climate change scenario
Climate change scenarios or socioeconomic scenarios are projections of future greenhouse gas (GHG) emissions used by analysts to assess future vulnerability to climate change. Scenarios and pathways are created by scientists to survey any long term routes and explore the effectiveness of mitigation and helps us understand what the future may hold this will allow us to envision the future of human environment system. Producing scenarios requires estimates of future population levels, economic activity, the structure of governance, social values, and patterns of technological change. Economic and energy modelling (such as the World3 or the POLES models) can be used to analyze and quantify the effects of such drivers.
Scientists can develop separate international, regional and national climate change scenarios. These scenarios are designed to help stakeholders understand what kinds of decisions will have meaningful effects on climate change mitigation or adaptation. Most countries developing adaptation plans or Nationally Determined Contributions will commission scenario studies in order to better understand the decisions available to them.
International goals for mitigating climate change through international processes like the Paris Agreement are based on reviews of these scenarios. For example, the Special Report on Global Warming of 1.5 °C was released in 2018 order to reflect more up-to-date models of emissions, Nationally Determined Contributions, and impacts of climate change than its predecessor IPCC Fifth Assessment Report published in 2014 before the Paris Agreement.

Purpose
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.A baseline scenario is used as a reference for comparison against an alternative scenario, e.g., a mitigation scenario. A wide range of quantitative projections of greenhouse gas emissions have been produced. The "SRES" scenarios are "baseline" emissions scenarios (i.e., they assume that no future efforts are made to limit emissions), and have been frequently used in the scientific literature (see Special Report on Emissions Scenarios for details).

Tools
Shared Socioeconomic Pathways
Representative Concentration Pathway
Factors affecting future GHG emissions
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. 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.
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.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.

Scenarios for different degrees of mitigation efforts
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.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.
An increase of global temperature by more than 2 °C has come to be the majority definition of what would constitute intolerably dangerous climate change with efforts to limit the temperature increase to 1.5 °C above pre-industrial levels per the Paris Agreement.  Some climate scientists are increasingly of the opinion that the goal should be a complete restoration of the atmosphere's preindustrial condition, on the grounds that too protracted a deviation from those conditions will produce irreversible changes.

Stabilization wedges
A stabilization wedge is an action which incrementally reduces projected emissions. The name is derived from the triangular shape of the gap between reduced and unreduced emissions trajectories when graphed over time. For example, a reduction in electricity demand due to increased efficiency means that less electricity needs to be generated and thus fewer emissions need to be produced. The term originates in the Stabilization Wedge Game. As a reference unit, a stabilization wedge is equal to the following examples of mitigation initiatives: deployment of two hundred thousand 10 MW wind turbines; completely halting the deforestation and planting of 300 million hectares of trees; the increase in the average energy efficiency of all the world's buildings by 25 percent; or the installation of carbon capture and storage facilities in 800 large coal-fired power plants. Pacala and Socolow proposed in their work, Stabilization Wedges, that seven wedges are required to be delivered by 2050 – at current technologies – to make a significant impact on the mitigation of climate change. There are, however, sources that estimate the need for 14 wedges because Pacala and Socolow's proposal would only stabilize carbon dioxide emissions at current levels but not the atmospheric concentration, which is increasing by more than 2 ppm/year. In 2011, Socolow revised their earlier estimate to nine.

Target levels of CO2
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.

450 ppm
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.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".

550 ppm
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.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.

Other greenhouse gases
Greenhouse gas concentrations are aggregated in terms of carbon dioxide equivalent. Some multi-gas mitigation scenarios have been modeled by Meinshausen et al.

Trends and predictions
Previous predictions
UNEP 2011 synthesis report
The United Nations Environment Programme (UNEP, 2011): 7  looked at how world emissions might develop out to the year 2020 depending on different policy decisions. To produce their report, UNEP (2011): 8  convened 55 scientists and experts from 28 scientific groups across 15 countries.
Projections, assuming no new efforts to reduce emissions or based on the "business-as-usual" hypothetical trend, suggested global emissions in 2020 of 56 gigatonnes CO2-equivalent (GtCO2-eq), with a range of 55-59 GtCO2-eq.: 12  In adopting a different baseline where the pledges to the Copenhagen Accord were met in their most ambitious form, the projected global emission by 2020 will still reach the 50 gigatonnes CO2. Continuing with the current trend, particularly in the case of low-ambition form, there is an expectation of 3° Celsius temperature increase by the end of the century, which is estimated to bring severe environmental, economic, and social consequences.The report also considered the effect on emissions of policies put forward by UNFCCC Parties to address climate change. Assuming more stringent efforts to limit emissions lead to projected global emissions in 2020 of between 49 and 52 GtCO2-eq, with a median estimate of 51 GtCO2-eq.: 12  Assuming less stringent efforts to limit emissions lead to projected global emissions in 2020 of between 53 and 57 GtCO2-eq, with a median estimate of 55 GtCO2-eq.: 12

National climate (change) projections
National climate (change) projections (also termed "national climate scenarios" or "national climate assessments") are specialized regional climate projections, typically produced for and by individual countries. What distinguishes national climate projections from other climate projections is that they are officially signed off by the national government, thereby being the relevant national basis for adaptation planning. Climate projections are commonly produced over several years by countries' national meteorological services or academic institutions working on climate change.
Typically distributed as a single product, climate projections condense information from multiple climate models, using multiple greenhouse gas emission pathways (e.g. Representative Concentration Pathways) to characterize different yet coherent climate futures. Such a product highlights plausible climatic changes through the use of narratives, graphs, maps, and perhaps raw data. Climate projections are often publicly available for policy-makers, public and private decision makers, as well as researchers to undertake further climate impact studies, risk assessments, and climate change adaptation research. The projections are updated every few years, in order to incorporate new scientific insights and improved climate models.
National climate projections form the basis of national climate adaptation and climate resilience plans, which are reported to UNFCCC and used in IPCC assessments.

Design
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). 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. Some countries have also produced more localized projections for smaller administrative areas, e.g. States in the United States, and Länder in Germany.
Various countries have produced their national climate projections with feedback and/or interaction with stakeholders. 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.

Examples
Over 30 countries have reported national climate projections / scenarios in their most recent National Communications to United Nations Framework Convention on Climate Change. Many European governments have also funded national information portals on climate change.
Australia: CCIA
California: Cal-Adapt
Netherlands: KNMI'14
Switzerland: CH2011 / CH2018
UK: UKCP09 / UKCP18For 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).

See also
Carbon budget
CMIP
Copernicus Programme
Ensemble forecasting
Sensitivity analysis
Uncertainty analysis
Scenario planning

References
External links
UNDP Climate Change Country Profiles - Introduction;UNDP Climate Change Country Profiles (61 countries)