The Truth About Alaskan Glaciers

 

The Alaskan glaciers are retreating, so it must all be our fault.

Right? Well, maybe not.

 

 

 

 

Dr Bruce Molnia works with the US Geological Survey and has spent the last 40 years studying glaciers in Alaska. In 2008 he published “Glaciers of Alaska”, a hugely comprehensive and impressive volume which attempts to map and measure every single glacier in Alaska. This work forms Chapter 8 in the USGS publication “Satellite Image Atlas of Glaciers of the World”. Molnia’s main task was to put on record the current size and extent of these glaciers, rather than to analyse past changes to them.

However, he does include a section called “Background and History”, which begins by stating “During the Little Ice Age, Alaska’s glaciers expanded significantly. The total area and volume of glaciers in Alaska continue to decrease, as they have been doing since the 18th century”. So in one simple sentence, we learn that the glacial retreat began long before modern man bought his first SUV. We also learn that the glaciers were much smaller before the Little Ice Age.

Although Molnia’s work does not go into the history in more detail, there are two other recent studies which tell us much more.

 

The first is the paper “Post Little Ice Age Rebound in the Glacier Bay Region” by Motyka,Larsen,Freymueller and Echelmeyer from the Geophysical Institute at the University of Alaska Fairbanks and published in 2007. The study, as the name suggests, attempts to measure glacial isostatic changes; in other words measuring how much the land around the Glacier Bay region in Alaska has risen as a result of retreating glaciers.

These measurements are based on three methods :-

1) GPS measurements over a period of about 5 years leading up to publication.

2) Tide Gauge Data

3) Raised shoreline surveys.

The authors found there to be very good correlation between the three different methods and also with earlier studies. The conclusion states very clearly “Dates for the initiation of emergence is estimated to have begun 1770±20 AD, the same period that Glacier Bay and other regional glaciers began retreating from their Little Ice Age maximums”. They go on to say “The GPS data show peak uplift rates of 30 mm/ yr in Glacier Bay, and also delineated a second center of rapid uplift east of Yakutat with peak rates of 32 mm/yr. These studies documented rapid and continuous total sea level changes of up to 5.7 m, and constrained the age of the ongoing uplift to less than 250 yrs”. This is quite significant, as it suggests that the rate of retreat over the last couple of centuries has not accelerated. (5.7m divided by 230 years = 25mm/year, but there was a further advance around 1890 – see below).

 They go on to make two more important points :-

1) “Overall, the newer sea level rates presented here are consistent with those determined earlier (Hicks and Shofnos, 1965) when the associated errors are considered (Larsen and others, 2004). We therefore conclude that both the pattern and magnitude of regional sea level rates have remained essentially constant at the level of measurement accuracy since the time of the earliest rate measurements. This finding is in agreement with the linear sea level rates over the entire permanent gage records at Sitka, Juneau, and Skagway (Larsen and others, 2003)”. Simply put, the rate of rebound (and therefore the rate of glacial retreat) has not been accelerating, either since the 1965 study, or during the period of the tide gauge records which date back to 1936.

2) “The pattern of sea level rates also agrees well with the pattern of uplift rates from GPS measurements in the Glacier Bay region (fig. 1)”. Again, this indicates that the rate of change, as measured by GPS during the last decade, has not been accelerating and remains similar to that of the 20th C.

 

The second paper “Tree ring dated Little Ice Age histories of maritime glaciers from western Prince William Sound, Alaska” was published in 1998 by Wiles, Barclay and Calkin. This study aimed to map glacial advances during the LIA by dating trees that had been overrun as glaciers advanced. Although the researchers only looked at the area around Prince William Sound, they do say “The detailed ‘Little Ice Age’ record from land terminating glaciers in western Prince William Sound is consistent on a timescale of decades with four other tree-ring-dated glacial histories from across the northern Gulf of Alaska. This coastal northeastern Pacific glacial record reveals the structure of the ‘Little Ice Age’ in the region and provides a strong basis for comparison with other proxy climate records spanning the past 1000 years”. In other words, the study is consistent with similar work undertaken in other parts of Alaska.

The results provide evidence of three separate glacial advances. The first was in the 12th and 13thC, followed by another in the 17th and early 18thC. The final advance was in the late 19thC. (These findings correlate closely with the historical record of the LIA in Europe and Greenland – see Brian Fagan’s book The Little Ice Age).

Some idea of where the glaciers had advanced from is given by dates and positions of the earliest trees discovered. A good example is shown at the Ultramarine Glacier, where remains of trees that were growing in 1306 have been found within yards of the current glacier. As the report says “The earliest record of growth is from a log at location 1 (Figure 10) that was growing by ad 1306; this requires the terminus of Ultramarine glacier to be up valley of this position at that time”. How much further up valley, we will only find out when the ice has retreated further!

The paper makes one other interesting comment:-

“The late LIA advance resulted in moraine building at nine of the study glaciers between ad 1874 and 1895. These moraines mark the greatest known extent reached by six of the study glaciers; moraines of earlier less extensive advances were presumably destroyed during this late LIA advance. At Billings, Taylor and Langdon-Kings Glaciers the c. 1890 moraine is the largest or only ridge nested within the early eighteenth-century moraine (Figure 3). Ice margins were generally close to the late LIA maximum position at the time of the first visits of scientific parties around the turn of the century”.

So we are making comparative judgments about the extent of today’s glaciers, that use as the “norm” the period when they were at or near their maximum for probably a millenium or more. Can this be sensible science?

 

Conclusions

So what do these three papers tell us about the Alaskan glaciers?

1) Glaciers advanced in three separate stages from around 1200 through to 1890.

2) There is evidence at some sites that the glaciers before these advances were smaller than they are now.

3) Glacial retreat began around 1770, but was interrupted by the late 19thC advance. Excluding this advance, the rate of retreat during the 19thC is not significantly different to the rate during the 20thC.

4) Retreat since 1900 has been near continuous and the rate of this retreat has not accelerated, at least since 1936.

 

 

Perhaps we need to keep a sense of perspective about these matters.

 

References

1) Glaciers of Alaska by Bruce Molnia.

2) Post Little Ice Age Rebound by Motyka et al.

3) Tree Ring Dated Little Ice Age Histories by Wiles et al.