Archive for September, 2012

early September Greenland ice reflectivity remains low, some melting remains active

Friday, September 7th, 2012

While ice sheet average temperatures are declining with the return of the cold season this September, ice sheet reflectivity (a.k.a. albedo) remains anomalously low (Fig. 1). The low albedo values reflect (pun alert) where snow accumulation has not yet covered the darkened surface. There remain some areas where melting remains active at the lowest elevations of the ice sheet (Fig. 2). Melt promotes or maintains low ice reflectivity.  Available sunlight in 2012 thus continues to heat the ice and snowpack more than it has in the period of observations beginning in 2000. Less heat will be required to maintain melting or bring the ice to the melting point in the future. It is easy to predict early melt onset in 2013 and a continuation of increasing ice sheet melt rates that contribute to the recently observed net ice loss from Greenland.

Fig. 1. Surface solar reflectivity retrieval from the NASA MODIS sensor on the Terra satellite

Fig. 2. Land surface temperature retrieval from the NASA MODIS sensor on the Terra satellite.

For more information about these analyses see http://bprc.osu.edu/wiki/Greenland_Ice_Albedo_Monitoring and http://bprc.osu.edu/wiki/Greenland_Ice_Surface_Temperature_Monitoring

 

continued retreat of Greenland’s most productive glacier

Wednesday, September 5th, 2012

In terms of ice flow discharge, one of Greenland’s most productive outlets from the inland ice sheet, if not the most productive glacier in the Northern Hemisphere, the Ilulissat glacier (also known as the Jakobshavn glacier) continues to retreat. The net area change at this glacier since late summer 2000 is a loss of 122 sq km, equivalent with 1.4 x Manhattan Is., retreating effectively 18 km (11.2 mi) in 12 years. In 2012, this glacier front lost an an area of 13 sq km, measured from August 2011 to August 2012. Thi’s year’s area loss is the largest since the 2007-2008 interval. A concern is that this and other major marine terminating glaciers, as they retreat, they accelerate, increasing their global sea level contribution. Indeed, once the ice shelf in front of this glacier disintegrated, by the end of summer 2003, it’s speed had doubled (Joughin et al. 2004).

Area changes at select Greenland marine-terminating glacier outlets are measured in consecutive annual end-of-melt-season NASA MODIS satellite images (Box and Decker, 2011). Here, the same approach is applied to updated our area change estimates to span the 12 annual intervals since year 2000.

Flying over Ilulissat glacier this July, it was stunning to notice how retreat has proceeded upstream into a northern tributary, producing effectively two main calving fronts to this ice sheet outlet. The faster stream from the west off the right side of the photo also remains in retreat. The glacier is based below sea level more than 75 km inland (Thomas et al. 2011).

On 22 July, 2012, the northern branch of the Ilulissat (a.k.a. Jakobshavn) glacier had retreated to a new minimum. It's arguably divided into two glaciers, one stream from the northeast (featured here) and a faster stream from the west off the right side of the photo. Photo - J. Box

The Ilulissat glacier is considered the most productive in the Greenland in terms of ice flow discharge into the ocean (see e.g. Rignot and Kanagaratnam, 2006), even the fastest continuously flowing glacier in the world.

This May 2002 view features the now gone Manhattan Is. sized ice shelf flowing out of the frame to the WNW. Photo – J. Box

Thomas et al. (2011) summarize key aspects of what is known of this glacier, including its retreat history since 1852, its doubling in speed in the 2000s:

Ilulissat glacier ” has a balance discharge (equivalent to total snowfall within its catchment basin) of about 30 km3 ice per year (Echelmeyer et al., 1991), and converges into a rapidly moving trunk ~4 km wide, that flows into a deep fjord on the west coast of Greenland. Until recently, a 15‐km floating glacier tongue was wedged between the fjord walls. VHF‐band radar surveys (J. Plummer et al., A high‐resolution bed elevation map for Jakobshavn Isbræ, West Greenland, submitted to Journal of Glaciology, 2011) show the fastest part of the glacier flowing in a deep trough, more than 1000m below sea level. Between 1850 and 1962, the calving front retreated ∼25 km up the fjord, and then stabilized to within 3 km until the mid‐1990s. During the 1980s and early 1990s, the glacier had a small positive mass balance [Echelmeyer et al., 1991]. Then, probably in 1997, the glacier began to thin (Thomas et al., 2003) at rates that increased to 15 m per year near the calving front, where its speed almost doubled to >12 km per year by 2003 as the floating tongue finally broke up, with continued increases since (Joughin et al., 2008), (Figure 1). Progressive retreat of the grounding line resulting from the rapid thinning reduced the basal and lateral drag acting on the glacier [Thomas, 2004], and by 2005 the glacier was thinning by >2 m per year at a distance of 50 km from the calving front, increasing to >5 m per year between 2005 and 2007.

Work Cited

  • Box, J.E. and D.T. Decker (2011) Greenland marine-terminating glacier area changes: 2000–2010, Annals of Glaciology, 52(59) 91-98. .PDF
  • Echelmeyer, K., T. Clarke, and W. Harrison (1991), Surficial glaciology of Jakobshavns Isbrae, west Greenland: Part I. Surface morphology, J. Glaciol., 37(127), 368–382.
  • Joughin, I., W. Abdalati, and M. Fahnestock (2004), Large fluctuations in speed on Greenland’s Jakobshavn Isbrae Glacier, Nature, 432(7017), 608–610, doi:10.1038/nature03130.
  • Joughin, I., I. Howat, M. Fahnestock, B. Smith, W. Krabill, R. Alley, H. Stern, and M. Truffer (2008), Continued evolution of Jakobshavn Isbrae following its rapid speedup, J. Geophys. Res., 113, F04006, doi:10.1029/2008JF001023.
  • Rignot, E. and P. Kanagaratnam (2006), Changes in the velocity structure of the Greenland Ice Sheet. Science, 311(5673), 986– 990.
  • Thomas, R. (2004), Force‐perturbation analysis of recent thinning and acceleration of Jakobshavn Isbræ, Greenland, J. Glaciol., 50(168), 57–66, doi:10.3189/172756504781830321.
  • Thomas, R., W. Abdalati, E. Frederick, W. Krabill, S. Manizade, and K. Steffen (2003), Investigation of surface melting and dynamic thinning on Jakobshavn Isbrae, Greenland, J. Glaciol., 49, 231–239, doi:10.3189/ 172756503781830764.
  • Thomas, R., E. Frederick, J. Li, W. Krabill, S. Manizade, J. Paden, J. Sonntag, R. Swift, and J. Yungel (2011), Accelerating ice loss from the fastest Greenland and Antarctic glaciers, Geophys. Res. Lett., 38, L10502, doi:10.1029/2011GL047304.