TOPICs 11/12: SHORTERM CLIMATE CHANGE
What is Climate?
1) Definition - A region's climate is the time-average of its yearly weather pattern. The time interval for averaging regional climate is usually several decades, but there is no magic limit.
2) Measurements related to climate:
a) temperature (daily and yearly cycle)
b) precipitation (and type; rain, snow, etc.)
c) wind variability
d) geographic effects (rain shadow, marine influence, etc.)
3) Seasonal Variation: The climate for a region depends critically on the yearly cycle of its temperature and moisture. Changes in 'seasonality' of climate factors can dramatically change a regio
4) Other factors that are important in defining a region's climate are the prevailing wind direction, seasonal humidity, geographic proximity to oceans and mountains, and cyclonic activity.
Dominant Climate Systems on Earth:
C) Factors Causing Climate Change:
1) Internal Interactions: Positive and negative feedbacks reflect the fact that the Earth's climate system is strongly interacting. The Atmosphere, hydrosphere, global heat budget, and hydrologic cycle, all strongly affect one another on a daily (and longer) time scale. We see the complexity of these interactions in our difficulty to predict weather.
a) Decadal Variability - Internal interactions cause climate variability on a variety of time scales. The shortest one is termed decada variability. Examples of decadal variability include the Dust Bowl Era (1930s) of central North America and ENSO (El Nino-Southern Oscillation) events.
b) Longer Term Variability - Climate changes on longer time scales include the Little Ice Age (ca. 1350-1850 AD), Medieval Warm Period (ca. 1100-700 AD), and recurring glacial ages.
2) External Forcing: The Earth System also receives inputs from extraterrestrial sources.
a) Milankovich Cycles - Variations in the Earth's orbital parameters (eccentricity of the Earth's orbit, obliquity of the Earth's rotation axis, precession of the Earth's rotation axis with respect to perhilion/aphelion) all affect the seasonality of solar insolation reaching the Earth. These effects are very long period, 20,000 years to more than 100,000 years per cycle.
b) Solar Variability - Solar activity can not be considered a constant. We can infer from geological studies that the Sun is evolving (slowly changing) just as are the Earth and other planets over periods of millions to billions of years.
1) ST relationships - this defines the pattern and effect of solar ionic particles as they interact with the Earth's atmosphere and magnetic field.
2) Changes in irradience - Historic observations of changes in sunspot number indicate that the Sun varies even on centennial scale. There have been probably three intervals within the last 1500 years when the Sun had no sunspots for 50-100 years. These must have had small (perhaps very small) changes in ST relationship and solar irradience associated with them.
c) Impacts - Meteorite impacts are impulse events that may kick enormous
quantities of dust into the upper atmosphere. Their long-term effects may
be analogous to large volcanic eruptions. Huge impacts far in the geologic
past are considered to have caused largescale biologic extinctions.
Climate Changes Over Historic Time
A. Decadal Variability
1. The climate for a region is only an average of the weather patterns that will prevail in any particular year. It is quite common to have intervals of a few to perhaps 10 years where the yearly weather patterns are quite different from the overall local climate. This is often referred to as decadal variability.
2. The 'Dust Bowl Era' of the 1930's, when much of central North America was a virtual desert with many crop failures, is an example of decadal variability. ENSO events are a larger spatial scale example of decadal variability.
3. For the most part, the source of decadal variability is unknown. But it is reasonable to presume that it reflects the complicated nature of the interrelationships of the many physical parameters that combine to make up climate.
Longer-Term Climate Variability
1. Historical records also indicate that there have been longer-term changes in climate over the last few thousand years. Four notable intervals are the:
a. post-glacial Altithermal of 3000-5000 BC
b. Iron Age cold epoch of 900-300 BC
c. Middle Ages climatic optimum of 1000-1200 AD
d. Little Ice Age of 1400-1850 AD.
2. These climate intervals are examples of centennial (102 years) to millenial (103 years) scale climate variability.
The Once and Future Glacial World (The world ofthe Wisconsin Glacial 12-71,000 years before present)
Ice?
continental glaciers extended over most of Canada, Alska, Greenland, Fennoscandia, and Northern Siberia
permanent ocean ice packs extended farther out to sea around Antarctica and in the North Atlantic Ocean (reaching southern Greenland and Scotland)
Sea Level
sea level was about 150 mlower than today
Baltic and North Seas were dry or covered by glaciers
very little continental shelf environment
Ocean Circulation
stronger thermal gradient between Equator and polar regions
ocean gyres were compressed toward equator by ice packs
stronger circulation
Ocean Chemistry
lower concentrations of greenhouse gases
excess carbon was stored in oceans (colder, can absorb more)