Towards critical white ice conditions in lakes under global warming

Abstract

The quality of lake ice is of uppermost importance for ice safety and under-ice ecology, but its temporal and spatial variability is largely unknown. Here we conducted a coordinated lake ice quality sampling campaign across the Northern Hemisphere during one of the warmest winters since 1880 and show that lake ice during 2020/2021 commonly consisted of unstable white ice, at times contributing up to 100% to the total ice thickness. We observed that white ice increased over the winter season, becoming thickest and constituting the largest proportion of the ice layer towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, a condition which occurs more frequently during warmer winters. Thus, under continued global warming, the prevalence of white ice is likely to substantially increase during the critical period before ice-off, for which we adjusted commonly used equations for human ice safety and light transmittance through ice.

Fig. 1. Sampling locations of seasonally frozen lakes and Northern Hemisphere winter air temperatures since 1880.

a Open access map from the International Permafrost Association (https://www.eea.europa.eu/legal/copyright) showing IceBlitz sampling locations with lake names during winter 2020/2021 (red dots). Also shown are locations of lakes for which some ice quality data from the literature are available (black dots). b Times series of winter and monthly mean Northern Hemisphere air temperatures (T) from 1880 to 2021, shown as anomalies over the base period 1951 to 1980. Lines are smoothing splines using a lambda of 0.05. Red dots represent air temperatures during the IceBlitz sampling campaign. Data for air temperature are from NASA GISS Surface Temperature Analysis (GISTEMP). c Examples of IceBlitz sampling occasions in Estonia and Russia during white and black ice conditions, respectively (photo courtesy: Margot Sepp and Oxana Erina).

Fig. 2. Seasonal accumulation of white ice in lakes.

a, b Boxplots showing the seasonal development of the thickness of white ice and the percentage of white ice observed in 31 Northern Hemisphere lakes during the IceBlitz sampling campaign in 2020/2021. Boxplots depict the minimum, first quartile, median, third quartile, and maximum. April values are not shown because too few measurements were available from that month. Colors represent the lake site-specific mean air temperature anomaly during December through March in 2020/2021 relative to the base period 1951–1980. Except for the lakes located on the Kola Peninsula (Northwest Europe), all lakes experienced warmer than normal winter air temperatures during the IceBlitz campaign. Eight lakes even had 3 °C warmer air temperatures compared to 1951–1980. c Seasonal development of total ice thickness and the thickness of black and white ice in Lake Oulujärvi, Finland during 2020/2021. The orange shaded area marks the time period when air temperatures varied around the freezing point, which is relevant for the formation of white ice. d Simplified, typical winter air temperature (T) curves representing a cold (blue line) and a warm (red line) winter (data taken from Weyhenmeyer et al.). The number of days when air temperatures vary around the freezing point increases when the seasonal cycle of winter air temperatures falling below 0 °C flattens during a warm year. In our conceptual figure, the number of days when air temperatures vary around the freezing point corresponds to ~15 days during a warm winter (marked in orange) compared to ~8 days during a cold winter (marked in blue).

Fig. 3. Lake ice conditions that are critical for ice stability and for the transmittance of photosynthetically active radiation.

a Variation in the estimated allowable load on ice depending on total ice thickness and ice quality. Pure black ice conditions were modeled using Eq. (1) with A = 17.5 kg cm⁻² and pure white ice conditions using Eq. (2) with A = 3.5 kg cm⁻². The dashed line represents estimates of the allowable load using Eq. (1) with A = 3.5 kg cm⁻², which is commonly used for ice safety guidelines. Black dots show estimates for the IceBlitz dataset using Eq. (2) with A = 3.5 kg cm⁻². The red and gray shaded areas mark the allowable load for an ice thickness of 10 cm and less under pure white ice (red) and pure black ice (gray) conditions. b Daily mean (8.00 a.m. to 8.00 p.m.) under-ice irradiance (Ed_z) in Watts (W) m⁻² in Lake Vendyurskoe during spring just before ice-off in relation to the thickness of snow and white ice (H) on the lake. Data were taken from Zdorovennova et al., measured during 1997–2020. Shown is the exponential decline of Ed_z with increasing H (black line). The red shaded area marks the light availability below a 10 cm thick snow and white ice layer on a lake.