Why are Ice and Snow Changing?
Summary ment Report
1
notes that it is highly likely (90 per cent
confidence) that humans have already contributed to a
Changes in ice and snow are influenced by variability rise in global temperatures due to an increase in green-
within the climate system itself and by external factors house gas forcing. Carbon dioxide (CO
2
), a primary
such as greenhouse gases, solar variability, and volcanic greenhouse gas, is now near 380 ppm (parts per million
dust – factors that act on time scales from months to of the atmosphere) and currently has a greater concentra-
hundreds of thousands of years. During the 21st cen- tion than during any of the previous interglacial warm
tury, the most important external influence on high lati- periods over the last 500,000 years. CO
2
is projected to
tude climate and on ice and snow conditions will be the reach 480 ppm by mid-century.
increase in greenhouse gases. Natural climate variability
will still impose regional, decadal, and year-to-year dif- In addition to external factors, there is a large and natu-
ferences, and feedbacks will become increasingly im- ral random aspect to climate change that produces dif-
portant in the climate system. Before 2050 the ice albedo ferences from year to year, decade to decade, and place to
feedback will accelerate the loss of Arctic sea ice. Warm- place. This variability is caused by instabilities in the air
er temperatures will reduce the area of snow cover and flow on the rotating Earth and this effect is greater near
produce an earlier melt in snow-covered regions. This the poles than near the equator. Examples of natural
reduced snow cover will itself speed up warming. variability are the warmer temperatures in the European
Arctic in the 1920s and 1930s, and the cooler tempera-
tures in the 1960s.
Forces that drive the climate system
When the climate trend from future greenhouse gas
Atmospheric climate, represented primarily by tempera- forcing is added to the natural range of climate variabil-
tures, precipitation, and winds, undergoes externally- ity, the result is a shift during the 21st century to over-
forced changes as well as natural, internal variations. all warmer temperatures, with many consequences for
the cryosphere. The Arctic will experience warmer high
External forcing factors include greenhouse gas fluctua- and low temperature extremes. The warmer average will
tions, dust from volcanic eruptions, and variations in the lead to a loss of sea ice and to earlier snow melt and
amount of solar radiation reaching the top of the atmos- river break-up – trends that are observed now. Globally,
phere. These changes in atmospheric conditions influ- the freezing level (also called snow line) in mountain-
ence the amount of ice and snow cover in a particular ous regions will continue to move up mountain slopes
region and the regional climate is affected by them in and larger proportions of precipitation will fall as rain
turn. In the 21st century the most significant change in rather than as snow. In Antarctica, where current warm-
external forcing for high latitude climate, and therefore ing trends are not widespread, models project that in-
the largest influence on ice and snow conditions, will be creased warming will affect the central parts of the huge
the increase in greenhouse gases. The IPCC 4th Assess- Antarctic ice sheet later in the century.
30 GLOBAL OUTLOOK FOR ICE AND SNOW