Polar and alpine environments - Fundamentals of the Physical Environment - page 593

Treeline
10°C July isotherm
Arctic circle

Bering Sea

Winter pack ice
Permanent pack ice

Pacific Ocean

Alaska

Arctic Ocean

Hudson
Bay

Greenland

0 1000

Iceland 0 1000

Atlantic Ocean km

km

Figure 24.1 The limits of the Arctic environment: the 10°C July isotherm, the treeline and the Arctic marine boundary.

month. For the Arctic this isotherm reflects latitude, ocean
currents and continentality ( Figure 24.1 ). It includes most
of the northern coasts and islands of Alaska, Canada,
Scandinavia and Russia, all of Greenland and Svalbard,
and most of Iceland. Anomalies occur where it is pushed
north by the warmer air masses associated with the North
Atlantic Drift (Scandinavia) and the Kuro Shio of Japan
(north-west North America), and where it is pushed south
by the cold Labrador current and the Bering current
(Bering Sea and Kamchatka).
As well as being an important ecological boundary the
treeline, the poleward limit beyond which trees do not
grow, has a considerable impact on the energy budget. As
we cross from boreal forest to tundra there is a sharp
decrease in net radiation. Part of this change is due to the
change in albedo of the surface as we move from the dark
forested area to the lighter-coloured tundra. Another
reason is that when snow falls it accumulates on the
tundra to give a complete whitish surface whilst snow

cover on the branches and trunks of the forest is more
broken.

Antarctic climate

For Antarctica the 10 C summer isotherm encircles the
whole of the continent and includes the tip of South
America and many islands in the southern ocean ( Figure
24.2 ). It seems to separate quite well those islands which
are treeless from those which are not. Another frequently
used boundary for Antarctica is the Antarctic Conver-
gence, a sharp boundary between Antarctic surface water
and slightly warmer and more saline subantarctic surface
water. It marks the point where colder Antarctic water
moving north and east sinks below subantarctic water
moving east and slightly southwards. It varies between
latitudes 45 S, being farther south in the Pacific

than in the Atlantic, and in the summer than in the winter
( Figure 24.2 ).

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