summarizes the mean 2004–2008 ice age/thickness relationship using the ICESat ice thickness dataset [25
]. For the entire 2004–2008 period (bottom right), there is a general increase in ice thickness with age of 0.36 m/year, with Rm
= 0.97 and Ra
= 0.48. First-year ice has a five-year mean thickness of 1.56 m, increasing for each class up to ~3.0 m for fifth-year and older ice. The mean ice thickness for the Arctic over this period was about 2.6 m. We note that 5+ year ice had the same thickness as 4th–year ice during spring 2008, with a mean of 2.52 m for both distributions. This phenomenon was observed in an earlier study [8
], which observed that a decline of 0.6 m in the mean sea ice thickness from 2004–2008 was entirely explained by the thinning of thick multiyear ice.
The age vs.
thickness relationships found for spring 2004 (~0.35 m/year), 2005 (~0.38 m/year), and 2006 (~0.38 m/year) were higher than found previously [6
], where computed slopes of 0.15 m/year, 0.24 m/year, and 0.17 m/year for 2004–2006, respectively were found. This is partly due to our technique of averaging together all ice at least 5 years or older into one ice age category, compared to the previous method of using ice categories up to 10 years old. A larger impact on the difference in age/thickness relationships between our study and the previous one is that the previous study’s authors reported slopes that excluded first-year ice [6
]. In 2004, the slope for ice aged 2–4 years in the previous ICESat results is ~0.18 m/year, and we compute 0.18 m/year for this same segment. The results are also similar for 2005 (0.29 vs.
0.28 m/year) and 2006 (0.21 to 0.25 m/year).
We note that the slope of the age/thickness line of best fit decreased from ~0.35 m/year in 2007 to ~0.28 m/year in 2008. There was therefore less thickness difference between younger and older ice in 2008 than in 2007. As noted previously, there was also a leveling off of ice thickness for ice four years and older in 2008. This change in the age/thickness relationship may in part reflect the extensive melt in 2007 that led to enhanced basal melting of the ice cover [14
]. While the data scatter is large, these results hint at a fundamental change in the thickness of the oldest ice in the year following the 2007 minimum.
The overall age/thickness relationships for 2009–2015, as well as for individual years, along with the IceBridge flight segments where ice thickness data was computed, are shown in Figure 3
. We find a more gradual slope for the age/thickness relationship in the IceBridge data than for ICESat: a 2009–2015 mean 0.15-m/year increase (Rm
= 0.95, Ra
From 31 March–25 April 2009, five IceBridge flights were conducted over sea ice off the coast of Northern Greenland, and two flights were focused on sea ice in the Beaufort Sea, which served as a limited sample in the inaugural campaign. We do not find a sea ice thickness/age relationship for this year. While this was an interesting initial campaign to view sea ice properties, the limited set of flights did not adequately sample all the ice age classes, with less than forty ice age cells overflown for each age class, significantly less than subsequent years (Table 1
). We also note that most of the thickness data was obtained over older ice near the Canadian Archipelago.
For the spring 2010 campaign there were eight IceBridge flights over the Arctic ice pack from 23 March–21 April. This campaign’s flight tracks covered significantly more of the Arctic than the previous year, overflying a substantial number of grid cells (Table 1
) for all ice age classes. We observe a relationship between ice age and thickness up through ice 5+ years old with an increase of about 0.13 m/year (Rm
= 0.96, Ra
= 0.29). We note that first and second-year ice thickness means are about the same (2.9 m), as are fourth-year and fifth-year and older ice (~3.3 m).
In 2011, nine IceBridge flights took place from 17–28 March. The results show an age/thickness relationship of 0.19 m/year (Rm
= 0.90, Ra
= 0.25). We see in Figure 2
that most of the sea ice flights acquired altimetry data near the coast of Greenland and the Northeast Canadian Archipelago, where ice tends to pile up and persist. For example, there were many more second-year and third-year ice pixels overflown, compared to first-year ice (Table 1
). Much of the first-year ice in this area was likely heavily deformed, as first-year ice thickness was computed up to 8 m [23
During 2012, there were 12 IceBridge flights over sea ice from 14 March to 10 April. We note an ice thickness vs.
age relationship of ~0.20 m/year (Rm
= 0.98, Ra
= 0.31), with first and second-year ice having about the same thickness as in 2010. The 2012 results span more sea ice over different regions than previous years (Figure 2
), which may contribute to the improved correlation in the mean (Rm
) over the correlation found for 2011. Furthermore, 2012 had the most ice age grid cells sampled, although 5+ year ice was sampled significantly less than younger age classes (Table 1
In 2013, 9 IceBridge flights were conducted over sea ice from 21–27 March. The age vs.
thickness relationship is found to be ~0.23 m/year (Rm
= 0.98 and Ra
= 0.42) although the difference in means for 2nd and 3rd year ice is minimal. Note again that regions bordering Alaska, Canada, and Greenland are sampled (Figure 2
), and that ice of each age is highly sampled (Table 1
). First-year ice is prevalent in the Beaufort and Chukchi Seas, and we note that the first-year ice thickness is considerably lower (~2.0 m) than in 2012 (2.8 m) and previous years. Furthermore, the 2013 overall mean thickness is nearly 80 cm thinner than the previous year. This likely relates in part to the chosen IceBridge flight paths, but may also reflect a thinning of the ice cover following the record September 2012 sea ice minimum [27
]. Thicknesses are dependent on specific flight tracks, which may or may not sample representative data for the entire Arctic. Furthermore, these thicknesses are from the quick-look dataset, which will be undergoing further quality control before final release. However, it is interesting and may be significant as a snapshot of the changing Arctic ice cover.
For 2014, 15 IceBridge flights, from 12 March to 28 April, overflew sea ice, obtaining a sufficient number of ice thickness observations to utilize 861 ice age grid cells (Table 1
). Ice thickness for all age classes exhibit a small variation, with an age/thickness relationship of only 0.16 m/year, with Rm
= 0.96 and Ra
= 0.21. However, we do note that the thickness does increase with age for each age class. In 2015, we do not observe a positive age/thickness relationship. We do note from Table 1
that most age classes, except for fifth-year and older ice, were not highly sampled.