whoever found his letter to measure the distance to the edge of the glacier again and send the result to him at Ohio State University In 2013 biologist Dr Warwick Vincent and his colleague Denis Sarrazin discovered the bottle and letter Following Walker’s wish they measured the distance to the glacier They discovered it was now 401 feet 122 m away 2 In the 54 years since Walker first measured it the glacier had retreated 233 feet 71 m It was shrinking Walker did not live long enough to receive a response from Vincent He died the same year he wrote the letter He was only in his twenties But Vincent remarked on Walker’s brilliance in leaving this message for the future In the 1950s he says It was unthinkable that this would melt 3 Walker was one of the first scientists to realize that the climate was warming ice was melting and future glaciers might look very different His data point a comparison of glacier change over time is valuable Vincent said It is very old very rare and from an isolated location Vincent and Sarrazin decided to carry on the tradition They photographed Walker’s letter replaced it and added a letter of their own They asked the next finder to measure the distance to the glacier again and report back to them MEASURING ICE CHANGES Finding Walker’s decades-old letter was lucky but scientists do not rely only on luck to learn about ice melt and glacier retreat So how do they know glaciers and ice sheets are retreating How do they know sea ice is melting How do they know how fast it is happening One way is by using satellite imaging Satellites in orbit around Earth monitor 6
October 1986 September 2002 Satellite photos taken decades apart show changes in Alaska’s Bering Glacier glaciers ice sheets and icebergs When scientists compare images based on data taken at different times they can observe changes in the extent of ice 7
Other scientists study ice cores to understand the history of climate They drill cores deep into ice sheets or glaciers and analyze gases trapped in bubbles within the ice These gases provide glimpses of the composition of past atmospheres They allow scientists to determine atmospheric temperature at those times Scientists have used ice cores to build a picture of Earth’s climate for the past 800,000 years An understanding of past climates can help predict future climates including rates of ice melt SATELLITES PHOTOGRAPH ICE MELT Surface ice on Greenland’s ice sheet is the top of the ice only The ice sheet is only a few meters thick at its edges but toward the center it can be 10,500 feet 3,200 m or almost 2 miles thick 4 On average melt occurs over approximately one-half of the ice sheet’s surface during the summer But in 2012 melt occurred over nearly all of the surface The melt progressed from approximately 40 percent on July 8 to 97 percent by July 12 5 Surface meltwater quickly refreezes over most of the ice sheet But near the coast much of it drains into the ocean The 2012 change was documented by three satellites measuring different ice properties and passing over Greenland at different times Lora Koenig a glaciologist who helped analyze the data says such extreme melts happen approximately every 150 years The last one was in 1889 But if they continue to happen as scientists expect it will be worrisome Koenig says 6 Scientists also make on-the-spot measurements of physical characteristics including temperature rates of glacier retreat and rates of sea ice melt Their tools range from sticks jammed in the ice to measure height to the latest scientific instruments To measure changes in land ice they travel over ice sheets often in bitter cold and high winds To measure sea ice they conduct cruises on research vessels dodging ice floes One reason for on-the-spot investigations is simply that they bring home the reality of climate change and ice melt much more clearly than photographs or pages of numbers Also those firsthand observations and measurements can validate the data from satellite images 8
MELTING SEA ICE Scientists at the National Snow and Ice Data Center in Boulder Colorado have been collecting precise measurements of the extent of melting Arctic sea ice since 1978 Nearly 40 years of data show a clear rapid melting trend Ice amounts are always at their minimum during the month of September at the end of summer melting Since measurements began total September ice coverage has decreased rapidly In the 1980s the Arctic contained approximately 773,200 square miles 2 million sq km of old ice or ice that is at least five years old By 2010 there were only 22,000 square miles 57,000 sq km of old ice a decrease of more than 97 percent 7 Researchers are trying to figure out why Arctic ice is melting so rapidly A 2015 international research expedition led by Harald Steen of the Norwegian Polar Institute surveyed a region around the North Pole on the RV Research Vessel Lance The group spent an entire season on the ship from January through the early summer ice breakup They seldom used engines instead drifting through ice floes and worked through six weeks of the 24-hour darkness of polar winter They explored nearby ice floes on foot skis and snowmobiles 9 DRONES MEASURE SEA ICE Christopher Zappa uses unmanned airborne vehicles or drones to study Arctic ice melt Drones get closer to the action than satellites and allow him to study much wider areas than when working from a research ship With drones he says we can study melting and other processes as they’re happening on a very fine scale 8 Zappa is an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory He and his colleagues launch drones from the Svalbard Archipelago a group of islands between Norway and the North Pole Each eight-foot 2 4 m drone carries up to 10 pounds 4 5 kg of scientific instruments They travel halfway to Greenland and back on each four-hour trip skimming the water surface to make close-up observations Different instrument packages measure variations in water and ice temperature the structure of disintegrating ice and ocean salinity or saltiness
