Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 328

Figure 11.4. Area-weighted mean observed temperatures over the latitude bands indicated.
The values are 9-year running means relative to the 1880-1890 mean (Shindell and Faluvegi,
2009).
regarding the impact of aerosols on global temperatures in the mid- to late 20th
century. It is dicult to resolve this important issue at this time.
There is an interesting correlation between temperature rise after 1978 and the
so-called El Nin˜ o-Southern Oscillation (ENSO) phenomena, as measured by the
Southern Oscillation Index (SOI). The SOI is a measure of large-scale fluctuations
in air pressure occurring between the western and eastern tropical Pacific during
El Nin˜ o and La Nin˜ a episodes. In general, the SOI corresponds to changes in
ocean temperatures across the eastern tropical Pacific. Prolonged periods of
negative SOI values coincide with abnormally warm ocean surface waters across
the eastern tropical Pacific typical of El Nin˜ o episodes. Prolonged periods of
positive SOI values coincide with abnormally cold ocean waters across the
eastern tropical Pacific typical of La Nin˜ a episodes. Estimates of the SOI have
been made that date back to the mid-19th century. These results indicate roughly
equal positive and negative fluctuations for many years. However, in the last
25 years of the 20th century, the trend of the SOI has been overwhelmingly
negative. It may well be that surface warming in the NH since 1978 has been
heavily influenced by abnormally warm surface waters across the Pacific
(Rapp, 2008). Douglass and Christy (2009) found almost perfect coincidence
between oscillations in satellite-measured global temperature and the ENSO index
since about 1978 (see Figure 11.5 ) . This would seem to suggest that warming since
1978 has been heavily influenced by the Pacific Ocean, rather than by increasing
CO 2 .
Satellite measurements indicate that the global average temperature of the
lower atmosphere oscillated in a narrow range after 1978 until the great El Nin˜ o
of 1998 which drove temperatures up sharply. In the aftermath of the 1998