Two summers: On the left, 1980; on the right, 2012.
(The picture is of the Arctic Ocean (with Iceland at the bottom and Alaska towards the top). It is about 3000 miles from side to side).
We tend to use ‘global warming’ and ‘climate change’ almost as synonyms, but that’s not quite right: the climate is changing, and one of the ways we see that change is as an increase in global mean temperature. We like global temperature as a measure partly because it is relatively well observed and understood (thanks, in a small part, to our contributions), but climate change is also showing itself in other ways, some of them more dramatic.
Every year in the Arctic, the sea-ice starts to melt in March and continues to retreat through the summer, reaching its minimum extent in September. Since 1979 we’ve been able to watch the change by satellite, and even over the 30-odd years of satellite observations we’ve seen some big changes, particularly in the summer ice coverage:
This is one reason why we are now concentrating on polar data. Arctic sea-ice is harder than global temperature – to measure, to understand, and to predict. So more observations are particularly valuable. And because changes in ice cover can be so large, we can make useful comparisons to modern records even with a limited set of ship observations: in 2012 the Northwest passage was clear of ice – it’s certain that William Parry, John Franklin, Roald Amundsen, and even our own Thetis, met very different conditions.