climate change scenario

File:Global CO2 emissions and probabilistic temperature outcomes of Paris.png in each of the four scenarios.

]]

The IPCC Sixth Assessment Report defines scenario as follows: "A plausible description of how the future may develop based on a [...] set of assumptions about key driving forces and relationships."IPCC, 2022: [https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Annex-I.pdf Annex I: Glossary] [van Diemen, R., J.B.R. Matthews, V. Möller, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, A. Reisinger, S. Semenov (eds)]. In IPCC, 2022: [https://www.ipcc.ch/report/ar6/wg3/ Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926.020{{rp|1812}} A set of scenarios shows a range of possible futures.

Scenarios are not predictions.{{rp|1812}} Scenarios help decision makers to understand what will be the effects of a decision.

The concept of pathways is closely related. The formal definition of pathways 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."{{rp|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 of fossil-fuel phase out or the reduction of fossil 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 terms scenarios and pathways are often used interchangeably.IPCC, 2023: [https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/syr/ Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 1-34, doi: 10.59327/IPCC/AR6-9789291691647.001{{rp|9}} The IPCC publications on the physical science basis tend to use scenarios more, whereas the publications on mitigation tend to use modelled emission and mitigation pathways as a term.{{rp|9}}

There are the following types of scenarios:{{rp|1813}}

• baseline scenarios
• concentrations scenarios
• emissions scenarios
• mitigation scenarios
• reference scenarios
• socio economic scenarios.

A baseline scenario is used as a reference for comparison against an alternative scenario, e.g., a mitigation scenario.{{cite web |author=IPCC |date=2007c |title=Annex. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz et al. Eds.] |url=http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm |url-status=dead |archive-url=https://web.archive.org/web/20181005032403/http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm |archive-date=2018-10-05 |access-date=2009-05-20 |publisher=Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.}} A wide range of quantitative projections of greenhouse gas emissions have been produced.{{citation |author=Fisher |title=Archived copy |access-date=2012-09-08 |archive-url=https://web.archive.org/web/20181116144321/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch3.html |archive-date=2018-11-16 |url-status=dead |at=[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch3s3-1.html Sec. 3.1 Emissions scenarios] |chapter=Chapter 3: Issues related to mitigation in the long-term context |chapter-url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch3.html |display-authors=etal}}, in IPCC AR4 WG3 (2007) The "SRES" scenarios are "baseline" emissions scenarios (i.e., they assume that no future efforts are made to limit emissions),{{citation |author=Morita |title=Archived copy |access-date=2012-09-08 |archive-url=https://web.archive.org/web/20130706010248/http://www.grida.no/climate/ipcc_tar/wg3/068.htm |archive-date=2013-07-06 |url-status=dead |at=[http://www.grida.no/climate/ipcc_tar/wg3/080.htm Sec. 2.5.1.1 IPCC Emissions Scenarios and the SRES Process] |chapter=Chapter 2, Greenhouse Gas Emission Mitigation Scenarios and Implications |chapter-url=http://www.grida.no/climate/ipcc_tar/wg3/068.htm |display-authors=etal}}, in IPCC TAR WG3 (2001). and have been frequently used in the scientific literature (see Special 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.{{cite web |author=Morita, T. |display-authors=etal |year=2001 |title=Greenhouse Gas Emission Mitigation Scenarios and Implications. In: Climate Change 2001: Mitigation. Contribution of Working Group III to the Third Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz et al. Eds.] |url=http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm |url-status=dead |archive-url=https://web.archive.org/web/20181005032403/http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm |archive-date=2018-10-05 |access-date=2010-01-10 |publisher=Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.}}

{{See also|Eco-economic decoupling|Sustainability}}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 increased energy 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.{{cite web

|year=2007

|title=Introduction. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz et al. Eds.]

|publisher=Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.

|author=Rogner, H.-H.

|url=http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm

|access-date=2009-05-20

|display-authors=etal

|archive-date=2018-10-05

|archive-url=https://web.archive.org/web/20181005032403/http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm

|url-status=dead

}} 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.{{cite web |author=Fisher, B.S. |display-authors=etal |year=2007 |title=Issues related to mitigation in the long term context. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz et al. Eds.] |url=http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm |url-status=dead |archive-url=https://web.archive.org/web/20181005032403/http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm |archive-date=2018-10-05 |access-date=2009-05-20 |publisher=Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.}}

• Carbon intensity: This is the CO₂ emissions per unit of TPES. Compared with other scenarios, Fisher et al. (2007) found that the carbon intensity was more constant in scenarios where no climate policy had been assumed. 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 and albedo. One of the dominant drivers in land-use change is food demand. Population and economic growth are the most significant drivers of food demand.{{cite web

|year=2007

|title="3.2.1.6 Land-use change and land-use management." In [book chapter]: "Issues related to mitigation in the long term context." In [book]: "Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz et al. Eds.]

|publisher=Print version: Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.. This version: IPCC website

|author=Fisher, B.S.

|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch3s3-2-1-6.html

|access-date=2010-03-18

|display-authors=etal

|archive-url=https://web.archive.org/web/20100425070512/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch3s3-2-1-6.html

|archive-date=2010-04-25

|url-status=dead

}}{{Dubious|Food demand citation|date=March 2010}}

In producing scenarios, an important consideration is how social and economic development will progress in developing countries. 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 the structural 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.), and trade patterns the degree of protectionism and the creation of regional trading blocks can affect availability to technology.

In the majority of studies, the following relationships were found (but are not proof of causation):

• Rising GHGs: This was associated with scenarios having a growing, post-industrial economy with globalization, 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 an ecologically 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:

• Needed emissions cuts
• Pledges
• Recent and currently remaining carbon budget

File:Greenhouse gas emission scenarios 01.svg. 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 which global warming is reduced by deliberate actions, such as a comprehensive switch to energy 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.Commonwealth of Australia, "Climate Change Mitigation Scenarios: Modeling report provided to the Climate Change Authority in support of its Caps and Targets Review," 2013. Retrieved 13 December 2018 from https://www.environment.gov.au/system/files/resources/a28424ae-cce9-48c9-aad2-56b3db0920a5/files/climate-change-mitigation-scenarios.pdf

The Paris Agreement has the goal to keep the increase of global temperature below 2{{nbsp}}°C, preferably below 1.5{{nbsp}}°C above pre-industrial levels to reduce effects of climate change.{{Cite web|last=UNFCCC|title=The Paris Agreement|url=https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement|url-status=live|access-date=18 September 2021|website=unfccc.int|archive-date=19 March 2021|archive-url=https://web.archive.org/web/20210319205057/https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement}} A typical mitigation scenario is constructed by selecting a long-range target, such as a desired atmospheric concentration of carbon dioxide ({{CO2}}), and then fitting the actions to the target, for example by placing a cap on net global and national emissions of greenhouse gases.

File:Global CO2 Pathways Using Remaining Carbon Budgets.svg

Contributions to climate change, whether they cool or warm the Earth, are often described in terms of the radiative forcing or imbalance they introduce to the planet's energy 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-million carbon dioxide equivalent" (ppm CO₂e), or the increment/decrement in carbon dioxide concentrations which would create a radiative forcing of the same magnitude.

The BLUE scenarios in the IEA's Energy 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.[http://climateprogress.org/2008/04/22/is-450-ppm-or-less-politically-possible-part-2-the-solution/ Is 450 ppm (or less) politically possible? Part 2: The Solution]

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 as China and India in a global cap-and-trade scheme initially operating in OECD and European Union countries. Also the less conservative 450 ppm scenario calls for extensive deployment of negative emissions, i.e. the removal of {{CO2}} from the atmosphere. According to the International Energy Agency (IEA) and OECD, "Achieving lower concentration targets (450 ppm) depends significantly on the use of BECCS".{{cite web

|title=OECD Environmental Outlook to 2050, Climate Change Chapter, pre-release version

|url=http://www.oecd.org/dataoecd/32/53/49082173.pdf

|publisher=OECD

|year=2011

|access-date=2012-01-16

}}

This is the target advocated (as an upper bound) in the Stern Review. As approximately a doubling of {{CO2}} levels relative to preindustrial times, it implies a temperature increase of about three degrees, according to conventional estimates of climate sensitivity. Pacala and Socolow list 15 "wedges", any 7 of which in combination should suffice to keep {{CO2}} levels below 550 ppm.{{cite journal|title=Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies|first1=S.|last1=Pacala|first2=R.|last2=Socolow|date=13 August 2004|journal=Science|volume=305|issue=5686|pages=968–972|doi=10.1126/science.1100103|pmid=15310891|bibcode=2004Sci...305..968P|citeseerx = 10.1.1.642.8472|s2cid=2203046}}

The International Energy Agency's World 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 on efficiency increases which deliver fuel cost savings of over $7 trillion.http://www.iea.org/weo/docs/weo2008/fact_sheets_08.pdf {{Webarchive|url=https://web.archive.org/web/20081117073843/http://www.iea.org/weo/docs/weo2008/fact_sheets_08.pdf |date=2008-11-17 }} World Energy Outlook 2008 Fact Sheet

Closely related to climate change scenarios are pathways, which are more concrete and action-oriented.

The IPCC assessment reports talk about the following types of pathways:{{rp|1810}}

• 1.5°C pathway
• Adaptation pathways
• Climate-resilient pathways
• Development pathways
• Emission pathways
• Mitigation pathways
• Non-overshoot pathways
• Overshoot pathways
• Representative Concentration Pathways (RCPs)
• Shared Socio-economic Pathways (SSPs)
• Transformation pathways

File:Global mean near-surface air temperature and thermosteric sea-level rise anomalies relative to the 2000–2019 mean for RCP climate change scenarios.webp) climate change scenarios{{Cite journal |last=Lyon |first=Christopher |last2=Saupe |first2=Erin E. |last3=Smith |first3=Christopher J. |last4=Hill |first4=Daniel J. |last5=Beckerman |first5=Andrew P. |last6=Stringer |first6=Lindsay C. |last7=Marchant |first7=Robert |last8=McKay |first8=James |last9=Burke |first9=Ariane |last10=O’Higgins |first10=Paul |last11=Dunhill |first11=Alexander M. |last12=Allen |first12=Bethany J. |last13=Riel‐Salvatore |first13=Julien |last14=Aze |first14=Tracy |date=January 2022 |title=Climate change research and action must look beyond 2100 |url=https://onlinelibrary.wiley.com/doi/10.1111/gcb.15871 |journal=Global Change Biology |language=en |volume=28 |issue=2 |pages=349–361 |doi=10.1111/gcb.15871 |issn=1354-1013|hdl=20.500.11850/521222 |hdl-access=free }}]]

{{excerpt|Representative Concentration Pathway}}

{{excerpt|Shared Socioeconomic Pathways}}

To explore a wide range of plausible climatic outcomes and to enhance confidence in the projections, national climate change projections are often generated from multiple general circulation models (GCMs). Such climate ensembles can take the form of perturbed physics ensembles (PPE), multi-model ensembles (MME), or initial condition ensembles (ICE).{{Cite journal |last=Parker |first=Wendy S. |date=2012 |title=Whose Probabilities? Predicting Climate Change with Ensembles of Models |journal=Philosophy of Science |language=en |volume=77 |issue=5 |pages=985–997 |doi=10.1086/656815 |issn=0031-8248 |s2cid=121314681}} As the spatial resolution of the underlying GCMs is typically quite coarse, the projections are often downscaled, 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 evaluate catchment areas of transboundary rivers.

Various countries have produced their national climate projections with feedback and/or interaction with stakeholders.{{Cite journal |last1=Skelton |first1=Maurice |last2=Porter |first2=James J. |last3=Dessai |first3=Suraje |last4=Bresch |first4=David N. |last5=Knutti |first5=Reto |date=2017-04-26 |title=The social and scientific values that shape national climate scenarios: a comparison of the Netherlands, Switzerland and the UK |journal=Regional Environmental Change |language=en |volume=17 |issue=8 |pages=2325–2338 |doi=10.1007/s10113-017-1155-z |issn=1436-3798 |pmc=6959399 |pmid=32009852 |doi-access=free}} 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 the United Nations Framework Convention on Climate Change. Many European governments have also funded national information portals on climate change.

{{cite book |last=Füssel |first=Hans-Martin |url= |title=How Is Uncertainty Addressed in the Knowledge Base for National Adaptation Planning?. In Adapting to an Uncertain Climate |publisher=Springer, Cham |year=2014 |isbn=978-3-319-04875-8 |location=pp. 41-66 |author-link=}}

• Australia: CCIA[https://www.climatechangeinaustralia.gov.au/en/climate-projections/ Climate Change in Australia]
• California: Cal-Adapt[https://cal-adapt.org California climate change scenarios and climate impact research ]
• Netherlands: KNMI'14[http://www.climatescenarios.nl KNMI'14 Pictures of the future - Climate scenarios]
• Switzerland: CH2011{{Cite web |title=Swiss Climate Change Scenarios CH2011 B |url=http://ch2011.ch/en/index.html |access-date=2018-08-23 |website=ch2011.ch}} / CH2018[http://www.ch2018.ch/en/home-2/ CH2018 - New Climate Scenarios for Switzerland]
• UK: UKCP09 / UKCP18[http://ukclimateprojections.metoffice.gov.uk/24125 UKCP18 Project announcement][https://www.metoffice.gov.uk/research/collaboration/ukcp UKCP18 Demonstration Projects (Met Office)]

For countries which lack adequate resources to develop their own climate change projections, organisations such as UNDP or FAO have sponsored development of projections and national adaptation programmes (NAPAs).[http://adaptation-undp.org UNDP - Supporting Integrated Climate Change Strategies][https://unfccc.int/topics/resilience/workstreams/national-adaptation-programmes-of-action/introduction UNFCCC - National Adaptation Programmes of Action - Introduction]

Decision processes, such as decisionmaking under deep uncertainty, may use multiple climate scenarios to evaluate vulnerabilities and function for actions under many different potential futures. {{Cite journal |last=Lempert |first=Robert J. |last2=Lawrence |first2=Judy |last3=Kopp |first3=Robert E. |last4=Haasnoot |first4=Marjolijn |last5=Reisinger |first5=Andy |last6=Grubb |first6=Michael |last7=Pasqualino |first7=Roberto |date=2024-07-03 |title=The Use of Decision Making Under Deep Uncertainty in the IPCC |url=https://www.rand.org/pubs/external_publications/EP70531.html |journal=Frontiers in Climate |language=en |volume=6}}

{{Portal|Climate change}}

• Carbon budget
• Climate sensitivity
• Climate stabilization wedge
• CMIP
• Copernicus Programme
• Decarbonization pathways

{{reflist}}

• [https://www.undp.org/publications/undp-climate-change-country-profiles UNDP Climate Change Country Profiles - Introduction];[https://www.geog.ox.ac.uk/research/climate/projects/undp-cp/ UNDP Climate Change Country Profiles (61 countries)]

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