Abstract
Water levels in the Laurentian Great Lakes have fluctuated dramatically over recent decades. Since 2015, each of the lakes has reached a record high, often following a recent record or near-record low. These exceptional swings have motivated examinations of changes in lake level variability, particularly given the known climate change-driven intensification of the hydrologic cycle. Recent studies have presented evidence of rising lake level variability and changing water balance components (i.e., overlake precipitation, overlake evaporation, and basin runoff), however a full characterization of trends in variability is needed. Here, we build on previous analyses by quantitatively answering the question: are trends in hydrologic interannual variability over the Great Lakes over recent decades – both lake levels and individual hydrologic components – statistically robust, or simply the result of random chance? Using two non-parametric trend tests, we find that interannual variability of lake levels is significantly increasing in Lakes Superior, Michigan-Huron, and Erie, while decreasing in well-regulated Lake Ontario. We also find robust increasing variability in overlake precipitation, overlake evaporation, and basin runoff for the vast majority of lakes. These results suggest that critical work must follow to both attribute causes of detected trends and to determine if trends will continue increasing in the future with continued anthropogenic climate change.
1. Introduction
Recent extraordinary shifts in Great Lakes water levels have prompted
questions about potential changes in year-over-year lake level
variability. Changes in the variability of Great Lake levels, namely,
how quickly lake levels fluctuate between higher and lower water levels,
can have dramatic environmental and societal impacts. Examples include
shifts in shoreline erosion patterns (Gronewold and Stow, 2014;
Davidson-Arnott, 2016), shipping costs (Millerd, 2010; Lindeberg and
Albercook, 2000; Wang et al., 2012), tourism and recreation (Wall, 1998;
Hartmann, 1990), and risks to critical infrastructure like water
resource management (de Loe and Kreutzwiser, 2000), hydropower (Meyer et
al., 2017), and toxic waste facilities (Environmental Law and Policy
Center, 2022). Researchers and the public alike have thus been
captivated by the rapid transition of Great Lake levels between record
low and high lake levels and the resultant impacts (e.g., Gronewold et
al., 2021; Egan, 2021). This interest is further motivated by the
observed and projected intensification of the hydrologic cycle due to
anthropogenic climate change (IPCC, 2021; Seager, 2014). Within this
context, Gronewold et al. (2021) presented evidence of rising lake level
variability and described the situation caused by this hydrologic cycle
intensification as a “continental-scale hydrological tug-of-war”
between changing water balance components.
Lake levels of large lakes are dominated by three net basin supply (NBS)
components: overlake precipitation, overlake evaporation, and basin
runoff, where the collective balance of these three components largely
determine Great Lakes levels (Δlake storage =
poverlake + rbasin -
eoverlake) (Gronewold et al., 2021). Note that we define
runoff here as the amount of water entering the lake from all incoming
river systems in a respective Great Lakes basin, excepting flow from any
upstream lakes. These components are all expected to change with the
amplification of anthropogenic climate change and trends in these
components have already been well observed. For instance, Javed et al.
(2019) find increasing evaporation, spatially mixed results on
precipitation, and no change in runoff, over Lakes Michigan and Huron.
Harp and Horton (2022) characterize an increase in wet day precipitation
intensity of ~5% over the U.S.-portion of the Great
Lakes basin from 1951 to the present. Looking forward, Mailhot et al.
(2019) found increases in net basin supply components with an
intensifying annual cycle, but claimed “no long-term changes can be
confidently estimated for lake levels.” Kayastha et al. (2022) used a
regionally downscaled model to project future Great Lake levels and
found a rise in both water levels and net basin supply components,
particularly overlake precipitation and runoff. The climate
change-driven increase in Great Lake levels was similarly projected by
Van De Weghe et al. (2022). These findings differ with earlier work by
Hayhoe et al. (2010) which projected falling Great Lakes levels based on
increasing evaporation rates with increasing regional temperatures.
Examining individual hydrologic components, Wang et al. (2018) project a
16% increase in lake evaporation by the end of the 21st century in a
high greenhouse gas emissions scenario (RCP8.5). However, despite
examination of trends of lake levels, little attention has been given to
statistically characterizing observed trends in variability for either
Great Lakes levels or their net basin supply components. Here, we
address this knowledge gap by providing a statistically rigorous
assessment of changes in interannual variability over the past five
decades.