Laboratory for Satellite Altimetry / Sea Ice and Polar Dynamics Science Team

Sea ice is an important indicator of climate change, and a key component of the polar climate system. Areal shrinkage of Arctic sea ice has been observed over the last 35 years [Parkinson, 2014; Comiso et al., 2008], and its decline is proceeding faster than forecasted [Stroeve et al., 2012]. A record minimum ice extent of 3.41 million square kilometers was reached in September 2012 [Parkinson and Comiso, 2013], and was thus 49 % of the average extent, measured between 1979-2000. These observed changes in the ice cover have related impacts on the regional Arctic and sub-Arctic climate, environment and ecosystems and directly affect natural resource exploitation, transport, commercial fisheries, and indigenous lifestyles. Historically, direct observations of sea ice thinning have been mostly limited to sparse submarine measurements of ice draft [Rothrock et al., 1999]. Satellite monitoring now offers dense, near-total coverage of the ice pack and provides observations of sea ice thinning on basin scales.

The main goal of the work conducted by the Sea Ice Research Group is to precisely determine changes in sea ice thickness and volume, and understand the nature of such changes. The specific objectives are to improve remote sensing techniques for monitoring sea ice thickness, obtain measurements over decadal scales, and support improvement of seasonal-to-decadal model predictions of the Arctic system. Laser and radar altimeters, onboard NASA and ESA satellites including ICESat, Envisat and CryoSat-2, provide synoptic measurements of Arctic sea ice freeboard, a proxy for ice thickness.

The latest analyses of satellite altimetry data sets from ICESat, Envisat and CryoSat-2 reveal a decline in the thickness of the Arctic sea ice pack over the last fifteen years. While NASA's ICESat ceased operations in October 2009, ESA's CryoSat-2 radar altimeter satellite has been operating successfully since 2010, and NASA plans the launch of ICESat-2 in 2017 [Abdalati et al., 2010], supporting continuity of the Arctic sea ice thickness time-series until at least the end of this decade. In addition, NASA's IceBridge mission, which commenced operations in March 2009, continues to conduct yearly (every March and April) airborne campaigns over the Arctic.

ICESat

ICESat provided, for the first time, a broad picture of the Arctic Ocean to 86 oN due to the near-polar inclination of the spacecraft. Observations from eleven ICESat campaigns revealed a rapid decline in sea ice freeboard between 2003 and 2008. The losses were strongest over the multi-year ice regions in the central Arctic, and north of Greenland and the Queen Elizabeth Islands, Canada. These changes in freeboard [Farrell et al., 2009] were also reflected in Envisat data that showed rapid thinning of the pack [Giles et al., 2008], and in ICESat data that showed a loss of sea ice volume across the Arctic basin between 2007 and 2008 [Kwok et al., 2009], associated with record sea ice minimum in September 2007.

CryoSat-2

The latest CryoSat-2 data is critical in order to understand the context of the ICESat-era decline in Arctic sea ice freeboard, thickness and volume [e.g. Farrell et al., 2009; Kwok et al., 2009]. In particular CryoSat-2 observations allow us to assess whether sea-ice volume loss continues into this decade. Combining ICESat laser altimetry from 2003- 2008 with CryoSat-2 radar altimetry from 2010-2012, Laxon et al. [2013] showed an overall decline in Arctic sea ice thickness and volume during the last decade. These observations are in line with numerical simulations of sea ice volume. Between the ICESat and CryoSat-2 periods the winter volume declined by 1479 km³. This is equivalent to a drop in Arctic sea ice volume of -9% in the winter between 2003 and 2012.

IceBridge

Yearly surveys of the Arctic ice pack by NASA's Operation IceBridge aircraft provide ice thickness measurements in two primary regions: the central Arctic, and the Beaufort/Chukchi Seas. The central Arctic remains dominated by multi-year ice with > 90% coverage, while the ice pack of the Beaufort Sea region is more seasonal in nature and is a mix of multi-year (~25%) and first-year ice (~75%). Ice thickness distributions for the central Arctic and the Beaufort/Chukchi Seas, along with the six-year mean and modal ice thicknesses, show that mean ice thickness currently remains stable in the central Arctic, at around 3.2m, with interannual variability of <= 0.6 m over the last six years. Average ice thickness in the Beaufort Sea region is significantly lower, at 2.0m, with interannual variability of ~1 m. There was a decrease in thickness in the winter of 2011, which persisted until 2014, when there was a slight increase in mean ice thickness due to the presence of a wide band of multi-year sea ice in the southern Beaufort Sea.

Validation Experiments

A critical step in exploiting satellite altimeter data for the effective monitoring of sea ice thickness is validation of these measurements. We validate satellite data by making comparisons with "ground-truth" observations from low altitude aircraft under-flights and in-situ measurements collected on the sea ice itself. Since 2002 the Sea Ice Research Group has conducted a series of airborne sea ice validation experiments jointly with NASA to assess how well sea ice elevation, and hence sea ice thickness, can be measured from space using satellite altimetry.