Category: glossary

Many dimensions to life and science

This post is timed to coincide with a meeting tomorrow, the Royal Meteorological Society’s “Communicating Climate Science”. If you are going, do come and say hello. If you aren’t, look out for me tweeting about it from 2-5.30pm BST.

On not blogging

I haven’t forgotten about you. I’ve still been churning over ideas and wanting to share them with you. I’ve thought of all of you that comment here, and those that silently lurk, whether friends, family, scientists, sceptics, passers-by, or a combination of these. But two big things this year have had to take priority over blogging (and the even more time-consuming process of moderating and replying to comments).

The first was a deadline. As some of you know well, the Intergovernmental Panel on Climate Change (IPCC) produces a report summarising the state-of-the-art in climate science research, and related topics, about every six years. They do this so policymakers have a handy (in practice, enormous and not very handy) reference to the evidence base and latest predictions. The IPCC set cut-off dates for including new research: one date for submission to journals, and another for acceptance after the peer-review process. The first of these dates was the 31st July this year. Translation: “try to finish and write up every piece of work you’ve ever started by this date”. Not every climate scientist chose to do this. But the project I work for, ice2sea, actually had it written into a contract with its funders, the European Union. We had no choice but to submit whatever was our current state-of-the-art in sea level predictions. I was a co-author of six papers* finished and submitted during June and July, and had several other studies on the go that didn’t make the deadline. So it was a rather intense time, and science had to take priority over talking about science.

The second was personal. I hesitated about whether to say this here. But part of my motivation for being a climate scientist in the public eye was to show the human side. And I also wanted to let you know that this blog is so important to me, has been so transformative, that it took something very big to keep me away. My husband and I separated two months ago.

I’m back, and I’m preparing for a big move. The US-based publisher and organisation PLoS (Public Library of Science) has invited me to be their climate blogger. It’s a fantastic opportunity to gain a big audience (more than 200,000 visitors per month, and a feed to Google News). I’m very happy to support PLoS because they publish open access journals, and because one of these (PLoS ONE) goes even further in its commitment to transparency in science. It will publish anything scientifically valid, whether or not it is novel. This might not sound important, or even a good idea, but it is an essential counter to the modern problem that plagues journals: that of only publishing new results, and not repeat studies. For the scientific method to work, we need studies that repeat and reproduce (or contradict) previous research. Otherwise we risk errors, chance findings, and very occasionally fraud, remaining unnoticed for years, or forever. I’m hosted at PLoS from the second week in December and will be posting twice a month.

The first post at PLoS will be a (long overdue) introduction to predicting climate change. It will probably be based around a talk I gave at the St Paul’s Way summer science school, at which I was the final speaker, which made Prof Brian Cox my warm-up act.

In other news, I talked about the jet stream and climate change live on BBC Wiltshire (9 mins), which was well received at the climate sceptic site Bishop Hill, and did a live Bristol radio show, Love and Science (1 hour). I also returned to my particle physics roots, with a Radio 4 interview about the discovery of the Higgs Boson (3 mins).

Our new(-ish) paper

Now the science bit. This is an advertisement for a paper we published in August:

Stephens E.M., Edwards T.L. and Demeritt D. (2012). Communicating probabilistic information from climate model ensembles—lessons from numerical weather prediction. WIREs Clim Change 2012, 3: 409-426.

It’s paywalled, but I can send a copy to individuals if they request it. Liz Stephens is a colleague and friend from my department at Bristol that did a great study with the UK Met Office and David Spiegelhalter on the interpretation of probability-based weather forecasts, using an online game about an ice cream man. I’ve never met David Demeritt, except in one or two Skype video calls. He’s interested in, amongst other things, how people interpret flood forecasts. I haven’t passed this post by them, but hopefully they will comment below if they have things to add or correct.

We noticed there was quite a bit of research on how well people understand and make decisions using weather forecasts, such as the probability of rainfall, and uncertainty in hurricane location, but not much on the equivalents in climate change. There have been quite a few papers, particularly in the run-up to the new IPCC report, that talk in general terms about how people typically interpret probability, uncertainty and risk, and about some of the pitfalls to avoid when presenting this information. But very few actual studies on how people interpret and make decisions from climate change predictions specifically. We thought we’d point this out, and draw some comparisons with other research areas, including forecasting of hurricanes, rain, and flooding.

Ensembles

The ‘ensembles’ in the title are a key part of predicting climate and weather. An ensemble is a group, a sample of different possibilities. Weather forecasts have been made with ensembles for many years, to help deal with the problem of our chaotic atmosphere. The most well-known explanation of chaos is the ‘butterfly effect’. If a butterfly stamps its foot in Brazil, could it cause a tornado in Illinois? Chaos means: small changes can have a big effect. A tiny change in today’s weather could lead to completely different weather next week. And in the same way, a tiny error in our measurements of today’s weather could lead to a completely different forecast of the weather next week. But errors and missing measurements are inevitable. So we try to account for chaotic uncertainty by making forecasts based on several slightly different variations on today’s weather. This is one type of ‘ensemble forecast’. It’s simply a way of dealing with uncertainty. Instead of one prediction, we make many. We hope that the ensemble covers the range of possibilities. Even better, we hope that the most common prediction in the ensemble (say, 70% of them predict a storm) is actually the most likely thing to happen. This gives us an estimate of the probability of different types of weather in the future.

Ensembles are at the heart of our attempts to describe how sure we are about our predictions. They are used to explore an uncertain future: what are the bounds of possibility? What is plausible, and what is implausible? Some climate prediction ensembles, like the weather forecast ensemble above, relate to the information we feed into the model. Others relate to imperfections in the models themselves. Some specific examples are in the footnotes below.**

The question we ask in our paper is: how should we express these big, complex ensemble predictions? There are too many dimensions to this problem to fit on a page or screen. Our world is three dimensional. Add in time, and it becomes four. There are very many aspects of climate to consider, such as air temperature, rainfall, air pressure, wind speed, cloud cover, and ocean temperature. We might have a prediction for each plausible input value, and a prediction for each plausible variation of the model itself. And one of these ensembles is produced for each of the different climate models around the world. Frankly, ensembles are TMI***.

To simplify or not to simplify

Scientists often think that the more information they can give, the better. So they dump all the raw ensemble predictions on the page. It’s a natural instinct: it feels transparent, honest, allows people to draw their own conclusions. The problem is, people are a diverse bunch. Even within climate science, they have different knowledge and experience, which affects their interpretation of the raw data. When you broaden the audience to other scientists, to policymakers, businesses, the general public, you run the risk of generating as many conclusions as there are people. Worse still, some can be overwhelmed by a multitude of predictions and ask “Which one should I believe?”

To avoid these problems, then, it seems the expert should interpret the ensemble of predictions and give them in a simplified form. This is the case in weather forecasting, where a meteorologist looks at an ensemble forecast and translates it based on their past experience. It works well because their interpretations are constantly tested against reality. If a weather forecaster keeps getting it wrong, they’ll be told about it every few hours.

This doesn’t work in climate science. Climate is long-term, a trend over many years, so we can’t keep testing the predictions. If we simplify climate ensembles too much, we risk hiding the extent of our uncertainty.

Our conclusions can be summed up by two sentences:

a) It is difficult to represent the vast quantities of information from climate ensembles in ways that are both useful and accurate.

b) Hardly anyone has done research into what works.

We came up with a diagram to show the different directions in which we’re pulled when putting multi-dimensional ensemble predictions down on paper. These directions are:

  1. “richness”: how much information we give from the predictions, i.e. whether we simplify or summarise them. For example, we could show a histogram of all results from the ensemble, or we could show just the maximum and minimum.
  2. “saliency”****: how easy it is to interpret and use the predictions, for a particular target audience. Obviously we always want this to be high, but it doesn’t necessarily happen.
  3. “robustness”: how much information we give about the limitations of the ensemble. For example, we can list all the uncertainties that aren’t accounted for. We can show maps in their original pixellated (low resolution) form, like the two maps shown below, rather than a more ‘realistic-looking’ smoothed version, like these examples.

Here’s the diagram:

The three ‘dimensions’ are connected with each other, and often in conflict. Where you end up in the diagram depends on the target audience, and the nature of the ensemble itself. Some users might want, or think they want, more information (richness and robustness) but this might overwhelm or confuse them (saliency). On the other hand, climate modellers might reduce the amount of information to give a simpler representation, hoping to improve understanding, but this might not accurately reflect the limitations of the prediction.

In some cases it is clear how to strike a balance. I think it’s important to show the true nature of climate model output (blocky rather than smoothed maps), even if they are slightly harder to interpret (you have to squint to see the overall patterns). Otherwise we run the risk of forgetting that – cough – all models are wrong.

But in other cases it’s more difficult. Giving a map for every individual prediction in the ensemble, like this IPCC multi-model example, shows the extent of the uncertainty. But if this is hundreds or thousands of maps, is this still useful? Here we have to make a compromise: show the average map, and show the uncertainty in other ways. The IPCC deals with this by “stippling” maps in areas where the ensemble predictions are most similar; perhaps the unstippled areas still look quite certain to the hasty or untrained eye. I like the suggestion of Neil Kaye, fading out the areas where the ensemble predictions disagree (examples of both below).


This brings us to the second point of our conclusions. The challenge is to find the right balance between these three dimensions: to understand how the amount of information given, including the limitations of the ensemble, affects the usefulness for various audiences. Do people interpret raw ensemble predictions differently to simplified versions of the same data? Do full ensemble predictions confuse people? Do simplifications lead to overconfidence?

There is very little research on what works. In forecasting rainfall probabilities and hurricanes, there have been specific studies to gather evidence, like workshops to find out how different audiences make decisions when given different representations of uncertainty. People have published recommendations for how to represent climate predictions, but these are based on general findings from social and decision sciences. We need new studies that focus specifically on climate. These might need to be different to those in weather-related areas for two reasons. First, people are given weather forecasts every day and interpret them based on their past experiences. But they are rarely given climate predictions, and have no experience of their successes and failures because climate is so long-term. Second, people’s interpretation of uncertain predictions may be affected by the politicisation of the science.

To sum up: we can learn useful lessons from weather forecasting about the possible options for showing multi-dimensional ensembles on the page, and about ways to measure what works. But the long-term nature of climate creates extra difficulties in representing predictions, just as it does in making them.

 

* Papers submitted for the IPCC Fifth Assessment Report deadline:

  • Ritz, C., Durand, G., Edwards, T.L., Payne, A.J., Peyaud, V. and Hindmarsh, R.C.A. Bimodal probability of the dynamic contribution of Antarctica to future sea level. Submitted to Nature.
  • Shannon, S.R., A.J. Payne, I.D. Bartholomew, M.R. van den Broeke, T.L. Edwards, X. Fettweis, O. Gagliardini, F. Gillet-Chaulet, H. Goelzer, M. Hoffman, P. Huybrechts, D. Mair, P. Nienow, M. Perego, S.F. Price, C.J.P.P Smeets, A.J. Sole, R.S.W. van de Wal and T. Zwinger. Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea level rise. Submitted to PNAS.
  • Goelzer, H., P. Huybrechts, J.J. Fürst, M.L. Andersen, T.L. Edwards, X. Fettweis, F.M. Nick, A.J. Payne and S. Shannon. Sensitivity of Greenland ice sheet projections to model formulations. Submitted to Journal of Glaciology.
  • Nick, F.M., Vieli, A., Andersen, M.L., Joughin, I., Payne, A.J., Edwards, T.L., Pattyn, F. and Roderik van de Wal. Future sea-level rise from Greenland’s major outlet glaciers in a warming climate. Submitted to Nature.
  • Payne, A.J., S.L. Cornford, D.F. Martin, C. Agosta, M.R. van den Broeke, T.L. Edwards, R.M. Gladstone, H.H. Hellmer, G. Krinner, A.M. Le Brocq, S.M. Ligtenberg, W.H. Lipscomb, E.G. Ng, S.R. Shannon , R. Timmerman and D.G. Vaughan. Impact of uncertainty in climate forcing on projections of the West Antarctic ice sheet over the 21st and 22nd centuries. Submitted to Earth and Planetary Science Letters.
  • Barrand, N.E., R.C.A. Hindmarsh, R.J. Arthern, C.R. Williams, J. Mouginot, B. Scheuchl, E. Rignot, S. R.M. Ligtenberg, M, R. van den Broeke, T. L. Edwards, A.J. Cook, and S. B. Simonsen. Computing the volume response of the Antarctic Peninsula ice sheet to warming scenarios to 2200. Submitted to Journal of Glaciology.

** Some types of ensemble are:

  1. ‘initial conditions’: slightly different versions of today’s weather, as in the weather forecasting example above
  2. ‘scenarios’: different possible future storylines, e.g. of greenhouse gas emissions
  3. ‘parameters’: different values for the control dials of the climate model, which affect the behaviour of things we can’t include as specific physical laws
  4. ‘multi-model’: different climate models from the different universities and meteorological institutes around the world

*** Too Much Information

**** Yes, we did reinvent a word, a bit. 

Limitless possibilities

Mark Maslin and Patrick Austin at University College London have just had a comment published in Nature called “Climate models at their limit?”. This builds on the emerging evidence that the latest, greatest climate predictions, which will be summarised in the next assessment report of the Intergovernmental Panel on Climate Change (IPCC AR5, 2013) are not going to tell us anything too different from the last report (AR4, 2007) and in fact may have larger uncertainty ranges.

I’d like to discuss some of the climate modelling issues they cover. I agree with much of what they say, but not all…

1. Models are always wrong

Why do models have a limited capability to predict the future? First of all, they are not reality….models cannot capture all the factors involved in a natural system, and those that they do capture are often incompletely understood.

A beginning after my own heart! This is the most important starting point for discussing uncertainty about the future.

Climate modellers, like any other modellers, are usually well aware of the limits of their simulators*. The George Box quote from which this blog is named is frequently quoted in climate talks and lectures. But sometimes simulators are implicitly treated as if they were reality: this happens when a climate modeller has made no attempt to quantify how wrong it is, or does not know how to, or does not have the computing power to try out different possibilities, and throws their hands up in the air. Or perhaps their scientific interest is really in testing how the simulator behaves, not in making predictions.

For whatever reason, this important distinction might be temporarily set aside. The danger of this is memorably described by Jonty Rougier and Michel Crucifix**:

One hears “assuming that the simulator is correct” quite frequently in verbal presentations, or perceives the presenter sliding into this mindset. This is so obviously a fallacy that he might as well have said “assuming that the currency of the US is the jam doughnut.”

Models are always wrong, but what is more important is to know how wrong they are: to have a good estimate of the uncertainty about the prediction. Mark and Patrick explain that our uncertainties are so large because climate prediction is a chain of very many links. The results of global simulators are fed into regional simulators (for example, covering only Europe), and the results of these are fed into another set of simulators to predict the impacts of climate change on sea level, or crops, or humans. At each stage in the chain the range of possibilities branches out like a tree: there are many global and regional climate simulators, and several different simulators of impacts, and each simulator may be used to make multiple predictions if they have parameters (which can be thought of as “control dials”) for which the best settings are not known. And all of this is repeated for several different “possible futures” of greenhouse gas emissions, in the hope of distinguishing the effect of different actions.

2. Models are improving

“The climate models…being used in the IPCC’s fifth assessment make fewer assumptions than those from the last assessment…. Many of them contain interactive carbon cycles, better representations of aerosols and atmospheric chemistry and a small improvement in spatial resolution.”

Computers are getting faster. Climate scientists are getting a better understanding of the different physical, chemical and biological processes that govern our climate and the impacts of climate change, like the carbon cycle or the response of ice in Greenland and Antarctica to changes in the atmosphere and oceans. So there has been a fairly steady increase in resolution***, in how many processes are included, and in how well those processes are represented. In many ways this is closing the gap between simulators and reality. This is illustrated well in weather forecasting: if only they had a resolution of 1km instead of 12km, the UK Met Office might have predicted the Boscastle flood in 2004 (page 2 of this presentation).

But the other side of the coin are, of course, the “unknown unknowns” that become “known unknowns”. The things we hadn’t thought of. New understanding that leads to an increase in uncertainty because the earlier estimates were too small.

Climate simulators are slow, as slow as one day to simulate two or three model years, several months for long simulations. So modellers and their funders must decide where to spend their money: high resolution, more processes, or more replications (such as different parameter settings). Many of those of us who spend our working hours, and other hours, thinking about uncertainty, strongly believe the climate modelling community must not put resolution and processes (to improve the simulator) above generating multiple predictions (to improve our estimates of how wrong the simulator is). Jonty and Michel again make this case**:

Imagine being summoned back in the year 2020, to re-assess your uncertainties in the light of eight years of climate science progress. Would you be saying to yourself, “Yes, what I really need is an ad hoc ensemble of about 30 high-resolution simulator runs, slightly higher than today’s resolution.” Let’s hope so, because right now, that’s what you are going to get.

But we think you’d be saying, “What I need is a designed ensemble, constructed to explore the range of possible climate outcomes, through systematically varying those features of the climate simulator that are currently ill-constrained, such as the simulator parameters, and by trying out alternative modules with qualitatively different characteristics.”

Higher resolution and better processes might close the gap between the simulator and reality, but if it means you can only afford the computing power to run one simulation then you are blind as to how small or large that gap may be. Two examples of projects that do place great importance on multiple replications and uncertainty are the UK Climate Projections and ClimatePrediction.net.

3. Models agree with each other

None of this means that climate models are useless….Their vision of the future has in some ways been incredibly stable. For example, the predicted rise in global temperature for a doubling of CO2 in the atmosphere hasn’t changed much in more than 20 years.

This is the part of the modelling section I disagree with. Mark and Patrick argue that consistency in predictions through the history of climate science (such as the estimates of climate sensitivity in the figure below) is an argument for greater confidence in the models. Of course inconsistency would be a pointer to potential problems. If changing the resolution or adding processes to a GCM wildly changed the results in unexpected ways, we might worry about whether they were reliable.

But consistency is only necessary, not sufficient, to give us confidence. Does agreement imply precision? I think instinctively most of us would say no. The majority of my friends might have thought the Manic Street Preachers were a good band, but it doesn’t mean they were right.

In my work with Jonty and Mat Collins, we try to quantify how similar a collection of simulators are to reality. This is represented by a number we call ‘kappa’, which we estimate by comparing simulations of past climate to reconstructions based on proxies like pollen. If kappa equals one, then reality is essentially indistinguishable from the simulators. If kappa is greater than one, then it means the simulators are more like each other than they are like reality. And our estimates of kappa so far? Are all greater than one. Sometimes substantially.

The authors do make a related point earlier in the article:

Paul Valdes of Bristol University, UK, argues that climate models are too stable, built to ‘not fail’ rather than to simulate abrupt climate change.

Many of the palaeoclimate studies by BRIDGE (one of my research groups) and others show that simulators do not respond much to change when compared with reconstructions of the past. They are sluggish, and stable, and not moved easily from the present day climate. This could mean that they are underestimating future climate change.

In any case, either sense of the word ‘stability’ – whether consistency of model predictions or the degree to which a simulator reacts to being prodded – is not a good indicator of model reliability.

Apart from all this, the climate sensitivity estimates (as shown in their Figure) mostly have large ranges so I would argue in that case that consistency did not mean much…

Figure 1 from Maslin and Austin (2012), Nature.

Warning: here be opinions

Despite the uncertainty, the weight of scientific evidence is enough to tell us what we need to know. We need governments to go ahead and act…We do not need to demand impossible levels of certainty from models to work towards a better, safer future.

This being a science and not a policy blog, I’m not keen to discuss this last part of the article and would prefer your comments below not to be dominated by this either. I would only like to point out, to those that have not heard of them, the existence (or concept) of “no-regrets” and “low-regrets” options. Chapter 6 of the IPCC Special Report on ‘Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX)’ describes them:

Options that are known as ‘no regrets’ and ‘low regrets’ provide benefits under any range of climate change scenarios…and are recommended when uncertainties over future climate change directions and impacts are high.

Many of these low-regrets strategies produce co-benefits; help address other development goals, such as improvements in livelihoods, human well-being, and biodiversity conservation; and help minimize the scope for maladaptation.

No-one could argue against the aim of a better, safer future. Only (and endlessly) about the way we get there. Again I ask, please try to stick to on-topic and discuss science below the line.

Update 14/6/12: The book editors are happy for Jonty to make their draft chapter public: http://www.maths.bris.ac.uk/~mazjcr/climPolUnc.pdf

 

*I try to use ‘simulator’, because it is a more specific word than ‘model’. I will also refer to climate simulators by their most commonly-used name: GCMs, for General Circulation Models.

**”Uncertainty in climate science and climate policy”, chapter contributed to “Conceptual Issues in Climate Modeling”, Chicago University Press, E. Winsberg and L. Lloyd eds, forthcoming 2013. See link above.

***Just like the number of pixels of a digital camera, the resolution of a simulator is how much detail it can ‘see’. In the climate simulator I use, HadCM3, the pixels are about 300km across, so the UK is made of just a few. In weather simulators, the pixels are approaching 1km in size.

 

 

A model of models

First, apologies for the delay after the overwhelmingly great start and my promises of new posts. I’ve been wanting to write for a week but had other urgent commitments (like teaching) I had to honour first. I hope to post once a week or fortnight, but it will be a bit variable depending on the day job and the interestingness of my activities and thoughts. I do have a lot of ideas lined up – I wouldn’t have started a blog if I didn’t – but at the moment it takes me time to set them down. I expect this to get faster.

Second, thanks for (mostly) sticking to the comments policy and making this a polite, friendly, interesting corner of the web.

Before I begin blogging about models, I ought to talk about what a model is. Aside from the occasional moment of confusion when describing one’s “modelling job” to friends and family, there are several things that might come to mind.

Model is a terribly over-burdened word. It can be an attractive clothes horse, a toy train, something reviewed by Top Gear, or a Platonic ideal. I will talk about three further meanings that relate to the sense of “something used to represent something else”: these are conceptual, statistical, and physical. They are distinct ideas, but in practice they overlap, which can add to the confusion.

A conceptual model is an idea, statement, or analogy that describes or explains something in the real world (or in someone’s imagination). It is ‘abstracted’, simpler than and separated from the thing it describes. In science, before you can do experiments and make predictions you must have an idea, a description, a concept of the thing you are studying. This conceptual model might include, for example, a tentative guess of the way one thing depends on another, which could then be explored with experiments.

A statistical model is a mathematical equation that describes the relationship between two or more things, ‘things’ being more commonly referred to as ‘variables’. A variable is a very broad term for something that varies (ahem), something interesting (or dull) that is studied and predicted by scientists or statisticians*: it could be the number of bees in a garden, the average rainfall in the UK, or the fraction of marine species caught in the North Atlantic that are sharks. A statistical model can often be represented in words as well as equations: for example, ‘inversely proportional’ means that as one variable increases a second variable decreases. The important thing about a statistical model is that it only describes and doesn’t explain.

A physical model is a set of mathematical equations that explains the relationship between two or more variables. It also refers to a computer program that contains these equations, and to help (or increase) the confusion, these computer models are often called simulators. By explain I mean that it is an expression of a theory, a physical law, a chemical reaction, biological process, or cause-and-effect: an expression not only of knowledge but understanding about the way things behave. The understanding might not be perfect – it might be a partial or simplified physical model – but the point is it attempts to describe the mechanisms, the internal cogs and wheels, rather than simply the outward behaviour.

Physical models are the main focus of this blog, but there are many interesting links between the three: physical models often incorporate statistical models to fill in the gaps where our understanding is poor; a statistical model may describe another model (conceptual or physical). There are myriad different types of physical model, and even more uses for them. In the next post, I will talk about a few physical models I use in my research.

A general note about my plans. I think it’s important to first set out some basic terms and concepts, particularly for those that not familiar with modelling, so please be patient if you are an expert. Before long I will also post more technical pieces, which will be labelled as such so as not to scare off non-experts. I’ll also start blogging about the day-to-day, such as interesting conference talks and random (mostly science-related) thoughts, rather than only pre-planned topics.

 

* The opposite of a variable is…a constant. These can be interesting too.