What are decadal climate predictions?

05 May 2022

As part of A4 we are working in workgroup 3 on the topic of decadal climate predictions. But how does it work?

When asked what decadal predictions mean a common easy answer is that it is something like a weather prediction just a few years ahead. That is of course not an accurate description, but it has at least a true core. To explain that, it is important to understand what weather predictions are about and what the differences are compared to things we often hear in the media about, especially in years when the IPCC publishes their assessment reports: climate projection.

What are weather predictions?

The aim of weather predictions are to predict specific events, like rain showers or storms, in time, space and amplitude, multiple days ahead. It tries to answers questions like: will there be a shower at 3 pm in Dublin next Tuesday, how heavy might it be and how sure can we be about it? Of course we understand from our daily experiences that uncertainties might be large, especially here in Ireland, and that usually the uncertainties decrease the closer we get to the event we like to predict. But how is it done? Core of every weather prediction is to know how the weather is at the start of the prediction. For this we need something like a photo of the atmosphere, which is done by doing a lot of observations around the world in a coordinated way. After all available observations are collected and quality controlled they are used as a training for a weather model. These models are physical equations, mainly for the atmosphere, translated into thousands of lines of computer code. The model tries to imitate the observations as good as possible („data assimilation“) and when it has done that we start the prediction of the weather by solving the equations over time. In the end we have for every few hours a predicted weather.

What are climate projections?

Unlike in weather prediction we do not need a photo of the weather in climate projections. Climate models are the larger cousins of weather models, as they include beside the atmosphere equations for things like ocean, soil, ice and the biological processes. It is an art to make them an adequate representation of our climate. This is achieved by running the model for a long time. When we are confident that the model is doing a good job, we start at a random day, determine it as a specific date in the past (often around 1850) and start the projection. Important here are the so called external forcings, including changes in sun, volcanoes and greenhouse gas emissions. For the past we can use for this realistic observations, but for the future we have to make assumptions about the Earth and society and create different scenarios. Main part of the analysis of the results is that we cannot take a single date as a representation of the true climate at that day and time. So we have to do the statistics of weather, like the famous 30-year means, and look how the model prediction have changed between a time period of today or in the past and another in the future.

What are climate predictions?

Climate predictions, like seasonal or decadal predictions, are a mix of both worlds. Like the weather predictions we use a photo of the atmosphere, but also for all the other components in the climate system, like the ocean. We use the climate models and teach them the observations before we start our predictions. The outcomes cannot be interpreted on a day by day basis, as we have to do statistics over longer time periods, as we have done with the interpretation of the results in the climate projections. But in our case the time periods are shorter. In case of seasonal predictions a couple of months, in case of decadal predictions a couple of years.

Why are we doing it?

While weather information are used in daily planing and preparations for private and public goods, like planing for the rescue services, climate predictions are an important tool for longer term adaptations. We do not deliver specific events for a specific time, but are able to estimate whether winters gets unusually cold or whether ocean temperatures will be unusually warm in the next years. With this planning, adaptation and investments can be supported for areas like fisheries or the energy sector. The climate projections focus then more on long term, large scale adaptation and mitigation leading to policies in various parts of our daily life. Predictions and projections are useful tools to make plans and decisions from hours to generations and decadal predictions are an important element in this, bridging a gap in a period decisive for many business decisions.