In this first worldwide synthesis of in situ and satellite‐derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice‐covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice‐free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

[1] Lake Superior summer (July -September) surface water temperatures have increased approximately 2.5°C over the interval 1979 -2006, equivalent to a rate of (11 ± 6) Â 10 À2°C yr À1 , significantly in excess of regional atmospheric warming. This discrepancy is caused by declining winter ice cover, which is causing the onset of the positively stratified season to occur earlier at a rate of roughly a half day per year. An earlier start of the stratified season significantly increases the period over which the lake warms during the summer months, leading to a stronger trend in mean summer temperatures than would be expected from changes in summer air temperature alone.Citation: Austin, J. A., and S. M. Colman (2007), Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback, Geophys. Res. Lett., 34, L06604,

A two-dimensional numerical model is used to study the response to upwelling-and downwelling-favorable winds on a shelf with a strong pycnocline. During upwelling or downwelling, the pycnocline intersects the surface or bottom, forming a front that moves offshore. The characteristics of the front and of the inner shelf inshore of the front are quite different for upwelling and downwelling. For a constant wind stress the upwelling front moves offshore at roughly a constant rate, while the offshore displacement of the downwelling front scales as because the thickness of the bottom layer increases as the front moves offshore. The geostrophic alongshelf ͙t transport in the front is larger during downwelling than upwelling for the same wind stress magnitude because the geostrophic shear is near the bottom in downwelling as opposed to near the surface in upwelling. During upwelling, weak stratification is maintained over the inner shelf by the onshore flux of denser near-bottom water. This weak stratification suppresses vertical mixing, causing a small reduction in stress at mid depth that drives a weak cross-shelf circulation over the inner shelf. For constant stratification, the inner shelf stratification and cross-shelf circulation are stronger. During downwelling on an initially stratified shelf, the inner shelf becomes unstratified because the very weak cross-shelf circulation forces lighter water under denser, driving convection which enhances the vertical mixing. As a result the stress is nearly constant throughout the water column and the cross-shelf circulation is slightly weaker than in the initially unstratified case. The downwelling response is essentially the same for the constant stratification and the two-layer cases. Model runs including the evolution of a passive tracer indicate that the inner shelf region acts as a barrier to cross-shelf transport of tracers from the coastal boundary to farther offshore and vice versa, due to strong vertical mixing and weak cross-shelf circulation in this region.

One of the most important physical characteristics driving lifecycle events in lakes is stratification. Already subtle variations in the timing of stratification onset and break-up (phenology) are known to have major ecological effects, mainly by determining the availability of light, nutrients, carbon and oxygen to organisms. Despite its ecological importance, historic and future global changes in stratification phenology are unknown. Here, we used a lake-climate model ensemble and long-term observational data, to investigate changes in lake stratification phenology across the Northern Hemisphere from 1901 to 2099. Under the high-greenhouse-gas-emission scenario, stratification will begin 22.0 ± 7.0 days earlier and end 11.3 ± 4.7 days later by the end of this century. It is very likely that this 33.3 ± 11.7 day prolongation in stratification will accelerate lake deoxygenation with subsequent effects on nutrient mineralization and phosphorus release from lake sediments. Further misalignment of lifecycle events, with possible irreversible changes for lake ecosystems, is also likely.

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

A 100-yr-long time series of water temperature measured just downstream of Lake Superior is used to produce proxy time series of open-lake temperature. This analysis suggests that open-water Lake Superior summer temperatures have increased by roughly 3.5uC over the last century, most of that warming occurring in the last three decades. Correspondingly, the length of the positively stratified season has increased from 145 d to 170 d. The observed amount of warming is greater than the observed change in regional temperature over the same time period by roughly a factor of two. The discrepancy can be understood in the context of reduced winter ice cover, and implies that spatially and temporally averaged ice cover in Lake Superior has decreased from 23% to 12% over the last century.Global average air temperatures have recently warmed beyond their natural limits in historic atmospheric records (IPCC 2007). However, the expected response of temperature of oceans and other water bodies is less clear. Water temperatures of large natural systems may respond to the atmospheric warming trend in unexpected ways, due to nonlinearities, geographic variability, and feedback mechanisms. Unlike air temperature records, such as the Goddard Institute for Space Science (GISS) database (Hansen et al. 1999), reliable century-scale records of measured water temperature are exceedingly rare (Nixon et al. 2004); long, continuous records in lakes more so.Some analyses of lake-water temperature trends over nearly a century have been performed in the hypolimnetic waters of tropical lakes such as Lake Tanganyika (O'Reilly et al. 2003;Verburg et al. 2003) and Lake Malawi (Vollmer et al. 2005). Due to the absence of strong seasonal variation in surface heat flux and a lack of seasonal overturn, the deep water of these lakes respond gradually, and roughly proportionally, to changes in climate, and can be reliably analyzed using relatively temporally sparse data. In contrast, mid-latitude lakes with large inter-annual and annual variability compared to the magnitude of a longterm trend (for instance, the Laurentian Great Lakes), require dense temporal coverage in order to extract a statistically significant trend. These sorts of long time series are especially important in large lakes, since it has been shown (Austin and Colman 2007) that the thermal response of a complex system like a large, seasonally ice-covered lake can significantly exceed the rate of temperature change experienced by the regional atmospheric climate. This can be explained in terms of a coincident reduction of winter ice cover, which in turns leads to earlier spring overturn and a longer warming season.One example of such a time series of daily water temperature has been collected in the St. Mary's River, just downstream of Lake Superior, at a pair of locations near Sault Ste. Marie (McCormick 1996) from 1906 to the present. These data (through 1992) were previously discussed (McCormick and Fahnenstiel 1999) along with several other long time series collected at ...

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[1] As part of an experiment to study wind-driven coastal circulation, 17 hydrographic surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999 with a small, towed, undulating vehicle. The cross-shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field. A measure of upwelling intensity based on the position of the permanent pycnocline is developed. This measure is designed so as to be insensitive to density-modifying surface processes such as heating, cooling, buoyancy plumes, and wind mixing. It is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress. This analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic observations. This relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling.