Global warming will continue, reaching 1.5°C in the first half of the 2030s and rising beyond 1.5°C, despite mitigation efforts today. The level of the temperature exceedance over 1.5°C will be in the range of 0.1°-0.2°C for several decades before the temperature declines to 1.5°C by the end of 2100, if global emissions peak in 2025 and reach net zero by 2050. This pathway would allow the world to stay within a carbon budget for limiting warming to 1.5°C by the end of this century. This pathway calls for prioritizing adaptation toward 1.5°C warming and enabling the capacity for negative emissions while pursuing rapid reductions in global emissions. Immediate reductions are necessary to reduce the temperature overshoot.
Impacts of Climate Change
The impact of current warming – the global average temperature increase of 1.1°C over the preindustrial period – is already severe and widespread. Weather and climate extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones are affecting every region. Approximately 3.3 to 3.6 billion people live in highly vulnerable areas to climate change. Between 2010 and 2020, human mortality from floods, droughts, and storms was 15 times higher in highly vulnerable regions, compared to regions with very low vulnerability. The extreme events worsened food and water insecurity in many locations in Africa, Asia, Central and South America, Small Islands and the Arctic, especially for small-scale food producers, low-income households and Indigenous Peoples. Crop yields in mid and low-latitude regions declined and agricultural productivity over the past 50 years slowed down due to climate change.
Ocean warming and ocean acidification have adversely affected food production from shellfish aquaculture and fisheries, resulting in an overall decrease in maximum catch potential. Many changes in the ocean, ice sheets and global sea level are irreversible. Sea level rise was already accelerating.[1] The average rate of sea level rise was 1.3 mm per year between 1901 and 1971, increasing to 1.9 mm per year in the ensuing 35 years, and then almost doubling to 3.7 mm per year in the latest 12-year period ending in 2018. Ocean acidification, ocean deoxygenation and global mean sea level will continue to increase in the 21st century.
Animal and human diseases, including zoonoses, are emerging in new areas. Approximately half of the species assessed globally have shifted polewards or, on land, to higher elevations. Species’ capacities for adapting to climate change through adjustments in geographic placement and shifting seasonal timing are not sufficient to cope with accelerating climate change. The impacts on some ecosystems in the mountains and Arctic of glaciers retreating and permafrost thaw are approaching irreversibility.
Global warming of 1.5°C will multiply risks to ecosystems and human systems. 3-14% of terrestrial ecosystems assessed will likely face a very high risk of extinction at a 1.5°C warming. Coral reefs are projected to decline by a further 70-90% at 1.5°C of global warming. Arctic ecosystems, dryland regions, small island development states, and Least Developed Countries face disproportionately higher risk. Many low-elevation and small glaciers around the world would lose most of their mass or disappear within decades to centuries at global warming of 1.5°C.
With 2°C warming, agricultural and ecological droughts will become more frequent and/or severe; the intensity of tropical cyclones will increase; and heat waves and drought become more frequent, occurring concurrently at multiple locations. The food security and diet quality will deteriorate particularly in sub-Saharan Africa, South Asia, and Central America. At sustained warming levels between 2°C and 3°C, the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly.
Global warming of 3°C will cause widespread systemic impacts, irreversible change, and many additional adaptation limits. The extinction risk for endemic species in biodiversity hotspots is projected to increase at least tenfold if warming rises from 1.5°C to 3°C. Flood damages are projected to be higher by 1.4-2 times at 2°C and 2.5-3.9 times at 3°C, compared to 1.5°C global warming with current level of adaptation.
Global warming of 4°C and above is projected to lead to far-reaching impacts on natural and human systems. The global burned area is projected to increase by 50-70% and the fire frequency by ~30% compared to today. About 4 billion people are projected to experience water scarcity. Every increment of global warming will increase risks from compound and cascading impacts through the food, energy, and water sectors as impacts from weather and climate extremes propagate through supply chains, markets, and natural resource flows. Beyond 4°C of warming, projected impacts on natural systems include local extinction of ~50% of tropical marine species and biome shifts across 35% of global land area.
Risks associated with tipping points for sea level rise, continued glacier melt, permafrost carbon loss, ice sheet instability, or ecosystem loss from tropical forests become high risk between 1.5°C-2.5°C and very high risk between 2.5°C-4°C. The Atlantic Meridional Overturning Circulation is very likely to weaken over this century for all considered scenarios, however, an abrupt collapse is not expected in this century.
Effectiveness of Adaptation
The current state of adaptation is no match for the severe, widespread adverse impacts of global warming. Most observed adaptation is fragmented, small in scale, incremental, and sector specific, prioritizing immediate and near-term climate risk reduction, e.g., through hard flood protection, which neglects the need for integrated adaptation strategy capable of managing complex risks from the food-energy-water-health nexus. Ecosystem-based adaptation can generate synergies for food security, health and well-being, livelihoods and biodiversity, sustainability, and ecosystem services. But its effectiveness declines as warming continues reaching 1.5.
Urban areas urgently need integrated adaptations to counter the rapidly escalating impacts of climate change. Urban areas are now home to 50% of the world’s population and account for 70% of global CO2 emissions. Between 2015 and 2020, urban populations globally grew by more than 397 million people, with more than 90% of this growth taking place in less developed regions where adaptive capacity is limited. By 2050 an additional 2.5 billion people are projected to be living in urban areas with up to 90% of this increase concentrated in the regions of Asia and Africa. And coastal and low-lying urban areas will face additional risks of limits to adaptation to sea level rise and storm surges due to continued rapid global warming. The high density of population and infrastructure in urban areas should work as leverage for integrated adaptation strategy to address the compounding risks from the food-energy-water-health nexus.
The effectiveness of adaptation will decrease with increasing warming. This is because increased warming reduces adaptive capacity of natural and human systems. Already at current warming, natural – autonomous and evolutionary – adaptation responses by terrestrial and aquatic ecosystems will increasingly face hard limits. At 1.5°C, humans face soft and hard limits to adaptation regarding the risks of heat stress and heat mortality. Measures such as disaster risk management, early warning systems, climate services, and risk spreading and sharing provide wide-ranging benefits to adaptation actions when combined with financial, institutional, and policy support and will lead to softening the soft limits to adaptations. Above 1.5°C, limits to adaptation spread to ecosystem-based adaptation, freshwater resources management, and measures to secure staple crops.
Currently, 60% of all adaptation measures are water-related adaptation options and their effectiveness declines with increasing warming. Most adaptation measures in agriculture – improving cultivars and agronomic practices, and changes in cropping patterns and crop systems – will become less effective from 2°C to higher levels of warming. Adaptation options for agroforestry and forestry lose their effectiveness at 3°C.
Emissions and Mitigation
The average annual GHG emissions in the last decade were higher than in any previous decade. However, the average annual GHG emissions growth between 2010 and 2019 (1.3% per year) was lower than that during the previous decade (2.1% per year) with the energy supply sector’s emissions growth rate declining from 2.3% to 1.0% per year; the industry sector, from 3.4% to 1.4% per year; and the transport sector remaining roughly constant at about 2% per year. The carbon intensity of primary energy declined by 0.3% per year between 2010-2019.[2] Energy efficiency – GDP per unit of primary energy – increased by 2% per year between 2010-2019. Land was a source of net negative emissions at an annual rate of -6.6 (±4.6) GtCO2 during 2010-2019.
Cumulative CO2 emissions from 1850 to 2019 were 2400 ±240 GtCO2. Of these, 42% occurred in the last 30 years between 1990 and 2019. The estimate of the total carbon budget for a 50% probability of limiting warming to 1.5 C is 2900 GtCO2.[3] Four-fifths of this was already used, leaving 500 GtCO2 as the remaining carbon budget from 2020.
Average per capita net anthropogenic GHG emissions in 2019 ranged from 2.6 tCO2-eq (Southern Asia) to 19 tCO2-eq (Europe) across ten regions with a global average of 6.9 tCO2-eq. In seven regions, the dominant source of GHG emissions is CO2 emissions from energy and industry. In three regions (Latin America and Caribbean, Southeast Asia and Pacific, and Africa), CO2 emissions from land use, land use change and forestry exceed fossil fuel-generated emissions. The 10% of households with the highest per capita emissions contribute 34-45% of global consumption-based household GHG emissions, while the middle 40% contribute 40-53%, and the bottom 50% contribute 13-15%. The share of emissions from urban areas increased from 62% to 67-72% between 2015 and 2020. Over 70% of emissions are from non-OECD countries.
By 2020, 56 countries had laws on reducing GHG emissions covering 53% of global emissions. By 2020, over 20% of global GHG emissions were covered by carbon taxes or emissions trading systems, although coverage and prices have been insufficient to achieve deep reductions. Equity and distributional impacts of carbon pricing instruments are barriers to carbon pricing. Economic and regulatory instruments resulted in avoided emissions of at least 1.8 GtCO2-eq per year.
At least 18 countries have sustained emission reductions – both production- and consumption-based CO2 emissions – for longer than 10 years since 2005. Energy supply decarbonization, energy efficiency gains, and energy demand reduction contributed to emissions reductions.
From 2010 to 2019, the unit costs of renewable energy and lithium-ion batteries declined at unprecedented rates with rapid deployment. Electricity from PV and the wind is now cheaper than fossil fuel-powered electricity in many regions, electric vehicles are increasingly competitive with internal combustion engines, and large-scale battery storage on electricity grids is increasingly viable. The mix of policy instruments including public R&D, funding for demonstration and pilot projects, and deployment subsidies enabled the decarbonization of electricity and passenger cars.
However, the industrial sector’s decarbonization faces technological challenges. For almost all basic materials – steel, cement, and chemicals – low- to zero-GHG intensity production processes are not yet market-ready.
More than 100 countries have pledged net zero CO2 emissions, covering more than two-thirds of global GHG emissions, but policies are insufficient to deliver on them. The continuation of policies implemented by the end of 2020, i.e., the absence of additional policies, will lead to global warming of 3.2°C by 2100. The best estimates of warming for 2081-2100 relative to 1850-1900 vary from 1.4°C in the very low GHG emissions scenario to 2.7°C in the intermediate GHG emissions scenario and 4.4°C in the very high GHG emissions scenario. The larger the overshoot, the more net negative CO2 emissions needed to return to a given warming level. For every tenth of a degree of overshoot, the required net negative emissions would be 220 GtCO2.[4]
Overshooting 1.5°C will result in irreversible adverse impacts on ecosystems, such as polar, mountain, and coastal ecosystems. Overshoot increases the risks that could increase GHG releases, making temperature reversal more challenging. Such impacts include increased wildfires, mass tree mortality, drying of peatlands, thawing of permafrost, and weakening natural land carbon sinks.
Carbon Dioxide Removal (CDR) can lower net CO2 or net GHG emissions and counterbalance ‘hard-to-abate’ residual emissions (agriculture, aviation, shipping, industrial processes) to help reach net zero CO2 or GHG emissions. Afforestation, reforestation, improved forest management, agroforestry, and soil carbon sequestration are CDR methods currently in use. Agroforestry has the potential to remove more than 3 Gt CO2 per year. The IPCC assessed that costs range from lower cost (e.g., -45 to 100 USD/tCO2 for soil carbon sequestration) to higher cost (e.g., 100-300 USD/tCO2 for direct air carbon dioxide capture and storage). Land-based CDR methods and coastal blue carbon management can enhance biodiversity and ecosystem functions, and local livelihoods.
Average annual investment requirements for 2020 to 2030 in scenarios that limit warming to 2°C or 1.5°C are a factor of three to six greater than current levels. Accelerated financial support for developing countries where 70% of global emissions occur is a critical enabler to limit warming to 1.5°C. Developing country’s high capital costs, high debt burden, and low institutional strength are barriers to overcome.
Sectoral Contributions to Mitigation
A combination of energy efficiency and conservation and a transition to low-GHG technologies and energy carriers is required to reduce GHG emissions in industry, transport, and buildings to net zero. Almost all electricity will be carbon-free in 2050. Increased electrification of energy end-use will increase the share of electricity in final demand to 50% in 2050 from the current share of 20%. The final energy demand would stay constant at the current level of 410 EJ for the next 30 years due to improved energy efficiency – double the past rate of efficiency improvement by 2030 – and behavioral changes. If combined with improved infrastructure design and access, behavioral change can have the most potential in developed countries for emissions reduction.
AFOLU mitigation options – reforestation, afforestation, reduced deforestation, and bioenergy – can deliver large-scale GHG emission reductions and enhanced CO2 removal. However, there are barriers, including the adverse impacts of climate change (release of soil carbon due to climate change), competing demands on land, conflicts with food security and livelihoods, and the complexity of land ownership and management systems. Further, land-based mitigation options – afforestation, production of biomass crops for bioenergy with carbon dioxide capture and storage, or biochar – can have adverse impacts on biodiversity, food and water security, local livelihoods and the rights of Indigenous Peoples, especially if implemented at large scales and where land tenure is insecure.
Concluding Thoughts
The aggregate effects of climate change mitigation on global GDP (excluding damages from climate change and adaptation costs) are small compared to global projected GDP growth. Even without accounting for non-economic benefits or the co-benefits of mitigation, the global benefits of limiting warming to 2°C exceed the cost of mitigation as assessed by the IPCC. Limiting global warming to 1.5°C instead of 2°C would increase the costs of mitigation, but also increase the benefits in terms of reduced impacts and related risks and reduced adaptation needs. The cumulative scientific evidence is unequivocal: climate change is a threat to human well-being and planetary health. Any further delay in global action on adaptation and mitigation will endanger a sustainable future for all.
Actions that prioritize equity, climate justice, social justice, and inclusion lead to more sustainable outcomes and co-benefits, and advance climate-resilient development. Equity, inclusion, and just transitions are key to progress on adaptation and accelerated mitigation. Eradicating poverty and providing opportunities for decent living in low-income countries in the context of achieving sustainable development objectives are possible without significant global emissions growth.
[1] IPCC Climate Change 2023: Synthesis Report, Summary for Policymakers, A.2.1.
[2] IPCC Climate Change 2022: Mitigation of Climate Change, Summary for Policymakers, B2.4.
[3] For every 1000 GtCO2 emitted by human activity, global mean temperature rises by 0.27°C-0.63°C (best estimate of 0.45°C). This relationship implies that there is a finite carbon budget that cannot be exceeded in order to limit warming to any given level.
[4] IPCC Climate Change 2023: Synthesis Report, p. 87.