assessment published in
CONSEQUENCES vol 5 no 1 1999, pp.
In late November of
1998, the Worldwatch Institute and Munich Re – the world's largest
reinsurer – issued a report which assessed the total losses, worldwide,
from storms, floods, droughts, and fires for the first eleven months
of that unusual year. The staggering sum, at that time, was a record
$89 billion: nearly 50 percent higher than the previous record of
$60 billion in 1996. In addition to material losses, these weather-related
events had taken an estimated 32,000 lives, while displacing 300
million people from their homes: more than the populations of Canada
and the United States combined. Three months earlier, LIFE magazine had chosen "WEATHER" for its cover story, noting that in
the preceding year and a half, an unusual run of weather-related
disasters around the world had cost thousands of lives and tens
of billions of dollars in damages. The extreme weather events that
the magazine described were but a sampling of what was to become
a staple of the daily news. And with ever mounting costs.
A Wild Ride
In early August, for example, major floods devastated parts of
Korea, and in August and September 1998, extensive monsoon-related
flooding struck heavily-populated eastern India and Bangladesh.
Widespread heavy rains in China, at about the same time, released
the mighty Yangtze River from its banks, with ensuing reports of
more than 3,000 deaths, some 230 million people homeless, and over
$30 billion in flood damage. In the summer of 1998 heat waves and
air pollution episodes plagued many regions of the world, particularly
in Egypt and other Mediterranean countries, and in southern Europe.
In New Zealand, record floods in July and October 1998 were the
worst in 100 years. But the costliest disaster of them all, in terms
of human life, struck the Caribbean in late October. Hurricane Mitch
caused the deaths of more than 11,000 people in Honduras, Nicaragua,
Guatemala and El Salvador, primarily through the extensive flooding
that followed prolonged and heavy rains.
A number of extreme-weather events with large human impacts fell
in the same period in the United States, and most of them were explained,
in news reports, as the expected impacts of the uncommonly strong
El Niño of 1997-98. In the southeastern states, tornado outbreaks,
and Florida floods (costing $1 billion in damage and taking at least
132 lives) were part of a pattern that led to the wettest winter
on record there. Torrential rains in February 1998 in California
brought flooding to many locales, with heavy mud slides and coastal
erosion. A winter ice storm in early 1998 wreaked havoc in New England
and southeastern Canada, leaving many communities without electric
power for several weeks. At the same time, the northern tier of
states experienced one of the mildest winters (1997-98) on record.
Lake Erie failed to freeze for only the third time on record.
The spring of 1998 brought heavy flooding to Iowa, Indiana and
other midwest states, and to some of New England, as part of generally
wetter than normal conditions from Idaho to the U.S. Northeast.
Ohio River flooding left 30,000 people without power.
Meanwhile drought enveloped the South. Extremely dry conditions
from April through June 1998 led to wildfires which destroyed many
structures, and in Florida alone, charred 485,000 acres – an area
that is half the size of Rhode Island. In Texas the drought hung
on through the summer months as well, bringing sweltering heat waves
that devastated agriculture in much of the state.
The drought in Texas (the number one U.S. cotton producer) and
the wetness in the winter and spring of 1998 in California (contributing
to the spread of soil fungus in the second-ranked cotton producing
state) resulted in a U.S. cotton crop that fell almost 25 percent
below that of 1997. In October, another switch occurred in Texas – from too little to too much rain – leaving parts of Texas inundated,
with twenty-two dead and reports of the worst flooding since the
late 1920s. In early November heavy rains triggered floods in Kansas
These are but a sampling of the extreme weather events that have
come our way in the past two or three years. Why so many, and what
brought them on? Can these questions be answered at all with any
certainty? How unusual were these extreme events, in the context
of "normal" weather variability or within the limits of what meteorologists
expect in El Niño years? Are we to anticipate more of the same in
years ahead, and how much need we be concerned?
The authors of the Worldwatch Institute and Munich Re report felt
certain that our own actions are in part responsible for the escalating
costs of weather-related disasters, through (i) a combination of
deforestation and land use change (which greatly affect water runoff
during heavy rains); (ii) the effects of growing population pressures
(which have led people to settle in flood plain and other regions
vulnerable to flooding); and (iii) our energy-related impacts on
climate itself. But can we tell whether human activities are really
The broadcasts and news stories that bring weather events to our
attention as they happen are of necessity cursory and incomplete.
With more extensive data and the benefit of hindsight we can hope
to make more responsible assessments of what happened and why. A
first step is to look for the telltale marks of probable causes,
to help us sift out significant changes in global and regional weather
from the chaff of more random variations.
Two probable causes
Two extremely likely suspects are potentially involved in what
occurred, and neither can be much of a stranger to anyone who has
watched or read or listened to the news: El Niño and global greenhouse
warming. The first and more certainly involved is the 1997-98 El
Niño and ensuing La Niña: a recurring, one-two punch to the global
climate system that is driven by distinctive warming and later,
cooling of surface waters in the tropical Pacific Ocean. El Niños
– the better known of the two – are set in motion at irregular intervals,
altering the course of weather and climate, around the world, for
up to a year or so. The most recent were in 1982-83, 1986-87, 1990-95,
We know from past occurrences that what might be called a "mini
global warming" accompanies each El Niño, with the highest air temperatures
typically occurring a few months after the peak in ocean surface
warming. The record-breaking and long-lasting El Niño of 1997-98
began in April of the first year and persisted until May of the
next. It almost certainly contributed in establishing 1998 as the
world's warmest year on record.
But the exceptional nature of the 1997-98 El Niño – more intense
in many ways than those well documented in the past – requires an
explanation, and suggests that it was aided and abetted, somehow,
by other climate forces. Most likely implicated is the warming of
the Earth – quite independent of El Niño – that is expected from
the accumulation of carbon dioxide and other greenhouse gases in
the atmosphere. Did "global warming" also show its hand in the run
of recent weather extremes, and did it amplify what might have been
a more run-of-the-mill El Niño?
Trying To Explain
In the early 1930s the U.S. experienced what is probably our best
remembered period of extreme weather events, in what came to be
known as the "Dust Bowl era." Extensive droughts, year after year,
took a particularly heavy toll on farms and farm families in the
bread basket of America, providing a backdrop for John Steinbeck's
novel, The Grapes of Wrath, that tells the human impacts
on displaced migratory farm workers. Some of the Dust Bowl years
still stand today, in certain localities, as the warmest on record.
Record-setting temperatures or precipitation amounts, and extreme
events of all kinds are normal features of climate: they occur,
somewhere, all the time, and always will. A year in which none was
noted, anywhere, would indeed be noteworthy. But there seem to have
been an uncommon number of such extremes in the late 1990s, particularly
when we look at the world as a whole. Or could it be the result
of more thorough reporting, or the existence of continuous TV news
or weather channels that must find something of interest to broadcast,
every hour of each day?
While we are indeed exposed to more and ever-wider coverage of
the weather, the nature of some of the records being broken suggests
a deeper explanation: that real changes are under way. In particular,
global mean temperature records – based on the average of continual
measurements taken at thousands of surface weather stations around
the world – were broken month after month in 1998, and by margins
never seen before.
The news media, reflecting valid public concerns, usually endeavor
to include an explanation for dramatic weather events. Their queries
are commonly answered by weather forecasters in terms of weather
phenomena themselves: such as "the drought was caused by a big stationary
anticyclone sitting over the region," or "the jet stream was displaced
south of normal." But these are more descriptions of what happened,
than explanations of why.
Why Does The Weather Change?
Such answers may be all that one can give, for often there is no
identifiable "cause," given that weather has a wide range of natural
variability, and is ever changing. Weather systems (such
as the large-scale rotating wind patterns called cyclones
and anticyclones) and weather phenomena (such as thunderstorms)
arise primarily from instabilities within the atmosphere:
where a small perturbation in the flow of air, for example, is rapidly
amplified. The energy that drives these instabilities comes most
often from the uneven way in which heat from the Sun is distributed
over the surface of the Earth and with height above the ground.
An example is the cyclones and anticyclones and associated cold
and warm fronts that arise from differences in the temperature of
the air in the tropics (where the Sun is more directly overhead)
and that at the poles of the Earth (where it is always lower
in the sky). The pole-to-equator temperature difference is the result
of the spherical shape of the planet, and the different angles at
which the Sun's rays enter the atmosphere and reach the surface.
The atmosphere responds to this unequal heating by continual attempts
to reduce the pole-to-equator temperature gradients. It does this
by generating, in the Northern Hemisphere, southerly winds to carry
warm air polewards and cold northerly winds to carry the colder
air to lower latitudes.
Another example of atmospheric instability is convection:
the process through which near-surface air – warmed by solar heating
– expands and rises, producing thermals which carry heat
upward to give birth and shape to the clouds, and sometimes thunderstorms
that bring rain.
When changes in solar radiation or any other component of the climate
system (such as the land surface, oceans, or glaciers) influence
the atmosphere in a systematic, as opposed to a random
way, local weather patterns are likely to be affected, and will
remain in an altered condition for seasons or years or longer. It
is these more persistent changes in the prevailing weather at any
place – which is to say, the climate – for which we can hope
to find specific causes.
The most rudimentary of all climate changes is the annual
cycle of the seasons, which results from the path of the Earth around
the Sun, and the 23 ½° angle at which the axis of rotation of the planet is inclined to
the plane of that orbit. The next best understood – and the next
most predictable climatic variation, thanks to quite recent research
accomplishments – is El Niño, which occurs on an irregular schedule
of a few years.
and La Niñas
In the early months of 1997, a familiar pattern of higher
sea surface temperatures (SSTs) in the central and eastern tropical
Pacific gave a clear signal that another El Niño was underway. It
was also obvious, given the magnitude of the warming, that it could
develop into a major event. For many months, peaking in December
1997, SSTs recorded in some of these tropical Pacific waters were
more than 5°C (9°
F) warmer than their average value. Over most of the ocean, by comparison,
departures from average for even a single month rarely exceed 2°C. Over continental areas of similar size – where the solid surface
of the ground is a less effective thermal buffer – departures of
5°From the normal
can sometimes be found for shorter periods, but rarely for more
than a month or two.
Records of past climate reveal that El Niños have been a recurring
feature of the Earth's climate for thousands of years. But while
we now know how their arrival is announced, and have set up monitoring
systems in the oceans to warn of their coming, we don't know exactly
when the next one, or the one after that, will first show its face:
beyond the knowledge that they appear, on average, about every three
to seven years.
El Niños are often preceded or followed by the opposite phase,
dubbed La Niña, when surface temperatures in the tropical Pacific
Ocean are systematically cooler than the long-term average.
A corresponding modulation in the general circulation of the global
atmosphere – called the Southern Oscillation – is also closely
allied with these ocean changes. The name which is widely used by
scientists to refer to the three together is the El Niño-Southern
Oscillation phenomenon, or ENSO.
El Niño is the warm phase of ENSO, and La Niña the cold. Both involve
the tropical oceans and the atmosphere, and the exchange of energy
between the ocean surface and the air above it. The ocean and the
atmosphere are linked together, interactively, and each affects
the other. Atmospheric winds push the ocean currents and help determine
the patterns of sea surface temperature. But SSTs at the same time
help determine the force and direction of winds and atmospheric
circulation, by adding heat or taking it away, chiefly by shifting
the places where tropical thunderstorms preferentially occur.
The warmest large-scale pool of ocean water in the world is normally
found in the vicinity of Indonesia – south of China and north of
Australia. El Niño shifts the location of the warm water pool over
3200 km (2000 miles) eastward, to more open waters near the International
Dateline or beyond. Meanwhile the normally dry zone over the eastern
equatorial Pacific – from mid Pacific islands to the arid coasts
of Chile and Peru and Ecuador – becomes much wetter.
When water vapor condenses and falls as rain, the heat that was
stored when the water was originally evaporated is released into
the air. Because of this, the major shift in the location of tropical
Pacific rainfall that accompanies an El Niño alters the heating
patterns of the whole atmosphere. Somewhat like a rock in a stream
of water, the placement of a new source of heat sets up waves in
the atmosphere that are as large as continents, and their effects
reach beyond the tropics into mid-latitude regions. In the end,
winds and storm tracks and the jet stream are all perturbed, in
ways that affect, to some degree, almost everyone on Earth.
When La Niña takes control, the situation changes. Areas with drier
than normal or even drought conditions during El Niño, such as Indonesia,
the Philippines, Australia, Southeast Asia, Hawaii, and parts of
Africa and Brazil, are apt to experience heavy rains during La Niña.
Meanwhile, areas that have experienced El Niño floods, such as Peru,
Ecuador, Uruguay and northern Argentina in South America, parts
of Africa, and southern parts of the U.S. in winter, are apt to
be drier than normal during the ensuing La Niña phase.
Impacts of the last El Niño
The most severe drought of the 1997-1998 El Niño struck Indonesia,
with the result that many of the fires commonly set to clear land
for agriculture raged out of control for weeks on end. So much ash
was carried into the air that respiratory problems were reported
as far as 1000 km (about 600 miles) away, and the loss of visibility
was held responsible for the crash of a commercial airliner. El
Niño-related drought and associated wildfires continued into 1998
in Brazil, Mexico, and Florida. As expected with El Niño, flooding
hit Peru and Ecuador but also Chile, and coastal fisheries were
Among the consequences of the 1997-98 El Niño was a persistent
Northern Hemisphere jet stream that in winter blew with particular
strength across the Pacific, over southern California, and then
across the southern states to Florida. Atmospheric disturbances
carried by these winds developed into major storms for the U.S.,
pummeling the West Coast and creating wet conditions from California
to Florida. Storms generated by the jet stream as it crosses the
North Pacific normally veer to the north and end up in the Gulf
of Alaska, or enter the North American continent in the vicinity
of British Columbia and the state of Washington. There they often
link up with frigid Arctic and Canadian air masses and bring these
down as recurring waves of individual cold fronts into the U.S.
When the jet stream instead veered south, the result in the
U.S. was relatively mild winter conditions over the northern states
that usually bear the brunt of these conditions, such that temperatures
in February in the Great Lakes area averaged more than 10°
C (18°F) above normal.
The pattern was not confined to North America. The jet stream in
the Southern Hemisphere was affected in a similar way, with similar
effects on South America, where it was then late summer. One of
the results was that the uncommonly warm February of 1998 set a
record for the greatest departure from the average global temperature
reading for any month on record.
A possibly unique development of 1997-98 El Niño conditions
was a marked warming of surface waters in other tropical
ocean basins. SSTs in the tropical Indian Ocean, for example, which
are customarily about 27°
C (80°F) exceeded
29°C (84°F); these waters were then warm enough to compete with the tropical
Pacific in their effect on the atmosphere. As a result, strong thunderstorms
developed over the western tropical Indian Ocean in October 1997,
to spread over the Horn of Africa region and bring torrential rains
to southern Somalia and Kenya for several months. Insect and disease
outbreaks followed the resulting floods, leading to the proliferation
of vector-borne diseases such as malaria, dengue fever, and Rift
Valley fever. As is also often the case in the aftermath of severe
storms, there and in other parts of the world, polluted local water
supplies, degraded hygiene, and increased incidence of cholera occurred.
In the spring of the year, the jet stream in the Northern Hemisphere
normally migrates northwards, carrying much of the potential for
stormy weather out of the reach of the central U.S. In 1998, however,
as had happened on previous El Niño occasions, the jet stream continued
to trace a course somewhat south of its normal position across the
United States. As a result, a more southerly-than-normal storm track
was established from Idaho to New England. This paved the way for
a sequence of flooding events in Iowa, flooding in the Ohio Valley,
and the wettest June on record in much of New England. It also diverted
storms, and more normal rainfall, from the southern states. Regional
weather patterns of this sort – wetter or colder in one area, dryer
or hotter in another – persist longer than usual in El Niño years.
By April 1998 conditions were rapidly drying out in the South, setting
the stage for subsequent drought and wildfires.
The 1997-98 El Niño
Specific weather events – such as a major snowstorm in February
in New England – cannot be attributed unequivocally to El Niño,
or indeed to any other climatic cause. The weather will always vary,
from day to day or week to week, and certainly from place to place
and, as noted earlier, unusual swings, this way or that, can always
be expected somewhere. What is definite is that El Niño, and La
Niña, change the odds that certain kinds of weather will occur in
a given place.
The most probable range of expected temperature, precipitation,
and other meteorological conditions can be calculated for a given
area, based on the unique perturbations that El Niño SSTs impose
on the general circulation of the global atmosphere. This is done
with the help of highly-detailed computer models of the climate
system. The results will depend to some degree on what meteorological
conditions were assumed to apply at the start of the calculation.
But by repeating the modeled calculation for a representative range
of these so-called "starting conditions," we can explore the scope
of possible outcomes. These are then compared with what the same
model projects in the absence of the El Niño perturbation.
The same models can be used to forecast the impacts of developing
El Niños, provided that reasonably reliable projections can be made
of sea surface temperatures. And indeed this can now be done, based
on our improved understanding of how El Niño works and given the
SST and sub-surface ocean observing system that is now in place
in the tropical Pacific Ocean.
The new knowledge and observing capabilities were put to the test
when the 1997-98 El Niño came into being, and the world profited,
for climate simulations and forecasts throughout the event were
more consistent and more skillful than had been the case before.
What happened in these years to the circulation of the global atmosphere,
for example – as seen in the changes in the jet stream and in storm
tracks – was close to what was forecast by El Niño models several
months in advance.
These successes can also help in identifying the cause of the extreme
weather of 1997-98. The accurate forecast, based on El Niño, of
many aspects of the winter weather patterns, is compelling evidence
that El Niño was indeed a major contributor. Attributing the consequences
– such as the proliferation of insects and other pests, or the outbreak
of disease – requires similar tests of a longer chain of events,
and is hence less certain. Nevertheless, the sorts of El Niño impact
chains that now seem likely include the warmer and wetter conditions
which favor the reproduction of malaria-carrying mosquitoes, with
consequences for public health; and the lack of more normal freezing
temperatures that would ordinarily kill off or delimit populations
of pests or fungi, with possible impacts across the spectrum of
The severe weather events that were attributed at the time to the
1997-98 El Niño dealt heavy blows to many geographic areas and commercial
sectors, while actually helping others. For instance, while it may
be possible to attribute the tornadoes in Florida to a more active
and southward-deflected storm track, it is possible that without
the interference from El Niño, equivalent tornado damage could have
been inflicted elsewhere, such as in neighboring Georgia, with equivalent
loss of life and property. A similar instance of deflected misery
can be found in the shift in 1997-98 hurricane activity from the
Atlantic basin to the Pacific.
One of many examples of a different kind was the mixed benefit
of the warmer winter of 1997-98: a boon to consumers but not for
the natural gas and heating oil industries. These all highlight
the need to identify what was clearly attributable to the 1997-98
El Niño and what was not. In doing this we need to consider both
the positive and negative impacts, in terms of human values as well
as economic terms.
The terms "global warming" or "enhanced greenhouse warming"
are commonly used to describe the human-induced change in global
climate that is projected to occur in the course of the next century
or two. Behind the projection is a continuing and well-documented
increase in the concentration of the atmospheric greenhouse gases
in the atmosphere. The most notable among them is carbon dioxide,
which accumulates in the air when fossil fuels – such as coal or
natural gas or gasoline – are burned, or when forests are cleared.
As a result of these human activities, the amount of CO2
that has accumulated in the air today is 30 percent greater than
the stable levels of pre-Industrial times. Most of this increase,
moreover, has come about since 1950.
Since these gases are the principal thermostat for the planet,
the question is not whether the global temperature will respond
to their increasing concentrations, but when, and by how much, and
whether other climate factors – such as clouds or ocean circulation
– might possibly compensate, and soften the blow.
There are many signs, including but not limited to meteorological
data, that the expected global warming may already be upon us. The
average surface temperature of the Earth has been gradually rising
for the last 100 years, and more steeply in the past two decades
(Figure 1). The last ten years are the
warmest decade on record. 1998 is far and away the warmest year
on record, and 1997 the second warmest.
The oft-cited value for the amount that the average surface
temperature of the Earth has warmed during the present century is
about 0.5°C (or 1°F.) These numbers, however, were derived for the period through
1995. As a result of a record-warm 1997, and an even hotter 1998,
the overall warming relative to last century is now closer to 0.8°
C (1.5°F). The melting
of glaciers over most of the world, diminishing sea-ice in polar
waters, and rising sea level are among the growing number of signs
that seem to confirm the reality of the increase in global temperature.
Nature responds to weather in ways that enable those who study
tree-rings and ice cores and other paleodata to extend our
knowledge of climate history far back into the past. A recent analysis
of the most extensive of such records produced a year-by-year temperature
history for much of the Earth for the last 1000 years: adding over
eight centuries to what we can glean from available thermometer
readings. In no year – or decade – of that long span was the average
temperature of the Earth as warm as it is now.
Hotter air temperatures are but one of many consequences when extra
heat is added to the lower atmosphere, as will happen with enhanced
global greenhouse warming. Another result is increased evaporation
of surface moisture. Hotter air can hold more water and, together
with enhanced evaporation, the inevitable result is a marked increase
in atmospheric moisture. In the U.S., for example, the average amount
of moisture in the atmosphere increased by 5 percent per decade
in the years from 1973 to 1993. This remarkable trend toward wetter
and wetter air is more than can be accounted for by global warming
alone, however, highlighting the need to account for El Niño-related
and other changes.
When the hydrologic cycle is predisposed in this way by global
warming, naturally-occurring droughts – such as those brought on
by El Niño – will set in quicker, plants will wilt sooner, and the
droughts will likely become more extensive and longer lasting. Moreover,
when there is little or no moisture in the soil to evaporate, all
the incident solar radiation goes into raising temperature, bringing
on sweltering heat waves of the sort that plagued Texas in 1998.
Further, over the Earth as a whole, any enhanced evaporation
must be balanced, somewhere, sometime, by an equivalent increase
in precipitation, for the global atmosphere can hold but so much
water. Outside the tropics, moreover, roughly 75 percent of the
water that falls as rain or snow comes from moisture that was stored
in the atmosphere when the storm began. Thus with more moisture
in the atmosphere, we can expect an enhancement of rainfall or snowfall
in precipitating weather systems, be they thunderstorms, or extra-tropical
rain or snow storms. Over the U.S., as in many other parts of the
world, the number of heavy rainfall events has been increasing throughout
the current century, adding to the potential for floods. The amount
of annual precipitation has also increased by about 10 percent in
the last 100 years. Thus, observations show that when it rains it
pours, harder than it would have under similar circumstances just
a couple of decades ago!
We should note that heavy rainfall is but one of several factors
that determine whether or not a flood occurs. The spatial extent
of the storm, the total amount of rain, and the rainfall rate are
also important, as are the nature and condition of the terrain on
which it falls. Important factors regarding the land include whether
melting snow is present; the precondition of natural drains, streams
and rivers; whether ice dams exist; and the degree of soil wetness.
Geophysical, topographical, and vegetation conditions are all involved.
Every bit as important, however, is whether human land use and structures,
such as ditches, dams, levees, and reservoirs, allow the runoff
to be channeled and ultimately managed. Because of these factors,
flood records in themselves are unreliable indicators of changes
What climate models say
How much might the Earth warm with added greenhouse gases, and
what can we expect in years ahead in terms of altered weather? Many
projections have been made, in scientific laboratories around the
world, using the best available climate models. Most commonly they
endeavor to foretell conditions through the next century or so,
to about the year 2100. But the models used, by necessity, include
assumptions that can have a major influence on what they project.
By far the most uncertain of these are the "what if" assumptions
regarding the often personal choices that people and governments
will make, around the world, in years to come. For example, the
projections of global warming models depend heavily on what each
assumes about future rates of energy production and fossil fuel
consumption. The projected rate of temperature increase will depend,
in addition, on how faithfully the model simulates natural processes,
including how clouds are depicted, or how vegetation responds. A
climate model's answer to questions asked of it is almost never
the single, simple answer that policy-makers want.
In spite of all the caveats and uncertainties, an ever-present
feature of every climate model is a projected increase in global
mean temperatures in coming decades and centuries. The consensus
projection for the so-called "mid-range" emissions scenario – in
which the CO2 content of
the atmosphere increases to twice the 1990 value by the year 2100 – is an increase in the mean temperature of 1.3°
C to 2.9°C above that
of 1990, with a "best estimate" of about 2°
C (4°F). In addition,
we need remember that other significant changes will accompany the
projected surface warming, including the more prevalent extremes
in rainfall that were noted above. Another major concern is the
rate at which the world's climate is expected to change in
response to enhanced greenhouse warming: more rapid than any natural
variation in the past 10,000 years.
Based on the close agreement between observed climate indicators
and what is predicted in global warming models, the Intergovernmental
Panel on Climate Change (IPCC) concluded in 1995 that "the balance
of evidence suggests that there is a discernible human influence
on global climate."
EL Niño and Global Warming:
A Potent Combination
The El Niño of 1997-98 was in many ways the largest yet recorded.
Was this by chance, or was it lifted to record levels by the help
of other hands? In attempting to answer that question we should
first note that El Niños seem to have marched to a different drummer
in the course of the last twenty years. Since 1976, there have been
more El Niños (7) and fewer La Niñas (4), when compared with the
historical record of the previous hundred years – in which the two
have occurred in more equal numbers. In the past two decades, moreover,
we have seen the two biggest El Niños on record – -the most recent,
and the previous record holder, 1982-83 – -as well as the longest
on record. The latter persisted for half a decade, from roughly
1990 to mid-1995, as three modest El Niños were blurred into one,
when sea surface temperatures in the equatorial Pacific failed to
fall below average conditions in between.
When the character of El Niño changes so dramatically, and for
so many years, we suspect that something else – most likely global
greenhouse warming – is also involved.
Before we make that claim, however, we need a plausible explanation
of how a long-term, global warming trend could have so marked an
effect on a recurring feature of shorter-term climate change; how
it might favor and accentuate El Niños, at the expense of the opposite,
La Niña phase; and what the combined effect should be, in terms
of our day-to-day weather.
Charging and discharging the Warm Water
How might El Niño be affected by global warming? The amount of
warm water in the tropics builds up prior to – and is then depleted
during – each El Niño. During the years of the cold La Niña phase,
clearer skies are more prevalent across the wide Pacific, allowing
radiation from the Sun to gradually warm the surface waters of the
tropical Pacific Ocean. The added heat is stirred and redistributed
by ocean currents, with most of it carried westward, where it accumulates
in the deep Warm Pool in the equatorial western Pacific.
At the start of an El Niño, warm water from this Pacific reservoir
is carried eastwards towards South America, to initiate the chain
of events on the world's weather that are now well known. After
their long eastward sweep along the equator, the warmer surface
waters are deflected north or south toward higher latitude by altered
ocean currents. Some of the heat that they have carried escapes
to the air, mainly by increased evaporation, which further cools
the ocean surface. When this added moisture falls out as rain –
usually hundreds or thousands of miles away – it contributes to
a general rise in surface temperature around the world that peaks
a few months after the start of a strong El Niño event.
It was not by chance that the record-breaking temperatures of the
first half of 1998 came just after the December 1997 peak of the
1997-98 El Niño, during the time when the Pacific Ocean was rapidly
dumping its excess heat. This suggests that ENSO events are most
simply an alternating sequence of storing up and then releasing
thermal energy – like repeatedly filling a bucket, then pouring
most of it out. The spacing of ENSO events would then be determined
by the time spent in recharging the system – that is, in accumulating
a sufficient volume of warm water in the tropics – plus the time
for the ensuing El Niño to run its course. An endless cycle of charging
and then discharging would also explain why El Niños are preceded
and followed by La Niñas.
The effects of global warming
If ENSO is fundamentally a process that redistributes heat within
the climate system, then tampering with the global thermostat –
by injecting more greenhouse gases into the air – will likely interfere.
Some possible explanations for the changed behavior of El Niños
in the past twenty years are that the Warm Pool in the tropical
western Pacific is expanding; and/or the recharge phase of El Niño
has speeded up; and/or the heat loss phase is less efficient. Any
of these could follow from warming and result in more frequent El
Niño events. With global greenhouse warming we should expect higher
temperatures in the upper layers of the ocean, and a steeper drop
in temperature beneath the surface, which would increase the magnitude
of swings between La Niña and El Niño.
One of the projected effects of global warming in climate models
are changes in ENSO patterns and frequency, but as yet, none of
the models simulates ENSO with sufficient fidelity to give confidence
to the results. Also, how clouds might change, and especially the
brightness of the ever-present convective clouds at equatorial latitudes,
is particularly uncertain and can influence the outcome. Similar
questions apply to the ocean, where changes that might alter the
slow stirring of water beneath the surface could act in ways that
are quite uncertain. Thus the question of how El Niño will change
with global warming is not yet answered.
This also highlights the need for more comprehensive climate models,
and particularly those that deal not only with El Niño and SSTs,
but changes in the composition of the atmosphere, as well, including
the greenhouse gases and solid pollutants that we add, and debris
from volcanic eruptions. The impacts of the El Chichon volcanic
eruption in Mexico in April 1982 – at the time of onset of the 1982-83
El Niño – have yet to be combed from the snarled climatic record
of these two overlapping events. Could this chance eruption explain
some of the differences between the 1982-83 and 1997-98 El Niños?
When someday we make reliable climate projections months or years
in advance, as now is done for the daily weather, the models employed
will of necessity include these and all other likely climate forcings.
They will also recognize that interannual variations, like ENSO
events, take place in a setting of decadal changes, and these, in
turn, against a back-drop of even longer-term variations, which
may be controlled by the world's oceans, and perhaps the Sun.
Changes in atmospheric circulation
Although the average temperature of the Earth has been generally
rising (Figure 1), the change, from place
to place, is not at all the same. What we know of the atmosphere – and particularly the nature of atmospheric waves – would seem
to guarantee a global inequality: some areas with greater-than-average
warming, others with less, and a few where the average temperature
has even dropped. The status of recent warming over the Earth, shown
in Figure 2, reveals that warming has
been largest over most of the northern continents, much less in
the eastern half of the United States, and absent altogether in
the North and South Pacific and North Atlantic, where ocean surface
temperatures have cooled somewhat.
It has also been demonstrated that the pattern of warmer winter
temperatures in the Northern Hemisphere arises in large part from
changes in atmospheric circulation. Some of these changes in flow
are linked to El Niño, whereby warming in the tropical Pacific is
spread along the coast of North America and throughout much of Canada
and Alaska. The cooling in the North and South Pacific can also
be attributed to the increase in El Niños.
Other changes were a wintertime increase in westerly winds in the
Atlantic ocean area, that contributed to warmer temperatures throughout
Europe and Asia and cooler ones over the western North Atlantic
and Greenland. Overall, the land has warmed more than the ocean,
in large part because of the way the atmospheric winds have changed.
It is not at all clear how the atmospheric circulation will respond
to further increases in greenhouse warming. Nor can we anticipate,
given these uncertainties, the meteorological consequences that
could follow alterations in so fundamental a feature of the atmosphere.
We should expect, however, that global warming will be manifested
through changes in natural modes of behavior of the climate system,
such as ENSO, by way of changes in the relative frequency and strengths
of El Niño and La Niña events. Moreover, the ENSO-caused floods
and droughts will be exacerbated by global warming.
One of the major impacts of El Niño involves the distribution and
intensity of hurricanes. In El Niño years, there is more vigorous
activity in the Pacific – especially the central and eastern portions
– and fewer and less energetic hurricanes in the Atlantic and Caribbean.
When the opposite, La Niña conditions prevail, as in 1995 and the
latter half of 1998, the Atlantic is again more prone to an active
hurricane season. What is not so clear is how global warming will
influence hurricanes. While hotter SSTs can fuel more vigorous storms,
with global warming the increase in air temperature will exceed
that of the sea, which should stabilize the atmosphere and decrease
the potential for tropical storms.
El Niño brings on floods and droughts in different parts of the
world, and global warming will likely exacerbate the extremes of
flooding and drought, no matter what their origin. Moreover, any
change in the average rainfall or temperature or other meteorological
variable will be accompanied by a disproportionate shift in the
What happens to the expected range of temperature readings – and particularly the extremes – when the mean surface
temperature warms but a small amount can be seen in Figure
3, for the simple case of a 5°
increase in a mean temperature of 50°F. The resulting change in the probable occurrence of any temperature
reading that is close to the mean value – which might be called
the silent majority – is very small. In contrast, at the two ends
of the bell-shaped curve of expected temperatures, corresponding
to the expected extreme readings, the impact is enormous. Temperature
readings in excess of 75°
F, for example, are more than twice as likely when the mean temperature
rises from 50° to only
55°F and the variability
remains constant. And the same is true for temperature readings
below 25°F, at the
other end of the expected spread.
Because of the natural variability of the weather – where
day-to-day swings of 20°F or more are not uncommon – most of us would never notice incremental
changes in the mean temperature, were it not for their effect
on the resulting temperature extremes.
Extremes in temperature or other meteorological indicators are
exceedingly important to both natural systems and to human systems
and infrastructure, in that they, and we, are designed to tolerate
a limited range of natural weather conditions. We readily adapt
and adjust to the variability and frequency of extremes that accompany
that span. But when extremes are reached that exceed the accustomed
spread, or when they are encountered more often, the limits of adaptation
or of linear response can easily be exceeded: as with the "straw
that breaks the camel's back."
Flooding is an example. With global warming, the 100-year floods
that are routinely considered in building and land-use practices – expected, on average, but once each century – may now become 50-
or 30-year floods. The change could be seen in floods that flow
over dams or break levees, inundating the surrounding countryside
and urban areas, resulting in drownings, water damage, and more
subtle impacts that include polluted drinking water.
The late-1998 flooding of the Yangtze River is a recent case, but
the same has also happened in the United States: in California in
early 1997, in the Red River valley in the Dakotas in spring 1997,
and in the Upper Mississippi Basin in 1993, to cite but three examples.
Insect and disease outbreaks often follow, especially in tropical
countries, where, as in some areas of Africa and South America,
cholera outbreaks and mosquito-borne diseases such as malaria, dengue
fever, and Rift Valley fever are often associated with flooding.
Short-term climate anomalies, such as were experienced with El
Niño, can serve as useful proxies, or analogs, for what might
happen, more often, in the course of future global greenhouse warming.
They can also teach us what we can do by way of mitigation, and
of particular importance in this regard are the forecasts that were
made of the 1997-98 event. Some of the relevant impacts of the 1997-98
El Niño are listed below.
- A rise of about six inches (fifteen cm) in sea-level along
the coast of California, which combined with storms to produce
particularly damaging coastal erosion. Sea level is expected
to rise around the world with global warming.
- Substantially higher than normal temperatures over land, particularly
in some areas in the winter of 1997-98, that mirrored the kinds
of changes that have been projected for global warming. Among
the economic impacts were major changes in the demand for heating
- Changes in precipitation patterns, leading to flooding in
some areas (such as Chile, Peru, California, and the southeastern
U.S.), and to drought in others (as in Indonesia and Central
America) that resulted in out-of-control fires, often arising
from slash and burn agriculture, in spite of ample warnings.
While changes in precipitation with global warming would differ
from those of the El Niño, the effects on agriculture, water
resources, and fires, for example, and how communities dealt
with them, could be much the same.
- The expected emergence of a number of significant, secondary
impacts. The fires in Indonesia, for example, brought respiratory
problems to areas that were 600 miles away. Some of the results
of California floods were an abundance of insects, rodents,
and snakes, an increase in soil fungus, and contamination of
domestic water supplies with consequences for human health.
Uncommon winter rains in the U.S. Southwest resulted in swarms
of grasshoppers in Arizona, and a surge in the population of
hantavirus-carrying rodents in New Mexico. Outbreaks of disease
occurred, such as Rift Valley fever in Kenya, cholera in Peru
and Tanzania, and malaria in Africa, which could have been anticipated
(and to some extent were.) All of these can provide lessons
for other climatic changes.
- Because prediction was possible and warnings were made, individuals
and institutions could respond in ways that lessened the negative
impacts. Some were unable to act; others chose not to. Projections
of impending climate change associated with global warming are
also now available, in large part through the continuing efforts
of the IPCC, and are being taken notice of by some communities
and countries more than others.
These and other examples demonstrate the potential benefit of enlightened
mitigation. For example, in some areas, including Australia, the
removal of litter and debris reduced the risk of drought-associated
fires. In some flood prone areas, drains and ditches were cleared,
roofs repaired, and dikes strengthened in anticipation of the forecast
heavy rains. Advanced plans were made for medical supplies, or for
meeting anticipated shortages in food and other commodities. In
retrospect, far more could have been done, given the overall accuracy
of what was forecast.
The modeled projections were not wholly on the mark, for in some
areas (and notably in Asia) the forecasts did not match what happened.
As a result, efforts to prepare were misdirected, and we can assume,
public faith in future projections of this kind was probably eroded.
In these experiences, good and bad, we have for future study a natural
experiment in how people, institutions, and governments can cope,
particularly in making decisions under uncertainty, and lessons
for longer-term climate change, whatever its cause.
A Personal Perspective on Recent Weather
To explain the weather disruptions over the past year or so, or
weigh the influences of El Niño or possible global warming, it is
important to have (1) a picture of what happened over the whole
Earth, as opposed to a single country, or region, or hemisphere;
(2) weather data from at least several months, to allow temporal
and geographic patterns to be discerned; and (3) a perspective that
encompasses all aspects of the weather.
In particular, it is misleading to think only in terms of temperatures
when considering the effects of global warming. The "air conditioning" effects of moisture are extremely important in making meaningful
projections of climatic change, as are regional differences in where
weather systems develop and persistent conditions apply. For example,
through the winter of 1997-98 in the U.S., the wet regions
in the South were quite distinct from the warm regions in
the North, and vice versa in the spring.
The floods and droughts that future El Niños will usher in are
likely to be exacerbated by global warming, whether or not El Niños
are themselves affected – in that these transient disturbances will
be imposed on a climate system that is in some ways already disturbed.
But the El Niño phenomenon could also be itself transformed as the
underlying climate warms, evolving, perhaps, in the direction of
more frequent and more energetic appearances, or in ways that we
can not as yet foresee.
Unquestionably, for fifteen consecutive months – from April 1997
until about July of the following year – the last and greatest El
Niño of this century exerted a dominant influence on all aspects
of prevailing weather patterns. Its heavy hand shifted weather patterns
on the global map, and held them there, relentlessly. Similarly,
after June 1998, the rapidly developing La Niña exerted major influence:
on the hurricane season and especially Asian flooding.
It is likely that global warming also contributed. Since the time
of the 1995 IPCC assessment, observational evidence has continued
to mount for a significant human influence on global climate. The
best assessment of the contribution of enhanced greenhouse warming
to the climate of today is that it is still small, but it is there.
The expected signals seem now to be emerging from the noise of background
variability, most clearly, perhaps, in the nature of weather extremes.
And while some changes arising from global warming could prove benign
or even beneficial, the economic costs of the impacts of more extreme
weather are substantial and clearly warrant further attention in
It may prove significant that the emergence of clear signs of global
surface warming, beginning in the late 1970s, is coincident in time
with the more frequent appearance of El Niños. A possible explanation
is that although present, the global warming influence was until
that time insufficient to perturb the normal working of the climate
system: that its capacity to affect the overall behavior of the
climate system was reached only after a certain threshold had been
passed. Were this what really happened, it would identify global
greenhouse warming as an abrupt, as opposed to a gradual, and thus
more easily accommodated climate change. It may take several years
to determine whether this perspective is the right one.