Changes in Antarctic sea ice are important indicators of climate change but it is vital to determine whether the changes are due to climate forcing or attributable to natural variability. This will be important for putting forward future climate change models.
De la Mare's work builds on research published in 1997 that was criticised. Other scientists claimed that there was no substantial retreat in sea ice beginning in the 1950s. "I now address these criticisms using available direct and proxy whaling data and show that my conclusions remain unchanged," he told environmentalresearchweb.
Whaling in the Antarctic has been carried out along the sea ice edge since the 1920s – originally for practical reasons so that whale factories without stern slipways could cut up the whales directly in the water. Such operations could only be performed in calm water, within the pack ice. As a result whalers discovered that the highest concentration of the much-sought after blue whales was found near the ice edge, so whaling fleets followed the ice as it retreated during the season.
In 1972, whalers began to target the minke whale, the smallest of the great whales, which are also found along the ice edge. So, once again whale catch locations provided information about ice edge retreat. Correlating these whaling positions with both early direct and modern satellite data showed strong relationships between whaling positions and ice edge in both pre-satellite and post-satellite whaling periods, says de la Mare.
"One of the criticisms of my earlier work was that it utilised data derived from ice charts compiled by the Joint Ice Centre (JIC) to determine sea ice extent for the modern era," explained de la Mare. "Scientists suggested that this data came from regions too far south, where there are problems in detecting sea ice during summer using passive microwave instruments – it is difficult to distinguish ice from open water at this time because of surface melting."
However, the whaling data analysis now puts an end to this controversy, backing up the JIC data analyses from both summer and winter. "Reassurance that the JIC data is reliable is an important conclusion," says de la Mare.
"I hope that the controversy of whether there has been a change in the sea ice extent from the 1930s to the 1980s can now be put to rest," he added. "Data from the various sources (JIC and whaling records) have all now been taken into account and they all support the idea that a significant change did occur."
De la Mare reported his results in Climatic Change.