Lakes & Climate Change
Kevin Rose | Miami University
Gases that trap heat in the atmosphere are known as greenhouse gases. The major greenhouse gases are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)1. At present, atmospheric CO2 is nearly 35% higher than preindustrial levels and is increasing1. Although these gases are released from natural activity, human activity is responsible for the increase of greenhouse gases in the atmosphere. According to both the European Environment Agency and the United States Environmental Protection Agency, CO2 emissions account for the largest share of total greenhouse gas emissions, equivalent to 80-85% of total emissions. Fossil fuel combustion used for transportation and electricity generation are the main source of CO2 emissions, contributing to more than 50% of total emissions.
Natural sources of greenhouse gases include lakes and other freshwaters such as rivers, streams, ponds, and wetlands as well as terrestrial landscapes such as forests and fields. Lakes are active, changing, and important regulators of (CO2), methane (CH4), and nitrous oxide (N2O).
Lakes play a much greater role in global carbon cycling than their area would otherwise predict. While lakes make up less than 2% of Earth’s surface area, they bury over three times more carbon in their sediments than all of the world’s oceans combined2. This means on a per area basis lakes bury over 100 times more carbon than the oceans. Small lakes that contain large amounts of algae tend to bury the most carbon; thus, the small drainage ponds, farm ponds, and recreational lakes around the world are important sites for carbon cycling and understanding global climate change3.
Despite the fact lakes bury huge amounts of carbon, they also tend to release more carbon dioxide to the atmosphere than they absorb, making them net sources of greenhouse gases. Most of the world’s lakes are supersaturated in CO2 and consequently release some of it to the atmosphere4. In fact, lakes and other freshwaters release almost as much CO2 as all the world’s oceans. This occurs because lakes generally drain large landscapes and the carbon from forests, fields, and lawns becomes concentrated in lakes where it can be buried or released into the atmosphere. Research studies show that the CO2 released from lakes comes from organism respiration — the breathing of bacteria, algae, zooplankton, fish, and other species5.
While methane is less common than CO2, it is a highly potent greenhouse gas. It has about 20 times more warming potential than CO2. Lakes contribute about 10% of total natural methane emissions, and they produce more methane than the oceans6. Many lakes and other freshwaters produce methane during warm summer conditions or when oxygen levels underwater drop. Most methane is produced in lake sediments when oxygen is no longer present due to different communities of bacteria that grow in environments without oxygen. In some lakes, bubbles can be seen rising from sediments; these bubbles are often methane produced by bacteria in oxygen deprived sediments6. Shallow areas around the shores of warm lakes are hot spots for methane production. Overall, lakes are important sites for carbon dioxide and methane production and release.
Lakes and other freshwater resources are also sources of nitrous oxide (N2O) cycling, another potent greenhouse gas that is produced in warm lakes by bacteria and other microbes. Within lakes, shallow sediments contribute most to N2O emissions7, while organisms in deep open waters may consume more N2O than they release. Lake shape may be an important predictor of N2O release, as shallow lakes with expansive shorelines may release more N2O than they produce compared with round deep lakes.
Is Hydroelectric Power a “Green” Energy Source?
Lakes and reservoirs are often built or used to generate power. In fact, so much water is retained behind dams that global sea level rise has been reduced by about 0.02 inches (0.55 mm) per year over the past 50 years8. Because fossil fuels are not used to produce hydroelectric power, lakes and reservoirs are often thought of as “green” energy sources. But lakes and reservoirs release potent greenhouse gases — particularly carbon dioxide and methane — into the atmosphere. If a hydroelectric dam releases enormous volumes of greenhouse gases, is it a “green” energy source?
Greenhouse gas emissions from freshwater lakes and reservoirs and their contribution to the increase of greenhouse gases in the atmosphere are at the heart of a worldwide debate concerning the electricity generating sector9. Hydropower represents about 20% of the world’s electricity generation capacity and on average emits 35 to 70 times less greenhouse gases per unit power generated than thermal power plants10. When reservoirs are first built for power generation, soils, plants, and trees are flooded. The decay of this plant and soil material can contribute to large emissions of CO2 and CH4 during the first few years after reservoir construction11. Studies show that 3-10 times more greenhouse gases are produced by newly formed reservoirs than from natural lakes of the same size in the first 2-5 years after a reservoir is constructed12.
Beyond the initial release of greenhouse gases, lakes and reservoirs also continue to release carbon dioxide and methane as they produce power. As methane and carbon dioxide enriched water passes through turbines, hydrostatic pressure drops and a large portion of the gas rapidly escapes to the atmosphere. In some regions, such as tropical reservoirs where methane production can be high, reservoirs can release more greenhouse gases than fossil fuel alternatives13.
While lakes and reservoirs used to generate hydroelectric power can release greenhouse gases, they typically release far smaller amounts than traditional fossil fuel based power plants. Hydroelectric is not a perfectly “green” energy source, but it is often much more environmentally friendly than alternative choices and can be part of an alternative energy solutions plan.
- IPCC, 2007
- Dean Gorham 1998
- Downing et al. 2008
- Sobek et al. 2005
- Del Giorgio et al. 1999
- Bastviken et al. 2004a
- Huttunen et al. 2003, Wang et al. 2006
- Chao et al. 2008
- Tranvik et al. 2010
- IAEA 1996, Table 1
- Rudd et al. 1993; Kelly et al. 1997
- Tremblay et al. 2005
- Fearnside 1995, 2006
Temperature & Ice
Kevin Rose | Miami University
The Formation of Lake Ice
Water has some unique thermal properties. For most substances, density increases as temperature decreases. However, peak density of water occurs in water at around 4oC and decreases at temperatures both above and below this (see Figure 1). Because ice is less dense than the underlying water, it floats. Without this unique property, life on Earth would probably not exist. Since ice floats, it forms a barrier and insulating layer between the warmer underlying water and the colder air above a lake. If ice were denser than water in the way that the solids of most materials are denser than their liquid states, ice would sink. Sinking ice would expose the underlying water to cold air temperatures which would again freeze the water and sink. Eventually this process would continue until a lake was completely frozen solid. Since most organisms cannot survive being frozen, this would create big problems for most aquatic species! Only in rare cases do lakes actually freeze solid – generally this only happens where lakes are very shallow and the climate gets extremely cold. Because the maximum density of water occurs at 4oC, the hypolimnion – the dense, bottom water layer of lakes – usually stays around 4oC and doesn’t change much throughout the year.
What happens under the ice of lakes? Water under the ice typically stays very cold, but above freezing. Most lakes that form ice at the surface stay at about 4oC most of the winter months. Most lake organisms (e.g., phytoplankton/algae, zooplankton, and most fish) are cold-blooded, meaning their body temperature varies with the temperature of the water. Therefore, the metabolism of organisms living under the ice slows way down compared to warmer summer months. Ice forms a barrier to the flux of gasses and energy flowing into and out from lakes. In some cases, oxygen levels can fall under the ice which can harm the organisms that live there. However, algae continue to photosynthesize and produce oxygen as long as there is enough light, so oxygen is being created as it is being consumed.
Ice as a Climate Indicator
Lake temperatures and ice formation on lakes have been recorded throughout history. Calendar dates of freezing and thawing of lakes has been recording in writings for hundreds of years. For example, Lake Suwa in Japan has an ice record dating back more than 550 years1. Often, these early measurements were made for religious, cultural, or practical reasons concerning transportation over ice vs. open water. In many cases, these records exist before modern thermometers. Scientists can use these records to estimate past climate and weather conditions, using lakes as sentinels of broader environmental and climate change. Year to year variability can be high due to local weather conditions, but long term records yield insightful data. Data show that ice is forming on lakes later and breaking up earlier than it did decades and centuries ago1. These simple records of ice provide a natural indicator of climate change that can be widely observed.
Does Ice Affect Climate Warming?
Water has a high specific heat capacity, about 3,000 times greater than air2. Thus, it takes much more heating or cooling to alter the temperature of a lake compared with the air. While changes in climate and weather may alter the air temperature rapidly, changes in water temperature usually occur slower. Slow changes in water temperature are especially common in larger lakes due to their high thermal mass. However, records show that in some lakes water temperature is changing faster than air temperatures – and ice may be the reason.
While water generally warms and cools slower than the atmosphere, research on lakes, temperature, and global climate change show this is not always true. A long-term data set collected from 1945 to present day on Lake Baikal in Russia shows that the water temperature is actually increasing about twice as fast as the air temperature3! Lake Baikal is the deepest, largest (by volume), and oldest lake in the world; because of its huge size, it has a large thermal mass (see Facts and Figures to learn more about the largest lakes of the world). Additionally, Lake Baikal is home to some unique animals, including the only species of freshwater seal in the world. This seal depends on the lake’s water temperature and ice conditions to survive. It’s been hypothesized that the increase in temperature is due to changes in ice on the lake. Affected by global climate change, ice now forms on the lake later in the year and disappears earlier in the year than it used to. The change in ice alters the color of the lake surface — ice and snow cover are generally white, while the lake water itself is a deep blue. The change in color along with changes in ice alters the heat absorbed from solar radiation, thereby increasing the rate of long-term warming of the world’s largest lake.
- Magnuson, J.J. et al., 2000, Historical Trends in Lake and River Ice Cover in the Northern Hemisphere, Science
- Molles, M.C., 2002, Ecology Concepts and Applications, New York
- Hampton, S.E. et al., 2008, Sixty years of environmental change in the world’s largest freshwater lake – Lake Baikal, Siberia, Global Change Biology
- Vallentyne J.R., 1957, Principles of Modern Limnology, American Scientist
- Lake Baikal: The Pearl of Russia Image Gallery. Pacific Environment. Retrieved online at: http://www.pacificenvironment.org/gallery.php?gal=6