4.1. Implications of Summer Melt and Firn Temperature Trends
 The record of MF developed from Penny Ice Cap cores testifies to an important increase in summer melt rates since the 1980s. On the whole, the data indicate that present-day summer melt rates are probably as great as or greater than they've been at any time in the period of instrumental records from the Canadian Arctic (1948–2010). Comparing the MF record with seasonal temperature anomalies for the eastern Canadian Arctic (Figure 2) shows that both summer and winter air temperatures in this region have risen markedly over the same period. Other signs of recent warming in the southern Baffin region are decreasing sea-ice cover trends over Hudson Bay, Davis Strait and southern Baffin Bay [Moore, 2006; Tivy et al., 2011]. At least some of the decadal sea-ice variability in this region is thought to be driven by the North Atlantic Oscillation [Stern and Heide-Jørgensen, 2003; Kinnard et al., 2006a]. The sea-ice cover decline has been largest since the mid-1990s [Tivy et al., 2011], which is consistent with the observed MF trend in Penny Ice Cap cores. Sea-ice cover reduction may contribute to increased glacier melt rates on southern Baffin Island by allowing greater oceanic heat fluxes to the atmosphere, particularly in late summer and autumn. The observed regional trends in air temperature and sea ice cover are consistent with the view that increased mass losses on QEI glaciers in the 21st century are driven by anomalously warm sea-surface temperatures in the northwest Atlantic, accompanied by increased southerly warm air advection to the High Arctic in summer via Baffin Bay [Sharp et al., 2011].
 An expected consequence of rising summer air temperatures in the eastern Canadian Arctic will be to increase the duration of the summer melt season on glaciers. This has been observed by Dupont et al.  on Barnes Ice Cap using spaceborne passive microwave measurements, as described earlier under section 2.6. In Figure 2f, we show the duration of the melt season at Penny Ice Cap summit since 1979, inferred with the same type of measurements and using the same detection algorithm. Based on these data, the length of the melt season has nearly doubled over this period, from 44 days in 1979 to 90 days in 2010. An increase in the duration of the melt season will not necessarily lead, on its own, to a greater volume of MF in the firn, but maintaining surface snow temperatures at 0°C for a greater length of time every summer will facilitate percolation of meltwater in the firn, carrying latent heat with it.
 Warming summer temperatures in the eastern Arctic may also have increased the frequency of rainfall events on Penny Ice Cap. This could enhance surface melt rates, as condensation may account for 30% of the surface energy supply at the ice cap summit during the melt season [Orvig, 1954]. Using the European Reanalysis Agency 1 dataset, Screen and Simmonds  found that rising summer temperatures in the Canadian Arctic since 1989 have been accompanied by a decrease in the snow to total precipitation ratio (SPR), which they ascribe almost entirely to changes in precipitation type (i.e., more frequent rainfall). For the east coast of Baffin Island, the change is most noticeable in late summer (August), with a decrease in SPR of 20 to 30%. Data from southern Baffin Island weather stations (Figure 1) show that the SPR in this region typically varies between ∼0.2 and 0.7 in summer (JJA; mean ±1σ) and is usually 1.0 (snowfall only) from October to May. A 20–30% decrease in late summer SPR over this region would translate into a relative decrease in the mean August SPR from 0.5 to ∼0.6–0.7. At Qikiqtarjuaq, where total precipitation in August averages ∼40 mm, such an increase would result in additional 1–3 mm of rainfall (on average) relative to the amount of snowfall in that month. The impact of such a small change is unlikely to be very significant at the summit of Penny Ice Cap, compared to the effect of temperature-driven surface melt. However, the effect of rainfall on surface melt rates and MF formation is presently impossible to quantify directly, due to the lack of direct precipitation data from the ice cap.
 A remarkable finding of this study is the 10°C warming of firn temperatures (10 m depth) that has occurred at the summit of Penny Ice Cap since the mid-1990s (Figure 5). This finding suggests that the regional warming of the late 1990s and 2000s resulted in a major infusion of latent heat by deep meltwater percolation into the upper firn layers. Interestingly, recent AWS air temperature recordings from the summit of Penny Ice Cap indicate that the MAT over the period 2007–11 has remained close to that measured in the 1990s, i.e. near −16°C. The MAT at the ice cap summit is apparently insensitive to the actual summer warming experienced in situ. This may result from the summertime surface radiative energy surplus being largely transferred into the firn by percolation and conduction. Indeed, Eley  found that the best predictor for early winter (October to December) firn temperatures at 7.5. m depth on Penny Ice Cap summit was the number of positive degree-days (PDD) in the previous summer. Furthermore to achieve the observed warming of 10°C since the 1990s, much of the latent heat transferred in the firn must be conserved through the winter, and this would be facilitated by deep percolation of meltwater [Van De Wal et al., 2002]. Winters in the eastern Canadian Arctic also warmed significantly, albeit irregularly, since the late 1980s (Figure 2), and this may have helped to maintain the present-day “warm” firn temperatures on Penny Ice Cap by reducing the firn-air temperature gradient.
 If an increasingly large amount of latent heat is transferred, and retained, into the firn with each successive summer of high melt, this may act as a feedback that amplifies the summer melt progress on the ice cap forced by regional air temperature trends. In effect, the firn layers are preconditioned by heat storage, such that at the onset of the following melt season, a lesser amount of energy is required to initiate surface melt. This scenario is consistent with the non-linearity between summer melt rates and air temperatures seen in Greenland and on Canadian Arctic ice caps [Fisher et al., 2011]. Some non-linearity is expected even in the absence of an internal feedback, because any large increase in summer mean temperature will lead to a proportionally greater increase in PDD, the amplification being a function of the variance in the temperature cycle [Reeh, 1989].
 The basal ice temperature under Penny Ice Cap, measured in the P96 borehole (D. Fisher, unpublished data, 1996), was ∼−12°C, slightly warmer than earlier estimates [Holdsworth, 1984; Fisher et al., 1998]. If the current rate of internal warming recorded in firn continues uninterrupted, the “warm wave” will gradually propagate through the ice cap by convection and conduction, and the basal ice temperature will eventually rise to the pressure-melting point (PMP). This is expected to alter the drainage of the ice cap and could lead to accelerated velocities of outlet glaciers, as observed in Greenland [e.g., Zwally et al., 2002]. However, spaceborne measurements of ice motion indicate that the velocity of Penny Ice Cap outlets glaciers has actually decreased by 10–20 m a−1 since the mid-1980s, which corresponds to an average slowdown of 25% per decade [Heid and Kääb, 2011]. Hence, there are, as yet, no indications of dynamical instabilities on Penny Ice Cap such as would be expected if basal temperatures were approaching the PMP.
 It is unclear at present if the recent regional warming in the eastern Canadian Arctic extends above an altitude of 2 km. Upper air soundings for southeastern Baffin Island are scarce, making it difficult to seek independent corroborating evidence. As an alternative, we used AWS data, weather station records, and Orvig's  report to compare air temperature lapse rates between Penny Ice Cap summit and the Baffin Bay coast for three different observation periods: summer 1953 (July only), 1992–2000 and 2007–2011 (Figure 9). Presently, the lapse rate is largest in October (∼0.75°C 100 m−1), and decreases through the winter to reach a minimum in February (∼0.15°C 100 m−1) when air temperatures are uniformly cold, or nearly so, from sea level to the summit of the ice cap. At the height of the summer melt period in July, the lapse rate is ∼0.50°C 100 m−1. We found that lapse rates in these months were not noticeably different for the three periods of observations considered (Figure 9). This suggests that the recent warming was felt uniformly over the ∼2000 m altitude range of Penny Ice Cap.
Figure 9. Mean temperature lapse rates compared for selected months between the summit of Penny ice cap and two coastal weather stations, (a) Qikiqtarjuaq and (b) Cape Dyer on southeastern Baffin Island (see Figure 1). The lapse rates in Figure 9a were computed using data from automatic weather stations on Penny Ice Cap (periods 1994–2000 and 2007–11; Eley  and this work), and from archived weather station data (Environment Canada, online bulletin, 2010). The lapse rates in Figure 9b also include air temperature recordings made at Penny Ice Cap summit during the 1953 AINA expedition [Orvig, 1954]. The period of overlapping data for 1953 is limited to July.
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 Supportive evidence may be found by examining summertime (JJA) air temperature anomalies at the 700 hPa level over Penny Ice Cap, calculated using overlapping datasets produced by the U.S. National Centers for Environmental Prediction (NCEP) and the European Reanalysis Agency (ERA40) reanalysis models over the period 1958–2010 (Figure 2) (data by A. Gardner, University of Michigan, 2011). The barometric pressure at Penny Ice Cap summit is ∼775 hPa [Orvig, 1954], hence temperatures anomalies at 700 hPa provide a good approximation of conditions at this altitude. As Figure 2 shows, the inferred trend and magnitude of JJA air temperature anomalies at 700 hPa in recent decades are closely comparable to the JJA surface air temperature anomalies for the eastern Canadian Arctic, which supports a relatively uniform warming of the lower troposphere. The linear trend in 700 hPa summer temperatures over the period 1979–2010 is 0.07°C a−1 which is comparable to the trend in surface JJA air temperatures over this period (Table 1), These findings are consistent with those by Graversen et al. , although their conclusions were contested on the grounds of heterogeneities in the observational datasets used [Grant et al., 2008]. We also note that a uniform warming over an altitude range of ∼2000 m would lead to a greater increase in surface melt rates at the low-altitude margins than at the ice cap summit, and this is supported by both our SBM measurements (Figure 6) and by repeat altimetry measurements [Abdalati et al., 2004; Schaffer et al., 2011].
 Our findings of rising melt rates on Penny Ice Cap are consistent with increased ice thinning rates observed there and on neighboring Barnes Ice Cap [Abdalati et al., 2004; Sneed et al., 2008; Schaffer et al., 2011]. While Sneed et al.  associated the thinning of Barnes ice Cap to regional warming, Abdalati et al.  suggested that it could be due to (unspecified) dynamic factors, rather than greater melt rates. The data presented here make it clear that enhanced surface melting could probably account for much or all of the observed net thinning, at least on Penny Ice Cap. However, some thinning may also result from reduced ice mass transfer rates to the ice cap margin, as suggested by glacier velocity observations of Heid et Kääb . The enhanced densification of the firn due to infiltration ice layer formation (Figure 7) on Penny Ice Cap will need to be accounted for in future attempts to infer ice-cap volume or mass changes using airborne or spaceborne altimetry data [e.g., Schaffer et al., 2011], as it implies an increase in internal mass accumulation. This effect will be quantified in future research.
4.2. The Holocene Record of Summer Melt on Penny Ice Cap
 The deep ice cores recovered in 1995–96 from Penny Ice Cap allow us to put recent melt trends, described in earlier sections, in the context of past millennia. Some features of the P95 MF record were presented by Grumet et al. , while Fisher et al.  offer a comparative analysis of ice-core MF records from several Canadian Arctic ice caps, including Penny. Here we focus our discussion on the significance of this record for the Holocene climate evolution of the southern Baffin Island region.
 Of the two long MF series developed from Penny Ice Cap, only that from the P96 core extends beyond 2 ka b.p. [Okuyama et al., 2003]. Figure 10a presents the P96 and composite (P95 and P2010–11) MF records, averaged over 100-year intervals. Owing to wind scouring, the ice accumulation rate at the P96 site is much lower (0.19 m a−1) than at the P95 site (0.37 m a−1), which results in much higher MF percentages, from 80 to >90% over the past century [Fisher et al., 1998; Okuyama et al., 2003]. The P96 record shows that for much of the early to mid-Holocene, between ∼10 and 4 ka b.p., the coring site experienced 100% melt, which implies possible discontinuities in the record if the summit of Penny Ice Cap experienced an overall negative mass balance (net wastage and runoff) at any time during this interval. The lack of a trend in the P96 MF during this period does not imply that regional temperatures were stable, but simply that surface melt rates at this site had reached a maximum (100%). As the δ18O record from the P96 core shows (Figure 10a), local temperatures rose to reach a maximum near 6 ka b.p., which is consistent with timing estimates for the Early Holocene Thermal Maximum (HTM) in this region obtained from syntheses of proxy data [Kaufman et al., 2009].
Figure 10. (a) Time series of δ18O (100-yr running mean) and melt % from the Penny Ice Cap P96 core [Fisher et al., 1998; Okuyama et al., 2003], compared with the extended melt feature percentage (MF %) record from the P95 coring site (MF in 100-year averages). Shading identifies the early Holocene Thermal Maximum [HTM]. Horizontal stippled line shows the MF % at the P95 coring site in the summer of 2010. Vertical stippled line identifies the inferred transition point between shear-dominated ice flow (SHEAR) and uniaxial vertical compression (VERT) regime in the P96 core [Okuyama et al., 2003]. (b) The past 300 years of melt % in the P96 and the composite P95/P2010–11 records, shown in annual averages (shaded) and 5-year averages (bold). The Little Ice Age cold interval (stippled line) in the southern Baffin Island region came to an end toward the late 19th century [Miller et al., 2005]. (c) The record of MF over the period AD 1700–1989 developed from two ice cores drilled at site J, southern Greenland [Kameda et al., 1995]. As in Figure 10b, the data are shown in 1-year (shaded) and 5-year averages (bold).
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 Thereafter, from ∼4 to 0.5 ka b.p., annual summer melt rates at the P95 and P96 sites declined, following the late Holocene climatic deterioration recorded by numerous proxies across the Arctic [Kaufman et al., 2009], and seen in the P96 δ18O profile. The decreasing MF trend in the late Holocene is steeper in the P96 than in the P95 record, and this difference is likely due to the decreasing altitude and accumulation at the P96 site (relative to P95) that resulted from topographic changes on Penny Ice Cap after it separated from the receding Laurentide Ice Sheet. On geological evidence, this separation is thought to have occurred gradually between 8 and 4.5 14C ka BP (A. Dyke, personal communication, 2011). Isostatic rebound following deglaciation [Kaplan and Miller, 2003] would have affected the P95 and P96 sites nearly equally and can not account for the difference in the MF % trends. The onset of Neoglaciation on Cumberland Peninsula is tentatively dated between 5 and 3 ka b.p. [Miller et al., 2010]. Okuyama et al.  measured ice fabric changes in the P96 core that suggests the establishment of the current accumulation regime at this site near the end of the period of very heavy melting (∼100%), ∼3 to 2 ka b.p.
 The two Penny Ice Cap MF records of the past 300 years (Figure 10b) both indicate that the late Holocene cooling trend that accompanied Neoglaciation came to an end in the latter half of the 19th century, coincident with the termination of the Little Ice Age (LIA). Since then, summer melt rates have risen steadily, but not monotonically, by ∼50–55% at the summit of the ice cap, with considerable interdecadal variability. The warming since the LIA was punctuated by decadal-scale variability, as attested, for example, by the cooler decades of the 1960s and 70s [Bradley, 1973; Jacobs and Newell, 1979]. The P96 MF record shows decadal variations of lesser amplitude than the P95 record, particularly after AD 1850. This is unsurprising, given that the P96 site, with its lower ice accumulation rate (0.19 m a−1), experiences summer melt rates close to 100%. As a result, MF variations recorded at this site are expected to be less sensitive to regional summer temperature fluctuations than those recorded at the P95 site.
 The last ∼300 years of the Penny Ice Cap summit (P95) MF record show remarkably good agreement with the MF % series developed by Kameda et al.  from two cores drilled in southern Greenland (Figure 10c) at a comparable latitude and altitude (Site J; 66°N; 2030 m). The good correspondence between these records argues for a coherence in the decadal to centennial temperature trends across the southern Baffin Island and southern Greenland region. Extrapolating MF observations (section 3.2.) using the Penny Ice Cap ice-core record indicates that recent melt rates on this ice cap are probably as high now as they have been since the mid-Holocene, ∼4 to 3 ka b.p. Using contemporary MAT and temperature lapse rates from Penny Ice Cap, we infer that the regional mean July temperature at sea level during the mid-Holocene may have been as warm as ∼8°C, i.e. ∼4°C warmer than 20th century averages for coastal weather stations along eastern Baffin Island [Miller et al., 2005].