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Geografia d'Europa: textos de suport |
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The 1995 warmth could not be attributed to El Niño/Southern Oscillation (ENSO) since the contrasting equatorial Pacific Ocean surface temperature anomalies were averaged to be near normal for the year. In the North Atlantic, however, sea-surface temperatures were considerably higher than the average of the 1961-1990 period.
There were many extreme climatic anomalies and weather events in 1995. Following prodigious rains in January and February, which caused massive flooding along major continental European rivers, the summer over the same region was of near record heat and, in the United Kingdom particularly, serious precipitation deficiencies caused widespread restrictions on water usage. There were examples of beneficial events, including the return of abundant rains to the drought-stricken Iberian Peninsula.
In 1995, the northern hemisphere was warmer than in all previous years, but the southern hemisphere was relatively less warm. Parts of Siberia were about 3°C warmer than usual in 1995, because the early part of the year was exceptionally warm, whereas 1995 was slightly cooler than normal over the Greenland/north-west North Atlantic region and over the mid-latitude North Pacific. It was the coldest year in Iceland since 1983. These features have recurred in many recent years. Also in 1995, the ocean surface was more than 1°C warmer than usual near the Azores.
Warm conditions characterized much of western Europe, reaching record or near record annual means. For instance, in the Netherlands, the mean, spatially-averaged temperature for 1995 reached 10.4°C, more than 1°C higher than the 1901-1987 average.
In central England, 1995, as a whole, was
not as warm as 1990 or 1949 due to a cold month of December. Taken together,
July and August, were the warmest ever in the record for central England
since 1659. Temperatures reached 34°C in some places and 30°C was
even reached in northern Scotland. Combined with this record summer warmth
was extreme dryness. Rainfall over the summer in England and Wales was
similar to that experienced in 1976, which was the driest in the series
since 1766. Spain also experienced hot, dry weather with temperatures soaring
to a record 47°C at Seville and Cordoba in July. Russia and adjacent
central Asia mean-while generally experienced their warmest year on record
in 1995. Weekly temperatures of 15°C above normal were common north
of the Arctic Circle from mid-February to the end of April. In early July,
temperatures soared to 47°C in parts of Turkmenistan and the Republic
of Kazakhstan. Abnormal warmth was evident during the last four months
of 1995 in most of the region. Central Europe began the year with abundant
rainfall and rapidly melting snow, which caused flooding and widespread
damage along major rivers, especially in the Netherlands.
The 1996 estimated global mean surface temperature anomaly was only about half the record experienced in 1995. The relative cooling over much of Eurasia can be attributed to dramatic shifts in the atmospheric circulation patterns, in particular a strong change in the phase of the North Atlantic Oscillation. Pronounced stratospheric ozone depletion was measured over Europe and the Arctic and north-eastern Atlantic Oceans during the January–March period. Perhaps the most predominant feature of 1996 was the extent and magnitude of heavy precipitation that affected many parts of the world. Most notably, southern Europe and parts of northern Africa and the Middle East were deluged with record rainfall amounts early in the year. The above-normal rainfall amounts in the Iberian Peninsula were in stark contrast to drier-than-normal conditions that characterized most of the previous decade.
Cooler-than-normal conditions prevailed over much of the land areas in the northern hemisphere and warmer-than-normal conditions pre-dominated over the oceans. It was a cold winter and spring over much of Europe and western Russia, although these features were compensated by the warmth over eastern Siberia, northern Africa, and the oceans.
Significant ozone depletion was observed
in measurements of the total column concentrations over the middle and
polar latitudes of the northern hemisphere in 1996. The ozone deficiencies
were greatest during the period from mid-January, to and including most
of March. Measured ozone concentration values below 250 m atm cm were recorded
on many days which contributed to monthly mean values being more than 20–30
per cent less than those during the 1957–1979 base period. The polar stratospheric
circulation vortex with its cold lower stratospheric temperatures (12–15°C
below normal) was dominant over the same
region at this time. Temperatures below
–78°C are known to facilitate the
generation of polar stratospheric clouds which, in the
presence of human-derived halogenated
compounds combined with the seasonal
increase in solar radiation, can cause
severe ozone destruction.
It was an extremely dry year from the UK
across central Europe to Russia. In Belgium,
the period from July 1995 to July 1996
was the driest since regular observations began in 1833. For England and
Wales, it was the third driest year since records began in 1766. In sharp
contrast to this dryness, the south-west parts of Europe and north-west
Africa received copious quantities of rain which brought much-needed relief
to a region that had been plagued by extreme dryness for the previous six
years. During the wet season, winter precipitation amounts were more than
double the normal annual amounts at several locations. For example, Málaga,
Spain, recorded 1 155 mm of rain compared to normal annual value of 583
mm. Despite the benefits of heavy rainfall to this drought-prone region,
excessive rainfall also resulted in some deaths and dislocations due to
local flooding.
The 1997 annual anomaly of 0.52 °C for the northern hemisphere was the second highest after 1995 since records began in 1860. One major contributing factor was the El Niño/Southern Oscillation (ENSO) episode with temperatures in the tropical belt being the second highest in the historical record. However, mid-latitude temperatures were also a major contributor as temperatures averaged above normal during the year over a large part of central and western Russia, western Europe, Alaska and the west coasts of the Americas.
Regional precipitation highlights with no apparent direct relationship to the ENSO included extremely heavy precipitation in July throughout central and northern Europe, especially over the south of Poland, Austria, the Czech Republic, Slovakia and eastern Germany. This caused the flood of the century in the Czech Republic, and widespread flooding on the Oder River in Poland and eastern Germany. It resulted in heavy damage, including more than 100 deaths in Poland and the Czech Republic, along with evacuations of more than 150 000 people and total costs of more than DM 10 000 million. Thousands of soldiers and emergency workers fought for more than two weeks to repair dykes to prevent more of the flooding, which had devastated thousands of homes.
The average CO2 concentration increase
at Mauna Loa during the 1980s and 1990s has been about 1.4 to 1.5 ppm per
year, but with significant year-to-year variability in the
growth rate. These fluctuations in growth
rate appear to be strongly influenced by the state of the ENSO, with increases
in the growth rate evident during cold episodes (e.g. 1988/89, 1995/96)
and decreases evident during warm episodes (e.g. 1982/83, 1991/92). The
very strong 1997/98 ENSO appears to have slowed the rate of increase by
the middle of 1997.
One of the important characteristics by
which glaciological change is judged is the “mass balance”, the difference
between ice and snow accumulating to a glacier, and melting from it. The
mass balance is one of the best indicators of glacier fluctuations because
it contributes important information about ranges of natural variability
and rates of change with respect to long-term energy-fluxes at the Earth’s
surface, and is therefore a key indicator for assessing trends. For three
swiss glaciers, the year 1997 was more or less balanced. Aletschglacier
and Silvrettaglacier had a slightly positive and the Griesglacier a slightly
negative net balance.
The global temperature in 1998 was the
warmest since reliable instrument records
began 139 years ago. A persistent El Niño
contributed to this record warm year. Compared to climatological standard
normals from the years 1961 to 1990, which was itself a warm era, record
high minimum temperatures were reported in Ireland, and a record warm year
in Cyprus. Annual temperatures near average were reported in only a
few countries, including Germany, Iceland
and Norway. It is remarkable that no significant areas reported temperatures
within the 10 per cent coolest on record.
In the lower troposphere (up to about six
kilometres), 1998 was the warmest year on record in each hemisphere. In
the lower stratosphere, radiosonde measurements
from about 400 land-based stations world-wide
indicated another very cold year
globally in 1998, although near-normal
temperatures were recorded in the northern high latitudes. Computer models
of the atmosphere show that increased carbon dioxide in the atmosphere
and depleted stratospheric ozone can both be associated with cooling in
the lower stratosphere.
Atmospheric concentrations of both carbon dioxide and methane, two principal green-house gases, continued to increase in 1998. Globally, carbon dioxide concentrations increased at a rate of 1.5 parts per million per year. Recent measurements indicate that the rate of increase of methane has slowed.
December 1997 to February 1998 was dominated
by anomalous warmth throughout most of Europe. Many places in Central Europe
experienced their warmest January and
February this century. Summer heat and
dryness plagued Ukraine and Kazakstan, and led to extensive fires in the
eastern Russian Federation. Only north-west Russian Federation, was colder
than normal. In June, a record-breaking heat wave in central Russia resulted
in extensive fires and more than 100 deaths.
In the period of September to November
1998, much of northern Eurasia was colder than normal. Europe and northern
Asia experienced bitter cold late in 1998, with excessive amounts of snowfall.
By contrast, the area of the Azores remained extremely warm.
In February, heavy snowfall disrupted communications, power supply and transportation across central Europe. The winter of 1998/99 saw some of the heaviest snowfall of the past 50 years in the Alps, where at least 50 people were killed in avalanches. In May, heavy rain and snowmelt caused extreme flooding of the river Danube and Lake Constance with losses estimated at DM1 billion. Heavy rains in early September led to flooding in eastern Bulgaria, which caused extensive property damage and the loss of 13 lives. Torrential rains in mid-November in the eastern Pyrenees led to mud slides and floods that took the lives of more than 20 people in southern France.
A series of severe December storms brought the highest wind speeds ever recorded in parts of Europe. Gusts reached 185 km/h on the island of Rømø, Denmark, (on 3 December); 173 km/h, Paris-Orly, France, (on 26 December); 213 km/h, Feldberg (1498m), Schwarzwald, southern Germany, (26 December); and 198 km/h, Saint Denis d’Oleron, France, (28 December). The storms resulted in the loss of life of over a hundred people and damage estimated in the billions of Euros. Numerous buildings and vast tracts of forest were damaged or destroyed, and transport and power were interrupted for days in some regions. The damage to France's electrical system was so great that it may take several years to fully restore it.
Although the globally averaged surface
temperature did not reach a record level in 1999, warmer than normal conditions
prevailed over many regions throughout the year. Some of the notable
extremes included cold as well as warm
events: