Marine environment in changing climate

The Baltic Sea is constantly changing. Some 9 000 years ago the Baltic Sea was a freshwater lake (The Ancylus Lake). As the sea level in the Baltic Sea has changed because of the land uplift in Scandinavia and increased global sea levels, the exchange of water between the Baltic Sea and North Sea has changed as well.

Results from research indicates that 6 000 years ago the cross section of Ă–resund (the Sound) was about two times larger than today. As a result the Baltic Sea probably had a higher salinity at the time, perhaps twice as high compared to today. There are also indications that the Baltic Sea may have been warmer and the river runoff lower than today. Thus, the large ancient climate changes most likely had significant impacts on the marine environment.

Climate changes take place also today and will continue to do so in the future. A new issue however, that needs to be taken into consideration, is human emissions of greenhouse gases. In accordance with air temperature observations on land, measurements from lightships during the last 100 years indicate that also the Baltic Sea has become warmer especially during the last 30 years. The temperature rise depends mostly on strengthening of the atmospheric greenhouse effect.

In order to compute the various possible scenarios on how the future of the Baltic Sea may develop, we use numerical models which couple the physical, chemical and biological processes in the ocean. A scenario is not a forecast since it depends on various assumptions on e.g. how human's greenhouse gas emissions around the world will develop in the future, or how nutrient supplies from land will develop in countries around the Baltic Sea. Accordingly it is important to study uncertainties in calculations of the future conditions using a number of model runs based on different assumptions, so-called ensemble runs.

Since the physical processes affect the marine ecosystem, the future climate changes may have great impacts on e.g. algal blooms, food chain and the species composition. For example, the surface water oxygen content is reduced when it gets warmer since the oxygen solubility in water decreases with an increased temperature. Oxygen deficiency, which is already a major problem for many species in the Baltic Sea deep water, can thus possibly worsen in a future climate.

In RCO-SCOBI, the model we mainly use, the ocean, the sea ice, the waves and the biogeochemical processes in water and sediments are linked together. Coupled effects between water and sediment are important to consider for simulations on longer time scale than a few days.

The efforts on model development and process studies as well as the use of the model to study the influence of climate and nutrient loads on the marine environment occurs in a number of national and international projects financed by Formas, VINNOVA, The Swedish Environmental Protection Agency, Baltic Sea 2020 Foundation, the Nordic Council of Ministers, the EU and the BONUS program. Examples of projects where SMHI participate are AMBER, BalticWay, ECOSUPPORT, and INFLOW within the BONUS program (www.bonusportal.org).

Phytoplankton concentrarions in present and future climate in the Baltic Sea based on models
Annual mean phytoplankton concentration [mgChl/m3] (0-10m). Present climate (upper panels), future climate according to ECHAM4/A2 for 2071-2100 (lower panels), reference conditions of nutrient supply during 1969-1998 (first column), nutrient load scenarios following the most optimistic (best) case (second column), the Baltic Sea Action Plan (third column), and the most pessimistic case assuming business as usual (fourth column). The nutrient load scenarios have been developed by the Baltic Nest Institute.