Snowfall on a major summit in North America's
highest mountain range has more than doubled since the beginning of the
Industrial Age, according to a study from Dartmouth College, the University of
Maine, and the University of New Hampshire.
The research not only finds a dramatic
increase in snowfall, it further explains connections in the global climate
system by attributing the record accumulation to warmer waters thousands of
miles away in the tropical Pacific and Indian Oceans.
The research demonstrates that modern snowfall
in the iconic Alaska Range is unprecedented for at least the past 1200 years
and far exceeds normal variability.
"We were shocked when we first saw how
much snowfall has increased," said Erich Osterberg, an assistant professor
of earth sciences at Dartmouth College and principal investigator for the
research. "We had to check and double-check our results to make sure of
the findings. Dramatic increases in temperature and air pollution in modern
times have been well established in science, but now we're also seeing dramatic
increases in regional precipitation with climate change."
According to the research, wintertime snowfall
has increased 117 percent since the mid-19th century in southcentral Alaska in
the United States. Summer snows also showed a significant increase of 49
percent in the short period ranging less than two hundred years.
The research, appearing in Scientific Reports,
https://www.nature.com/articles/s41598-017-18022-5
is based on analysis of two ice cores
collected at 13,000 feet from Mount Hunter in Denali National Park. According
to the authors, accumulation records in the separate samples taken from just
below the summit of the mountain known as "Denali's Child" are in
nearly complete agreement.
"It is now glaringly clear from our ice
core record that modern snowfall rates in Alaska are much higher than natural
rates before the Industrial Revolution," said Dominic Winski, a research
assistant at Dartmouth and the lead author of the report. "This increase
in precipitation is also apparent in weather station data from the past 50
years, but ice cores show the scale of the change well above natural
conditions."
Once the researchers established snowfall
rates, they set out to identify why precipitation has increased so rapidly in
such a short amount of time. Scientific models predict as much as a 2 percent
increase in global precipitation per degree of warming because warmer air holds
more moisture, but this could not account for most of the dramatic increases in
Denali snowfall over the studied period.
The research suggests that warming tropical oceans
have caused a strengthening of the Aleutian Low pressure system with its
northward flow of warm, moist air, driving most of the snowfall increases.
Previous research has linked the warming tropical ocean temperatures to higher
greenhouse gas concentrations.
The analysis includes a series of dramatic
graphs that demonstrate extreme shifts in precipitation and reinforce the
global climate connections that link snowfall in the high reaches of the North
American continent with warm tropical waters. As noted in the paper, this same
atmospheric connection accounts for a decrease in Hawaiian precipitation.
"Everywhere we look in the North Pacific,
we're seeing this same fingerprint from warming tropical oceans. One result is
that wintertime climate in the North Pacific is very different than it was 200
years ago. This doesn't just affect Alaska, but Hawaii and the entire Pacific
Northwest are impacted as well," said Winski.
The research builds on a recent study using
the same ice cores that showed that an intensification of winter storm activity
in Alaska and Northwestern Canada, driven by the strengthening Aleutian Low,
started in 1740 and is unprecedented in magnitude and duration over the past
millennium. The new record shows the result of that increase in Aleutian Low
storm activity on snow accumulation.
For this analysis, researchers were able to
segment the ice core records by seasons and years using markers like magnesium
from spring dust to separate winter snow from summer snow. To account for snow layers
getting squeezed and thinned under their own weight, the researchers applied
four separate equations used in other studies, and in all cases the corrected
record shows at least a doubling of snowfall.
According to the paper, while numerous snow
accumulation records exist, "to our knowledge, no other alpine ice core
accumulation record has been developed with such a thorough characterization of
the thinning regime or uncertainties; all of the thinning models produce a
robust increase in accumulation since the mid-19th century above late-Holocene
background values."
The researchers note that the findings imply
that regions that are sensitive to warming tropical ocean waters may continue
to experience rain and snowfall variability well outside the natural range of
the past millennium.
"Climate change can impact specific
regions in much more extreme ways than global averages indicate because of
unexpected responses from features like the Aleutian Low," said Osterberg.
"The Mount Hunter record captures the dramatic changes that can occur when
you get a double whammy from climate change - warming air combined with more
storms from warming ocean temperatures."
However, the researchers also note that the
regional findings do not necessarily mean that the same level of snowfall
increases will occur elsewhere throughout the mid- and high latitudes.
"Scientists keep discovering that on a
regional basis, climate change is full of surprises. We need to understand
these changes better to help communities prepare for what will come with even
more carbon dioxide pollution in the air," said Osterberg.
As part of the analysis, the authors suggest
that current climate models underestimate the sensitivity of North Pacific
atmospheric connections to warming tropical ocean temperatures. They argue that
refining the way the modeled atmosphere responds to tropical ocean temperatures
may improve rain and snowfall predictions in a warming world.
This research was supported by the National
Science Foundation Paleoclimate Program (P2C2).