The differences between seasonal and climate forecasting
We create forecasts for a few hours ahead to more than a century into the future, and everything in between. While our ultimate aim is to produce all of these through a single forecasting process, we currently have to approach timescales in a slightly different way — using our knowledge and technology in the best way possible, in order to get accurate results.
Seasonal forecasts are the newest part of this process and are very different from other types of predictions — not only in the way they are created but also in the information they provide.
- Weather forecasts — give a detailed view of what the weather will do over the coming hours and days.
- Seasonal forecasts — aim to show how temperature and rainfall for a given season will differ from long-term averages.
- Climate projections — look at how the long-term averages themselves will change.
Seasonal forecasts are still a developing area of meteorology, only becoming possible in recent years due to advances in technology. They pose a particular challenge for forecasters because they have to take seasonal variability into account — these are the complex variations in our weather which make the same season different from one year to the next.
Climate forecasting is different from seasonal forecasting because the projections look at how long-term averages will change, not how seasons or years will differ from that average. While this is a subtle point, it is crucial to understanding the fundamental differences between the types of forecast.
Another difference between the two is that seasonal forecasts use current weather observations to create a starting point for their predictions. Because the forecast is sensitive to the initial conditions, any errors in the observations can have a huge effect on the outcome. Climate predictions don’t rely on these observations and, so, bypass this source of potential error.
Because of the chaotic nature of seasonal variability and the potential for errors in the starting conditions, seasonal forecasts can only be delivered in terms of probabilities.
“Climate forecasting is different from seasonal forecasting because the projections look at how long-term averages will change, not how seasons or years will differ from that average.”
Forecast models are run through our supercomputer numerous times; each will give slightly different results. The number of results which predict warmer or cooler than average temperatures, or more or less than average rainfall, are counted and then the probabilities of each outcome are calculated.
Due to the probabilistic nature of the forecast, it’s impossible to say whether a single seasonal forecast is correct or not. If the least likely outcome of a forecast happens, it doesn’t necessarily mean the probabilities were wrong — it may be simply that the least likely outcome occurred.
Adam Scaife, Met Office Head of Seasonal to Decadal Forecasting, explains: “The odds just tell us how likely an outcome is, they don’t say what is going to happen. Because of this, the only way we can judge the accuracy of seasonal forecasts is by looking at a large number of forecasts.”
Judging the accuracy of climate forecasts is also difficult — mainly because we won’t know if they are right for several years, or decades, to come.
There are two factors that strongly reinforce their credibility, however:
- Climate forecasts are started up to 150 years in the past and run on to about 100 years in the future. From this, we know that, in the years leading up to the present day, the output from the model accurately generates our climate history. This gives us reassurance that the models are correctly recreating our climate at least until the present day.
- We can find reassurance in climate forecasts in terms of probability. Every scientifically viable climate model ever run has predicted substantial climate warming towards the end of this century. So that is a 100% probability from the models that the Earth will warm further by 2100, if greenhouse gases continue to increase in the atmosphere due to human activities. While there are, indeed, differences in the amount of predicted warming, they generally fall within a range of 2–6 °C by the end of the century.
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