Ice Data

From Early “Citizen Scientists” Confirms Warming Since Industrial Revolution

by Adam Hinterthuer, Limnology News – Number 25, Fall 2016

Woodblock print of Lake Suwa, Japan, site of the earliest “citizen scientist” ice records in history. Photo courtesy: Brooklyn Museum, Online Collection

In 1442, fifty years before Columbus “sailed the ocean blue,” Japanese priests began keeping records of the annual freeze date of a nearby lake. Along a Finnish river, starting in 1693, local merchants recorded when the ice broke up each spring. They are the oldest inland water ice records in human history and, according to a study published this year in Nature Scientific Reports, the recordkeeping of these historical “citizen scientists” reveals increasing trends toward later ice-cover formation and earlier spring breakup since the start of the Industrial Revolution.

“These data are unique,” says John Magnuson, director emeritus at the Center for Limnology. “They were collected by humans viewing and recording the ice event year after year for centuries, well before climate change was even a topic of discussion.”

Magnuson and Sapna Sharma, (Postdoc 2009-11, Vander Zanden), who is now an associate professor at York University, co-led the study.

The records from Lake Suwa in the Japanese Alps, says Magnuson, were collected by Shinto priests observing a legend about a male god who crossed the frozen lake to visit a female god at her shrine. A local merchant began data collection on Finland’s Torne River because the river, and its frozen-or-thawed status, was important to trade, transportation, and food acquisition.

Ice seasonality, or when a lake or river freezes over in winter or thaws again in spring, is strongly related to climate, says Magnuson. And, while such a long-term dataset is remarkable in and of itself, the climate trends they reveal are equally notable. “Even though the two waters are half a world apart and differ greatly from one another,” he says, “the general patterns of ice seasonality are similar.” For example, from 1443 to 1683, Lake Suwa’s annual freeze date was moving almost imperceptibly to later in the year – at a rate of 0.19 days per decade. From the start of the Industrial Revolution, however, that trend grew 24 times faster, to 4.6 days per decade. On the Torne River, there was a corresponding trend for earlier ice break-up in the spring, as the speed with which the river moved toward earlier thaw dates doubled.

“Although there are local factors that are influencing both systems,” says Sharma, “climate changes associated with increasing carbon dioxide emissions and air temperatures are important, perhaps overarching, factors explaining the trends.”

John Magnuson at Lake Mendota, which is also seeing later “ice on” and earlier “ice off” dates. Photo courtesy: Clean Lakes Alliance

In recent years, she notes, both waters have exhibited more extreme ice dates corresponding with increased warming. Before the Industrial Revolution, Lake Suwa froze over 99 percent of the time. More recently, it does so only half the time. On the Torne River, early ice break up used to occur in early May or later 95 percent of the time. Now those “extreme” dates are primarily in late April and early May.

The consequences of less ice span ecology, culture and economy. For example, Sharma says, “decreasing ice cover erodes the ‘sense of place’ that winter provides to many cultures, with potential loss of winter activities such as ice fishing, skiing, and transportation.” In addition, less ice cover can lead to more evaporation and lower water levels while warmer water contributes to more algal blooms and impaired water quality.

The team of researchers are planning follow-up studies to better understand how lake and river ecosystems are impacted as the number of days they spend “on ice” continues to melt away.