Baltic Sea future climate


It is difficult to make pronouncements about the future of the Baltic Sea. We still know too little about the processes that drive the climate. We have only just begun to understand the interplay between the geosphere, the biosphere, and human activity. The earth is not a machine that can be easily described in computers and manuals.

 

We are inclined to believe that the traditional scientific disciplines have the answers, and truly there is a sea of information there. But if we are serious in our ambitions to understand climate systems, with their complex flows of air, water, and chemical substances, many researchers from many different disciplines will have to work together.

 

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Figure 1. The Baltic Sea from Christian Island, spring 2004. Photo: Karin Wesslander. People have always been fascinated by the future. Artists, writers, scientists, and visionaries perhaps sense something beyond the horizon. Mainly, we are guessing. We are prisoners of the present. But time is approaching with its invitation to attend.

 

Forecasts should be taken with a grain of salt and preferably come with a "Best Before" label. Model errors also grow with the length of the forecast. For meteorologists, this is a matter of a mathematical initial problem - a matter of thoroughly understanding the conditions from which the models start their calculations. Climate researchers, on the other hand, tend to ignore the starting conditions and regard forecasts as solely a problem of boundary values. The term refers to that factors outside the actual calculation area determine the calculation. Changes in solar radiation are one example, but volcanic eruptions and increased quantities of greenhouse gases are also often regarded as boundary conditions. Climate forecasts are both an initial and a boundary value problem. Future simulations are meaningless if we fail to take the changes and the conditions into account. Land and seas undergo constant change. Greenhouse gases and volcanic eruptions affect the earth. There are no simple connections here. The biosphere dynamic is a complex nonlinear system that we have only begun to sniff out with our mathematical models.

 

The scare reports on air temperature and water level come one after the other: "Sweden will be four degrees warmer in a hundred years," "Göteborg Transformed into the Venice of the North," "Skåne Ravaged by Drought and Norrland Drowned in Rain."

 

So, what can we say about the future? Researchers are very much inclined to work with various types of scenarios, or more accurately put, a type of sensitivity study that varies one or two factors at a time. But a scenario is not a forecast. The models are incapable of realistically describing the direction of the wind and how much it rains. That doesn't stop less scrupulous forecasters from uncritically adding their own climate model's trend to the actual observations of the day. Linear functions and straight answers, in other words, but they have nothing to do with science. The truth is that we know very little about the future. Climate forecasts are still the art of the educated guess.

 

Yet another problem with climate forecasts is that they report calculations not previously measured. The models are adapted using data from the current climate, but describe changes that have not yet been observed. Our research team has worked with models of the Baltic Sea and we have been able to show how water level, salinity, temperature, and ice vary under given conditions. We have access to direct observations and are thus able to develop realistic models.

 

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Figure 2. A model calculation showing how salinity in the central Baltic Sea can vary with variations in freshwater inflow from rivers and precipitation. The dashed field represents actual values in the last hundred years. Redrawn from Omstedt and Hansson (2006).

 

The freshwater inflow for the last hundred years has hovered around a mean value of 16,000 m3/s. What happens if the freshwater supply increases? The curve in the diagram shows that if it were tripled, the Baltic Sea would be transformed into a freshwater sea. Can we rely on that and how are we supposed to know? The calculations show that the Baltic Sea is sensitive to variations in freshwater inflow. But it is highly unrealistic to conceive of an increase of several hundred percent. Our calculations indicate that the Baltic Sea will remain a brackish sea in the future. At the same time, we are convinced that our results will be discussed and called into question when new knowledge comes to light.

 

What is going to happen to the Baltic Sea tomorrow? The only thing we can be sure of is that the future has a surprise or two in store for us. Is there an answer to the question, then? If we want to be able to say anything about the next hundred years, we should adopt a historical perspective. A time scale of two hundred years back in time immediately illustrates the uncertainty.

 

In the year 2100, the mean temperature in Stockholm will have changed by -0.1 to +0.7 ºC. The Baltic Sea's water level will be 0.3 to 8.5 cm above the current mean and the maximum ice cover will have declined by 5 to 20%. The freshwater inflow will not have changed to any significant degree and thus the salinity of the Baltic Sea will not have changed either.

 

Clearly, nothing spectacular. Is it a reasonable forecast? It is based on the assumption that the climate a hundred years from now will be similar to the climate today and it follows the trend in observed series. Dramatic changes in ocean circulation, particularly of the Gulf Stream, were not included. Nor were unusual effects of solar radiation, greenhouse gases, or volcanic eruptions - phenomena that can drastically change forecast conditions, but about which we know little. As said, forecasts have a limited shelf life. As we know more, we make new and better predictions.

 

With all due respect to forecasts, we need thorough knowledge of the current climate to understand what is waiting for us around the corner. We learn a great deal about the sea and the large-scale circulation of the atmosphere by tracking variations in the water level. We can use salinity to read how the water balance between precipitation and river inflow is changing throughout the Baltic Sea. Temperature and ice distribution give us an understanding of the heat balance and rapid indications of abnormal changes. The Baltic Sea is quite simply keeping a watchful eye on the climate of Northern Europe.

 

Measuring water level, salinity, temperature, and ice distribution is perhaps our most important task right now. We also need more in-depth research in order to geologically and historically map the climate, trace the causes of change, develop better models, and understand the consequences for biological life. The more we know about the Baltic Sea yesterday and today, the more we can say about the Baltic Sea tomorrow. Our job is to piece a puzzle, solve an enigma, and get to know a sea.

 

References:

  • Omstedt, A., and D., Hansson, 2006. The Baltic Sea ocean climate system memory and response to changes in the water and heat balance components. Continental Shelf Research 26, 236-251.


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