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RCP 8.5: Why did the climate change model get it wrong?
00:08:57
How an important climate model was too pessimistic about the future
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Transcript29 lines
- [00:00]
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- [00:36]
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- [01:09]
Tim Harford
Hello and thanks for downloading the More or Less podcast with a program that looks at the numbers in the news, in life and in statistical models predicting climate catastrophe, I'm Tim Harford. Whether we like it or not, global warming is happening. The global temperature has already gone up and it's going to go up more because the atmosphere is already full of carbon dioxide and other greenhouse gases and we're continuing to add to that stock. Quite how much it will increase by is a very important question for all of us. Until relatively recently, during much of the 2010s and into the 2020s, many scientists claimed that if we just kept on with business as usual, then by the end of the century global temperatures would increase by nearly 5 degrees centigrade. That, in case you're wondering, is a big number since pre industrial times. So far they're up by a bit over 1 degree. This projection was based on something called RCP 8.5, a statistical scenario used by scientists to model the future of the climate. You can still find scientific papers published in 2025 that make the same claim. However, there's a good case that RCP 8.5 should never have been used as a business as usual scenario. And in hindsight it doesn't look like an accurate vision of the future at all. So what's going on?
- [02:40]
Dr. Zeke Hausfather
So I am Dr. Zeke Hausfather. I'm a climate scientist and the climate research lead at Stripe, which is A financial technology company. Back in 2020, I published a piece in Nature, the world's sort of foremost scientific journal, along with a researcher in Norway named Glenn Peters. And in it, we essentially argued that the climate science community had been making a mistake for the last decade, and that mistake is that they had been conflating the worst case scenario with the most likely outcome.
- [03:06]
Tim Harford
Climate modeling is an important part of figuring out climate change. But in order to work out how much the temperature is going to rise in the coming decades, you need to know how much greenhouse gas we're going to emit, how much coal and oil will burn, and how many cows will burp methane, and so on. This is not an easy question.
- [03:25]
Dr. Zeke Hausfather
It depends on a huge amount of factors, on how much population grows, on how wealthy we get, on how fast different technologies develop and change in price. And so climate scientists and energy system modelers have put together scenarios where they essentially say, here's a number of different outcomes that could happen this century in terms of our emissions, and then those outcomes. Those scenarios are used by climate modelers to run climate model simulations.
- [03:50]
Tim Harford
RCP 8.5 was one of those scenarios.
- [03:54]
Dr. Zeke Hausfather
RCP stands for representative Concentration pathway, essentially an illustrative example of the concentration of CO2 in the atmosphere and other greenhouse gases that might occur by the end of the century. And 8.5 is, in technical jargon, the watts per meter squared radiative forcing. But really it's just a measure of how much heat is being trapped by greenhouse gases by the end of the century.
- [04:18]
Tim Harford
Our story starts at the turn of the 2000 and tens in the run up to a key international climate report from the ipcc, the Intergovernmental Panel on Climate Change. There was a big effort to upgrade the existing scenarios with some better models. There were all kinds of RCPs imagining different levels of international action on climate change.
- [04:39]
Dr. Zeke Hausfather
You know, RCP5 in particular. The paper that published the scenario in 2011 described it as the 90th percentile of no policy baseline outcomes, essentially the 9 out of 10 worst outcomes in a world where we don't care about climate change.
- [04:54]
Tim Harford
In this scenario, the model predicted the world might be roughly 4.5 degrees warmer by the end of the century, although with some big uncertainties on either side. This was a scenario which the modellers thought was at the extreme edge of what was possible if the world did not nothing to mitigate climate change.
- [05:13]
Dr. Zeke Hausfather
It had some pretty extreme assumptions, like it had the world burning five times more coal by the end of the century than they were burning in 2010, you know, it had global emissions tripling.
- [05:23]
Tim Harford
In one of the imaginary realities that leads to this outcome. Oil becomes scarce and there's a widespread use of coal as a fuel for cars.
- [05:32]
Dr. Zeke Hausfather
You can actually turn coal into what we call syn fuel, essentially synthetic petrol. South Africa actually did this at pretty large scales during the apartheid era.
- [05:42]
Tim Harford
Indeed, some analysts think the model predicted we'd burn more coal than it's realistic to think could be extracted from the ground. But the real problem isn't that the researchers who created RCP 8.5 were transparent that it was a worst case scenario if the world didn't act on climate change.
- [06:01]
Dr. Zeke Hausfather
That information was a little lost in translation between the energy system modelers and the broader climate science community. And so for a decade or so after that, there was increasing use of this high end scenario. RCP5 as business as usual is the term a lot of people refer to it as, and it was never intended to be business as usual. It was sort of the worst case of the universe of business as usual outcomes, but not necessarily the most likely outcome.
- [06:28]
Tim Harford
In major reports from the UN and IPCC down, RCP 8.5 was used as the business as usual case to contrast with what would happen if the world acted to cut climate emissions. This was where we were probably heading if we did nothing. And Zeke says there was arguably some justification for this interpretation early on.
- [06:49]
Dr. Zeke Hausfather
You know, global emissions that increased by a third in just the prior decade between 2000 and 2010. You know, China was building two new coal plants a day. You know, the idea that the 21st century could be dominated by coal didn't seem unrealistic. There was also a number of papers published at the time that argued that actual global emissions were tracking at or above RCP 8.5 levels, which was actually true through almost 2015 or so.
- [07:15]
Tim Harford
However, RCP 8.5 did not age well.
- [07:19]
Dr. Zeke Hausfather
Obviously today, when solar is the cheapest form of new energy in the world, and almost all energy, or electricity at least, being added globally, is coming from renewables, it does seem crazy, but that's a very new phenomenon. And even the proponents of clean energy technology 15 years ago did not expect things to get as cheap as they have as quickly as they have.
- [07:42]
Tim Harford
Instead of shooting up, coal use has plateaued, renewables have boomed, and a world where we run cars on coal seems like a strange and alarmist fairy tale. RCP 8.5 was always meant to describe an unlikely sequence of events, but I.
- [08:00]
Dr. Zeke Hausfather
Think it has moved in the last 15 years from the worst case scenario to an implausible or I would even say impossible scenario. Given the progress we've made, it no.
- [08:10]
Tim Harford
Longer makes sense to talk about business as usual as a world where we don't act on climate change at all and don't invest in renewable energy. While recent talks in Brazil showed that the world is not in full agreement yet, if we just carry on at our current trajectory, we're not going to hit anything like that level of fossil fuel use.
- [08:31]
Dr. Zeke Hausfather
It's hard to predict the future, and any prediction I make will probably be wrong. But based on the estimates we have today of where the world is likely headed, if we don't enact stronger climate policies than we have right now, we'll probably have emissions be flat for the next few decades and then falling gradually by the end of the century. As just technology gets cheaper, fossil fuels potentially get more expensive in that world. We end up somewhere between two and a half and three degrees warming. And a world of, you know, close to three degrees warming also has a lot of really severe impacts, particularly on natural systems and in places that, you know, don't have the adaptive capacity that we have in rich countries.
- [09:13]
Tim Harford
Since Zeke and others pointed out the problems with RCP 8.5, he says the big climate organizations have largely stopped using it to talk about business as usual. But the Ghost of RCP 8.5 is still haunting the discussion. It still pops up in the occasional graph or calculation and appears sporadically in scientific papers. But Zeke is confident that serious climate scientists have largely left it behind.
- [09:41]
Dr. Zeke Hausfather
I see this as the natural way that science advances. It's a messy process, it is often slower than we'd like to, but it is ultimately self correcting.
- [09:51]
Tim Harford
Thanks to Zeke Hausfather. And that's it for this week. But do get in touch if you've seen a number you think we should take a look at. Our email address is more or lessbc.co.uk until next week. Goodbye.
- [10:10]
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