Forcing scenarios

The climate of the future will continue to experience natural variability, much as it has in the past. However, the background change in mean climate, already being driven by human activities, will continue at a rate that is determined primarily by current and future emissions of greenhouse gases and aerosols. Because future emissions are uncertain, for many applications it is advisable to use a range of plausible scenarios from high to low emission pathways.

What are scenarios? The evolution of scenarios over time

Future climate projections are dependent on external drivers of climate change. These include greenhouse gas and aerosol emissions, land-use change, solar variations, and volcanic activity. The latter two are referred to as natural drivers as opposed to human-caused or anthropogenic drivers, which are related to projected changes in socio-economic conditions such as population growth, energy consumption, energy sources, and climate policy.

Scenarios are a set of possible future pathways for humanity that cover a range of possible choices and outcomes. Since it is impossible to predict how humanity will change in the future, as human actions and decisions have a large impact on future changes to climate, scientists construct a range of plausible future scenarios. These future socio-economic pathways are translated into scenarios of greenhouse gas emissions or concentrations, aerosol amounts, and land-use change that serve as input to climate models to project changes to the Earth’s climate according to each set of conditions and climate drivers. Neither the IPCC nor CMIP attaches a likelihood or possibility of occurrence to any scenario. Scenarios are meant to give the public and policymakers insight into how current and future human actions affect Earth's climate. Just as climate models have been improved over time, the scenarios used to project future conditions have also gone through several iterations.

The use of scenarios for projecting changes in future climate began with the IPCC scenarios, named Scenarios A through D, for the IPCC’s First Assessment Report (FAR) (1990). The scenarios estimated emissions from 1990 to 2100 and the resulting atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and trichlorofluoromethane (CFC-11). The IPCC scenarios were soon replaced by the first official set of scenarios, the IS92 scenarios, used for the IPCC’s Second Assessment Report (SAR) (1996). The IS92 scenarios were the first to estimate future concentrations for the full suite of greenhouse gases for the period 1990 to 2100. These scenarios were used in fairly simple climate models to make projections of future climate change.

The first improvement to the original IS92 scenarios were the Special Report on Emissions Scenarios (SRES) scenarios, used for the First (1996) and Second Phases (1997) of the Coupled Model Intercomparison Project (CMIP1 & CMIP2) and IPCC’s Third Assessment Report (TAR) (2001), as well as the Third Phase of the Coupled Model Intercomparison Project (CMIP3) (2005-2007) and the IPCC’s Fourth Assessment Report (AR4) (2007). Scientists and experts from a broad range of backgrounds and specialties developed six SRES scenario families which covered a wide range of potential climate drivers including demographic, technological, and economic developments.

The SRES scenarios were replaced by Representative Concentration Pathways (RPCs) for the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5) (2008-2013) and IPCC Fifth Assessment Report (AR5) (2014) (CMIP4 was skipped to align CMIP and IPCC AR numbering). RCPs describe the concentration of greenhouse gases in the atmosphere over time and are labelled according to the Earth’s simulated total radiative forcing in 2100. Radiative forcing is essentially the change in the difference between incoming solar radiation from the sun, and outgoing energy radiated back into space by Earth. By convention, a positive radiative forcing means more energy is retained within the climate system, leading to climate warming, whereas a negative radiative forcing means more energy is leaving the system than is retained, leading to climate cooling. RCP’s are plausible scenarios for humanity’s emissions of greenhouse gases based on plausible future developments but have no specified associated socio-economic conditions.

The latest iteration of scenarios, used for CMIP6 (2016-2021) and IPCC Sixth Assessment Report (AR6) (2021), are the Shared Socio-economic Pathways (SSPs).

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