A reader recently asked about a news item regarding recent results from the CryoSat-2 satellite mission. One of its purposes is to measure sea ice thickness throughout the Arctic. By combining that with data for sea ice concentration, one can estimate the total volume of Arctic sea ice.
The news item is titled “Arctic Sea Ice Up from Record Low.” It tells of findings reported at the recent meeting of AGU (the American Geophysical Union), when a team studying the CryoSat-2 data announced that Arctic sea ice volume had increased substantially from October 2012 to October 2013. In 2012, October sea ice volume averaged about 6000 km3, but in 2013 that figure rose to about 9000 km3 — a 50% increase in a single year.
Unfortunately the CryoSat-2 data only go back to 2010, and even more unfortunately the satellite isn’t expected to have a very long lifetime (only a few more years). That means that in order to place the recent changes in perspective, we need sources of information besides just CryoSat-2.
This can be especially tricky because no data set, including CryoSat-2, is perfect. As stated in Laxon et al. (2013, GRL, 40, 1–6, doi:10.1002/GRL.50193),
The absolute thickness estimates from CS-2 may be subject to biases from a number of different sources. Our assumption that the radar penetrates to the snow-ice interface is still the subject of investigation and may also introduce errors into bias our thickness estimates [Willatt et al., 2011]. Additional uncertainties may be introduced due to uncertainties in our assumed snow loading and ice/water densities, employed when converting freeboard to thickness. For this reason, it is important to compare our CS-2 retrievals with other sources of co-incident ice thickness data. We use three independent data sets that allow us to verify the CS-2 retrievals over a wide area, including both first- and multiyear ice, and over an entire ice growth season.
One source for more perspective PIOMAS, which estimates ice volume by combining observations with a computer model of the ice. Here’s a graph of the two data sets over the 2010/2011 and 2011/2012 ice-growth seasons:
Clearly, the PIOMAS data are consistently lower than the CryoSat-2 data. But the important thing is that they both show similar patterns of change over time, although there are differences even in the patterns, including the seasonal changes.
Nonetheless, the PIOMAS data are sufficiently representative of the changes to enable us to get that perspective we’re seeking. Here’s the PIOMAS data for October of each year from 1979 through 2013:
Note that PIOMAS also indicates a substantial increase in October Arctic sea ice volume since last year, enlarging by 42% (rather than the 50% suggested by CryoSat-2). To put it in perspective we need to pay attention to what led up to 2013. The overall decline is evident.
Last year’s difference between the raw data value and the smoothed value (shown in red in the graph) is the largest positive difference on record, although not by much. Some might regard this as raising the possibility that Arctic sea ice loss is starting to level off, but the most recent October value really is well within expectation even given the existing trend, so it’s certainly too early to draw such a conclusion.
One conclusion we can draw, with high confidence, is that the news announcement has been subject to “spin” by fake “skeptics” of global warming.