power generation, such as renewables; changing food systems, such as diet changes away from land-intensive animal products; electrifying transport and developing 'green infrastructure', such as building green roofs, or improving energy efficiency by smart urban planning, which will change the layout of many cities." As another example, an increase of forestation by 10,000,000 square kilometres (3,900,000 mi²) by 2050 relative to 2010 would be required.

The pathways also assume an increase in annual investments in low-carbon energy technologies and energy efficiency by roughly a factor of four to ten by 2050 compared to 2015.

Carbon dioxide removal

The emission pathways that reach 1.5 °C contained in the report assume the use of carbon dioxide removal (CDR) to offset for remaining emissions. Pathways that overshoot the goal rely on CDR to remove carbon dioxide at a rate that exceeds remaining emissions in order to return to 1.5 °C. However, understanding is still limited about the effectiveness of net negative emissions to reduce temperatures after an overshoot. Reversing an overshoot of 0.2 °C might not be achievable given considerable implementation challenges. The report highlights a CDR technology called bioenergy with carbon capture and storage (BECCS). The report notes that apart from afforestation/reforestation and ecosystem restoration, "the feasibility of massive-scale deployment of many CDR technologies remains an open question", with areas of uncertainty regarding technology upscaling, governance, ethical issues, policy and carbon cycle. The report notes that CDR technology is in its infancy and the feasibility is an open question. Estimates from recent literature are cited, giving a potential of up to 5 GtCO2 per year for BECCS and up to 3.6 GtCO2 per year for afforestation.

Solar radiation management

The report describes several proposals for solar radiation management (SRM). It concludes that SRMs have potential to limit warming, but "face large uncertainties and knowledge gaps as well as substantial risks, [...] and constraints"; "the impacts of SRM (both biophysical and societal), costs, technical feasibility, governance and ethical issues associated need to be carefully considered." An analysis of the geoengineering proposals published in Nature Communication confirmed findings of the SR15, stating that "all are in early stages of development, involve substantial uncertainties and risks, and raise ethical and governance dilemmas. Based on present knowledge, climate geoengineering techniques cannot be relied on to significantly contribute to meeting the Paris Agreement temperature goals".