This week Prof. Ed Hawkins from the Department of Meteorology and NCAS-Climate gave a University of Reading public lecture discussing the science of climate change. A plethora of research was presented, all highlighting that humans are changing our climate. As scientists we can study the greenhouse effect in scientific labs, observe increasing temperatures across the majority of the planet, or simulate the impact of human actions on the Earth’s climate through using climate models.
Fig. 1, presented in Ed Hawkins’ lecture, shows the global mean temperature rise associated with human activities. Two sets of climate simulations have been performed to produce this plot. The first set, shown in blue, are simulations controlled solely by natural forcings, i.e. variations in radiation from the sun and volcanic eruptions. The second, shown in red, are simulations which include both natural forcing and forcing associated with greenhouse gas emissions from human activities. The shading indicates the spread amongst climate models, whilst the observed global-mean temperature is shown by the solid black line. From this plot it is evident that all climate models attribute the rising temperatures over the 20th and 21st century to human activity. Climate simulations without greenhouse gas emissions from human activity indicate a much smaller rise, if any, in global-mean temperature.
However, whilst there is much agreement amongst climate scientists and climate models that our planet is warming due to human activity, understanding the local impact of anthropogenic climate change contains its uncertainties.
For example, my PhD research aims to understand what controls the location and intensity of the Intertropical Convergence Zone. The Intertropical Convergence Zone is a discontinuous, zonal precipitation band in the tropics that migrates meridionally over the seasonal cycle (see Fig. 2). The Intertropical Convergence Zone is associated with wet and dry seasons over Africa, the development of the South Asian Monsoon and the life-cycle of tropical cyclones. However, currently our climate models struggle to simulate characteristics of the Intertropical Convergence Zone. This, alongside other issues, results in climate models differing in the response of tropical precipitation to anthropogenic climate change.
Figure 3 is a plot taken from a report written by the Intergovernmental Panel on Climate Change (Climate Change 2013: The Physical Science Basis). Both maps show the projected change from climate model simulations in Northern Hemisphere winter precipitation between the years 2016 to 2035 (left) and 2081 to 2100 (right) relative to 1986 to 2005 under a scenario where minimal action is taken to limit greenhouse gas emissions (RCP8.5) . Whilst the projected changes in precipitation are an interesting topic in their own right, I’d like to draw your attention to the lines and dots annotated on each map. The lines indicate where the majority of climate models agree on a small change. The map on the left indicates that most climate models agree on small changes in precipitation over the majority of the globe over the next two decades. Dots, meanwhile, indicate where climate models agree on a substantial change in Northern Hemisphere winter precipitation. The plot on the right indicates that across the tropics there are substantial areas where models disagree on changes in tropical precipitation due to anthropogenic climate change. Over the majority of Africa, South America and the Maritime Continent, models disagree on the future of precipitation due to climate change.
How should scientists present these uncertainties?
I must confess that I am nowhere near an expert in communicating uncertainties, however I hope some of my thoughts will encourage a discussion amongst scientists and users of climate data. Here are some of the ideas I’ve picked up on during my PhD and thoughts associated with them:
- Climate model average – Take the average amongst climate model simulations. With this method though you take the risk of smoothing out large positive and negative trends. The climate model average is also not a “true” projection of changes due to anthropogenic climate change.
- Every climate model outcome – Show the range of climate model projections to the user. Here you face the risk of presenting the user with too much climate data. The user may also trust certain model outputs which suit their own agenda.
- Storylines – This idea was first shown to me in a paper by Zappa, G. and Shepherd, T. G., (2017). You present a series of storylines in which you highlight the key processes that are associated with variability in the regional weather pattern of interest. Each change in the set of processes leads to a different climate model projection. However, once again, the user of the climate model data has to reach their own conclusion on which projection to take action on.
- Probabilities with climate projections – Typically with short- and medium-range weather forecasts probabilities are used to support the user. These probabilities are generated by re-performing the simulations, each with either different initial conditions or a slight change in model physics, to see the percentage of simulations that agree on model output. However, with climate model simulations, it is slightly more difficult to associate probabilities with projections. How do you generate the probabilities? Climate models have similarities in the methods which they use to represent the physics of our atmosphere and therefore you don’t want the probabilities associated with each climate projection due to similarity amongst climate model set-up. You could base the probabilities on how well the climate model simulates the past, however just because a model simulates the past correctly, doesn’t mean it will correctly simulate the forcing in the future.
There is much more that can be said about communicating uncertainty among climate model projections – a challenge which will continue for several decades. As climate scientists we can sometimes fall into the trap on concentrating on uncertainties. We need to keep on presenting the work that we are confident about, to ensure that the right action is taken to mitigate against anthropogenic climate change.