How climate change is affecting our freshwaters

Ms Adrian, as a co-author of the forthcoming IPCC Report, you follow both the scientific and political discussions. What role do fresh waters play in the current debate?

Inland waters are rarely explicitly mentioned alongside oceans and land, and are typically assigned to land in global considerations. This was also the case at the COP26 conference. Of course, the diversity of ecosystems cannot be taken into account in detail; however, explicitly considering inland waters – as major sources of fresh water and as ecosystems undergoing major changes in the context of global warming - is very important, I think.

It is also a matter of making politicians and society more aware of the essential services of fresh waters. People are directly affected by the warming of inland waters, and lakes in particular can warm up more than the air. In fact, the surface water has already exceeded the 2-degree target for the long-term annual average in the past decades.

Warming leads to more nutrients, algal blooms and anoxia. This can have an impact on drinking water supplies, because surface waters play a crucial role as a resource worldwide. In addition, lakes lose their recreational value if water quality deteriorates further. When lakes become warmer, they mix less frequently, the oxygen content decreases, which in turn becomes problematic for fish and other organisms. But it is not only a question of quality but also of quantity: entire regions are affected by water shortages and declining groundwater levels. All this is happening not only in the Mediterranean, but near our doorsteps. Most importantly, ecosystems in general have an inherent value, which deserves protection.

Ms Jähnig, the climate crisis is masking the biodiversity crisis. In your opinion, to what extent are the various ecological crises still viewed in isolation from each other? And how does one affect the other?

Indeed, both crises are closely linked - and this is not always taken into account. In the final document of the COP26 conference, the link is only briefly mentioned once. Habitats are lost due to climate change, for example because water bodies temporarily dry up. This is not a marginal issue because about half of all watercourses worldwide dry up temporarily. And this trend will continue. Animals that can only tolerate a small temperature range have to move to other regions - which is not always possible. The deterioration in water quality that Ms Adrian mentions also plays a role, of course. In addition, climate change leads to the spread of alien species that compete with or harm native animals.

One point that is very close to my heart is resolution of the conflict that is sometimes perceived between the goals of climate and biodiversity protection in hydropower development. Hydropower is a renewable energy source, but not an environmentally friendly one. So-called small hydropower in particular contributes extremly little to the energy transition, but has a significant impact on the ecological status of water bodies and their organisms. For this reason, 65 experts from 30 scientific institutions recently issued a joint statement urging federal policymakers to end support for inefficient small hydropower plants from Germany’s Renewable Energy Sources Act (EEG) or tax revenues.

Climate protection and biodiversity conservation often go hand in hand. Rivers that are given more space, for example, significantly contribute to flood protection and at the same time support species-rich and diverse habitats. This is because so-called refugial spaces such as backwaters, pools or root structures can be created, which facilitate recolonization after periods of strongly fluctuating water levels. 

The COP26 framework decision recognises "that the impacts of climate change will be much smaller with a temperature increase of 1.5 degrees compared to two degrees" and pledges to "continue efforts to limit the temperature increase to 1.5 degrees". Mr. Gessner, from the point of view of a limnologist: A good decision?

Yes, the stronger commitment to limit warming is a good result, in principle. But now it is important to take effective measures to achieve this goal, which the international community is still hesitant about.

But let me also point out that the threshold of 1.5 or 2 degrees cannot be directly applied to fresh waters. As Ms Adrian said, many lakes have already exceeded these marks even today. For example, the surface water of Lake Stechlin in northern Brandenburg has warmed by about 2 degrees since the late 60s. And our long-term data from well-studied Lake Müggelsee in Berlin show an increase in the average summer water temperature of even 0.6 degrees per decade between 1978 and 2020.

In addition to these long-term temperature trends, at least three other factors require attention. First, the mixing regime: At mid-latitudes, the entire lake water column is normally completely mixed twice a year. This happens in spring and autumn, whereas the surface and deep water layers are neatly separated in the summer and winter.  Warming will lead to continued mixing throughout the winter in some of our larger lakes, including Lake Stechlin, as the models of our lake physicists show. These lakes will hence fundamentally change their character in the future. Here, a temperature increase of 1.5 or 2 degrees in winter makes a big difference.

Coupled to this temperature increase are influences on lake chemistry and biology. Consider oxygen conditions. This is the second important aspect I wanted to address. If the summer stratification phase is prolonged, oxygen will be increasing depleted in the deep water. And when that depletion is complete, as is now observed in Lake Stechlin below 40 metres depth, impacts on fish and other organisms are dramatic, and nutrient dynamics in the lake are also greatly affected.

In addition, we must not forget extreme weather events. This is the third point. An increase in both the frequency and intensity of such events is projected for the future. This includes both heat waves and storms. Heat waves can trigger mass developments of potentially toxic blue-green algae, as well as fish mortality. Storms can also trigger such developments, as we have observed in Lake Stechlin and were also able to reproduce in a large-scale field experiment. An important outcome of these observations is that clear-water lakes, which have been little influenced by humans or have been restored, are particularly vulnerable.