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The primary drivers of the recent accelerated warming of the Laurentian Great Lakes from 1982 to 2012 are explored through observations, remote sensing, and regional climate model experiments. The study focuses on the abrupt warming from 1997 to 1998 as a proxy for the long-term warming trend. The lake surface warming has been heterogeneous in both space and time, ranging from moderate warming in late spring over the southern lakes and shallow areas of the northern lakes to strong warming in mid-summer over the northern, deep lake areas. The greatest lake warming between 1997 and 1998 occurs over the deepest areas of Lake Superior during mid-summer, primarily arising from enhanced heat accumulation during the mild winter of 1997/1998 and amplified by greater incoming surface solar radiation and air temperature during the spring of 1998, according to model experiments. The mild winter condition, together with the increased solar radiation and air temperature during spring, causes an earlier onset of springtime stratification, resulting in enhanced heat absorption by surface water and thereby contributing to lake surface warming during the subsequent summer in 1998 compared with 1997. In contrast, the modest peak warming over southern lakes and shallow areas of northern lakes from 1997 to 1998 is a rapid response to synchronous increases in solar radiation and air temperature during May between the 2 yr. Changes in antecedent wintertime lake ice cover are found to have played only a minor role in the accelerated warming trend of the Laurentian Great Lakes.

Paleolimnological approaches were used to assess the ecological and environmental implications of diatom assemblage compositional changes recorded over the last , 200 yr from four sites in the Lake of the Woods (LoW), Ontario, Canada. Comparisons between a reference site (Whitefish Bay) and three disturbed sites (Bigstone Bay, Paleolimnological Project site No. 1, and Forrest Island) provide insights into the effects that multiple stressors (dam construction, total phosphorus [TP] changes, and recent warming) have had on the ecology of this highly complex freshwater system. Overall patterns of diatom compositional changes from highresolution 210Pb-dated sediment cores revealed a strong temporal coherence ca. 1910 and then again over the last few decades among all sites. Hydromanagement activities at the turn of the 19th century and recent warming over the last few decades played key roles in the LoW diatom changes. Diatom compositional changes at all sites were significantly related to trends in nearby air temperature records over the past century and to changes in lake ice phenology over the past , 40 yr from Whitefish Bay. Turn-of-the-century hydromanagement activities do not appear to have had long-term effects on diatom-inferred TP (DI-TP), particularly at the disturbed sites. Clear decreases in DI-TP over the last few decades were evident at all sites, but at the reference site these decreases were well below pre-disturbance levels, which we link in part to recent warming. Substantial changes in climate will amplify the effects of multiple stressors on lake water properties that have major implications on algal communities.

A generalized empirical approach to the problem of the prediction of intermittent lake ice cover for climate- change studies is presented and applied to historical ice data from Muggelsee, a shallow lake in northern Germany that experiences either no ice cover, intermittent ice cover, or continuous ice cover, depending on the severity of the winter. The approach presented allows the total duration of ice cover in any winter to be estimated in terms of an air-temperature probability function. Advantages over traditional empirical methods based on the mean air temperature during a fixed period or on the computed number of negative degree-days are: (1) it is valid for both continuous and intermittent ice cover; (2) it is not necessary to specify a fixed time period for integration in advance; (3) the results emerge in time units, allowing them to be directly compared with the measured duration of ice cover without prior calibration; and (4) it is based on statistical properties of the ambient air temperature that are routinely forecast by climate models, making it easy to apply predictively. Regional climate model forecasts for northern Germany imply that for Muggelsee, the percentage of ice-free winters will increase from ,2% to over 60% by the end of the current century.

Records of freezeup and breakup dates for Grand Traverse Bay, Michigan, and Lake Mendota, Wisconsin, are among the longest ice records available near the Great Lakes, beginning in 185 1 and 1855, respectively. The timing of freezeup and breakup results from an integration of meteorological conditions (primarily air temperature) that occur before these events. Changes in the average timing of these ice-events are translated into changes in air temperature by the use of empirical and process-driven models. The timing of freezeup and breakup at the two locations represents an integration of air temperatures over slightly different seasons (months). Records from both locations indicate that the early winter period before about 1890 was - 15°C cooler than the early winter period after that time; the mean temperature has, however, remained relatively constant since about 1890. Changes in breakup dates demonstrate a similar 1.0-l .5”C increase in late winter and early spring air temperatures about 1890. More recent average breakup dates at both locations have been earlier than during 1890-l 940, indicating an additional warming of 1.2”C in March since about 1940 and a warming of 1 . 1°C in January-March since about 1980. Ice records at these sites will continue to provide an early indication of the anticipated climatic warming, not only because of the large response of ice cover to small changes in air temperature but also because these records integrate climatic conditions during the seasons (winter-spring) when most warming is forecast to occur. Future reductions in ice cover may strongly affect the winter ecology of the Great Lakes by reducing the stable environment required by various levels of the food chain.

Top (Top et al. 1983, 1985) has found that in laboratory-grown ice and in Arctic ice floes 50% of the gas is retained in the ice upon freezing. As Top points out in his comment, Bari and Hallett (1974), whom we cite in Wharton et al. (1986), actually discussed how and when the ice-rejected dissolved gas gets back into the ice. We did not misquote this reference. Careful examination of the Bari and Hallett paper will reveal that in their experiments “bubbles cease abruptly” at freezing rates below about 3 pm sl. The freezing rate of the ice in the Antarctic lakes is estimated to be 30 cm yrl which corresponds to 0.009 pm s-l, well below the limit of bubble formation. The rate of freezing is an important variable in the exclusion of gas when water freezes. Hence, we do not expect a significant amount of gas to be retained in the Antarctic lake ice. This is consistent with observations of the ice cover. The results from laboratory experiments and Arctic floes reported by Top are not relevant since much higher freezing rates occur in these relatively thin ice environments. Indeed, the thick, stable, perennial ice cover of an Antarctic lake is unique in nature. Therefore, it is not too surprising that Arctic experience led Top to an incorrect conclusion. In conclusion, we feel that our original assumption of 100% exclusion is indeed representative of the conditions in the Antarctic lake. Top’s other comments are based on his incorrect estimate of the gas retention and hence do not apply.