What is RCP8.5? Unpacking misleading claims about climate scenarios

How do scientists understand what climate change might bring? In short, they ask themselves ‘what if’ questions. What if the world carries on its current path? What if the world makes drastic cuts to its emissions? Or – what happens if the world builds out more and more fossil fuels, causing emissions to rise and rise?

Climate scientists can answer each question by modelling its results. These scenarios range from moderate, complying with the Paris agreement, to extreme climate change. For example, a world where emissions keep on rising is the basis of a high-emissions scenario that climate scientists call RCP8.5. 

Now, climate scientists are updating this high-end RCP8.5 scenario to account for the latest data – and this update has spawned many claims attacking climate science.

Perhaps the loudest voice is U.S. President Donald Trump, who claimed “the United Nations TOP Climate Committee just admitted that its own projections (RCP8.5) were WRONG! WRONG! WRONG!”

Many other websites picked up on the same announcement: claiming that RCP8.5 contained “climate change lies”; that it was “junk science”; or that scrapping it shows climate advocates have exaggerated the risks of climate change.

In this article, we’ll examine what climate scientists have actually said about RCP8.5. First, let’s describe what RCP8.5 is – and what it is not.

RCP8.5 is not a prediction

RCP8.5 (which is sometimes called SSP8.5 or SSP5-8.5) is not a prediction. It’s not like a weather forecast; it’s not a guess at what will happen in our own future. Instead, RCP8.5 is a ‘what if’ scenario.

A scenario like RCP8.5 supposes that the world’s greenhouse gas emissions follow a certain ‘pathway’. Climate scientists can use scenarios like this to model what could happen if the world’s emissions follow that pathway.

When scientists use a scenario, they’re asking ‘what if?’. They’re not projecting that our future will play out like that scenario.

Because future emissions aren’t fixed, climate scientists want to understand what could happen in different possible futures. To do this, they work with multiple ‘what-if’ scenarios. RCP8.5, then, is just one scenario in a set called Responsible Concentration Pathways (RCPs). There are four common RCPs: RCP2.6, RCP4.5, RCP6.0, and RCP8.5^[1]. 

Climate scientists designed the RCPs in the early 2010s. Each RCP lays out a different ‘what if’ pathway for the rest of the 21st century^[1]. Their numbers don’t stand for temperature (they refer to something called radiative forcing) but RCPs with higher numbers do include more global warming^[2] (Figure 1):

• RCP2.6 asks: what if we drastically cut emissions? Emissions drop from the 2020s and reach net-zero before 2100. This complies with the Paris agreement, which pledges to limit global warming to 1.5C since the Industrial Revolution.
• RCP4.5 asks: what if emissions stay flat for decades? It’s a ‘middle-of-the-road’ scenario in which we don’t cut emissions until after 2050, leading to about 2.5C of warming by 2100.
• RCP6.0 asks: what if emissions keep rising until the second half of the century? In this scenario, we only reduce emissions in the late 21st century, allowing about 3C of warming by 2100.
• RCP8.5 asks: what if we don’t cut emissions at all? Instead, the world continues to use more and more fossil fuels, and emissions keep on rising. This scenario sees as much as 5C of warming by century’s end.

As you can see, RCP8.5 is at the high-emission end. It was not intended to be a ‘likely’ scenario^[3]. It’s also not the only scenario to include dramatic climate change – so do more moderate scenarios like RCP4.5 and RCP6.0, as we’ll cover in the next section.

The world’s emissions aren’t following RCP8.5, but climate change is still a serious problem

Although ‘what if’ scenarios can’t be right or wrong, we can still compare them to the emissions that we’ve observed in real life. This lets us understand what scenarios are more likely to play out in our own future.

Climate scientists designed the RCPs in the early 2010s, based on what we knew about the climate and about our emissions at the time. Let’s compare them to emissions since then (Figure 2):

In the early 2010s, the world actually seemed on course for RCP8.5^[4]. We saw a rapid rise in emissions in the 2000s. This was partly thanks to a surge in coal, the most greenhouse-gas-intensive fossil fuel. Low-emissions alternatives like solar and wind were far more expensive then. Transitioning away from fossil fuels did not seem practical.

The world looks different today. Many cities and countries have taken measures to reduce their emissions. We’ve built unprecedented and growing numbers of low-emissions solar panels and wind turbines, which are by some measures now cheaper than fossil fuels. Indeed, our coal use has also plateaued, and there are signs it’s starting to decline.

As a result, the world’s emissions have flattened. They aren’t falling, but they aren’t rapidly rising like RCP8.5 says. Instead, they’re closer to more modest emissions scenarios, such as RCP4.5 (Figure 2). We’re now on course for about 2.5 to 2.9C of warming by 2100.

Because we aren’t seeing RCP8.5’s rapid emissions increases, some climate scientists believe that this high-emissions ‘what-if’ is now less and less plausible with each passing year^[5].

If RCP8.5 is now improbable, what does this tell us? Climate scientists say it does not mean climate change is not serious. Thanks to climate models using scenarios like RCP4.5, we know that even 2.5C of global warming will have grave consequences: a loss of agricultural land, more heat waves, more severe weather, damages to coral reefs and other ecosystems, and sea level rise^[6].

In fact, because the world’s emissions have flattened but not fallen, the other end of the RCP range – RCP2.6, which imagines dramatic cuts to emissions in the 2020s – is now just as unlikely.

Instead, it tells us that the world’s choices can change our emissions pathway. Again, the RCPs are not predictions.

Claims about RCP8.5 miss the mark

With all that in mind, what exactly has happened to RCP8.5, and what do claims like Trump’s get wrong?

For one, Trump’s claim that RCP8.5 is a product of ‘the United Nations top climate committee’ is incorrect. The United Nations’s Intergovernmental Panel on Climate Change (IPCC) doesn’t create these scenarios. Several other websites have also falsely stated that the IPCC was responsible for RCP8.5.

Instead, the RCPs are made by a separate group called the Coupled Model Intercomparison Project (CMIP). Their work does help the climate scientists who compile the IPCC’s reports, but the IPCC has no involvement in creating CMIP’s scenarios.

CMIP first created the RCPs in the early 2010s, but as we’ve seen, the world has changed since then. It’s a natural part of science to look at new information, and CMIP’s scenarios are no exception. Therefore, earlier this year, CMIP proposed a new set of scenarios^[7].

CMIP also didn’t ‘admit that its projections were wrong’. For one, as we’ve seen, RCP8.5 is not a prediction. It’s a ‘what if’ scenario, and it doesn’t make sense to call it ‘wrong’ or ‘right’.

Instead, CMIP updated RCP8.5, or SSP8.5, into a more modest high-end scenario that’s more likely to happen in our own future^[7]. Instead of assuming that emissions would continue to rise unchecked, this new high-end scenario assumes that countries will roll back their climate pledges and return to fossil fuels. In this scenario, emissions start to rise again from our current plateau, but they’re still lower than under the old RCP8.5.

According to climate scientists, simply because the world is tracking a less severe scenario than RCP8.5 does not mean climate change is not a threat to ecosystems and human livelihoods. In fact, at the same time, CMIP also updated its low-end scenarios to account for the fact that the world hasn’t reduced emissions^[7].

Conclusion

It’s misleading to call RCP8.5 a ‘lie’, as many have done, or to use its update to say that climate scientists are ‘exaggerating’. RCP8.5 isn’t a prediction, and it was never meant to be. It was a scenario that helped climate scientists understand what might happen if greenhouse gas emissions kept on rising.

According to climate scientists, the update to RCP8.5 tells us instead that the world’s emissions pathway isn’t fixed. Greenhouse gas emissions didn’t keep on rising. While they aren’t falling to comply with the Paris agreement, they have flattened, thanks to human actions.

References

• 1 – Van Vuuren et al. (2011) The representative concentration pathways: an overview. Climatic Change.
• 2 – IPCC (2013) Climate Change 2013: The Physical Science Basis. Fifth Assessment Report of the Intergovernmental Panel on Climate Change.
• 3 – Riahi et al. (2011) RCP 8.5 – A scenario of comparatively high greenhouse gas emissions. Climatic Change.
• 4 – Peters et al. (2012) The challenge to keep global warming below 2°C. Nature Climate Change.
• 5 – Hausfather and Peters (2020) Emissions – the ‘business as usual’ story is misleading. Nature.
• 6 – IPCC (2023) Climate Change 2023: Synthesis Report. Sixth Assessment Report of the Intergovernmental Panel on Climate Change.
• 7 – Van Vuuren et al. (2026) The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7). Geoscientific Model Development.