Worldwide alteration of lake mixing regimes in response to climate change

Lakes hold much of Earth’s accessible liquid freshwater, support biodiversity and provide key ecosystem services to people around the world. However, they are vulnerable to climate change, for example through shorter durations of ice cover, or through rising lake surface temperatures. Here we use a one-dimensional numerical lake model to assess climate change impacts on mixing regimes in 635 lakes worldwide. We run the lake model with input data from four state-of-the-art model projections of twenty-first-century climate under two emissions scenarios. Under the scenario with higher emissions (Representative Concentration Pathway 6.0), many lakes are projected to have reduced ice cover; about one-quarter of seasonally ice-covered lakes are projected to be permanently ice-free by 2080–2100. Surface waters are projected to warm, with a median warming across lakes of about 2.5 °C, and the most extreme warming about 5.5 °C. Our simulations suggest that around 100 of the stud- ied lakes are projected to undergo changes in their mixing regimes. About one-quarter of these 100 lakes are currently clas- sified as monomictic—undergoing one mixing event in most years— and will become permanently stratified systems. About one-sixth of these are currently dimictic—mixing twice per year—and will become monomictic. We conclude that many lakes will mix less frequently in response to climate change

Rapidly expanding lake heatwaves under climate change

Lake heatwaves – prolonged periods of hot surface water temperature in lakes – have recently been shown to increase in intensity and duration, with numerous potential implications for aquatic ecosystems. However, an important physical attribute of lake heatwaves that has not yet been investigated is their spatial extent, and how it varies within a warming world. Here, we show that the spatial extent of lake heatwaves, defined as contiguous regions within a lake that simultaneously experience extreme warm conditions, is increasing in the largest group of freshwater lakes on Earth, The Laurentian Great Lakes. We show that the maximum spatial extent of lake heatwaves is sensitive to inter-annual variations in winter ice cover and the timing of stratification onset in spring. Notably, we find that a lengthening of the warm summer season and, in turn, an overall increase in surface water temperature, stimulates the development of larger lake heatwaves. On average, our results suggest that the mean spatial extent of lake heatwaves has increased two-fold since 1995. We anticipate this rapid expansion of lake heatwaves to have widespread implications for heat-related impacts on aquatic species

Climate velocity in inland standing waters

Inland standing waters are particularly vulnerable to increasing water temperature. Here, using a high-resolution numerical model, we find that the velocity of climate change in the surface of inland standing waters globally was 3.5 ± 2.3 km per decade from 1861 to 2005, which is similar to, or lower than, rates of active dispersal of some motile species. However, from 2006 to 2099, the velocity of climate change will increase to 8.7 ± 5.5 km per decade under a low-emission pathway such as Representative Concentration Pathway (RCP) 2.6 or 57.0 ± 17.0 km per decade under a high-emission pathway such as RCP 8.5, meaning that the thermal habitat in inland standing waters will move faster than the ability of some species to disperse to cooler areas. The fragmented distribution of standing waters in a landscape will restrict redistribution, even for species with high dispersal ability, so that the negative consequences of rapid warming for freshwater species are likely to be much greater than in terrestrial and marine realms

Impact of the 2018 European heatwave on lake surface water temperature

In 2018 Europe experienced the warmest May-October (Northern Hemisphere Warm Season) since air temperature records began. In this study, we ran model simulations for 46,557 lakes across Europe to investigate the influence of this heatwave on surface water temperature. We validated the model with satellite-derived lake surface temperatures for 115 lakes from 1995 to 2018. Using the validated model, we demonstrated that, during May-Oct 2018, mean and maximum lake surface temperatures were 1.5°C and 2.4°C warmer than the base-period average (1981-2010). A lake model experiment demonstrated that, on average, the increase in air temperature was the dominant driver of surface water temperature change. 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 our results in the context of projected 21st century climate change, we then ran the lake model with input data from state-of-the-art climate model projections under three emissions scenarios. Under the scenario with higher emissions (Representative Concentration Pathway 8.5), we demonstrated that by the end of the 21st century, the lake surface temperatures that occurred during the heatwave of 2018 will become increasingly common across many lake regions in Europe

Severe Lake Heatwaves Attributable to Human‐Induced Global Warming

Much of the focus of global warming impacts on lakes have focused on changes in mean temperature. However, lakes are also highly vulnerable to thermal extremes. Such extremes occur, by definition, during lake heatwaves. Heatwaves in lakes have occurred globally in recent decades and have had severe negative impacts. However, unlike their atmospheric counterparts, it is currently unknown to what extent lake heatwaves are altered by human-induced climate change. Here, we estimate the human contribution to lake heatwaves, specifically focusing on the most severe events. We demonstrate that the occurrence probabilities of severe lake heatwaves increase substantially due to human influence. Our analysis suggests that 94% of severe heatwaves observed during the satellite data-taking period have an anthropogenic contribution. Globally, we suggest that severe heatwaves are 3 and 25- times more likely in a 1.5°C and 3.5°C warmer world, respectively, compared to a world without anthropogenic influence

Earlier ice loss accelerates lake warming in the Northern Hemisphere

How lake temperatures across large geographic regions are responding to widespread alterations in ice phenology (i.e., the timing of seasonal ice formation and loss) remains unclear. Here, we analyse satellite data and global-scale simulations to investigate the contribution of long-term variations in the seasonality of lake ice to surface water temperature trends across the Northern Hemisphere. Our analysis suggests a widespread excess lake surface warming during the months of ice-off which is, on average, 1.4 times that calculated during the open-water season. This excess warming is influenced predominantly by an 8-day advancement in the average timing of ice break-up from 1979 to 2020. Until the permanent loss of lake ice in the future, excess lake warming may be further amplified due to projected future alterations in lake ice phenology. Excess lake warming will likely alter within-lake physical and biogeochemical processes with numerous implications for lake ecosystems

Global reconstruction of twentieth century lake surface water temperature reveals different warming trends depending on the climatic zone

Lake surface water temperatures (LSWTs) are sensitive to climate change, but previous studies have typically focused on temperatures from only the last few decades. Thus, while there is good appreciation of LSWT warming in recent decades, our understanding of longer-term temperature change is comparatively limited. In this study, we use a mechanistically based open-source model (air2water), driven by air temperature from a state-of-the-art global atmospheric reanalysis (ERA-20C) and calibrated with satellitederived LSWT observations (ARC-Lake v3), to investigate the long-term change in LSWT worldwide. The predictive ability of the model is tested across 606 lakes, with 91% of the lakes showing a daily root mean square error smaller than 1.5 °C. Model performance was better at mid-latitudes and decreased towards the equator. The results illustrated highly variable mean annual LSWT trends during the twentieth century and across climatic regions. Substantial warming is evident after ~ 1980 and the most responsive lakes to climate change are located in the temperate regions