– The world is getting warmer and especially the Baltic Sea region is getting warmer. And all parameters that depends directly on temperature are seriously affected. But there are larger uncertainties in parameters related for example to the water cycle, says Professor Markus Meier, Leibniz Institute for Baltic Sea Research Warnemünde (IOW)/ Swedish Meteorological and Hydrological Institute (SMHI).

Markus Meier has coordinated the work behind the comprehensive fact sheet Climate Change in the Baltic Sea 2021, launched by HELCOM and Baltic Earth earlier this fall. The joint expert network EN-CLIME involves more than 110 scientists from around the Baltic Sea, and the fact sheet is aiming to compile the latest scientific knowledge on how climate change has already affected the Baltic Sea and what we can expect for the future.

– The uncertainty is large, but we have now more and larger model ensembles. That means that we have better possibilities to estimate this uncertainty. We can say for some parameters that the models definitely don’t agree – we don’t know what will happen – and for other parameters we are even more sure, says Markus Meier.

Temperature will rise

When it comes to rising temperature, both in the air and in the water, the confidence in the future predictions is high. The air temperature has already increased in the Baltic Sea region, especially in winter and spring, and more so than the global average. By the end of this century, air temperature could have increased by 1.5 degrees, compared to the period 1970-1999, if the Paris agreement is fulfilled (RCVP 2.6), and 4.5 degrees in a worst-case scenario (RCP 8.5).

– The largest warming is going on in the northeast in the winter time, and the land is warming more than the sea, concludes Markus Meier.

The sea surface temperature is expected to increase by between 1.1 degrees (RCP2.6) and 3.2 degrees (RCP8.5) by the end of the century, and certainty is high that the maximum sea ice extent will decrease.

Professor Markus Meier, Leibniz Institute for Baltic Sea Research Warnemünde (IOW)/ Swedish Meteorological and Hydrological Institute (SMHI).

Regional differences in runoff from land

Less confident is the scientific community on the changes that relate to the water cycle, such as precipitation, runoff from land and, by extension, salinity in the Baltic Sea. Until now, no significant trend has been detected when it comes to changes in runoff from the whole Baltic Sea catchment, but there are regional differences; there has for example been a clear increase of discharge from the north of Sweden to the Bothnian Bay.

No overall trend in salinity has so far been detected either, but it’s noticeable that there has been an increase in the gradient: the salinity in the north has decreased the last decades compared to the one in south.

In the future projections, precipitation is expected to increase in the whole region in the winter time, but what will happen in the summer is more uncertain.

– But clear is that the north will be getting wetter, also in the summer, says Markus Meier.

Future salinity – more unclear than previously thought

As increased precipitation leads to more freshwater to the Baltic Sea, which could result in an overall decrease in salinity. But the expected increase in sea level (although counteracted by land uplift in the north) and changes in wind patterns could on the other hand lead to a larger water exchange with the North Sea – with more saline water reaching the Baltic Sea. As there are large uncertainties in the projections for all these parameters, what will happen to salinity in the future is also uncertain.

–­ Salinity has been expected to decrease, but these predictions seem now to be less certain than one earlier thought, says Markus Meier.

–­ What we learned in recent years is that the sea level rise is quite important and it varies a lot. This has not been studied in detail in earlier assessments.

Measures against eutrophication have effect

The changing climate could also impact the eutrophication effects in the Baltic Sea, such as the hypoxic bottom areas.

In the projections that include climate change, the oxygen situation in the sea will improve, particularly if the nutrient load reductions in the Baltic Sea Action Plan are implemented, but not as fast as it would have in an unchanged climate.

– Despite climate change, the Baltic Sea Action Plan works. There is an effect from the warming, but it’s not completely counteracting the efforts, says Markus Meier.

Knowledge about extreme events is important

Climate change could also have an impact on the occurrence of natural hazards, caused by extreme weather events.

– Knowing about an average change of climate is of course important, but knowing about possible changes in extreme events is even more important for society’s ability to adapt, says Professor Anna Rutgersson, Uppsala University, one of the authors behind a review over natural hazards and extreme events in the Baltic Sea region.

I general, Sweden is relatively spared from extreme events compared to more southern European countries. What is extreme, however, depends on what you are worried about; when it comes to fatalities extreme temperature, droughts and forest fires are most severe, whereas the highest costs are connected to storms and flooding.

Anna Rutgersson, Uppsala University.

Short-term and long-term extremes

Apart from causing human fatalities and economic losses, ecosystem degradation could also be a consequence and societal implications of extreme events include shipping, forest fires, coastal flooding and dams and infrastructure.

Wind storms, extreme waves, snow canons and river floods are examples of shorter timescale extreme events.

– But longer-term events can also cause large problems for society, says Anna Rutgersson and takes heat waves, phytoplankton blooms, ice season and drying as examples.

– We have the magnitude of the extreme, but we also have the extent of a certain condition that can make an extreme event problematic for society.

Impacts of climate change

Some, but not all, extreme events are impacted by climate change. There has, for example, been an increase in the number of days defined as a heat wave from 1950 to 2020, and a decrease in the number of days defined as a cold spell.

– The climate change scenarios indicate that this is projected to continue in the future, says Anna Rutgersson.

The number of days with heavy precipitation has also increased in the last decades. In the climate models, particularly the heavy precipitation is projected to increase in intensity in the future. Heavy ice conditions are expected to decrease.

Cyanobacteria blooms could also be defined as an extreme event. The last three decades have shown an increase in harmful cyanobacteria blooms, and this increase is projected to continue in a future changing climate, says Anna Rutgersson.

Varying effects on drought and flooding

The occurrence of drought as well as of river flooding, varies greatly within the Baltic Sea region.

– North of 59°N, there has been a decrease in drought conditions and in the south there has been an increase.

In the future, drought is expected to further decrease in the north (except for in the spring) and increase in the in some of southern regions in the spring and summer. River flooding is expected to decrease in the spring and increase in the winter.

– The climate in the Baltic Sea region is highly variable – it is so now and it will continue to be so in the future. This makes climate change signals harder to detect and is why we have relatively low confidence levels on some of the parameters, says Anna Rutgersson.

Moderator Gun Rudquist, Stockholm University Balic Sea Centre.

What is on your wish list for Christmas from the policy makers, asks moderator Gun Rudquist the participating scientists.

– A global carbon dioxide tax, says Anna Rutgersson.

– Climate change is global problem so the solutions have to be global. But adapting to a changing climate is something we need to do on a national and regional level. Coastal protection and coastal city protection have to be addressed better on a national level and not be left to the local authorities to deal with.

– I like very much this dialogue between scientists and policy makers, and that HELCOM started the collaboration with Baltic Earth and other scientific network is very good. So my wish is that this will go on and perhaps even be intensified, says Markus Meier.

See the webinar here