FRANCESCA RIBAS. Researcher in beach dynamics, from the Fluid Dynamics and Geophysical and Technological Applications research group (DF-GeoTech), Universitat Politècnica de Catalunya · BarcelonaTech
Sandy beaches occupy 31% of the coasts of our planet and have considerable value at social and economic level and from an ecological perspective. In addition, they are vital to protect against storms the cities and infrastructure that we often build close to the sea.
It is known that beaches will suffer more frequent episodes of erosion and flooding due to global warming. In any case, mathematical models are needed to make quantitative predictions and validate mitigation measures. Although these predictions still have a lot of associated uncertainty, in recent years great advances have been made.
How does the water and sand move on beaches?
Sandy beaches are very dynamic, and their behaviour is extremely complex. Waves generated by wind travels long distances and transport energy until they reach the coast. Waves spread above the average sea level, which changes depending on the astronomical tide and the meteorological tide. The latter depends on atmospheric pressure and is responsible for the rise in sea level that occurs during a storm.
When waves break on the beach, all their energy is transformed into currents and causes sediment transport that can be very intense. As a result, the morphology of the beach changes (that is, the position of its bed) and this in turn affects the waves and currents.
Storms produce changes in beaches in a question of hours. The zone that is usually above the water floods due to the meteorological tide and the waves erode the sand in this area and deposit it in the underwater part. Subsequent waves with less energy have the capacity to move the sand towards the coast again, which leads to a degree of recovery of the beach in a matter of weeks. In addition, sand is transported longitudinally along the coast.
However, we still do not know exactly all the complex processes of transporting sediments on beaches. Therefore, we should use data to calibrate models of the evolution of beaches, that is, optimise the results by comparing them with historical observations of real beaches.
Consequently, quantitative predictions can only be made at regional level and it is vital to have a good observational database. Even so, most existing mathematical models have a limited prediction capacity, especially at the scale of months to decades.
Consequences of global warming
First, the sea level is rising at an accelerated rate in all oceans and open seas as a result of climate change, as shown by data from the last century.
Global rise in sea level in the period 1900-2005 (purple) and forecasts up to 2100 corresponding to two emissions scenarios: the most optimistic in blue and the most pessimistic in red. Source: IPCC
The two main reasons for this rise are the melting of glaciers and the thermal expansion of water in the oceans, effects that will continue to act throughout the twenty-first century. Forecasts of the sea level rise are made periodically for different CO₂ emissions scenarios, depending on the global response to the climate emergency in the next few decades. According to these forecasts, at the end of the century, the average sea level could be between 45 cm and 85 cm above the level at the start of the twenty-first century.
At the same time, global warming could cause changes in the patterns and intensity of storms, but the increases that are measured vary dramatically in the different seas and oceans. In some regions, a drop has even occurred.
It is forecast that throughout the twenty-first century the intensity of storms could continue to increase, although also with great geographic variability (Figure 2). For example, according to these forecasts, there could be an increase of over 2 m in the wave height in the Antarctic Ocean, while the Mediterranean Sea would not experience changes. In addition, the uncertainty associated with these wave predictions is still very high.
Change forecast for the end of the twenty-first century (compared to the start of the century) in significant wave height during extreme storms in the most pessimistic emissions scenario. Average values of the results of different models. Lobeto, H., Menendez, M. & Losada, I.J. Future behavior of wind wave extremes due to climate change. Sci Rep
How will the beaches be affected?
Currently, a significant percentage of sandy beaches on our planet are being eroded. Climate change is already playing a certain role, but quantifying it is complicated because there are other effects of human activity that have had a more relevant influence during the last century. For example, the urban development of coasts and rivers and the construction of infrastructure, such as ports and reservoirs, have been factors that are responsible for erosion in many zones.
However, we know that the accelerated increase in sea level throughout the twenty-first century will have an enormous impact. It will cause an increase in flooding episodes and erosion and retreat of the coastline, even if the number of storms does not increase. The rise in sea level will lead to direct retreat of the coastline due to flooding alone. In addition, storms will act on areas of beaches that today are always above water level and will cause net erosion, with a loss of sediment to the sea.
Can we quantify these effects?
To quantify the effect of climate change on beaches, mathematical models of the evolution of beaches are required with a demonstrated capacity to predict on the scale of decades. Recent studies show that some models could reproduce around ten years of regional observations, after calibrating them with data.
Subsequent calculations with these models up to 2100 predict a retreat in the coastline from dozens of metres up to a hundred metres, depending on the type of beach and the climate change scenario. Although natural beaches have the capacity to adapt to a higher sea level by migrating towards the land, a high percentage of them have urban developments and could disappear if mitigation measures are not implemented.
Any exercise of modelling should include an analysis of the uncertainty associated with each prediction. Scenarios of sea level rises vary depending on CO₂ emissions. In addition, there is considerable uncertainty about future climate conditions (waves and meteorological tide). Finally, there are unknown factors in the formulae for sediment transport in the models.
The most recent studies already include this type of analysis and indicate that lack of knowledge of sediment transport dominates uncertainty in predictions for the first decades, while the existence of different sea level scenarios dominates the uncertainty of predictions for the second half of the twenty-first century (Figure 3).
Forecasts of the total area of dry beach in the twenty-first century in a region of 17 km of the southern coast of Holland in the most optimistic scenario (blue) and the most pessimistic (red). The bands indicate the uncertainty associated with the values of the parameters of sediment transport. Author provided.
To sum up, although we already know that the rise in sea level will lead to the coastline retreating, we need mathematical models to quantify this and to be able to validate potential mitigation measures. Advances in recent years show that sufficient prediction capacity can be achieved, controlling for uncertainties. In addition, work is being carried out to achieve more accurate formulae for sediment transport, obtain good observational data and continue to improve models. All this enables us to act with more knowledge to protect these environments that are so valuable and vulnerable.
This article was originally published in The Conversation.