World’s Lakes: Sentinels of Climate Change

View of Blüemlisalp and Oeschinen lake, Bernese Alps

As climate scientists scour the Earth’s surface looking for indications of climate change impacts, freshwater lakes and reservoirs are becoming the sentinels of choice for many investigations. Although they make up a small percentage of the planet’s surface area, such bodies of water–small to large–are providing clues to past climate fluctuations, as their sediments and “catchments” (the total chemical and biological material that results from the presence of the body of water) often record ancient climate shifts and impacts and offer indicators of current climate change.

About twice as much terrestrial carbon is received by inland waters as reaches the Earth’s oceans. In total, according to recent calculations, inland waters may bury (“sequester”) up to four times the carbon as do the worlds oceans. The sediments of lakes and natural reservoirs preserve many climate signals through both mineral deposition (in the surrounding or submerged sediments) and their “outgassing” of GHGs–making them “hot spots” of carbon cycling (and thus climate regulation).

Of course, water quantity is perhaps the most obvious “signal” presented by lakes. Currently, freshwater lake levels in North America are declining, and may indicate that at some point in this century, these surrounding areas may be rendered much drier than in previous centuries. In Wisconsin, smaller lake water levels rise and fall in step with the nearby Great Lakes ( called the Laurentian Great Lakes by scientists), which suggests that larger climate oscillations are driving both (note: it is much easier to gauge the water quantity /quality of a small body of water than a large one).

Lake water and bed sediments also preserve a record of water quality, in general. Lake sediment can preserve a record of prehistoric climate change (for example, in the height and position of their beach ridges) and can be used to deduce the state of water supply in past millennia. Pollen and seeds preserved in lake sediments give scientists a glimpse into the hydrological past and can paint a picture of the larger wet and dry cycles that impact a given region.

Lakes world-wide are relatively shallow (90% world wide are considered “small”) but have sufficient light penetration and nutrients to permit high levels of biological diversity and productivity. Some lakes, of course, can become highly saline over time, and some become eutrophic (too much nutrient in the water, causing an over-growth of, for example, algae, which can diminish bio-diversity rapidly). Currently, the biodiversity of many freshwater ecosystems is perilous. During the 20th Century alone, over 120 species of freshwater animals (vertebrates and invertebrates) have gone extinct, and anywhere from one fifth to half of current species (from mussels and crayfish to amphibians and fish) became endangered by the end of the last century.

Lakes also release a good deal of natural methane (CH4) gas and carbon dioxide (CO2) that was formerly mineralized and “trapped” in their sediments. It is calculated that the release of these GHGs into the atmosphere equals the amount absorbed by oceans. Thus, climate models that calculate oceanic absorption of GHGs will need to include data from inland lakes if they are to give accurate, long-term forecasts.

Lakes are also a good indicator of invasive species impacts–although manmade lakes and reservoirs attract the bulk of invasive plants and animals. It is believed that these artificial bodies accelerate the spread of invasive species by decreasing the distance to the nearest water “stepping stone”–typically a natural, freshwater body. Currently, the Great Lakes are in the throes of an “invasional meltdown” due to the aggressive growth and spread (via Eurasian ships) of some 55 species of non-native plants and animals–including the water hyacinth, which is considered to be one of the world’s most damaging invasive plant species.

Lastly, inland lakes and reservoirs offer an important sentinel function in regards to climate change impacts to forest and wetland habitats. Lower water levels and overall drying of lakes are signs of an increased potential for wild fires. For example, currently Canadian lake drying is “predictive” of a 70 to 100+ % increase in the burned area resulting from forest fires (and indicate increasingly dryer conditions as this century unfolds).

The long-term outlook for reservoirs and lakes is not good. It is believed, however, that proper reading and analysis of the many indicators (in water and sediments) offered by these bodies can give us valuable insight into future climate change trends. Climate researchers advocate global lake observation networks to supplement experimental data.

This is something that even the average, non-scientist can help with and participate in…The sentinels of climate change are all around us, we just need to know how to read the signs.

Reference for this article: Sentinels of Change (Williamson, Soros, and Schindler, Science, Feb. 19, 2009)

photo: Public Domain