Factsheet 6: CMIP5 and RCPs or CMIP6 and SSPs?

CMIP5 models informed the previous Fifth Assessment Report (AR5) from the Intergovernmental Panel on Climate Change (IPCC) (released in sections between 2013 and 2014). CMIP5 models considered several different trajectories for future changes in the concentration of atmospheric greenhouse gases (GHGs) described using Representative Concentration Pathways (RCPs).

An RCP describes a trajectory for future GHG concentrations based on assumptions about how different natural processes and human activities may change the rate of emissions (and sequestration) of GHGs.

There are four standard RCPs applied in climate modelling. The number included in the name for each RCP refers to the increase in climate forcing in the year 2100, resulting from the level of atmospheric GHGs (in watts per square meter) – a bigger number indicates more climate warming associated with higher concentrations of GHGs.

• RCP2.6 – describes a pathway to a low GHG concentration where it peaks somewhere in the middle of the century and then declines to 420 parts per million (ppm) as GHG emissions are substantially reduced over time with ambitious mitigation efforts. It is aligned with mitigation efforts aiming to limit the increase of global mean temperature to 2°C.
• RCP4.5 – describes a pathway where GHG concentrations stabilise at 540ppm by 2100.
• RCP6.0 – describes a pathway where GHG concentrations stabilise shortly after 2100 at 660ppm by 2100.
• RCP8.5 – represents a future with little reduction in GHG emissions where GHG concentrations reach 940ppm by 2100.

In their design, no RCP is inherently more or less likely than the others and they are not forecasts. However, an RCP may appear to be more or less likely depending on how policy and other parameters evolve in comparison to their assumptions or how actual GHG concentrations track against the modelled trajectories over time.

Queensland Future Climate provides high-resolution CMIP5 projections based on two RCPs - RCP4.5 to represent a moderate emissions future and RCP8.5 to represent a high emissions future.

CMIP6 models represent a new generation of climate models and are highlighted in the most recent Sixth Assessment Report (AR6) from the IPCC (released in sections from 2021 to 2023). In comparison to CMIP5, CMIP6 includes more models based on the latest understanding of ocean and atmospheric processes (such as improved descriptions of cloud processes and biogeochemical cycles). CMIP6 models also feature a wider range of climate sensitivities (the average change in global mean temperature in response to a change in climate forcing). CMIP5 models use a reference period of 1986 to 2005 while CMIP6 models use a reference period of 1995-2014.

CMIP6 models also employ a new way to describe future GHG trajectories using Shared Socioeconomic Pathways (SSPs). The SSPs are based on five different narratives that describe the broad trends that could shape society in the future, including population growth, economic growth, technological development, education and urbanisation. SSPs are intended to span the range of plausible futures from a sustainable future to one of rapid and unconstrained economic growth.

For climate modelling, the SSPs are combined with a range of mitigation targets described using a measure of the resulting increase in ‘climate forcing’ as used for the RCPs. Climate modelling initiatives focus on a set of five combined scenarios.

• SSP1-1.9 – This is the most optimistic of the scenarios where global GHG emissions are cut aggressively to net zero by about 2050. This scenario aligns most closely with the goal of keeping warming below 1.5°C at 2100 (although there may be a bit of ‘overshoot’ where temperatures rise beyond the 1.5°C goal temporarily before dropping below the goal by the end of the century).
• SSP1-2.6 – Under this scenario, global emissions are reduced but don’t reach net zero until after 2050 and warming stabilises at about 1.8°C by 2100.
• SSP2-4.5 – This scenario has emissions steady at roughly the same as current levels before starting to fall in the middle of the century and net zero is not reached by 2100, with temperatures rising by about 2.7°C by the end of the century.
• SSP3-7.0 – Under this scenario, both emissions and temperatures continue to rise, the latter by about 3.6°C by 2100.
• SSP5-8.5 – This high-growth, energy-intensive scenario has emissions roughly doubling by 2050 and global average temperatures rising by about 4.4°C by the end of the century.

The figure below illustrates the projected global mean surface temperature change to 2100 under these five scenarios (relative to the period from 1850 to 1900). Keep in mind that local or regional changes in mean temperature could be different from changes in global mean temperatures, and that climate hazards usually result from extreme events rather than mean values.

The CMIP6 projections for Queensland do not include data for the SSP5-8.5 scenario. Why not? RCP8.5 was considered to be a good representation of a high range emissions scenario for the CMIP5 suite of models that could be used to inform the plausible upper bounds of climate change for risk assessments. Based on more recent data and improved understanding of how the main drivers of future emissions are likely to change over time, there is a common view among climate scientists that the SSP5-8.5 scenario is very unlikely to be realised, largely because of the changing face of energy generation and rapid electrification in other sectors in response to changing economics. As a result, SSP3-7.0 is now considered to be good representation of a high-end emissions scenario within the CMIP6 suite of models (i.e. because emissions higher than described in SSP3-7.0 are now considered very unlikely).

Based on this, the organisations developing high-resolution downscaled climate projections for Australia agreed to concentrate their resources on the same three scenarios that are expected to cover the most likely range of possible future climates. For more information, please refer to the Climate Projections Roadmap for Australia.

CMIP6 models can provide advantages in the description of possible future climates, particularly for more sophisticated analyses where the outputs of climate models are used as inputs for further modelling. Examples include regional downscaling simulations or analyses that focus on implications for particular climate hazards or systems (such as extreme events or ecological modelling etc).

However, for the purposes of climate risk assessments and adaptation planning, CMIP6 models are unlikely to provide any substantial differences in relevant climate parameters that would have a material effect on risk classification or decisions on adaptation options.

Both CMIP5 and CMIP6 projections can continue to be applied in climate risk assessments with confidence.

In most cases, CMIP6 projections will be preferred as they represent the latest available climate projections information, and we recommend that all new climate risk assessments, adaptation plans and policies be developed based on CMIP6 projections.

However, there may be cases where the use of CMIP5 projections is warranted. Examples could include:

• reviewing or updating an existing risk assessment or adaptation plan developed using CMIP5 projections
• for compliance with policy or other requirements that specify particular RCPs should be applied
• where the application requires the consideration of a very high emissions scenario even where this may be considered unlikely to eventuate (e.g. for the consideration of low-likelihood, high impact events on long-lived critical infrastructure).

It might not be a good idea to mix CMIP5 and CMIP6 data in the one analysis. There are some differences in the way variables are presented, particularly with different reference periods, that will make interpretation difficult. A climate risk assessment methodology will help you select the most appropriate scenario to use for your application given your interests, decision lifetime, risk tolerance etc. You can find more information on this in the separate factsheet on climate science for risk assessments.

High-resolution projections data and information based on CMIP6 projections available here.

High-resolution projections data and information based on CMIP5 projections available here.

Climate Projections Roadmap for Australia and the National Partnership for Climate Projections.

This factsheet was developed with support from the NESP Climate Systems Hub.