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Scenarios

Scenarios can be thought of as stories of possible futures. They allow the description of factors that are difficult to quantify. In the context of climate change scenarios are used for the future development of factors such as governance, social structures, future population growth, technical development and agriculture. These descriptions are essential to model the future climate.

Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs)

For the IPCC Fifth Assessment Report (AR5) an other approach has been taken. Basically, the socioeconomic scenarios (called Shared Socioeconomic Pathways - SSPs) have now been decoupled from the GHG concentration scenarios (now called Representative Concentration Pathways - RCPs). This change of approach stems from the recognition that the SRES scenario's do not cover the range of uncertainties that models could represent, e.g. that even high growth scenarios may be realised at low emissions, assuming that sufficiently 'green' technologies will become available. Also, instead of the linear development model outlined in the figure, now a parallel procress has been adopted to speed up the process, facilitate more iterations and improve the interactrions between especially the mitigation (WGIII) and adaptation (WGII) communities.

Four RCPs have been defined, named after the radiative forcing that eventually will be reached: RCP2.6, RCP4.5, RCP6 and RCP8.5. They work both forwards towards climate modelling and backwards to analyse what future world development is needed to achieve a certain level of antrophogenic influence on the climate. This allows development of mitigation scenarios. It is these names that you will come across mostly when looking for future climate data.

Below the emission pathways for the main GHGs and air pollutants are shown. Though conceptually different the RCPs can be mapped, at least approximately onto the SRES scenarios in terms of CO2 emission development. Thus, RCP8.5 would be more or less equivalent to the SRES-A1FI, RCP6 to the SRES-A1B and RCP4.5 to the SRES-B1scenario. The RCP2.6 has no equivalent in the SRES scenarios; it has been developed primarily as a scenario that would probably meet the two degree warming limit considered by many to prevent 'dangerous interference with the climate system'. Conversely, the RCP8.5, 6 and 4.5 scenarios would likely lead to a global warming of six, four and three degrees (Kelvin or Celcius), respectively.

These RCP can be associated with SSPs in a matrix as conceptualised in the below picture and further explained in the link at the bottom of this page. The whole idea of new scenarios was to enable the IAV community to be able to utilize the suite of climate model products generated under CMIP5. However, the SSPs do not necessarily produce reference scenarios that closely coincidel with the RCP-based climate model ensemble calculations. The scenario matrix approach introduces climate policies to constrain RCP-based reference scenarios to climate forcing levels consistent with the RCPs. Thus, emissions mitigation policies can be introduced to create RCP replications. Of course, not every RCP can be replicated starting from every SSP reference scenario. More on SSPs through the link at the bottom of this page.

SRES scenarios

The SRES scenarios, from the Special Report on Emission Scenarios, were used in the IPCC Third Assessment Report (TAR), published in 2001, and in the IPCC Fourth Assessment Report (AR4), published in 2007. There are different socioeconomic scenarios used for future world development, for example A1, B2 or A1B. They are input to an Integrated Assessment Model (IAM) that calculates future emission of greenhouse gases and aerosols and creates scenarios. The emission scenarios go into a biochemical model that translates the emissions to atmospheric concentration scenarios and radiative forcing.

The SRES chain of scenarios and models. Image source: SMHI Rossby Centre.

Global and regional climate scenarios

The greenhouse gas concentrations are used in a global climate model (GCM). Other inputs to a GCM are topography, physiography, vegetation and land cover, scenarios of other factors and a representative initial state of the atmosphere and oceans for starting the simulation. The GCM produces global climate scenarios of a range of atmospheric and oceanic variables at a pre-defined temporal resolution. The most common variables are related to temperature and precipitation, for which both mean conditions and extremes are derived. The global climate scenarios typically have a spatial resolution of 100-300 km.

To get higher resolution and a more detailed results the global scenarios can be used as input to a regional climate model (RCM). The RCM also use topography, physiography and land cover etc as inputs, usually more detailed compared to the GCM input. The RCM produces regional climate scenarios for a predefined area of the globe.

Impacts scenarios

The global or the regional climate scenarios can be used for Impacts, Adaptation & Vulnerability (IAV) studies, for example hydrology, water resources, ecosystems and infrastructure. Often an additional Impact model is used, depending on the study requested.

Related links

 

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