The glaciers of Alaska and northwestern Canada are a major contributor of fresh water to the world's oceans. They have long been considered important contributors to global sea level, but their remoteness has complicated efforts to measure changes in ice mass. As described in a new perspective article, published in the May 27 issue of Science, satellite measurements of Earth's gravitational field and improved on-site measurements have been used to generate global maps of water-mass variation, confirming the large role Alaska glaciers play in global sea-level regulation. Climate change advocates would claim that global warming will melt Alaska's glaciers, flooding low lying countries, but determining the rate at which the glaciers are melting is not a simple thing. Far from solving the puzzle, the new observations are revealing unexpected complexities in the magnitude and rate at which Alaska glaciers respond to climate.
Nearly 30 years ago, the first regional assessment suggested that Alaska's glaciers accounted for more than one-third of the total sea-level contribution from glaciers and ice caps. This total excluded ice sheets and was based on data from only three glaciers and a largely intuitive regional extrapolation scheme—in other words, guesswork. The small number of glaciers under continuous monitoring is primarily due to the rugged and remote nature of Alaska. The 49th state's varied geography is also a complicating factor, according to perspective author Anthony A. Arendt, from the University of Alaska's Geophysical Institute.
Alaska's unique climatic and geologic environment set the stage for complex patterns of glacier change. Five mountain ranges rise up from the Gulf of Alaska to create enormous gradients in snow accumulation and air temperature that drive rapid exchanges of mass between the atmosphere and glacier surfaces. Much of the ice formed at high elevations is discharged at sea level through tidewater glaciers, which can advance and retreat over time scales that are often independent of climate. Further inland, glaciers experience drier conditions and more extreme fluctuations in temperature than their coastal counterparts, resulting in different sensitivities of these glaciers to climate. Glacier surges, volcanic events, and rapid rates of erosion alter the dynamics and geometry of ice in ways that are difficult to predict.
Before the recent efforts using aircraft with LIDAR (light detection and ranging) and gravity sensing satellites, researchers had monitored the Gulkana and Wolverine glaciers. Measured biannually since 1965, and these records show unique responses of glaciers to interior and maritime climate conditions. During the mid-1970s, Alaska experienced an increase in precipitation along with widespread warming. The Wolverine Glacier gained mass while the Gulkana Glacier lost. More recently, since the early 1990s, both glaciers have been steadily losing mass.
Hubbard glacier calving.
In the early 1990s, the University of Alaska, Fairbanks, glaciology group began flying light aircraft equipped with GPS and LIDAR to measure the elevation of more than 100 glaciers. These readings were then compared with elevations from maps or from earlier altimetry measurements. Additional measurements of surface elevation from satellite laser and radar altimetry, and stereo photogrammetry, have provided maps of glacier surface-elevation change covering the entire region.
In 2003, NASA and the German Aerospace Center launched the Gravity Recovery and Climate Experiment (GRACE), which measures glaciers by sensing the gravitational variation they cause. GRACE observations also show how quickly Alaska glaciers can change from year to year. “Such interannual variability suggests that researchers should be cautious about extrapolating future behavior from short time series, and highlights the need for long-term records,” Arendt states.
Effects of glaciers on Earth's gravity field.
The weight of the glaciers themselves can cause changes in elevation. In the wet maritime climate of southeast Alaska, huge seasonal variations in glacier mass force the Earth's crust and upper portions of the mantle to flex elastically, producing centimeter-scale vertical crustal displacements. Since glacier ice has been diminishing since the end of the last glacial period, around 14,000 years ago, the sea level along the coast of Alaska and northern Canada has been dropping as the lithosphere rebounds.
The complex dynamics of glaciers has been revealed by these more modern measurement efforts. Despite what some climate change alarmists would have us believe, it is not as simple as a warming climate melting all the glaciers. For example, airborne altimetry showed that the Guyot Glacier in Icy Bay thinned by over 100 m at its terminus between 2007 and 2009, while the nearby Yahtse Glacier terminus thickened by 80 m over the same period.
It's not all melting.
Does this mean that Alaska's glaciers are not melting? No. On average, Alaska glaciers have been losing mass more rapidly since the mid-1990s. But glacial melting is not constant and can vary cyclically over decades and centuries. Also, glaciers have been melting since the beginning of the Holocene warming and will continue melting until Earth begins to slip into the next frigid glacial period. Then we will not be worrying about global warming but global cooling, as glacial ice expands and sea levels drop. The real lesson here is that blanket statements about glaciers melting, or anything else having to do with climate change, are always wrong when examined closely. Climate is complex and always changing, as are its side effects—simple answers only reflect the inadequate minds that espouse them.
Be safe, enjoy the interglacial and stay skeptical.