To understand the Arctic system and the profound changes taking place, we need to become familiar with some of the basic climate drivers and features of the region, starting with solar energy, polar night, and the midnight sun. Attention then turns to patterns of temperature, precipitation and cloudiness and how they vary across the Arctic.

A defining feature of the Arctic is that it is a largely ice-covered ocean surrounded by land. While by area, it is the smallest of the world's oceans, it is by far the most interesting of them all. This week, we focus on the Arctic Ocean starting with some basics - its shallow seas and deep basins, and then move on to discuss the Arctic Ocean's floating ice cover. As we will learn, the Arctic sea ice cover, which ranges from a thin veneer to more than five meters thick, grows and shrinks with the seasons. The ice cover is in near constant motion, with ice flows squeezing together to form pressure ridges and pulling apart to form leads. Unlike other oceans, the Arctic Ocean has an intriguing vertical structure, with fresh cold water at the top and warm salty water below.

The Arctic lands are much more than treeless, windswept tundra. Greenland, the largest island in the world, is mostly covered by an ice sheet up to 3,200 meters thick. It is losing mass and contributing to sea level rise. While much of the Arctic lands are indeed covered by tundra, the Arctic is also home to large swaths of forest and extensive mountain ranges. Some very large rivers drain into the Arctic Ocean, discharging massive amounts of fresh water. Almost all of the Arctic lands are covered with perennially frozen ground called permafrost. However, this permafrost is now thawing, changing the landscape and putting infrastructure at risk.

While blizzards, whiteouts and extreme winter cold are a part of living in the Arctic, summer thunderstorms are common over land. Arctic weather is varied and fascinating, but the Arctic region plays a bigger role in the climate system by helping to shape weather patterns across the Northern Hemisphere. Arctic weather and climate is also shaped by teleconnections, such as the North Atlantic Oscillation, that link weather conditions across widely separated locations. Another important framework for understanding the Arctic is its energy budget, which refers to the flows of energy into and out of the Arctic and between the atmospheric ocean.