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Summary
- What is the difference between weather and climate?
Weather is the state of the atmosphere for short time whereas climate represents average
weather conditions over a long time period. For example, weather might give us a
warm day in January but climate tells us that January is typically a cold month.
- What is the principal control on global climate
patterns?
The amount of incoming solar radiation (insolation). Temperatures increase with
more insolation and decrease with less insolation. Insolation is greatest at the
equator and least at the Poles. Consequently, temperatures decrease with increasing
latitude.
- Why is the Northern Hemisphere colder in winter than in
summer?
The principal reason for the seasonal differences in climate around the globe is
the tilt of Earth's axis. Earth rotates around an axis that is tilted 23.5 degrees to
vertical. Insolation is greatest when the Sun is directly above a location on Earth and
decreases as the angle of the Sun's rays becomes more oblique. The axial tilt places the
Sun directly overhead at the Tropic of Cancer in the Northern Hemisphere during the Summer
solstice (June 21). Likewise, the Sun's rays strike the Northern Hemisphere more obliquely
when the Sun lies over the Tropic of Cancer in the Southern Hemisphere during the Winter
solstice (December 21).
- What drives the global atmospheric circulation system?
The contrast between insolation at the equator (more insolation) and the Poles
(less insolation) generates a heat gradient that results in rising air at the equator and
sinking air at the Poles. This simple convection model is distrupted into three separate
cells by the effect of Earth's rotation. Winds associated with the convection cells make
up the circulation system.
- How is circulation in the Hadley convection cell related
to climate over the equator and tropics?
The atmosphere above the equator receives 2.5 times more incoming solar radiation
than the atmosphere above the poles. This warmed air rises and the humidity of the air
increases as it cools during its ascent. This results in condensation, cloud
formation, and precipitation. Equatorial regions are characterized by warm temperatures
and heavy rainfall (e.g tropical climate). This air then moves north or south before
beginning to descend above the tropics (20-35o latitude). The descending air
becomes warmer and dryer as it approaches Earth's surface, preventing condensation and
resulting in clear skies over the tropics. The descending air flows toward the equator,
forming the last leg in the convection cell. These winds are deflected to the west (right)
in the Northern Hemisphere and to the east (left) in the Southern Hemisphere due to the
Coriolis effect and are known as the trade winds.
- How are the convection cells related to cloud cover?
Clouds form where ascending (cooling) air occurs between adjoining convection cells
but clear skies occur when air descends (warms). Ascending air is found above the equator
and at the Polar front (between the Ferrel and Polar cells) and these regions are
characterized by cloudy conditions. In contrast, clear skies are located in regions of
descending air such as the Tropics below (between the Ferrel and Hadley cells) and
over the Poles.
- Why are continental temperatures more extreme than
temperatures for the oceans at the same latitude?
The mixing of ocean currents results in less extreme temperatures in oceans
adjacent to land masses. High temperatures in the deserts of North Africa may be 10oC
more than the adjacent Atlantic Ocean. Likewise, the extreme cold (-50oC) of
the Antarctic interior is not matched in the waters of the Southern Ocean where
temperatures remain a few degrees above freezing.
- Where are temperature and precipitation greatest?
The highest average global temperatures are typically found between the Tropics and
the equator and decrease progressively toward the Poles. Precipitation is also
greatest along the equator and is typically least at the Tropics and the Poles.
- How are climate regions identified in the Koppen-Geiger
classification system?
The system considers three parameters: 1. average monthly temperatures; 2. average
monthly precipitation; and, 3. total annual precipitation.
- Can climate regions be differentiated by latitude?
Four are typically identifiable by latitude. Beginning at the equator and
moving toward the Poles the climate regions in order are: tropical, mesothermal,
microthermal, and polar. Dry and highland climates are specific cases.
- How did climatic changes influence the Viking's
colonization of Greenland?
Viking settlement of Greenland occurred when temperatures in the North Atlantic
region rose approximately 900 years ago at the start of the Medieval Warm Period.
Unfortunately, temperatures began to decline about 500 years later at the beginning of the
Little Ice Age and the Vikings were not able to adapt to living in a colder climate.
- How has the climate of the Northern Hemisphere changed
during the recent geologic past?
The climate of the Northern Hemisphere was dominated by the presence of a massive
continental ice sheet during the last two million years. This period was known as an ice
age and was divisible into long (~100,000 year) cold intervals (glacials) and short
(~20,000 year) warm intervals (interglacials). Glacials were up to 8o C colder
than the interglacials and warm temperatures correlated with periods of higher carbon
dioxide concentrations. Global temperatures increased rapidly approximately 10,000 years
ago as the world entered the most recent interglacial (the Holocene). The rise of
civilization occurred during the Holocene.
- How do we know what the climate was like in the past as
there were no instruments (or people) to measure climate parameters?
Climate fluctuations during the history of the earth can be determined from the analyses
of a variety of proxy records, data that can be interpreted to give indirect information
on past climates. Paleoclimatic data comes in a variety of forms, some give information on
long-term climates (oceanic microfossils), while others provide precision in the recent,
short-term climate record (tree rings, pollen).
- How can we determine climate characteristics in the
long-term geologic record, stretching back hundreds of thousands, or even millions of
years?
Changes in temperature over millions of years can be determined using oxygen
isotopes (oxygen atoms with different numbers of neutrons). Two isotopes of oxygen, 16O
(more abundant) and 18O (less abundant), are present in ocean water. These
isotopes are preserved in the ice of Greenland and Antarctica and are incorporated into
the skeletons of microscopic organisms that dwell in the oceans. The ratio of 18O/16O
in ancient ice or in organism’s skeletons can be compared with standard values. The
difference can be used to estimate the temperature of the air in which the ice (snow) was
precipitated or the temperature of the water in which the organisms grew. The ratio acts
as a paleothermometer for ancient climates. The 18O/16O ratio is
higher at lower temperatures (when oceans are enriched in 18O), and decreased
as temperatures increased.
- How can we determine climate
characteristics in the short-term geologic record over the last 10,000 years?
Short-term climatic changes can be identified on some of the longest historical
records (Norse saga, European agricultural records, Chinese weather descriptions),
archeological discoveries, tree-ring evidence, pollen characteristics, and oxygen isotope
records of coral. Pollen reveals plant assemblages that are linked to climate patterns and
tree ring research provide evidence of wet and dry years.
- How do current temperatures compare with those of the
geologic past?
Average global temperatures have flucuated in the geologic past. Earth was warmer
than today for the majority of the last 60+ million years but was cooler during the recent
ice age that ended approximately 10,000 years ago.
- Why have climates changed throughout the geologic past?
The cause of long-term global climate changes has to be linked to processes that
operate on a global scale over millions of years. The most likely causes are associated
with the changing locations of continents and oceans that would in turn affect atmospheric
and oceanic circulation patterns. More rapid plate motions may be linked to warmer
climates and widespread uplift associated with continental collisions may have contributed
to global cooling events.
- What causes variations in climate over intervals of
thousands of years?
Short-term climate fluctuations are related to variations in the earth’s orbit
(Milankovitch cycles) that cause the amount of incoming solar radiation to vary. These
variations result from changes in the shape of Earth's orbit and changes in the
magnitude and direction of tilt of Earth's axis.
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