Is the trend a harbinger of global warming, or merely the result of interannual variabilty?
The record is not a simple one. Passive microwave sensors measure surface brightness temperatures, from which sea ice concentrations are mathematically determined. Derived ice concentrations are generally accurate to about five percent, except in summer, when melting conditions can affect the signal, leading to underestimations of ice. To complicate matters, the passive microwave record is actually a combination of data collected by different sensors on two satellite systems. Comparing these essentially different data sets requires compensating for differences in instrument sensitivity, operations, and resolutions. “Merging data raises important issues,” says NERSC research scientist Martin Miles. “There’s an overlap between the two data sets of one month. You can use that overlap to try to adjust algorithms so that the values you get from one sensor will match the other. “Initially, we decided to treat the two time series as separate. It’s a very safe, conservative way of assessing ev
