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Human activity is estimated to have brought about a warming of 1.0°C above pre-industrial levels. If current trends continue, a warming of 1.5°C is likely to be reached between 2030 and 2052. Some regions, like the Arctic, experience greater warming than the average, and warming of land tends to be greater than over oceans. Changes in weather and climate extremes have been observed in the last decades, a period over which global temperatures have increased by 0.5°C.
Impacts have already been observed, and many ecosystems have already changed. Future impacts will be larger if global warming exceeds 1.5°C before going back down than if doesn’t go over. Some impacts, like the loss of ecosystems, might be irreversible. Adaptation and mitigation are already underway, but an increase of both would reduce the risk of impacts from climate change.
Warming from emissions of greenhouse gasses emitted through human activities (CO2 but also previously chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs), methane, some solvents, …) that has happened since the pre-industrial period will continue to cause changes in the climate for centuries to come, by increasing for instance droughts and the acidification1 and level of the seas.
If there were no more greenhouse gases emissions from now on, global mean temperatures increase would be unlikely to reach 1.5°C. This is of course not a realistic possibility, since current human activity will continue to cause the emission of greenhouse gases in the foreseeable future unless decisive action is taken.
The maximum temperature rise would then depend on the total amount of greenhouse gases that are released into the atmosphere by human activity. Reducing net carbon dioxide (CO2) emissions to zero and maintaining that level would stop warming within several decades. On a longer time scale, a net negative level of CO2 emissions would be required to prevent further warming, to reverse ocean acidification, and to minimize sea-level rise.
1 Water acidification has a significant effect on aquatic ecosystems such as plant and animal plankton or coral reefs
The risks to natural and human systems posed by a global planet warming of 1.5°C are higher than at the current level of 1°C, and would be even greater at 2°C. Specific risks depend on the rate and amount of warming, on the location and the situation of human populations, which includes the level of development, vulnerability, and on the adaptation and mitigation measures that are put in place.
Clear differences between the current situation of 1°C mean temperature rise and a future situation where the global mean temperature has warmed up to 1.5°C or 2°C are shown by climate models. Besides higher mean temperatures in most areas with risks being of course higher at 2°C than at 1.5°C, these differences include hot extremes in inhabited regions, changes in precipitations, with some regions experiencing heavier precipitations and other regions experiencing more severe droughts. At 1.5°C global warming, hot days in temperate regions would be warmer by 3°C, and by 4°C at 2°C global warming. Extreme cold nights would be less cold, by as much as by 6°C with a 2°C global warming.
Sea levels will continue to rise for a long time after the global mean temperature is stabilized, since it takes a long time for heat to be distributed down into the depths of the ocean. The higher the global warming, the faster the rise and the less opportunity there is for populations living in coastal areas and on small islands to adapt. If warming reaches tipping points where the Greenland or the Antarctic ice sheets are destabilized and that large portions of them detach from the bedrock, it could result in an increase of sea levels by several meters, a situation that would persist for hundreds if not thousands of years. This is something that could happen already at a global warming level between 1.5 and 2°C.
Impacts on ecosystems and biodiversity, whether on land or in the oceans, are projected to be more severe at 2°C than at 1.5°C global warming. This includes risks of species extinctions, of forest fires and of transformation of ecosystems. High-latitude ecosystems, like the tundra and boreal forest, are particularly at risk of important changes since there is more warming in those regions and that the permanently frozen ground in many areas of tundra is going to thaw. The probability that the Arctic Ocean becomes ice-free during the summer is once per century at 1.5°C and once per decade at 2°C global warming.
Risks to health, livelihoods, food security, water supply, human security, and economic growth are projected to increase with global warming of 1.5°C and increase further with 2°C. Poverty and disadvantage are expected to increase in some populations as global warming increases; 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.
More adaptation will thus be needed at 2°C than at 1.5°C, but there are limits to adaptation strategies and at some point, some strategies will fail, even more so for the systems and people who are the most vulnerable.
For limiting temperature rise to 1.5°C limit, the total “carbon budget” involved should not trespass about 2,800 Gt CO2eq. In the meantime, as of 2017 the budget reached already 2,200 Gt CO2 eq and about 42 Gt CO2 more per year are still emitted. But there are still significant uncertainties on these numbers, including potential larger methane release from thawing permafrost in the Arctic region, the fact that mitigation actions could not sufficiently limit greenhouse gases emissions and uncertainties about the climate response to these emissions.
Based on the outcome of current nationally stated mitigation ambitions as submitted under the Paris Agreement, estimates of the global carbon emissions would lead to global greenhouse gas emissions in 2030 of 52–58 Gt CO2eq/year.
However, 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. Avoiding overshoot and reliance on future large-scale deployment of carbon dioxide removal (CDR) can only be achieved if global CO2 emissions start to decline well before 2030. The lower the carbon emissions in 2030, the lower the challenge in limiting global warming to 1.5°C after 2030 with no or limited overshoot.
All but one pathways that could limit global warming to 1.5°C with no or limited overshoot require a decline in global greenhouse gas emissions to below 35 Gt CO2 eq/yr in 2030, while half of available pathways fall only within the 25–30 Gt CO2 eq/yr range which is a 40–50% reduction from 2010 carbon emissions levels. The challenges from delayed actions to reduce greenhouse gas emissions include the risk of cost escalation, lock-in in carbon-emitting infrastructure, stranded assets, and reduced flexibility in future response options in the medium to long term.
In a ‘business as usual scenario’, where emission pathways reflect current nationally stated mitigation ambition until 2030, the result is a global warming of about 3°C by 2100, with global warming continuing afterwards.
Limiting global warming requires addressing the total cumulative emissions of greenhouse gases by human activity since the pre-industrial period. These 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.
In short, there are two main general ways to limit the global warming to 1.5°C:
With statistical high confidence, overshoot trajectories will result in higher impacts and associated challenges compared to pathways that limit global warming to 1.5°C with no or limited overshoot. In order to prevent such overshoot of the 1.5°C rise limit, CO2 emissions need to decline by 2030 by about 45% of the 2010 emissions levels, and reach net zero emissions by 2050. For limiting the global warming to 2°C, the decline needed is 25% by 2030, with net zero emissions reached by 2070. Such levels of CO2 emissions reduction would involve, among others, “decarbonization” of energy, of transportation and industries by reducing the use of fossil fuels, as well as via carbon dioxide capture.
Greenhouse gases other than CO2, such as methane emitted from agriculture or the waste sector through organic decomposition, will also need to be reduced significantly.
One issue is the mitigation of solar radiation change related among others to the decrease of the “albedo effect” of the pack ice. Solar radiation modification (SRM) measures2 may be theoretically effective in reducing an overshoot, but they face large uncertainties and knowledge gaps, so these measures are not included in any of the available assessed pathways. The deployment of these measures faces also substantial risks and institutional and social constraints related to governance, ethics, and impacts on sustainable development. They also do not mitigate ocean acidification due to the increased CO2 concentration in water.
2 Such solar radiation modification measures have as objective to increase the reemission of the heat captured by the planet, for example via aerosols that would be spread in the atmosphere. Such reemissions occur naturally via the ice surfaces (albedo effect) but these surfaces are reduced due to the melting of ice shelves.
See for example : en.wikipedia.org/wiki/Solar_radiation_management
If mitigation and adaptation synergies are maximized while trade-offs are minimized, the avoided climate change impacts on sustainable development, eradication of poverty and reducing inequalities would be greater if global warming was limited to 1.5°C rather than 2°C. Sustainable development, which balances social well-being, economic prosperity and environmental protection is indeed closely linked to climate change impacts and responses.
The 17 United Nations Sustainable Development Goals (SDGs), adopted in 20153, provide an established framework for assessing the links between global warming of 1.5°C or 2°C and development goals that include poverty eradication, reducing inequalities, and climate action.
3 www.un.org/sustainabledevelopment/development-agenda/
There are 4 domains to which priority of action should be given:
Energy systems: Lower energy use is one of the major ways to reduce carbon emissions. Fossil energy sources with lower emissions or with carbon capture are also needed to take a larger part of the energy mix, including renewables, as well as nuclear power. There are still challenges for renewable energies, but solar and wind energy and electricity storage technologies have substantially improved over the past few years, and the political, technical and economic feasibility or these systems has also improved.
Industry: CO2 emissions from industry need to be about 65–90% lower in 2050 relative to 2010, in order to reach 1.5°C temperature rise limit. Such reductions can be achieved through combinations of new and existing technologies and practices, including electrification, hydrogen, sustainable bio-based feedstocks, product substitution, and Carbon Capture, Utilization and Storage (CCUS) technologies.
Infrastructure: The urban and infrastructure system transitions would imply changes in land and urban planning practices, as well as deeper emissions reductions in transport and buildings.
Land use: Transitions in global and regional land use can be found in all pathways, but their scale depends on the pursued climate change mitigation objectives. Depending on the pathway, land can be used as agricultural land for energy crops, pastures could be transformed into forests or other possible changes of use. However, such large transitions would pose profound challenges for sustainable management of the various demands on land for human settlements, food, livestock feed, fiber, bioenergy, carbon storage, biodiversity and other ecosystem services.
All pathways that consider to limit global warming to 1.5°C by 2100, with limited or no overshoot, consider the recapture of exceeding carbon emissions in any shape or form. Carbon dioxide or CO2 recapture or removal (CDR) would compensate for residual emissions or contribute to achieve net negative emissions to return global warming to 1.5°C following an overshoot. While it should be on the order of 100–1000 Gt CO2 over the 21st century, by mid-century the present potential of carbon recapture, would be limited to remove only about 9 Gt CO2/yr.
Large CO2 emissions overshoots would of course require greater amounts of CDR, but limitations on the speed, scale, and societal acceptability of CDR deployment would determine the real ability to return global warming to below 1.5°C following such overshoots. Concretely, reversing warming after an overshoot of 0.2°C or larger during this century would require upscaling and deployment of CDR at rates and volumes that might not be achievable, given the considerable implementation challenges.
Existing and potential CDR measures differ indeed widely in terms of maturity, potentials, costs, risks, co-benefits and trade-offs. These include a series of options among which:
Furthermore, the understanding of global carbon cycle and climate system at the planet level is still limited as is the effectiveness of net negative emissions or carbon recapture to reduce temperatures after they peak.
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