Page 347 - Earth's Climate Past and Future

CHAPTER 17 • Climatic Changes Since the 1800s 323

FIGURE 17-20 Pacific Decadal
Oscillation (PDO) Like El Niños,
positive PDO years have warm
temperatures off the west coast of
the United States but cool
temperatures in the northwest
Pacific ocean. PDO oscillations can
persist for many years to decades.
(Adapted from N. J. Mantua et al.,
“A Pacific Interdecadal Climatic
Oscillation with Impacts on Salmon
Production,” Bulletin of the American
Meteorological Society 78 [1997]:
1069–79, and from K. E. Trenberth
and J. W. Hurrell, “Decadal
Atmosphere-Ocean Variations in the
Pacific,” Climate Dynamics 9 [1994]:
303–19.)
A

2

PDO index 0

–2

B 1940 1960 1980 2000
Year

IN SUMMARY, it is premature to dismiss the possibility Review Questions
that short-term oscillations have played some role in
the global trend toward greater warmth since the 1. How do ice sheets that melted many thousands of
late 1800s, but it is equally premature to conclude years ago complicate efforts to determine the
that they have. As the many climatic sensors now in global sea level rise during the past century?
place register year after year of record or near-record
warmth, the current warming trend looks more and 2. What is the urban heat island effect? How does it
more like a long-term anthropogenic trend rather complicate attempts to synthesize trends of
than a response to natural short-term oscillations. regional, hemispheric, or global temperature
change?
3. Name four kinds of satellite evidence that support
Key Terms a gradual warming of high northern latitudes in
the last two decades.
tide gauges (p. 310) Pacific Decadal
peripheral forebulge Oscillation (PDO) 4. If Arctic sea ice has retreated by 25% and thinned
(p. 311) (p. 321) by 40% in the last 50 years, what has been the
urban heat island North Atlantic percentage loss in its volume?
(p. 314) Oscillation (NAO) 5. Why is it difficult to determine whether or not ice
(p. 322)
radiosondes (p. 320) sheets are growing or shrinking?