inside or below the permafrost (methane hydrates). We Changes from melting ice and snow
thus risk a situation where global warming melts the affect people’s homes and livelihoods
permafrost, which in turn adds extra greenhouse gas- worldwide
es to the atmosphere, in all likelihood amplifying the
warming. On the other hand, a considerable melting of Sea-level rise is one of the most obvious consequences
the deep permafrost is necessary before the store of fro- of melting ice on land (Chapter 6). The global sea level is
zen methane could be affected, and that will take many currently rising by about 3 mm per year mostly because
years. During that time, the warming may cause the seawater expands as it gets warmer and because melting
boreal forest to expand across the tundra, which will re- glaciers and ice sheets add fresh water to the oceans
3

move carbon from the atmosphere. But tree crowns ab- (Chapter 6). The IPCC
3
projects that the sea level may
sorb more heat from solar radiation than the flat, white rise by as much as half a metre in this century, mainly
tundra, which can again increase warming
2
. Thus, what caused by the thermal expansion of seawater. There is,
the net effect will be on the global climate from these
processes is unknown.
Malekula Islands, Vanuatu.
Another factor that may affect the global distribution
Photo: Topham Picturepoint TopFoto.co.uk
of heat is a change in the major ocean currents caused
by melting of ice, excess warming of ocean waters and
their freshening. One of the main factors driving the
ocean circulation is the formation of deep, dense wa-
ter in the Greenland Sea, the sea near Baffin Island in
eastern Canada, and in the Weddell Sea in Antarctica
3
.
Water becomes heavier as it gets saltier and colder. The
cold and saline water in these areas sinks and flows
along the bottom of the world’s oceans while the warm-
er water flows closer to the surface of the ocean to these
colder areas, where it releases its warmth, and becomes
colder and more saline. This thermohaline circulation
(Figure 2.1) forms a major system of ocean currents,
which is also called the Great Ocean Conveyor Belt. The
North Atlantic Current is a part of this system. Ther-
mohaline circulation may be affected by melting and
freezing processes, such as reductions in the extent
and thickness of sea ice (Chapter 5) and input of light-
er fresh water from melting glaciers (Chapter 6). The
IPCC
3
projects a 25 per cent reduction in this century
of the North Atlantic Current because of a weakening of
the deep water formation.
CHAPTER 2 WHY ARE ICE AND SNOW IMPORTANT TO US? 23