Hypothetical representation of potential future conditions
This article is about the theory behind developing and using scenarios. For actual predictions of future emissions and global warming temperatures, seeclimate change mitigation.
Aclimate change scenario is a hypothetical future based on a "set of key driving forces".[1]: 1812 Scenarios explore the long-term effectiveness ofmitigation andadaptation.[2] Scenarios help to understand what the future may hold. They can show which decisions will have the most meaningful effects onmitigation andadaptation.
Closely related to climate change scenarios arepathways, which are more concrete and action-oriented. However, in the literature, the termsscenarios andpathways are often used interchangeably.[3]: 9
Many parameters influence climate change scenarios. Three important parameters are the number of people (and population growth), their economic activity and new technologies.Economic andenergy models, such asWorld3 andPOLES, quantify the effects of these parameters.
Climate change scenarios exist at a national, regional or global scale. Countries use scenario studies in order to better understand their decisions. This is useful when they are developing their adaptation plans ornationally determined contributions. International goals for mitigating climate change like theParis Agreement are based on studying these scenarios. For example, the IPCCSpecial Report on Global Warming of 1.5 °C was a "key scientific input" into the2018 United Nations Climate Change Conference.[4] Variouspathways are considered in the report, describing scenarios for mitigation of global warming. Pathways include for example portfolios for energy supply andcarbon dioxide removal.
Four climate change scenarios, based on 2015 data.[5][6] Left: emissions pathways following the scenarios of (1) no policy, (2) current policy, (3) meeting the governments’ announcements with constant country decarbonization rates past 2030, and (4) meeting the governments’ announcements with higher rates of decarbonization past 2030. Right: global temperatures, depending on the amount ofgreenhouse gases emitted in each of the four scenarios.
TheIPCC Sixth Assessment Report definesscenario as follows: "A plausible description of how the future may develop based on a [...] set of assumptions about key driving forces and relationships."[7]: 1812 Aset of scenarios shows a range of possible futures.
Scenarios are not predictions.[7]: 1812 Scenarios help decision makers to understand what will be the effects of a decision.
The concept ofpathways is closely related. The formal definition ofpathways is as follows: "The temporal evolution of natural and/or human systems towards a future state. [...] Pathway approaches [...] involve various dynamics, goals, and actors across different scales."[7]: 1810
In other words: pathways are a roadmap which list actions that need to be taken to make a scenario come true. Decision makers can use a pathway to make a plan, e.g. with regards to the timing offossil-fuel phase out or the reduction offossil fuel subsidies.
Pathways are more concrete and action-oriented compared to scenarios. They provide a roadmap for achieving desired climate targets. There can be several pathways to achieve the same scenario end point in future.
In the literature the termsscenarios andpathways are often used interchangeably.[8]: 9 The IPCC publications on the physical science basis tend to usescenarios more, whereas the publications on mitigation tend to usemodelled emission and mitigation pathways as a term.[8]: 9
There are the following types of scenarios:[1]: 1813
baseline scenarios
concentrations scenarios
emissions scenarios
mitigation scenarios
reference scenarios
socio economic scenarios.
Abaseline scenario is used as a reference for comparison against analternative scenario, e.g., a mitigation scenario.[9] A wide range of quantitative projections ofgreenhouse gas emissions have been produced.[10] The "SRES" scenarios are "baseline" emissions scenarios (i.e., they assume that no future efforts are made to limit emissions),[11] and have been frequently used in the scientific literature (seeSpecial Report on Emissions Scenarios for details).
Climate change scenarios can be thought of as stories of possible futures. They allow the description of factors that are difficult to quantify, such as governance, social structures, and institutions. There is considerable variety among scenarios, ranging from variants of sustainable development, to the collapse of social, economic, and environmental systems.[12]
The following parameters influence what the scenarios look like: future population levels, economic activity, the structure of governance, social values, and patterns of technological change. No strong patterns were found in the relationship between economic activity and GHG emissions. Economic growth was found to be compatible with increasing or decreasing GHG emissions. In the latter case, emissions growth is mediated by increasedenergy efficiency, shifts to non-fossil energy sources, and/or shifts to a post-industrial(service-based) economy.
Factors affecting the emission projections include:
Population projections: All other factors being equal, lower population projections result in lower emissions projections.
Economic development: Economic activity is a dominant driver of energy demand and thus of GHG emissions.
Energy use: Future changes in energy systems are a fundamental determinant of future GHG emissions.
Energy intensity: This is the total primary energy supply (TPES) per unit of GDP.[13] In all of the baseline scenarios assessments, energy intensity was projected to improve significantly over the 21st century. The uncertainty range in projected energy intensity was large.[14]
Carbon intensity: This is the CO2 emissions per unit of TPES. Compared with other scenarios, Fisheret al. (2007) found that the carbon intensity was more constant in scenarios where no climate policy had been assumed.[14] The uncertainty range in projected carbon intensity was large. At the high end of the range, some scenarios contained the projection that energy technologies without CO2 emissions would become competitive without climate policy. These projections were based on the assumption of increasing fossil fuel prices and rapid technological progress in carbon-free technologies. Scenarios with a low improvement in carbon intensity coincided with scenarios that had a large fossil fuel base, less resistance to coal consumption, or lower technology development rates for fossil-free technologies.
Land-use change:Land-use change plays an important role in climate change, impacting on emissions,sequestration andalbedo. One of the dominant drivers in land-use change is food demand. Population and economic growth are the most significant drivers of food demand.[15][dubious –discuss]
In producing scenarios, an important consideration is howsocial and economic development will progress indeveloping countries.[14] If, for example, developing countries were to follow a development pathway similar to the current industrialized countries, it could lead to a very large increase in emissions. Emissions do not only depend on the growth rate of the economy. Other factors include thestructural changes in the production system,technological patterns in sectors such as energy,geographical distribution of human settlements and urban structures (this affects, for example, transportation requirements), consumption patterns (e.g., housing patterns, leisure activities, etc.), andtrade patterns the degree ofprotectionism and the creation of regionaltrading blocks can affect availability to technology.
In the majority of studies, the following relationships were found (but are not proof of causation):[12]
Rising GHGs: This was associated with scenarios having a growing,post-industrial economy withglobalization, mostly with low government intervention and generally high levels of competition. Income equality declined within nations, but there was no clear pattern in social equity or international income equality.
Falling GHGs: In some of these scenarios,GDP rose. Other scenarios showed economic activity limited at anecologically sustainable level. Scenarios with falling emissions had a high level of government intervention in the economy. The majority of scenarios showed increased social equity and income equality within and among nations.
Predicted trends for greenhouse gas emissions are shown in different formats:
Scenarios of globalgreenhouse gas emissions. If all countries achieve their current Paris Agreement pledges, average warming by 2100 will go far beyond the target of the Paris Agreement to keep warming "well below 2°C".
Climate change mitigation scenarios are possible futures in whichglobal warming is reduced by deliberate actions, such as a comprehensive switch toenergy sources other than fossil fuels. These are actions that minimize emissions so atmospheric greenhouse gas concentrations are stabilized at levels that restrict the adverse consequences of climate change. Using these scenarios, the examination of the impacts of different carbon prices on an economy is enabled within the framework of different levels of global aspirations.[16]
TheParis Agreement has the goal to keep the increase of global temperature below 2°C, preferably below 1.5°C above pre-industrial levels to reduceeffects of climate change.[17] A typical mitigation scenario is constructed by selecting a long-range target, such as a desired atmospheric concentration ofcarbon dioxide (CO2), and then fitting the actions to the target, for example by placing a cap on net global and national emissions ofgreenhouse gases.
This figure depicts the rates at which global CO2 emissions must decline after 2024 to limit the global temperature increase to 1.5, 1.7, or 2.0 degrees Celsius without relying on net-negative emissions.[18]
Contributions to climate change, whether they cool or warm theEarth, are often described in terms of theradiative forcing or imbalance they introduce to the planet'senergy budget. Now and in the future,anthropogenic carbon dioxide is believed to be the major component of this forcing, and the contribution of other components is often quantified in terms of "parts-per-millioncarbon dioxide equivalent" (ppmCO2e), or the increment/decrement in carbon dioxide concentrations which would create a radiative forcing of the same magnitude.
The BLUE scenarios in the IEA'sEnergy Technology Perspectives publication of 2008 describe pathways to a long-range concentration of 450 ppm.Joseph Romm has sketched how to achieve this target through the application of 14 wedges.[19]
World Energy Outlook 2008, mentioned above, also describes a "450 Policy Scenario", in which extra energy investments to 2030 amount to$9.3 trillion over the Reference Scenario. The scenario also features, after 2020, the participation of major economies such asChina andIndia in a global cap-and-trade scheme initially operating inOECD andEuropean Union countries. Also the less conservative 450 ppm scenario calls for extensive deployment ofnegative emissions, i.e. the removal of CO2 from the atmosphere. According to theInternational Energy Agency (IEA) and OECD, "Achieving lower concentration targets (450 ppm) depends significantly on the use ofBECCS".[20]
This is the target advocated (as an upper bound) in theStern Review. As approximately a doubling of CO2 levels relative topreindustrial times, it implies a temperature increase of about three degrees, according to conventional estimates ofclimate sensitivity.Pacala and Socolow list 15 "wedges", any 7 of which in combination should suffice to keep CO2 levels below 550 ppm.[21]
TheInternational Energy Agency'sWorld Energy Outlook report for 2008 describes a "Reference Scenario" for the world's energy future "which assumes no new government policies beyond those already adopted by mid-2008", and then a "550 Policy Scenario" in which further policies are adopted, a mixture of "cap-and-trade systems,sectoral agreements and national measures". In the Reference Scenario, between 2006 and 2030 the world invests $26.3 trillion in energy-supply infrastructure; in the 550 Policy Scenario, a further $4.1 trillion is spent in this period, mostly onefficiency increases which deliver fuel cost savings of over $7 trillion.[22]
Global mean near-surface air temperature and thermosteric sea-level rise anomalies relative to the 2000–2019 mean for RCP (Representative Concentration Pathway) climate change scenarios[23]
Different RCP scenarios result in different predictedgreenhouse gas concentrations in the atmosphere (from 2000 to 2100). RCP8.5 would result in the highest greenhouse gas concentration (measured as CO2-equivalents).
Representative Concentration Pathways (RCP) are climate change scenarios to project futuregreenhouse gas concentrations. These pathways (ortrajectories) describe future greenhouse gas concentrations (notemissions) and have been formally adopted by theIPCC. The pathways describe different climate change scenarios, all of which were considered possible depending on the amount of greenhouse gases (GHG) emitted in the years to come. The four RCPs – originally RCP2.6, RCP4.5, RCP6, and RCP8.5 – are labelled after the expected changes inradiative forcing values from the year 1750[24][25] to the year 2100 (2.6, 4.5, 6, and 8.5 W/m2, respectively).[26][27][28] The IPCCFifth Assessment Report (AR5) began to use these four pathways forclimate modeling and research in 2014. The higher values mean higher greenhouse gas emissions and therefore higherglobal surface temperatures and more pronouncedeffects of climate change. The lower RCP values, on the other hand, are more desirable for humans but would require more stringentclimate change mitigation efforts to achieve them.
In the IPCC'sSixth Assessment Report the original pathways are now being considered together withShared Socioeconomic Pathways. There are three new RCPs, namely RCP1.9, RCP3.4 and RCP7.[29] A short description of the RCPs is as follows: RCP 1.9 is a pathway that limitsglobal warming to below 1.5 °C, the aspirational goal of theParis Agreement.[29] RCP 2.6 is avery stringent pathway.[29] RCP 3.4 represents an intermediate pathway between thevery stringent RCP2.6 and less stringent mitigation efforts associated with RCP4.5.[29] RCP 4.5 is described by the IPCC as anintermediate scenario.[30] In RCP 6, emissions peak around 2080, then decline.[31] RCP7 is abaseline outcome rather than a mitigation target.[29] In RCP 8.5 emissions continue to rise throughout the 21st century.[32]: Figure 2, p. 223
For the extended RCP2.6 scenario, global warming of 0.0 to 1.2 °C is projected for the late 23rd century (2281–2300 average), relative to 1986–2005.[33] For the extended RCP8.5, global warming of 3.0 to 12.6 °C is projected over the same time period.[33]
Predicted atmospheric CO₂ concentrations for different shared socioeconomic pathways (SSPs) across the 21st century (projected by MAGICC7, a simple/reduced complexity climate model). Each data point represents an average of simulated values generated from fiveintegrated assessment models.[34]
There are also ongoing efforts to downscaling European shared socioeconomic pathways (SSPs) for agricultural and food systems, combined withrepresentative concentration pathways (RCP) to regionally specific, alternative socioeconomic and climate scenarios.[41][42]
To explore a wide range of plausible climatic outcomes and to enhance confidence in the projections, national climate change projections are often generated from multiplegeneral circulation models (GCMs). Suchclimate ensembles can take the form ofperturbed physics ensembles (PPE),multi-model ensembles (MME), orinitial condition ensembles (ICE).[43] As the spatial resolution of the underlying GCMs is typically quite coarse, the projections are oftendownscaled, either dynamically using regional climate models (RCMs), or statistically. Some projections include data from areas which are larger than the national boundaries, e.g. to more fully evaluatecatchment areas oftransboundary rivers.
Various countries have produced their national climate projections with feedback and/or interaction with stakeholders.[44] Such engagement efforts have helped tailoring the climate information to the stakeholders' needs, including the provision of sector-specific climate indicators such as degree-heating days.
Over 30 countries have reported national climate projections / scenarios in their most recent submissions to theUnited Nations Framework Convention on Climate Change. Many European governments have also funded national information portals on climate change.[45]
For countries which lack adequate resources to develop their own climate change projections, organisations such asUNDP orFAO have sponsored development of projections andnational adaptation programmes (NAPAs).[53][54]
Decision processes, such asdecisionmaking under deep uncertainty, may use multiple climate scenarios to evaluate vulnerabilities and function for actions under many different potential futures.[55]
^Meinshausen, M., Nicholls, Z. R. J., Lewis, J., Gidden, M. J., Vogel, E., Freund, M., Beyerle, U., Gessner, C., Nauels, A., Bauer, N., Canadell, J. G., Daniel, J. S., John, A., Krummel, P. B., Luderer, G., Meinshausen, N., Montzka, S. A., Rayner, P. J., Reimann, S., . . . Wang, R. H. J. (2020). The shared socio-economic pathway (SSP) greenhouse gas concentrations and their extensions to 2500. Geoscientific Model Development, 13(8), 3571–3605.https://doi.org/10.5194/gmd-13-3571-2020Archived 2023-04-16 at theWayback Machine
^Parker, Wendy S. (2012). "Whose Probabilities? Predicting Climate Change with Ensembles of Models".Philosophy of Science.77 (5):985–997.doi:10.1086/656815.ISSN0031-8248.S2CID121314681.
^Füssel, Hans-Martin (2014).How Is Uncertainty Addressed in the Knowledge Base for National Adaptation Planning?. In Adapting to an Uncertain Climate. pp. 41-66: Springer, Cham.ISBN978-3-319-04875-8.{{cite book}}: CS1 maint: location (link)
