(2028-04-19) Carbon Brief Explainer How Shared Socioeconomic Pathways Explore Future Climate Change

Carbon Brief Explainer: How ‘Shared Socioeconomic Pathways’ explore future climate change. They show that it would be much easier to mitigate and adapt to climate change in some versions of the future than in others. They suggest, for example, that a future with “resurgent nationalism” and a fragmentation of the international order could make the “well below 2C” Paris target impossible.

The SSPs were initially published in 2016, but are only now just starting to be used in the next round of climate modelling – known as the Coupled Model Intercomparison Project version 6, or CMIP6 – in preparation for the IPCC’s sixth assessment report.

Global population (left) in billions and global gross domestic product (right) in trillion US dollars on a purchasing power parity (PPP) basis.

All SSPs project dramatic growth in the global economy, with global GDP in 2100 between four and 10 times larger than it was in 2010

This growth is one of the primary drivers of future CO2 emissions

Energy use in the SSP baselines

While the SSP baseline scenarios all represent worlds without new policies to address climate change, they differ significantly in how they see global energy use changing. Wait, are all 5 "baseline"?

Combining SSPs and mitigation targets

While the baseline SSP scenarios portray a range of outcomes in the absence of additional climate policy, researchers also wanted to examine how different levels of climate mitigation and adaptation would fit into the future described by each SSP.

With the release of the SSPs, modellers have expanded the range of mitigation targets that they are considering. The IPCC fifth assessment report focused on RCP2.6, RCP4.5, RCP6.0 and a high-end no-mitigation RCP8.5 pathway. The SSPs have added RCP1.9, RCP3.4 and are planning to add RCP7.0.

The combination of five SSPs and six RCPs is shown in the figure below. (RCP7.0 is not shown as the runs are not yet complete.)

Combination of SSP and RCP model runs in the SSP database, with RCPs listed in order of increasing mitigation and SSPs in the (rough) order of increasing mitigation difficulty. Ratios in cells indicate the number of models that succeeded in making the scenario “work” out of the total number of models available for the SSP.

Each box in the figure shows the number of models that were able to successfully reach the RCP target, out of the total number of models available for a given SSP

To ascertain whether the underlying socioeconomic factors in an SSP allow for the level of mitigation necessary to meet RCP targets, models used shared policy assumptions about limits to international cooperation

For example, SSP1 and SSP4 see it as possible for there to be “global collaboration” on climate policies by the year 2020. The more fossil fuel-driven SSP2 and SSP5 worlds have delays in establishing global action

For land use, which is an important and difficult-to-regulate source of emissions, SSP1 and SSP5 allow effective international cooperation to reduce emissions. SSP2 and SSP4 allow some more limited efforts to reduce emissions from deforestation and agriculture, while SSP3 generally assumes it will not be possible to encourage individual countries to avoid deforestation.

The differences between SSPs affect the ability of scenarios to have large near-term mitigation of greenhouse gas emissions. While SSP1 and SSP4 allow for quick global action in reducing emissions beyond those already agreed to in the nationally determined contributions (NDCs) under the Paris Agreement, other scenarios, such as SSP3 and SSP5, find that even these existing commitments are challenging to achieve in full.

While the rapid technological development in SSP4 makes it easier to attain more modest mitigation targets, the high inequality makes it more difficult to attain very strong emission reductions, particularly for land-use emissions in poorer countries.

Negative emissions in the SSPs

All scenarios in the SSP database that keep warming below 2C incorporate some bioenergy with carbon capture and storage (BECCS). However, the degree to which they rely on BECCS – or on negative emissions more broadly – to meet the goal varies both by model and SSP.

No single ‘business as usual’

One important takeaway is a shift in the definition of “business as usual”. Instead of a single worst-case scenario, the SSPs present a wide range of future emissions possible in the absence of climate policy

If any SSP can be said to be characteristic of current conditions it is SSP2, where social, economic and technological trends do not shift markedly from historical patterns.