|
Almost everyone has heard about global warming, and what is thought
to be causing this rapid climate change. Over the last one hundred years
the Earth's climate has warmed on average by approximately 0.3 to 0.6
degrees Celsius (1). This recent climate change, it would seem, has been
caused largely by human beings. (2). This has been brought about by changing
land use, and even more importantly, rapid increases in certain gases
in the Earth's atmosphere. These gases are called "greenhouse gases"
because they effectively trap heat in the Earth's atmosphere. Most climatologists
now think that increases in greenhouse gases are the principal reason
for the most recent climate changes. Carbon dioxide has, for example, increased
by
over thirty percent in the last one hundred years with higher amountsof
tropospheric ozone, nitrous oxide and vapour - due to a more active
hydrological cycle in the tropics - contributing to the warming
as well.
Much larger increases than those indicated above have occurred in
some
areas such as the high latitudes of North America, Asia and Europe.
The
pattern of precipitation has also changed within this century. Much
of
North America, parts of Australia, Europe and Asia have become wetter,
and
critical areas of Africa and South America have become significantly
drier.
Though it is thought that most of the climate changes of this century
are
the result of man, some of the effects may be due to natural variability.
The key is separating climate change attributable to man and that
due to
natural variability. One possible natural cause is thought to be
variable
energy output from our own sun. Relatively small changes in solar
output
are amplified by the Earth's climate system (3). During the Little
Ice Age
for example, an estimated 0.24% reduction in solar irradiance, and
forced a
whole series of responses within the climate system called feedbacks,
may
have been responsible for between one to two degree Celsius colder
temperatures over the Northern Hemisphere. As to climate change
since the
Little Ice Age, as much as three-quarters of the over one half degree
Celsius warming from 1600 to 1800 may be attributable to natural
causes
(4). But from 1800 to the present, the natural component is thought
to have
dropped to approximately one half, and since 1970 less than one
third is
attributable to natural causes (5). Complicating the picture further
are
occurrences such as the Southern Oscillation (ENSO) phenomena in
the
tropics, which may be responding to both human induced and naturally
caused
influences.
Some believe that recently there has been an increased frequency
and
strength of El Ninos. During the El Nino phase of ENSO, British
Columbia
tends to have milder winters.
Another very important feature in this part of the world occurs
in the form
of a fifty to seventy year oscillation-and is a phenomenon called
the
Pacific Decadal Oscillation (PDO).
This is a long-lived El Nino like pattern of Pacific Ocean variability.
The
Pacific Decadal Oscillation is manifested in both oceanic and atmospheric
changes in circulation.
Shifts in the pattern have occurred over very regular intervals
within the
past one hundred years or so. A deepening or strengthening of the
Aleutian
low-pressure area in the winter and early spring months over the
North
Pacific, results in changes in atmospheric circulation in this part
of the
world. During the time when the Aleutian low is well developed,
prominent
trough development occurs in the north central Pacific Ocean, approximately
two and half to three times more frequently than ridging or high
pressure
development (6). Associated weather with the trough, or positive
phase of
the PDO is usually mild with slightly drier winters, at least in
coastal
areas of British Columbia. During the opposite part of the phase,
the ridge
or negative phase, cold winters are favoured in western North America.
That
is not to say that colder weather cannot occur with the positive
phase or
that milder weather does not happen in the negative portion of the
cycle.
The cause of the PDO is still being debated. Variations in the amount
of
solar radiation, even very small increases, may warm tropical sea
surface
temperatures, which may in turn lead to a deepening of the Aleutian-low
pressure area and hence result in the positive phase of the PDO
(7).
Shifts from the positive to negative, and back to the positive phase
have occurred a
number of times over the past one hundred years or so. The mid-1970's
saw the
shift from cold (negative) to the warm (positive) phase (8). Prior
to then, a change
from a warm to a cold phase occurred in the mid-1940's, preceded
by shifts
in the mid-1920's and early 1890's. It is thought by some scientists
that
in the late 1990's the PDO once again shifted back to the cold phase;
however this is far from certain.
Over the past century in this part of the world, there have been
many
discernible changes in the climate. A recent provincial government
report
has shown that minimum temperatures particularly have been increasing
throughout British Columbia. The greatest increases, at least in
the
southern parts of the province, have been observed in the spring
and winter
months (9). Precipitation has also been on the increase in most
parts of
the province by as much as 2-4% per decade (10).
The principal purpose of this website is to attempt to determine
not only
the climate trends to date, but isolate the most likely causes for
both
temperature and precipitation variations in British Columbia . In
order to
do this, the climate stations with the longest-term records have
been
examined. Climate stations from coastal and interior B.C. sites
have been
analyzed by looking at overlapping thirty-year normal (average)
monthly or
seasonal temperature and precipitation records (the longest-term
records
extend from the 1890's to the present). As these long-term stations
represent between one-and-a-half to sometimes two full cycles of
the PDO,
temperature and precipitation records can be examined for both the
early
and later cold and warm phases of the cycles. The climate elements
analyzed
here include: winter precipitation, snow amounts, winter temperatures,
last
spring freeze, and growing season temperature and precipitation. |
