Explainer: IPCC Scenarios

The third part of the  Intergovernmental Panel on Climate Change’s Sixth Assessment report is here, detailing precisely how we must mitigate climate change.

A lot of climate research relies on the IPCC’s “scenarios” to make predictions – from how much aluminium we’ll need in solar panels, through to the likelihood of the Sydney Opera House being inundated by the rising sea level.

What are these scenarios, how are they calculated, and what do the current ones say? Read on to find out.

Since 1990, the IPCC has outlined various possibilities for the future of Earth’s climate, based on how much more greenhouse gases (GHGs) are emitted.

These possibilities range from the very pessimistic (rapid increase in GHGs), through to the optimistic (sudden decrease, and eventual  negation of GHG emissions through carbon capture and other means). They also take into account anthropogenic aerosols and land use, both of which have a slight cooling effect (but not enough to offset the warming from GHGs).

The expected average temperature rise across the globe is then calculated for each scenario.

These predictions and calculations rest on decades of data collection, modelling, and analysis from thousands of researchers around the world. So far, they’ve been broadly accurate – and improvements in observations, artificial intelligence and big data make them better each year.

The scenarios are regularly updated. The most recent update was released last August, with the first part of the Sixth Assessment Report.

This report updated the five different scenarios which had been used in previous reports, making projections from 2015 to 2100. They have decidedly un-catchy names – Shared Socioeconomic Pathways, or SSPs:

• SSP1-1.9: emissions rapidly decline to net zero by about 2050, and become negative after that
• SSP1-2.6: emissions decline to net zero by about 2075, and become negative after that
• SSP2-4.5: emissions rise slightly, before declining after 2050, but not reaching net zero by 2100
• SSP3-7.0: emissions rise steadily to become double their current amount by 2100
• SSP5-8.5: emissions rise steadily, doubling by 2050 and more than tripling by the end of the century

Each scenario has an associated global temperature rise.

Under SSP1-1.9, the IPCC expects a long-term rise of 1.0°C to 1.8°C on industrial levels by the end of  this century, while the best estimate is a 1.4°C rise. Temperatures would be slightly higher in the middle of the century but would then cool thanks to negative emissions. (Note that the Earth is currently already 1°C hotter than preindustrial levels, so in the best-case scenario, using the most optimistic predictions, the world  won’t be any hotter in 2100 than it is now.) This scenario is the only one compatible with the Paris Agreement on climate change which was ratified at the end of 2015.

Under SSP5-8.5, the worst-case scenario, there is a projected rise of 4.4°C by 2100, with a range from 3.3°C to 5.7°C.

Source: IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. In Press.

The IPCC doesn’t make projections about which of these scenarios  is more likely, but other researchers and modellers can. The Australian Academy of Science, for instance, released a report last year stating that our current emissions trajectory had us headed for a 3°C warmer world, roughly in line with the middle scenario.

Climate Action Tracker predicts 2.5 to 2.9°C of warming based on current policies and action, with pledges and government agreements taking this to 2.1°C.