Monitoring Change in the Arctic - Satellite-based Applications on Climate Change - page 139

clouds have a warming effect over Greenland and the North Pole area (north of
80 N). In winter, clouds have a warming effect on the surface almost everywhere
over the Arctic region north of 60 N, but in summer the cloud warming effect only
holds for Greenland and the western part of the central Arctic region as a result of
the high surface albedo.
9.6 Sea Ice
The Arctic Ocean and surrounding seas occupy an area of about 14 million km 2 ,
most of which are ice covered in late winter. The pack ice in the Arctic is a mixture
of young and old floes that are highly variable in thickness and extent. Sea ice is a
key component of the climate system for several reasons. It limits exchanges of heat
and moisture between the ocean and the atmosphere. The large difference in
reflectivity of ice and ocean results in more heat being absorbed by the ocean
instead of being reflected back into the atmosphere. When ice extent declines, this
ice-albedo feedback will amplify the effect of warming in high latitudes. The
interannual fluctuations and long-term trends in ice thickness and extent have
important effects not only on Arctic climate but also on global climate change
through complex feedbacks.
Visible/IR satellite sensors can provide information on ice extent, concentration,
motion, and melt, but coverage is limited due to frequent cloud cover. While AVHRR
provides a long time series for recent climate studies, MODIS, operating since 1999,
has more robust spectral information and higher spatial resolution. Dual-polarized,
multifrequency passive microwave radiometers are more commonly used for ice
studies than visible/IR imagers (Cavalieri et al. 1999 ;DrobotandAnderson 2001 ).
They provide near-complete daily coverage of the polar regions under all sky
conditions for characteristics such as concentration, extent, motion, and melt. These
time series extend back to late 1978 from the Nimbus-7 Scanning Multichannel
Microwave Radiometer (SMMR) through a series of Defense Meteorological Satel-
lite Program (DMSP) Special Sensor Microwave/Imager (SSM/I) sensors, providing
more than a quarter century record that can track interannual trends and variability.
For climate studies, however, caution must be exercised because the time series
combine data from several different satellites and sensors. Slight variations in
instrumentation and orbits necessitate inter-sensor calibration for a consistency.
The derived ice characteristics are generally accurate in areas where the emissivity
of sea ice is predictable and well defined (as in dry and relatively thick seasonal
and perennial ice), but the errors can be considerable in newly formed ice and in
areas of melt. Additionally, the spatial resolution of these products (10-25 km) is
such that detailed information on the ice cover, such as deformation, melt-pond
and lead formation, and ridging, cannot be obtained. Figure 9.10 shows the seasonal
cycle of Arctic sea ice extent from passive microwave sensors for recent years
with low ice extents.