Monitoring the melt season length of the Barnes Ice Cap over the 1979–2010 period using active and passive microwave remote sensing data Florent Dupont, 1,2 * Alain Royer, 1 Alexandre Langlois, 1 Alicia Gressent, 1,2 Ghislain Picard, 2 Michel Fily, 2 Patrick Cliche 1 and Miroslav Chum 1 1 Centre d’Applications et de Recherches en Télédétection (CARTEL), Université de Sherbrooke, Sherbrooke, Québec, J1K 2R1, Canada 2 UJF–Grenoble 1/CNRS, Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE) UMR 5183, Grenoble, F-38041, France Abstract: The Barnes Ice Cap (BIC) located on Baffin Island (Nunavut, Canada) is one of the most southern ice caps of the Canadian Arctic Archipelago. Observational data provide evidence of increased melting, thinning and contour recession due to recent climate warming in the Arctic. The duration of the summer melt season for the BIC, over the period 1979–2010, was derived using a threshold algorithm for 19 GHz horizontal polarization brightness temperature data; the passive microwave satellite measurements included data from the quasi-daily Scanning Multichannel Microwave Radiometer and the Special Sensor Microwave Imager. Our results show the melt season lengthened by 33% from 65.6  6 days at the beginning of the period (1979–1987) to 87.1  7.8 days towards the end (2002–2010). The interannual variations of the number of melt days were in agreement with those derived from active microwave backscatter data from the QuikSCAT scatterometer for the overlapping 2000–2009 period. In addition, elevation change data from the ICESat altimeter confirmed the thinning of the BIC at a mean rate of 0.75 m/year for the 2003–2009 period. For the 32-year period that we analysed, correlations with summer and annual air temperature and annual sum of positive days were examined for both the North American Regional Reanalysis and the Clyde River Automatic Weather Station data. Correlations with land surface temperature data from MODIS were also examined over the last decade. The results of these investigations showed that these climate indicators did not adequately explain the observed melt variations for the BIC. Ground-based snow and ice measurements collected near the BIC summit during a 10-day field campaign in March 2011 provided insights onto the surface properties and confirm the relevance of the remote sensing invariant threshold algorithm used for melt detection. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS Barnes Ice Cap; surface melting; Arctic climate change; remote sensing; SMMR; SSM/I; QuikSCAT Received 30 August 2011; Accepted 26 April 2012 INTRODUCTION The Barnes Ice Cap (BIC) is one of the most southern ice caps of the Canadian Arctic Archipelago (CAA) and is located on Baffin Island (Nunavut, Canada). Its geograph- ical range extends from 69.27  to 70.21  N and from 71.45  to 74.38  W. The BIC surface area ranges between 5671 km² (Abdalati et al., 2004) and 5900 km² (Sneed et al., 2008), and its maximum width is approximately 140 km. It is a relatively flat terrestrial ice cap reaching more than 1100 m a.s.l. at the summit plateau, and its margins terminate on land at approximately 500 m a.s.l. (Figure 1). Recent studies have shown that the rate of climate warming is faster in the Arctic than the rest of the world (Comiso, 2003; ACIA, 2004; Lemke et al., 2007; Kuzmina et al., 2008; Zdanowicz et al., 2012). For the subregion of the CAA, a warming trend has been observed of more than 2  C since 1975 (Environment Canada), which is signifi- cantly higher than the global trend (Serreze et al., 2009). Recently, significant changes have been observed in the CAA’s ice caps (Abdalati et al., 2004; Gardner et al., 2011). Several studies of the BIC have focused on measure- ments of elevation and contour changes (Jacobs et al., 1993, 1997; Abdalati et al., 2004; Sneed et al., 2008; Gardner et al., 2011). Observational data for the BIC show evidence of increased melting, thinning and contour recession due to recent climate warming in the Arctic. Sneed et al. (2008) showed acceleration in the mean thinning rate over the south dome of the BIC from 0.2 m/year for the 1970–1984 period to 1.1 m/year for the 2004–2006 period. For the southern part of the CAA, Gardner et al. (2011) showed a rate of glacier mass loss that significantly increased by 58% (+14 Gt/year) between 2004–2006 and 2007–2009 in direct response to warmer summer temperatures. The cumulative change in glacier mass of the entire CAA would actually contribute to a mean increase of 0.17  0.02 mm/year to sea-level rise (Gardner et al., 2011). Our study extends in time this analysis over a longer period and complements previous studies with the analysis of melt duration. The present study provides insight as to if the mass loss is due to an increase of the summer melting period. *Correspondence to: Florent Dupont, Centre d’Applications et de Recherches en Télédétection (CARTEL), Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada. E-mail: Florent.Dupont@lgge.obs.ujf-grenoble.fr HYDROLOGICAL PROCESSES Hydrol. Process. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9382 Copyright © 2012 John Wiley & Sons, Ltd.