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 he 1990s were, globally, the warmest decade since
instrumental measure started in the 1860s. Recent scientific
evidence based on pre-instrumental proxy climate data, mostly
from sites in the northern hemisphere, indicates that the 1990s
were the warmest decade and the 1900s the warmest century during
the last 1,000 years. The seven warmest years globally in the
instrumental record have occurred this decade, with the warmest
being 1998. The 1999 global mean combined land-surface air and
sea surface temperature is estimated to be in the order of 0.3
to 0.4 ºC above the 1961-1990 normal, which will be the
5th warmest in the 140-year record, according to the World Meteorological
Organization (WMO).
 1999
will have been the 21st consecutive year with an above normal
global surface temperature, and the ten warmest years have
all occurred since 1983. The four warmest years were 1998 (0.58
ºC above normal), 1997 (+0.44), 1995 (+0.38) and 1990 (+0.35).
The high temperature of 1999 is remarkable because it occurred
despite the typical cooling influence of the tropical Pacific
La Niña, which persisted throughout the year. The global
mean annual temperatures at the end of the 20th century are almost
0.7 ºC above those recorded at the end of the 19th century.
The regional
temperature patterns (January through October) show most
of the globe with above average temperatures, except for the
Northeast Pacific, equatorial east and central Pacific, western
South America and northernmost parts of Eurasia. In central England,
where the instrumental record extends back 341 years, it is likely
that 1999 will be the warmest year ever. Records in Japan indicate
that 1999 will be the 3rd warmest year in their 102-year record.
In Canada, 1999 will likely be the second warmest since 1948
(1998 was the warmest). In June/July, Russia experienced one
of its longest heat waves of the century, with record temperature
departures from normal exceeding 5 °C in June in central
and northwestern regions. Drought and heat led to the outbreak
of numerous forest fires. In parts of northern and central Europe,
September was the warmest in this century. Northern Germany recorded
departures from normal of up to 4.5 °C. In September, Norway
had departures from normal up to 5 ºC and, in November,
up to 6-7 ºC. By contrast, a cold snap in late January of
this year brought some of the coldest temperatures experienced
in Norway and Western Russia since the late 1800s.
The major
1997/98 El Niño gave way to La Niña conditions
(colder than average equatorial Pacific sea surface temperatures)
by mid-1998, which continued through late 1999. The La Niña
is expected to continue well into 2000 according to most climate
models. Such long-lived cold episodes don't occur often but this
La Niña is not unique. Extended cold episodes characterized
the periods 1954 to 1957, 1973 to early 1976 and late 1983 to
early 1986. The evolution of the 1998/99 La Niña has been
similar to, but much stronger than, the 1983 to 1986 cold episode,
which followed the extremely strong El Niño in 1982/83.
The ozone
depletion over Antarctica during the austral spring was again
very strong last year. This was only the second season in the
last 20 years that ozone-hole values (<220 m atm. cm) covered
an area greater than 10 million km2 for more than 93 consecutive
days (compared to 100 days in the record year, 1998). Over northern
middle latitudes, ozone values were 4 - 8% lower than the pre-1976
averages. Above the Arctic, the lower stratosphere was unusually
warm with no ozone destruction during the northern winter-spring
season of 1999.
Extremely
heavy precipitation had devastating
consequences in many parts of the world. A severe super-cyclonic
storm with winds of up to 250 km/h crossed the coast in Orissa,
India on October 29, 1999. This may prove to have been the worst
cyclone of the century in the Orissa region, and is responsible
for as many as 10,000 deaths, for rendering millions homeless,
and for extensive property damage. Scores of people in Japan
and China have been killed this year by floods, landslides and
storm surges caused by tropical cyclones. Two serious flooding
events following torrential rains late this year have rendered
hundreds of thousands of people in Vietnam homeless, and hundreds
dead.
In February, heavy snowfall occurred across
Central Europe disrupting traffic connections and cutting off
roads and villages, especially in Hungary, Slovakia, Bulgaria,
Ukraine, Austria, Switzerland and France. Regionally, the heaviest
snowfall in decades was recorded. Destructive avalanches occurred
across the Alps; in western Austria, southern Switzerland and
eastern France, at least 50 people lost their lives. In May,
heavy rain and thawing of the extreme deep snow cover in the
higher parts of the Alps caused extreme flooding of the river
Danube and Lake Constance. Losses amounted to one billion German
Marks.
Excessive precipitation
fell in the USA's Pacific Northwest during the November 1998-March
1999 period, which resulted in a very deep snowpack in many of
the mountainous areas in the region. Many areas reported 150%
to 200% of their normal seasonal precipitation totals, with Mt.
Baker in Washington State setting a new record of 28.96 m for
the most snowfall ever measured in the United States in a single
season. In western Canada, at Tahsta Lake, British Columbia,
Canada's greatest one-day accumulation of snow ever, reaching
145 cm, was recorded this year. Parts of the central and eastern
Canadian Prairies received more then half of their normal yearly
rainfall in May alone, submerging some of the country's most
productive farmland.
In the July to October period, heavy rains and
flash floods wreaked havoc in parts of western Africa. Thousands
were left homeless, hundreds dead, and there was extensive property
damage across large areas. Also in October, torrential rains,
landslides and flooding devastated parts of Mexico, leaving hundreds
dead. New Cale-donia has reported a warm and wet year, thanks
to the relatively strong La Niña influence early in the
year. Finally a rare snow event occurred in and around Buenos
Aires in August.
Overall, the 1999 Atlantic hurricane season
was very active with 12 named storms, 8 of which became hurricanes
and 5 of which became intense hurricanes. Only one of these twelve
storms formed prior to 19 August. The Atlantic hurricane seasons
in 1995, 1996 and 1998 were also very active. During each of
these periods, the regional circulation features known to be
conducive to increased tropical storm and hurricane activity
were strongly linked to a much larger scale climate signal associated
with La Niña conditions.
A number of areas were plagued by drought conditions.
One of the worst droughts in eastern US history intensified during
spring and peaked in late summer before increasing rainfall,
aided by three tropical cyclones, significantly eased conditions
later in the year. Lack of precipitation damaged crops and forced
the imposition of substantial water use restrictions. In parts
of eastern United States, April - July 1999 was the driest such
period since records began in 1895 and the July 1998 - July 1999
period was among the driest 3% of all 13-month periods on record.
At the end of 1999, the Great Lakes water levels are below the
80-year average, and for Lakes Michigan and Huron, the drop in
water levels was the largest year-to-year decline since record-keeping
began in 1860. Nova Scotia, in Eastern Canada, is experiencing
drought for the third successive year. In Hawaii, rainfall amounts
have been below normal for well over a year in most areas.
In Australia, there was well below average precipitation
over central and southeastern areas and record drought over the
last three years in parts of southeastern Australia. In Argentina,
precipitation has been less than normal in eastern coastal regions.
Drought and heat led to the outbreak of numerous forest fires
early in 1999, and in October. Serious droughts were reported
this year in the Middle East as well.
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