supporting the need to conduct ecosystem comparisons to understand the causes
of observed changes in particular ecosystems.
Concerns have recently been raised that global warming is likely to affect El
Nin o events by altering the background climate, as ENSO involves large
redistribution of heat in the tropical Pacific. Models suggest that increased
greenhouse gases are likely to increase the frequency of El Nin os as well as the
intensity of cold periods, but the level of greenhouse gases that will cause such
changes is debated. From past records we know that ENSOs have existed for at
least 130 000 years, but their strength seems to have increased in the 20th
century. At shorter time scales it has been noted that in the cooler and drier
tropical ocean of the late 19th century ENSO cycles lasted about 10—15 years,
which were replaced by strong shorter cycles (3 years) coinciding with the
warming step in the early 20th century. Although three decades of weak
interannual variability followed until 1950, the results suggest that ENSO may
respond to further global warming in ways that we still do not understand and
that may be more complex than we anticipate. Recent results suggest that ENSO
events can also interact with changes in the Earth’s orbit to trigger rapid changes
in climate. Periodic and gradual changes in the shape of the Earth’s path
around the Sun and the tilt of its axis are thought to cause radical climate shifts
such as the ice ages. These orbital variations can alter ENSO’s pulse, locking it
into step with the yearly cycle of seasons. During such a deadlock average
temperatures across the globe fall significantly. The authors believe that this
changeover between normal (2—7 year periodicity resulting from heat and energy
moving between atmosphere and oceans) to locked ENSO happens in a matter of
decades. The current ENSO cycle seems to be in a phase close to that in which
sudden switches can occur. If correct, ENSO and climate change may be closer
that we currently perceive them to be.
Case Study: The Pacific Decadal Oscillation
The Pacific Decadal Oscillation describes a decadal pattern of climate variability
in the Pacific. It has similar climatic fingerprints to El Nin o, but a significantly
different temporal behaviour (Figure 6). PDO eras persist for 20—30 years, while
ENSO events are typically 6—18 months long. Warm phases of the PDO are
characterized by anomalously cool temperatures in the central North Pacific,
and unusually warm temperatures along the west coast of the Americas. At the
same time these conditions favour low pressures over the North Pacific and high
over western North America and the subtropical Pacific, enhancing counterclockwise wind stress over the North Pacific. Only two full cycles have been
A. V. Fedorov and S. G. Philander, Science, 2000, 288, 1997—2002.
M. Collins, J. Clim., 2000, 13, 1299—1312.
A. W. Tudhope, C. P. Chilcott, M. T. McCulloch, E. R. Cook, J. Chapell, R. M. Ellam, D. W. Lea,
J. M. Lough and G. B. Shimmield, Science, 2001, 291, 1511—1517.
F. E. Urban, J. E. Cole and J. T. Overpeck, Nature, 2000, 407, 989—993.
A. C. Clement, M. A. Cane and R. Seager. J. Clim., 2001, 14, 2369—2375.
S. R. Hare, N. J. Mantua and R. C. Francis, Fish. Habitat, 1999, 24, 6—14.
N. J. Mantua, S. R. Hare, Y. Zhang, J. M. Wallace and R. C. Francis, Bull. Am. Meteorol. Soc., 1997,
78, 1069—1079.
Influence of Climate Variability and Change on Marine Ecosystems
73
of observed changes in particular ecosystems.
Concerns have recently been raised that global warming is likely to affect El
Nin o events by altering the background climate, as ENSO involves large
redistribution of heat in the tropical Pacific. Models suggest that increased
greenhouse gases are likely to increase the frequency of El Nin os as well as the
intensity of cold periods, but the level of greenhouse gases that will cause such
changes is debated. From past records we know that ENSOs have existed for at
least 130 000 years, but their strength seems to have increased in the 20th
century. At shorter time scales it has been noted that in the cooler and drier
tropical ocean of the late 19th century ENSO cycles lasted about 10—15 years,
which were replaced by strong shorter cycles (3 years) coinciding with the
warming step in the early 20th century. Although three decades of weak
interannual variability followed until 1950, the results suggest that ENSO may
respond to further global warming in ways that we still do not understand and
that may be more complex than we anticipate. Recent results suggest that ENSO
events can also interact with changes in the Earth’s orbit to trigger rapid changes
in climate. Periodic and gradual changes in the shape of the Earth’s path
around the Sun and the tilt of its axis are thought to cause radical climate shifts
such as the ice ages. These orbital variations can alter ENSO’s pulse, locking it
into step with the yearly cycle of seasons. During such a deadlock average
temperatures across the globe fall significantly. The authors believe that this
changeover between normal (2—7 year periodicity resulting from heat and energy
moving between atmosphere and oceans) to locked ENSO happens in a matter of
decades. The current ENSO cycle seems to be in a phase close to that in which
sudden switches can occur. If correct, ENSO and climate change may be closer
that we currently perceive them to be.
Case Study: The Pacific Decadal Oscillation
The Pacific Decadal Oscillation describes a decadal pattern of climate variability
in the Pacific. It has similar climatic fingerprints to El Nin o, but a significantly
different temporal behaviour (Figure 6). PDO eras persist for 20—30 years, while
ENSO events are typically 6—18 months long. Warm phases of the PDO are
characterized by anomalously cool temperatures in the central North Pacific,
and unusually warm temperatures along the west coast of the Americas. At the
same time these conditions favour low pressures over the North Pacific and high
over western North America and the subtropical Pacific, enhancing counterclockwise wind stress over the North Pacific. Only two full cycles have been
A. V. Fedorov and S. G. Philander, Science, 2000, 288, 1997—2002.
M. Collins, J. Clim., 2000, 13, 1299—1312.
A. W. Tudhope, C. P. Chilcott, M. T. McCulloch, E. R. Cook, J. Chapell, R. M. Ellam, D. W. Lea,
J. M. Lough and G. B. Shimmield, Science, 2001, 291, 1511—1517.
F. E. Urban, J. E. Cole and J. T. Overpeck, Nature, 2000, 407, 989—993.
A. C. Clement, M. A. Cane and R. Seager. J. Clim., 2001, 14, 2369—2375.
S. R. Hare, N. J. Mantua and R. C. Francis, Fish. Habitat, 1999, 24, 6—14.
N. J. Mantua, S. R. Hare, Y. Zhang, J. M. Wallace and R. C. Francis, Bull. Am. Meteorol. Soc., 1997,
78, 1069—1079.
Influence of Climate Variability and Change on Marine Ecosystems
73