Topic 12: Overview of Climate Variability

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.

Evidence For Paleoclimate Variability Over Geologic Time Scales

A. The historic record of climate is very brief when we think of the geological time scale. We have no direct quantitative records of the Earth's climate regime(s) over the millions to billions of years of its history because these were prehistoric time intervals with no direct records.

B. In spite of the lack of actual climate records in prehistoric time, we know that there have been repeated ice ages and other unusual climate regimes in our recent past.

1. For example, there was a great ice age (the Wisconsin Ice Age of North America; the Wurm Ice Age of Europe) 12,000-71,000 years ago. What were regional climates like during the last glacial?

2. There was also an interglacial interval of conditions even warmer than today between 71,000-128,000 years ago.

3. We need to better understand the climatic history before historic time in order to properly assess the possible range of climates we might expect in the future.

4. We can attempt to build up a picture of preshistoric climate through the use of proxy indicators or proxies. Proxies are evidence of a climate pattern that is preserved in sedimentary rocks. It may be fossils, which are biologic evidence of the types of communities that lived in a particular area at a particular time. It may also be other types of chemical or inorganic evidence.

C. Fossils as Proxy Records of Climate

1. The most obvious proxy data for past climates are fossils - the remains of plants and animals that have been fossilized. Particular biological organisms will grow under particular conditions of temperature, moisture, etc.

2. On land, the most often used fossils are pollen from plants that are deposited in lakes. By coring lakes, dating the sediments, and looking at the changes in pollen types through time, we can estimate how local climate has changed.

3. In the oceans, the most commonly used fossil proxies are plankton which are very sensitive to changing temperature and sea water salinity.

D. Isotopes as Proxy Records of Climate

1. Most elements in nature occur as a mixture of stable isotopes. Examples are:

a. 1H, 2H (3H, tritium, is radioactive)

b. 12C, 13C (14C is radioactive)

c. 16O, 17O, 18O

2. Because of their different masses, each of the stable isotopes behaves slightly differently when forming more complex chemicals, for example organic chemicals, or when changing phase, for example during evaporation. These differences are related to temperature and other environmental parameters.

3. Therefore, if we can measure changes in the ratio of stable isotopes and understand what causes their changes in ratio, we can then get quantitative proxy measurements of past climatic change.

E. Sediments as Proxy Records

1. One can also estimate climatic change simply from the stratigraphic record of changes in sediment type in lakes or oceans.

2. Variations in clastic sediment grain size or percentage of biogenous sediment can tell of regional changes in paleocenogrpahic/paleoclimatic conditions.

III. Causes of Longterm Climatic Change

A. Milankovich Cycles: Long-term changes in the orbital parameters of the Earth's lead to periodic changes in the amount and timing of insolation at the Earth's surface. The periodicities of these orbital changes are collective called the Milankovich cycles and extend from about 20,000 to 400,000 years in duration. These cycles are thought to be the source of the glacial/interglacial cycles that have dominated the global climatic regime for the last 2 million years.

B. Plate Tectonics: On an even longer time scale, the configuration of the continents and ocean basins on the surface of the Earth can affect climate. When continents are at the poles or when continents surround the poles creating closed ocean basins, then there is a larger temperature gradient from the poles to the equator. This leads to large polar ice caps and colder climates.