Climate of extremes - Antarctica: Global Science from a Frozen Continent - page 134

Figure 4.29 Time series of South Pole carbon dioxide

concentration. (Credit: Ralph Keeling, Scripps
Institution of Oceanography)

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1/1/57 1/1/65 1/1/73 1/1/81 1/1/89

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1/1/97 1/1/05 1/1/13 1/1/21

One type of what-if scenario that has been explored using global climate system
models is how the global temperature during the twentieth century would have
varied without the observed increase in greenhouse gases. Results from these
experiments clearly demonstrate that the observed warming during the twentieth
century can only be reproduced in the climate system model simulations when the
observed increases in greenhouse gases are included in the models. Without these
increases in greenhouse gases little change in global temperatures is simulated by the
models, inconsistent with the observed changes in global temperatures.
A second what-if question is: what is the impact of changing the amount of
greenhouse gases in the atmosphere over the next century? Evaluation of global
climate system model simulations, and other types of models, forced with a range
of possible changes in greenhouse gas concentrations, indicate that the global
average temperature is likely to warm between 1.1 and 6.4ºC by the year 2100.
Based on the IPCC Fourth Assessment Report, it is likely that surface temperatures
and precipitation in the Antarctic will increase during the twenty- rst century.

In the Antarctic reliable weather observations extend back only to the middle
of the twentieth century, unlike mid-latitude locations in the northern hemisphere
where reliable weather observations can extend back several centuries. Based on
a combination of Antarctic weather observations and satellite data surface
temperatures over the continent have warmed by 0.12 C per decade from 1957
to 2006. This warming is greatest in West Antarctica, which has warmed 0.18 C
per decade from 1957 to 2006. Precipitation data over the Antarctic is even more
limited than temperature data, and no trends in Antarctic precipitation have
been observed during the second half of the twentieth century.
Global climate system model projections of climate change in the Antarctic
indicate warming of between 1.4 and 5ºC during the twenty- rst century. This

warming is expected to be relatively uniform across the continent, in contrast to the
very localised warming observed during the twentieth century. Precipitation is also
expected to increase over the Antarctic continent. The IPCC Fourth Assessment
Report indicates that precipitation is likely to change between

- 2% and +35% over