Geo 14

Glacial Ages

Glacial Ages The term *"ice age"* typically invokes images of a frozen world, covered in snow and ice, in a time when woolly mammoths and sabre-toothed tigers roamed the Earth. However, *scientists use the term ice age or glacial age to describe any geological period in which long-term cooling takes place and ice sheets and glaciers exist.* That means we are *currently in the midst of an ice age right now!* More specifically, we are in an *interglacial (warm period) within a glacial age.* Cold periods within a glacial age are called *glacials or glaciations*, and are characterized by cooler temperatures and advancing glaciers. Glacial ages *come and go over millions of years*. Interglacial *periods*, like the one we are in now, are typically *spaced apart by hundreds of thousands of years*. Based on observed patterns, we should be swinging back to an "*icehouse Earth.*" However, since the industrial revolution, *increased carbon dioxide (CO2) levels in the atmosphere (largely due to the burning of fossil fuel), are pushing Earth toward a warmer climate.* In fact, we now see that this increase in *CO2 is warming Earth at a rate ~100 times faster* than Earth has seen through slow natural swings.

Today's Climate on Earth

Throughout its history, Earth has experienced several periodic swings in climate. For example, Earth was entirely ice-free and temperatures were hot enough for turtles and palm trees to thrive at the poles during the *Early Eocene Climatic Optimum* around *49 million years ago.* On the other hand, during the *Last Glacial Maximum*, which occurred between *26,500 and 19,000 years ago*, ice sheets covered nearly one third of Earth's surface. *Today, we are somewhere in between extremes*. Snow and ice exist year round near the poles and seasonally at lower latitudes. Glaciers cover about 10% of Earth's surface and can be found on every continent except Australia

Climatic Patterns

When we look at temperature on a regional or global scale over the course of many years, climatic patterns emerge.

Written Documentation and Descriptive Accounts of Weather

Written documentation and descriptive accounts of the weather make up the second general category of evidence for determining climate change. Weather phenomena commonly described in this type of data includes the *prevailing character of the seasons of individual years, reports of floods, droughts, great frosts, periods of bitter cold, and heavy snowfalls.* Large problems exist in the interpretation of this data because of its *subjective* nature.

Physical Properties: ice cores

isotopic analysis of oxygen content in ice cores gives history based on ratio of heavier or lighter isotopes. Oxygen 16 and Oxygen 18. Ratio is dependent on global temperature. Bands / layers can be counted to determine age within a decade. In Antartica and Greenland. Gives info about ice ages and interglacial periods. Also contain bubbles of captured atmosphere. Can be tested for co2 levels historically. allows us to see how carbon dioxide and climate are related.

Differential climate change

local averages change at different rates in different places while the global number remains to be increasing.

The sun and the sun's energy

sun spots - more = more energy fluctuates every 11 years sun tilt, axis, distance all change over long time scales. Eccentricity - the oval aphelion / perihelion orbit changing becoming more exaggerated and less exaggerated over tens of thousands of years. precession changes on the shortest scale, obliquity in between, and eccentricity is longest. Milankovich scale helps determine ice ages.

History of Climate

A wide range of evidence exists to allow climatologists to reconstruct the Earth's past climate. This evidence can be grouped into three general categories.

Earth's Climatic History

Climatologists have used various techniques and evidence to reconstruct a history of the Earth's past climate. From this data, they have found that during most of the Earth's history global temperatures were probably 8 to 15 degrees Celsius warmer than today. In the last billion years of climatic history, warmer conditions were broken by glacial periods starting at 925, 800, 680, 450, 330, and 2 million years before present. The period from 2,000,000 - 14,000 B.P. (before present) is known as the *Pleistocene or Ice Age*. During this period, large glacial ice sheets covered much of North America, Europe, and Asia for extended periods of time. The extent of the glacier ice during the Pleistocene was not static. The Pleistocene had periods when the glacier retreated (interglacial) because of warmer temperatures and advanced because of colder temperatures (glacial). During the coldest periods of the Ice Age, average global temperatures were probably 4 - 5 degrees Celsius colder than they are today.

Meteorological Instrument Records

Common climatic elements measured by instruments include *temperature, precipitation, wind speed, wind direction, and atmospheric pressure.* However, many of these records are temporally quite short as *many of the instruments used were only created and put into operation during the last few centuries or decades*. Another problem with instrumental records is that large areas of the Earth are not monitored. Most of the instrumental records are for locations in populated areas of Europe and North America. *Very few records exist for locations in less developed countries (LDCs), in areas with low human populations, and the Earth's oceans*. Over the last half century many meteorological stations have been added in land areas previously not covered. Another important advancement in developing a global record of climate has been the recent use of *remote satellites.*

Changes in the obliquity (tilt) of Earth's axis

Earth is slightly *tilted*—that's why we have *seasons*. As Earth orbits the sun, one hemisphere will be tilted toward the sun for a period of time (summer) and tilted away from the sun six months later (winter). *Today, Earth's rotational axis is tilted at about 23.5 degrees from vertical.* However, this tilt *oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle.* Variations in the obliquity of Earth's rotational axis *result in changes in the severity of seasonal changes.* When the tilt is larger, the extremes between summer and winter temperatures are greatest. When the tilt is smaller, the average temperature difference between winter and summer is less drastic. It is believed that it's actually these *periods of smaller tilt that promote the growth of ice sheets*. When Earth's axis is less tilted, winters are relatively warmer and summers are relatively cooler. This means that there is more moisture in the air in winter and therefore more snowfall. It also means that there is less summer melting, so more of the winter snow accumulation will last through the warmer months.

Changes in Earth's "Wobble" (Precession)

Earth's axis of rotation behaves like a spinning top that is slowing down, wobbling in a circle over time. *Earth's axis wobbles between pointing at Polaris (what we now call the North Star) and pointing at the star Vega (which would then be considered to be the North Star).* Every year, this wobble *causes Earth to travel slightly farther than one full orbit each year*. This means that on today's date next year, Earth will be a little bit further in its orbit than it is right now. This is called *precession*. Earth's axis completes a full cycle of precession approximately *once every 26,000 years.* *Because Earth's orbit isn't perfectly circular, the distance between the Earth and sun (and the average temperature) will be slightly different each year on the same date.* Precession *can cause significant changes in climate due to greater contrast in seasons*. For example, when Earth's axis is pointed at Vega, the winter solstice in the northern hemisphere coincides with Earth being at its farthest distance from the sun (aphelion), and the summer solstice coincides with Earth being at its closest distance from the sun (perihelion). Just like with variations in obliquity and eccentricity, the more drastic seasons brought on by precession will impact glaciation.

1550 to 1850 AD

From 1550 to 1850 AD global temperatures were at their *coldest since the beginning of the Holocene*. Scientists call this period the *Little Ice Age*. During the Little Ice Age, the *average annual temperature of the Northern Hemisphere was about 1.0 degree Celsius lower than today*. During the period *1580 to 1600*, the western United States experienced one of its *longest and most severe droughts* in the last 500 years. *Cold weather* in Iceland from *1753 and 1759* caused *25% of the population to die* from crop failure and famine. Newspapers in *New England* were calling *1816* the *year without a summer.*

3000 to 2000 BC

From 3000 to 2000 BC a *cooling trend occurred.* This cooling caused* large drops in sea level* and the emergence of many *islands (Bahamas) and coastal areas* that are still above sea level today. A *short warming trend* took place from *2000 to 1500 BC,* followed once again by colder conditions. Colder temperatures from *1500 - 750 BC* caused renewed ice growth in continental glaciers and alpine glaciers, and a *sea level drop of between 2 to 3 meters below present day levels.*

Physical and Biological Data

Many types of physical and biological data can provide *fossil evidence* of the effects of fluctuations in the past weather of our planet. Scientists refer to this information as "*proxy data*" of past weather and climate. Examples of this type of data include tree ring width and density measurements, fossilized plant remains, insect and pollen frequencies in sediments, moraines and other glacial deposits, marine organism fossils, and the isotope ratios of various elements. *Scientists using this type of data assume uniformity in the data record*. Thus, the response measured from a physical or biological character existing today is equivalent to the response of the same character in the past. However, past responses of these characters may also be influenced by some other factor not accounted for. Some common examples of proxy data include: *Glacial Ice Deposits*. Fluctuations in climate can be determined by the analysis of *gas bubbles trapped in the ice* which reflect the state of the atmosphere at the time they were deposited, the *chemistry* of the ice (concentration or ratio of major ions and isotopes of oxygen and hydrogen), and the *physical properties* of the ice. *Biological Marine Sediments.* Climate change can be evaluated by the analysis of *temporal changes* in fossilized marine fauna and flora abundance, *morphological changes* in preserved organisms, coral deposits, and the *oxygen isotopic concentration* of marine organisms. *Inorganic Marine Sediments*. This type of proxy data includes *clay mineralogy, aeolian terrestial dust, and ice rafted debris.* *Terrestrial Geomorphology and Geology Proxy Data.* There are a number of different types of proxy data types in this group including *glacial deposits, glacial erosional features, shoreline features, aeolian deposits, lake sediments, relict soil deposits, and speleothems* (depositional features like stalactites and stalagmites). *Terrestrial Biology Proxy Data*. Variations in climate can be determined by the analysis of biological data like *annual tree rings, fossilized pollen and other plant macrofossils, the abundance and distribution of insects and other organisms, and the biota in lake sediments*.

Paleoclimatology data

Paleoclimatology data are *derived from natural sources* such as tree rings, ice cores, corals, and ocean and lake sediments. These *proxy climate data* extend the archive of *weather and climate information hundreds to millions of years.* The data include geophysical or biological measurement time series and some *reconstructed climate variables* such as temperature and precipitation. NCEI provides the paleoclimatology data and information scientists need to understand natural climate variability and future climate change. We also operate the World Data Service for Paleoclimatology, which archives and distributes data contributed by scientists around the world.

Variations in the shape of Earth's orbit (eccentricity)

The gravitational pull of other planets orbiting the sun causes the *shape of Earth's orbit* to be *elliptical rather than perfectly circular*. *Eccentricity (e), which ranges from 0 to 1,* is a measure of how much an ellipse deviates from a perfect circle (how flattened the circle is). An orbit with an eccentricity of *0 is perfectly circular*, and an orbit with an eccentricity of *1 is a parabola (no longer a closed orbit)*. *The shape of Earth's orbit ranges from nearly circular (e = 0.005) to slightly elliptical (e = 0.058) and back again about every 100,000-400,000 years.* Changes in eccentricity are important to *determining periods of glaciation because they determine the distance between the Earth and sun*, and therefore how much radiation is received at the Earth's surface during different seasons. When the orbit is nearly circular, the distance between Earth and the sun (and therefore the amount of solar energy reaching Earth) remains relatively constant throughout the year. However, when the orbit is more elliptical, the distance between the Earth and sun (and the amount of energy reaching Earth) fluctuates between seasons, resulting in slightly warmer or cooler temperatures. *Today, Earth's orbit has an eccentricity of 0.017.*

12,000 BC

The most recent glacial retreat is still going on. We call the *temporal period* of this retreat the *Holocene epoch*. This warming of the Earth and subsequent glacial retreat began about 14,000 years ago (12,000 BC). The warming was shortly interrupted by a sudden cooling, known as the *Younger-Dryas*, at about *10,000 - 8500 BC*. Scientists speculate that this cooling may have been caused by the release of fresh water trapped behind ice on North America into the North Atlantic Ocean. The release *altered vertical currents* in the ocean which exchange heat energy with the atmosphere. The *warming resumed by 8500 BC*. By *5000 to 3000 BC* average global temperatures reached their *maximum level* during the Holocene and were 1 to 2 degrees Celsius warmer than they are today. Climatologists call this period the *Climatic Optimum.* During the Climatic Optimum, many of the Earth's great ancient civilizations began and flourished. In Africa, the Nile River had three times its present volume, indicating a much larger tropical region.

1850 to Present

The period 1850 to present is one of general warming. Figure 7x-1 describes the global temperature trends from *1880 to 2006.* This graph shows the yearly temperature anomalies that have occurred from an average global temperature calculated for the period *1951-1980*. The graph indicates that the anomolies for the *first 60 years* of the record were *consistently negative*. However, beginning in *1935 positive anomalies* became more common, and from *1980 to 2006* most of the anomolies were between *0.20 to 0.63 degrees Celsius higher than the normal period (1951-1980) average.* In the 1930s and 1950s, the central United States experience two periods of *extreme drought.* In the seventeen year period from *1990 to 2006*, ten of the *warmest* years in the last 100 years and possibly since the Little Climatic Optimum have occurred. Proxy and instrumental data indicate that *2005 was the warmest year globally in 1200 years of Earth history.* Many scientists believe the warmer temperatures of the 20th and 21st centuries are being caused by the human enhancement of the Earth's *greenhouse effect.*

900 to 1200 AD

The period 900 - 1200 AD has been called the *Little Climatic Optimum*. It represents the warmest climate since the Climatic Optimum. During this period, the *Vikings* established settlements on *Greenland and Iceland.* The *snow line* in the *Rocky Mountains* was about 370 meters above current levels. A *period of cool and more extreme weather followed* the Little Climatic Optimum. A *great drought* in the *American southwest* occurred between *1276 and 1299*. There are records of floods, great droughts and extreme seasonal climate fluctuations up to the *1400s*.

700 BC to 800 AD

The period from 750 BC - 800 AD saw *warming up to 150 BC*. Temperatures, however, did not get as warm as the Climatic Optimum. During the time of *Roman Empire (150 BC - 300 AD)* a cooling began that lasted until about *900 AD*. At its height, the cooling caused the Nile River (829 AD) and the Black Sea (800-801 AD) to freeze

Milankovitch Cycles

We know that Earth's climate has been highly variable over time, but what processes are behind these climatic swings? Serbian astrophysicist *Milutin Milankovitch* is credited for developing one of the most significant theories relating changes in Earth's orbit to long-term changes in climate, including ice ages. Milankovitch's theory is based on cyclical variations in three aspects of Earth's orbit that result in changes to the seasonality and location of solar radiation reaching Earth. These Milankovitch cycles include: obliquity, eccentricity, and procession.

Boundary conditions

incoming energy surface of earth' atmosphere