How to love uncertainty in climate science

This is the script of my TEDxCERN talk, a 12-13 minute talk I did from memory. When the video is put online in a week or so, you’ll be able to follow along and see where I fluffed it improvised. A shorter version appeared on Vice News under the headline “There Is Some Uncertainty in Climate Science — And That’s a Good Thing”.

I used to be a particle physicist. Sadly, I left before it became cool to be a particle physicist.

Here’s one of the collisions I observed for my PhD at Fermilab:

And in that previous life, the stringent criteria for being “certain” about a new discovery, like the Higgs boson that made headlines at CERN, is the 5 sigma confidence level.

Here’s the famous “bell curve”, with the sigma levels shown along the bottom:

You can see that 5 sigma is way out in the tails, with a very, very low probability of occurring by chance. It means we think there is only a one in 3.5 million chance that the signal could have been seen if there no Higgs.

Now that I’m a climate scientist, I dream of such certainty! We’re studying an enormously complex planet. I’m going to talk about some of the reasons there’s uncertainty in climate science, some of the problems that’s causing between science and society, and what I think we can do about it.

I’ll start with some things we’re certain about. The earth’s energy budget is out of balance: there’s more energy going in than coming out, so the planet is storing it up. That’s not unusual in itself, only that we are helping tip the scales. The extra energy means the atmosphere and the surface of the ocean have warmed, making the hottest days warmer and more frequent, and the coldest days less frequent.

As the oceans heat up they expand, and ice on land – in glaciers, and the Greenland and Antarctic ice sheets – has also been melting into the oceans, and breaking off in ice bergs, faster than it has been replaced by new snow. So global average sea level has risen. We’re confident our activities have been the dominant cause of warming since the middle of the last century.

How about the future? We predict more of the same. We’re confident the world will get warmer, shifting the hottest and coldest days further, and that rainfall will become heavier in some places, such as the wet tropical regions. Global average sea level will continue rising, making the extreme highs in sea level higher and more frequent.

But it’s not only climate scientists that are certain. Not everyone knows this, but more and more climate sceptics agree with us too. Yes, there are people who don’t believe CO2 is a greenhouse gas, and likely never will. But in my experience many sceptics in the blogosphere, media and politics absolutely agree we are having an effect on climate. They question only the details, such as how fast that warming will be, or how we should reduce the risks.

How do we know what we’re talking about? The big picture predictions come from our observations of the planet and our fundamental physical understanding, some of which is 200 years old. But the details – exactly how fast temperatures and sea levels will rise, and which parts of the world will experience heavier rainfall – must come from computer models.

Here’s a map of the world in a climate model:

The model’s about 15 years old, but it’s still used. You can see the world has been simplified: it looks blocky, almost like a very early digital camera.

We need to use computer models because we don’t have a miniature earth to play with. It’s not only climate science with this difficulty. If you want to study, say, the evolution of galaxies, it’s a bit easier to write computer code than to create a hundred million stars… At the heart of climate models are basic laws of physics, like Newton’s laws of motion, and over time we’ve added more and more physics, chemistry, biology and geology.

But a model can never be perfect: it is by definition a simplified representation of reality. There’s a great saying by this statistician George Box, who sadly died last year: “All models are wrong, but some are useful”. I think this is so important I named my blog after it.

Not only are all models simplified, but their predictions partly depend on the numbers you plug in. And we can’t always know what those numbers should be: say, if they’re hard to measure in the real world. So there’s uncertainty because of the simplifications and unknown inputs of our computer models.

A second reason is that the very definition of climate has uncertainty at its heart. People often think of weather and climate as the same thing, but they’re not. Weather is the state of the atmosphere: the temperatures, rainfall and pressures we can measure with instruments. Climate is different. We can think of climate as “the probability of different types of weather occurring”.

The fact that climate is a statement of probability means two important things. First, that climate is inherently uncertain. A probability is a statement of uncertainty. “We predict the weather will mostly be X, sometimes Y and occasionally Z”.

Second, it means that climate is a long-term thing. That’s because to estimate a probability you need a lot of data. If you were flipping a coin to see if it’s fair, 50:50 heads or tails, you’d have to do it a lot of times before you could be sure. In the same way we need around 30 years or more of weather records to get just one data point of climate. So that fact that climate is a probability means that it’s uncertain and that we need a lot of data to test our models.

My research focuses on both sources of uncertainty I’ve mentioned: the limitations of computer models, and testing their predictions.

Uncertainty doesn’t mean we don’t know anything. In fact I’d argue that uncertainty is the engine of science, because it drives our search to understand the universe. But misunderstanding and misrepresentation of uncertainty is damaging the relationship between climate science, the media and society, because climate science is both complex and highly politicised.

The first problem uncertainty brings is the extra difficulty for the expert in explaining their results, and the non-expert in understanding them. For example, over the past 17 years or so there has been a slowdown, even a pause, in the rate of warming of the atmosphere. We’re confident the climate is still changing, because the ocean is still warming, the land losing ice, sea level rising, and we predict the atmosphere will start to warm again after this temporary blip.

We think there are several contributing factors to this pause, including a change in the movement of heat around the planet, a dip in the brightness of the sun, reflection of the sun by pollution and volcanic eruptions. But because we need to use computer models to understand it, and because 17 years is not that long when it comes to climate, we don’t know the exact contributions of each. Clearly this is not simple, sound bite science.

The second problem is that scientists in any area of cutting edge research will disagree with each other. If the media or public don’t expect that it can cause confusion, and, worse still, because climate science is politicised, these disagreements are often sold as proof of “unreliable science”, an argument to ignore scientists until it’s all “sorted out”.

For example, some scientists predict global average sea level rise under the highest greenhouse gas emissions scenario will likely be 20 to 30 inches by the end of the century. Others predict it will very likely be 3 to 5 feet, or possibly over 6 feet. That’s a big difference! The reason is that the two groups look at the problem in quite different ways – the first use methods based in physics, the second statistics. That’s an interesting story to tell, because we don’t yet know the best approach.

We might like to think of science as a neat, orderly book of facts, but really it’s like searching for the right path in a fog. It takes time to find out which was the right one.

The third problem is that scientific uncertainty allows people to spin our results. We had a press conference for project I worked in called ice2sea, which made predictions of global average sea level rise using the physics-based methods. Some journalists reported our results as “Sea level rise to be less severe than feared” because they compared us with those higher, statistical studies. Other journalists reported the same press conference as “Risk from rising sea levels worse than feared”, because they chose to compare us to the previous report of the Intergovernmental Panel on Climate Change which, like us, used the physics methods, but didn’t tally every possible part of sea level rise. One website chose to go with “The End of London as we know it…”.

It’s no wonder the public are confused. Every media outlet tells the story it wants to tell.

But we as scientists haven’t always helped. We haven’t always sold the idea of uncertainty as not only inevitable but even exciting, and we’ve sometimes over-simplified our communication. That pause in warming of the atmosphere surprised the media and public, even though scientists always expected this kind of thing could happen in the short term. That’s because we focused too much on talking about the long-term average predictions, which smooth out the year-to-year changes.

We’ve also done a bad job at being available. How many climate scientists can you name? Where do you get your climate science from: interviews with scientists? More likely the media, politicians, and activists, whether environmental or sceptical. We’ve mostly kept our head below the parapet, for fear of attracting fire in communicating complex science in a politicised atmosphere. There are certainly days I hide away. But we need to be braver.

Things are changing. There are more of us online than ever, giving interviews and talks, and trying to explain the nuances of the science.

But we can do better. My colleagues Ed Hawkins, Doug McNeall and I wrote a journal article about the slowdown in atmospheric warming called “Pause for thought”, which we’re proud to say was the first journal article to use Twitter handles for the author contact information. We called on our colleagues to join us online and in the media, because the more that do, the easier it gets: the less we have to speak outside our comfort zone, and the more we can support each other.

I’d especially love more female scientists to get out there. In our society, men are often rewarded for being competitive. I like to think if there were more women involved, it might help naturally move things from a climate debate to a climate conversation.

And for that conversation I’d like to invite you, to the public, to come and find us. There are now hundreds of climate scientists on Twitter, and the small number of us that blog is growing. But we’re mostly engaging with those who are already passionate, the environmentalists and dissenters. We’d like to talk to more in the middle ground: the fence-sitters, the understandably confused.

So I’m curating a Twitter list of climate scientists: a directory of active researchers – physical scientists, computer scientists and statisticians – who are studying climate change and its impacts. You can find it from my Twitter profile, flimsin.

So far I’ve added 250 climate scientists. If you’re a climate scientist, or know one, tweet me to add to the list. And if you’d like to ask a climate scientist a question – to discuss a news article, or explain their results – then just read through the biographies and find some scientists to ask.

I hope this list will grow, and start conversations that help us deal better with uncertainty in climate science – perhaps even with the messy business of science itself. So if you’re confused about climate … puzzled about the pause … surprised about sea level … or just uncertain about uncertainty … please come and find us. We’d love to talk.

With enormous thanks to TED coach Michael Weitz and Head of TEDxCERN Claudia Marcelloni De Oliveira for helping me make this talk more accessible and clear. Thanks to Vice News for suggestions to improve the readability of my article, and to Jonty Rougier, Ed Hawkins and Doug McNeall for useful comments and encouragement.