Introduction
An Historical First: Rain at Greenland Summit Station
Some of you may recall the report that on August 21, 2021, rain was observed at the highest point on the Greenland Ice Sheet, 3250 m (10663 ft) above sea level. Not only that, but temperatures were above freezing for eight consecutive hours for only the fourth time in the historical record going back to 1950, but the third time since 2012. This observation is clearly related to global warming, which is amplified in the polar regions. On that day, there was an estimated 7 billion tons of rainfall on the ice sheet, resulting in a loss of surface ice mass the next day that was seven times above the mid-August average, according to the National Snow and Ice Data Center.
What the Climate Models Had Been Telling Us, and What They Say Now
Until recently, it was thought that by around the turn of the century (2090-2100), the Arctic would switch from more snow than rain to the opposite in the summer and fall(Climate Model Intercomparison Project (CMIP) 5). More recent research (CMIP6) has been done using better climate models which are better able to simulate Arctic (and global) precipitation and offer insights into the ways rain and snow patterns may shift over the coming decades. They now suggest that this transition will now most likely occur between 2060-2070, about 20-30 years earlier, and:
… at a lower level of global warming, potentially under 1.5 °C, with profound climatic, ecosystem and socioeconomic impacts.
McCrystall, M.R., Stroeve, J., Serreze, M. et al. New climate models reveal faster and larger increases in Arctic precipitation than previously projected. Nat. Commun.12, 6765 (2021). https://doi.org/10.1038/s41467-021-27031-y
Retrospectives of the 1979-2005 period using reruns of CMIP5 and CMIP6 models show a modest increase of 0.1mm/day, consistent with what’s been observed, so at least over the past 26 years the climate models are reasonably representing the climate at the time. Additionally, spatial patterns of high and low precipitation are consistent with observations. Incidentally, the ratio of snow to total precipitation dropped over that retrospective period by a statistically significant −6% across the observational record.
summary of model results
Changes in the ratio of rain and snow are seen in the graphics below, taken from the Nature Communications article previously referenced. The top graphic shows DJM (winter) and MAM (spring), while the bottom graphic shows JJA (summer) and SON (autumn).
In winter, Arctic area-averaged total precipitation increases by about 0.6 mm/day (2.83” for the season) in CMIP6, and by about 0.5 mm/day (2.43” for the season) in the older CMIP5. There is a significant difference in the rain versus snow partitioning of precipitation between the old and new climate simulations; in the old CMIP5, rain increases at the same rate as snow through 2100. In CMIP6, 0.3 mm/day more rain than snow occurs by 2100, while only 0.1 mm/day more water equivalent of snow occurs. Spring shows only half of the change in total precipitation for the simulations, but with more rain than snow in both simulations after is somewhat higher than in winter for both simulations. The proportional increase in rain is largest in the CMIP6 runs.
For the summer season, Arctic total precipitation increases by about 0.3 mm/day in CMIP6 (0.25 mm/day in CMIP5) by 2100. We see that the proportion of precipitation that falls as rain continues to increase for the summer season (by 0.65 mm/day in CMIP6 and 0.5 mm/day in CMIP5). Snow water equivalent decreases significantly in both global warming climate runs, by 0.25 mm/day in CMIP5 and 0.3 mm/day in CMIP6. Finally, autumn total precipitation increases by about 0.8 mm/day (3.78” for the season) in CMIP6, and by about 0.6 mm/day (2.83” for the season) in the older CMIP5. The largest difference between CMIP6 and CMIP5 occurs in autumn for rainfall (1.05 mm/day and 0.8 mm/day) and for snow water equivalent (0.4 mm/day vs about 0.18 mm/day). This is likely because of less Arctic sea ice leading to warmer temperatures at least for the first part of autumn.
Implications of Earlier Shift to More Arctic Rain Than Snow
Impact of Increased Precipitation and Rain on Reindeer/Caribou
So, what are these “profound climatic, ecosystem and socioeconomic impacts? One is the increase in Arctic precipitation resulting from global warming and its amplification in the Arctic. One resulting ecosystem impact has already been occurring and is devastating: the depletion of caribou herds in Lapland and the Arctic Archipelago because of starvation. Why are they starving? Because global warming has thrown the caribou out of sync with their environment. Additionally, the vegetation that the caribou need in the winter has been more difficult to forage because of deeper than normal snow (increased precipitation generally), or rain freezing on the snow to make it virtually impenetrable (more cold season rain). According to the Yale School of the Environment:
Thirty-four of the 43 major herds that scientists have studied worldwide in the last decade are in decline, with caribou numbers plunging 57 percent from their historical peaks. Some populations have fallen precipitously: The Bathurst herd in Canada’s central Arctic has plummeted from a peak of 472,000 in 1986 to 32,000 today — a drop of 93 percent.
Finally, deep snow or ice make it more difficult for caribou to outrun their biggest predator, wolves.
Many changes resulting from warming are conspiring against the caribou, from increased insect populations (that torment them while they are trying to eat) to replacement of lichens with small shrubs and the introduction of new infectious diseases. These negative environmental changes are certainly affecting many other Arctic species, including human beings.
Impacts of More Rainfall on Snow Fields, Glaciers/Ice Sheets and Permafrost
Snow cover and snow depth are important modulators of climate. A shorter snow season results in more absorption of incoming solar radiation. This in turn causes earlier Arctic green-up (or green-up where previously there was none), generally warmer soils to a further depth, and further thawing of permafrost with increasing methane release and damage to infrastructure such as roads, buildings, and pipelines. Furthermore, more rain and less snow results in lower accumulations of snow mass to add annually to existing ice sheets, including Greenland. Rain falling on snow, even if it doesn’t result in bare surfaces, changes the characteristics of the snow, including reduction of its albedo, again increasing absorption of solar radiation.
All things considered, it seems likely there will be increases in the proportion of rain to snow over the rest of the 21st century. The timing and degree of that increase is the only thing in doubt, and that will depend on the mean global temperature and that of the Arctic.