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Investigating Climatic Extreme Events in Lakes

Periodic Reporting for period 1 - IntEL (Investigating Climatic Extreme Events in Lakes)

Reporting period: 2018-11-01 to 2020-10-31

Extreme climatic events, such as storms, floods, and heat waves, can have a major influence on lake ecosystems. Evidence is now growing that the frequency and severity of extreme weather events are increasing as a result of directional climate change, and there is a growing realization that predicting the effects of future climatic conditions on lake ecosystems must explicitly incorporate extreme events. Understanding the impact of extreme weather is important because of the negative effects they can have on ecosystem services that lakes provide, such as the provision of safe water for drinking and irrigation, and economic benefits such as fisheries and tourism. As a result of EU funding initiatives, Europe has been at the forefront of high-frequency lake monitoring in recent decades and there now exists a globally unique long-term lake data archive of key parameters needed to investigate how extreme climatic events are critically altering freshwater resources across the continent. Using these globally unique data, combined with sophisticated lake modelling tools, Investigating Climatic Extreme Events in Lakes (IntEL) aimed to improve our understanding of how lakes are responding to the increased occurrence of extreme events. The ability to investigate the influence of climatic extremes in lakes across Europe will result in a fundamental change in our understanding of the consequences of climatic extremes, and will produce a step-change in the scale of investigation that has previously been possible.
The science in this inter-disciplinary project included a retrospective analysis of past extreme events in lakes (such as the 2018 European heatwave) to understand commonalities and differences across lake types (e.g. lake location and depth), and provide insight into the consequences of the increased occurrence of climatic extremes in the future. Using high frequency data from monitoring buoys to calibrate an ensemble of lake models, the project contributed to the generation of global lake projections within the ISIMIP Lake Sector. These simulations were then used by the researcher to investigate the intensity and duration of lake stratification under climate change and the increased occurrence of extreme events. This study is the first to use an ensemble of lake and climate model projections to simulate lake thermal responses to climate change, and the manuscript is currently under review.

To investigate the thermal response of 46,557 lakes to the 2018 European heatwave, a lake model, validated with satellite derived observations from 155 lakes from 1995 to 2018, demonstrated that the 2018 heatwave had a considerable influence on lake surface temperatures across the continent. Overall, the increase in air temperature had the strongest influence. However, in some lake regions, other meteorological forcing had a greater influence. Notably, higher than average solar radiation and lower than average wind speed exacerbated the influence of the heatwave on lake surface temperature in many regions, particularly Fennoscandia and Western Europe. To place the results in the context of projected 21st century climate change, the lake model was then run with input data from state-of-the-art climate model projections under three climate change emissions scenarios. Under the scenario with highest emissions (Representative Concentration Pathway 8.5) it was demonstrated that by the end of the 21st century, the lake surface temperatures that were recorded during the heatwave of 2018 will become increasingly common across many lake regions in Europe.

A one-dimensional lake model was also used to simulate the response of lakes to the increased occurrence of extreme events under climate change. Specifically, the researcher developed a new metric for identifying extreme events in lakes, termed lake heatwaves. In this work, an ensemble of climate model projections was used, under different climate trajectories, to force a lake model from 1901 to 2099. Lake heatwaves were defined as a period in which daily lake surface temperatures exceed a local and seasonally varying 90th percentile threshold, relative to a baseline climatology, for at least five days. The project investigated how lake heatwave intensity and duration responded to climate change and simulated lake heatwave events from 1901 to 2099. Metrics were derived for duration and intensity of lake heatwaves. In addition, an intensity-based lake heatwave category was used to define the relative strength of each lake heatwave, where each event was classed as being Moderate, Strong, Severe, or Extreme. The work demonstrated that lake heatwaves will become more intense and longer lasting by the end of the 21st century.
This study is the first to use an ensemble of lake and climate model projections to simulate lake thermal responses to climate change. The assessment the 2018 European heatwave demonstrated that the event had a considerable influence in lake surface temperatures across the continent. This work, particularly the manuscripts under review in high impact journals, link directly to the ISIMIP lake sector, which will contribute to the upcoming IPCC report (AR6) and will have a potential influence on policy making. The project therefore addressed fundamentally important questions of both national and international significance, and will benefit directly statutory bodies, the water industry (specifically, water supply companies), and various public groups, including individuals who use lakes recreationally and scientifically interested individuals. Notably, lake thermal extremes will have knock-on effects on freshwater bodies, including the increased potential for the occurrence of toxic algal blooms and mass mortality events (e.g. fish kills). Understanding the occurrence and consequences of thermal extremes in lakes is important because of the negative effects they can have on the ecosystem services that lakes provide, such as the provision of safe water for drinking, recreational use, and economic benefits such as fisheries and tourism. Algal blooms (which could worsen with the increased occurrence of lake thermal extremes) threaten the quality of water and, in turn, are a cause for concern to the industry. As water supply companies, for example, are required to guarantee the production of safe drinking water irrespective of water quality, treatment plants often use a permanent high dosage of water treatment chemicals. This essentially safeguards the potential influence of algal blooms on water quality. Anticipating algal blooms, as a result of lake thermal extremes, would therefore be a great benefit to water supply companies.
Lake surface temperature in the 2018 heatwave (a, b) & drivers of those changes in temperature (c).