Special Report on Global Warming of 1.5 °C
The Special Report on Global Warming of 1.5 °C (SR15)[note 1] was published by the
Its key finding is that meeting a 1.5 °C (2.7 °F) 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 that "limiting global warming to 1.5 °C compared with 2 °C would reduce challenging impacts on
Global warming will likely rise to 1.5 °C above pre-industrial levels between 2030 and 2052 if warming continues to increase at the current 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 necessary steps to limit global warming.
Even assuming full implementation of conditional and unconditional
Pathways (i.e. scenarios and portfolios of mitigation options) that would allow such reduction by 2050 permit only about 8% of global electricity to be generated by gas and 0-2% by coal (to be offset by
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
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
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
"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."
Human activities (
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 exact value of the this budget is not assessed in the report, but estimates of 400–800 GtCO2 (gigatonnes of CO2) remaining budget are given (580 GtCO2 and 420 GtCO2 for a 66% and 50% 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
Current emissions deplete 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).
Current nationally stated mitigation ambitions, as submitted under the
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 yer, 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% until 2030 and by 100% until 2075.
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.
Various pathways are considered, describing scenarios for mitigation of global warming, including portfolios for energy supply and
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
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.
The emission pathways that reach 1.5 °C contained in the report assume the use of
There are two main groups of geoengineering types in the report,
As for SRM, the report focuses on