I followed one of the items that "caught the eye" of Judith Curry at the weekend. It was an article by denier statistician to the stars, William M. Briggs. Yes, he's the bloke who coauthored a dumb article with Christopher Monckton of all people. That in itself tells you more than you need to know about him.
Anyway, William M. Briggs might shine brightly for some stars but he's no futurist. William wrote an article (archived here) under the headline:
There Is No Difference Between A Forecast, A Scenario, or A Projection
He's wrong of course. And not just in having a superfluous comma. Even I make that mistake, from time, to time :)
I thought I'd take a peek at what he wrote underneath his headline to see just how wrong he would get. This is what I found:
This is a tad incoherent...Well, his first clause was correct but that's about all he got right.
People trying to escape the implication of a bad forecast often claim their forecast wasn’t a forecast but a projection or scenario. The implication is that a bad forecast means a (possibly beloved) theory is no good. Therefore, if the forecast wasn’t a forecast, but a projection or scenario, the theory can still be admired (or funded).
This won’t do. Forecasts are scenarios are projections. And bad forecast-scenario-projections means bad theories.
There is a lot wrong with the above. I'll list a few things:
- If a forecast is bad it's bad.
- A projection isn't a forecast.
- A scenario isn't a forecast.
- A projection isn't a scenario.
- A forecast isn't a scenario.
So what is the difference between a forecast, a projection and a scenario? It's not even subtle.
What is a forecast?
A forecast is an informed prediction, usually based on statistics or on previous observations of what happened under the same conditions. It is an estimation (not a guess) of what is going to happen in the future to something or the other.
A weather forecast is an informed estimate of what the weather will be like in a particular locality over the coming day or maybe anything up to a week. It is based on current meteorological conditions combined with a knowledge of atmospheric science. For example, if radar images show a storm moving across the country it is likely that the storm will affect locations ahead of it in its current trajectory. The weather bureau will issue a severe storm alert. It will forecast the storm and give an estimate of its path and time of arrival.
Today there's a frost warning out for parts of Victoria, Australia. The frost hasn't happened yet but the models used to forecast weather indicate it's likely to happen overnight. That's a forecast. It's not conditional upon anything except known science and current conditions. The current conditions are as good as current observations, which these days are pretty accurate. The known science is better than it was a decade ago and much better than it was a century ago. Plus there has been a vast improvement in recent years in the tools used to estimate how the weather will progress. Computers are a lot more powerful than they used to be.
What is a projection?
A projection isn't a forecast. It is not a statement of likelihood without any conditions except current observations and known science. A projection starts to bring in "what ifs". A projection typically looks further ahead in time and therefore it has to make assumptions about more and more variables. In the case of climate projections, assumptions are made about variables such as human behaviour, technological advances, government policies and international agreements. It's not just that any one variable could work out differently. The variables can affect each other so the interplay of differences in variables can have a multiplier effect.
Good projections will list the assumptions. They don't need to be extensive. For example, with climate projections, the various variables could be represented by an assumption about future emissions of carbon dioxide. The future emissions of carbon dioxide will depend on a lot of things, such as economic activity, the rate of adoption of renewable energy technology, the rate of adoption of energy efficient appliances, government policy to curb carbon emissions and so on. But all of these can be collapsed into an assumption about future emissions. That allows a projection of other effects for different assumptions about future emissions. Such as how quickly ice will melt, how quickly seas will rise, how quickly global surface temperatures will rise and by what amount. None of the projections are right or wrong in themselves. However they can be right or wrong in the context of the assumptions. In other words, if the projection is that temperatures will rise by 3C if emissions are such that CO2 doubles, then that will be evident some time after CO2 doubles - if emissions stop after doubling.
What is a scenario?
Scenarios are a bit different from projections and forecasts. Scenarios are imaginary futures. They are typically used by the military when developing strategies. They are also used by corporations in planning. The idea is to imagine a future and describe it in terms that have meaning for the context in which they'll be used.
A not-for-profit providing services for the homeless might think of a future in which there is an economic downturn, combined with local disasters (such as a major fire or earthquake) and a "small government" government that has reduced or cut completely any support for the homeless or for low income housing. It might also come up with other scenarios. The scenarios don't have to be completely realistic. They are most certainly not forecasts. Nor are they projections. By their nature they will be somewhat simplistic. They are used to test the limits for decision-making purposes. To test the impact of decisions that are made today on what could be the result of those decisions if today's conditions changed tomorrow.
If scenarios are described in sufficient detail, they can be used to make projections. They can't be used to make predictions or forecasts. That would be shifting into magical thinking territory. There are too many variables embodied in a scenario to be as certain about the impact as one would be about a forecast. However they can be used as the basis for projecting what a future would be like under that scenario.
In the example I chose about the homeless, one might liken it to the great depression when the economy took a dive, government was small and the main roads were filled with swaggies (or hobos) looking to make a few bob in the next town or the one after. Some good folk helped by serving soup to desperate people. It made a difference.
Climate projections and scenarios
So what does that mean for climate projections and scenarios? The IPCC in its latest report didn't use the word scenarios. It used the word "pathways". It's not that different. There was effort put into describing and modeling different scenarios as integrated assessment models or IAMS, none of which are very realistic. They included assumptions about economic activity, technological advances, population trends etc. However at least when they are simplified down they allow projections of climate change. Without them, then climate projections would be based on even more rubbery assumptions.
The science of climate is well understood. More than sufficiently well to know that we must cut emissions of carbon dioxide. It is more difficult to forecast how human society will respond over the next few years. Will we rise to the challenge of mitigation? Will we adopt renewable energy technology quickly enough? What about other factors that could impact the future? What if there is another economic meltdown or another world war or a major pandemic or a massive volcanic eruption? How would they affect future emissions of carbon dioxide?
My forecast, prediction and scenario
I don't know if William M Briggs really doesn't know the difference between a forecast, a projection and a scenario or if he's just pretending to be dumb. He did say that his article was the basis of an abstract he was submitting to the upcoming AGU Fall meet later this year. If it's anything like his blog article, I imagine a scenario in which his abstract elicits a grunt before being dropped into the waste paper basket. I predict that scenario will come to pass. In fact I forecast that his abstract won't win a guernsey. In the unlikely event that it does, I forecast it will gain nothing but mockery from anyone who matters.
Let's see how that forecast, prediction and scenario pan out :)