Over the last few decades, the Arctic Ocean has experienced profound changes. Its summer sea ice is shrinking dramatically, both in thickness and extent. Ever warmer pulses of Atlantic water are circulating within the Arctic basins. Pacific waters are bringing in record amounts of oceanic heat. Freshwater storage in the Arctic Ocean is displaying considerable variability. There are early signs of ocean acidification, and some waters are already corrosive to carbonate minerals important to marine life. Surface air temperature and pressure fields are exhibiting patterns different from those of the last several decades.