How Do We Know About the Temperature of the Past? Are Scientists Just Making This Stuff Up?

How do we know about the temperature of the past? Are scientists just making this stuff up?

Goals:

Students will be able to define proxy data and provide specific examples of sources of proxy data.

Students will understand that proxy data provides historical information about climate.

Resources:

(background on proxies from Randy’s page)

(purchase tree cookie)

Tree rings

Photos

Activity

Boring a tree ring coreSawing a tree cross section

Tree rings

Background on Tree Rings

Trees contain some of nature's most accurate evidence of the past. Their growth layers, appearing as rings in the cross section of the tree trunk, record evidence of floods, droughts, insect attacks, lightning strikes, and even earthquakes.

Each year, a tree adds to its girth, the new growth being called a tree ring. Tree growth depends upon local conditions such as water availability. Because the amount of water available to the tree varies from year to year, scientists can use tree-ring patterns to reconstruct regional patterns of drought and climatic change. This field of study, known as dendrochronology, was begun in the early 1900s by an American astronomer named Andrew Ellicott Douglass.

A tree ring consists of two layers:

• A light colored layer grows in the spring
• A dark colored layer in late summer

During wet, cool years, most trees grow more than during hot, dry years and the rings are wider. Drought or a severe winter can cause narrower rings. If the rings are a consistent width throughout the tree, the climate was the same year after year. By counting the rings of a tree, we can pretty accurately determine the age and health of the tree and the growing season of each year.

Modern dendrochronologists seldom cut down a tree to analyze its rings. Instead, core samples are extracted using a borer that's screwed into the tree and pulled out, bringing with it a straw-size sample of wood about 4 millimeters in diameter. The hole in the tree is then sealed to prevent disease.

Computer analysis and other methods have allowed scientists to better understand certain large-scale climatic changes that have occurred in past centuries. These methods also make highly localized analyses possible. For example, archaeologists use tree rings to date timber from log cabins and Native American pueblos by matching the rings from the cut timbers of homes to rings in very old trees nearby. Matching these patterns can show the year a tree was cut, thus revealing the age of a dwelling.

To investigate the extent, speed, and effects of historical climate changes locally and globally, scientists rely on data collected from tree rings, ice cores, pollen samples, and the fossil record. Computers are used to detect possible patterns and cycles from these sources. In dendrochronology, large databases allow scientists to compare the ring records of many trees, construct maps of former regional climates, and reveal when, where, and how quickly the climates changed. These historical records are extremely valuable as we struggle to understand the extent and nature of any possible future climate change.

Fire scars on a tree

Coral

Short overview video of proxy and corals

Lake Sediments

Pollen

Reading with bags of pollen and dirt, very abbreviated version of activity

Evidence found in the fossil record indicates that in the distant past, the earth's climate was very different than it is today. There have also been substantial climatic fluctuations within the last several centuries, too recently for the changes to be reflected in the fossil record. Since these changes are important to understanding potential future climate change, scientists have developed methods to study the climate of the recent past.
Although human-recorded weather records cover only the last few hundred years or so, paleoclimatologists and paleobotanists have found ways of identifying the kinds of plants that grew in a given area, from which they can infer the kind of climate that must have prevailed. Because plants are generally distributed across the landscape based on temperature and precipitation patterns, plant communities change as these climatic factors change. By knowing the conditions that plants preferred, scientists can make general conclusions about the past climate.

How do paleobotanists map plant distribution over time? One way is to study the pollen left in lake sediments by wind-pollinated plants that once grew in the lake's vicinity. Sediment in the bottom of lakes is ideal for determining pollen changes over time because it tends to be laid down in annual layers (much like trees grow annual rings). Each layer traps the pollen that sank into the lake or was carried into it by stream flow that year.

To look at the "pollen history" of a lake, scientists collect long cores of lake sediment, using tubes approximately 5 centimeters (cm) in diameter. The cores can be 10 m long or longer, depending upon the age of the lake and amount of sediment that's been deposited. The removed core is sampled every 10 to 20 cm and washed in solutions of very strong, corrosive chemicals, such as potassium hydroxide, hydrochloric acid, and hydrogen fluoride. This harsh process removes the organic and mineral particles in the sample except for the pollen, which is composed of some of the most chemically resistant organic compounds in nature. Microscope slides are made of the remaining pollen and examined to count and identify the pollen grains.

Because every plant species has a distinctive pollen shape, botanists can identify from which plant the pollen came. Through pollen analysis, botanists can estimate the composition of a lake area by comparing the relative amount of pollen each species contributes to the whole pollen sample. Carbon dating of the lake sediment cores gives an approximate age of the sample.

Scientists can infer the climate of the layer being studied by relating it to the current climatic preferences of the same plants. For example, they can infer that a sediment layer with large amounts of western red cedar pollen was deposited during a cool, wet climatic period, because those are the current conditions most conducive to the growth of that species.

Why are scientists who study climate change interested in past climates? First, by examining the pattern of plant changes over time, they can determine how long it took for plant species to migrate into or out of a given area due to natural processes of climate change. This information makes it easier to predict the speed with which plant communities might change in response to future climate change. Second, by determining the kinds of plants that existed in an area when the climate was warmer than at present, scientists can more accurately predict which plants will be most likely to thrive if the climate warms again.

Packrat Middens (from windows to the universe

Packrat Middens

Packrats, as their name implies, constantly collect all kinds of materials from their surroundings. Their collections, called "middens", provide clues to the past climates of the region. Packrat middens are clumps of vegetation, insects, remains of vertebrates, and other materials cemented together by crystallized packrat urine (referred to as amberat). These rock-hard deposits can be more than 20,000 years old.

Several species of packrats live in the arid deserts of western North America. Before scientists began examining middens, little was known about the past climates of desert regions of North America, for other paleoclimate proxies such as tree rings, fossil pollen, ice and coral cores, and lake and ocean sediments are either absent from these regions or are too sparse to provide adequate data. The peculiar middens turned out to be gold mines of data for climate researchers!

Materials encased in middens are often remarkably well-preserved. Scientists are often able to sequence DNA from vegetation in middens, providing them with extraordinarily detailed insights to the evolution of plant communities, which in turn are good indicators of climatic conditions. Other astonishing artifacts have been discovered in middens. A midden from Utah contained a bone from a camel that, though once widespread in North America, had gone extinct more than 12,000 years ago.

Packrats are not the only animals that produce middens. These peculiar structures are also created by four families of rodents in South America (including a close relative of the Chinchilla), a stick-nest rat in Australia, a hyrax (a distant relative of elephants) in Africa and the Middle East, and a rock-dwelling vole in central Asia. Middens from these creatures are helping scientists reconstruct the former extent of vegetation in arid regions around the globe, and to infer the corresponding climate histories of those regions.

A sample of packrat midden from Red Creek (RC2) that dates to 3320 B.P. Needles of lodgepole pine were recovered from this midden found in the lower basin.

Feces midden

Ice: it’s more than just frozen water

Time: 1-2 60 minute periods

Goals:

Students will make observations and gather data about simulated ice core samples.

Students will make inferences about climate change based on their data.

Students will analyze historical climate data (specifically CO2 levels) to identify trends, patterns, and outliers.

Lesson Sequence:

Resources:

NASA lesson plan

(show short version as intro)

(ice core drilling photos)

(map of cores)

(need to cut back from 6 min – show drilling

(need to condense)

(video has Jim White in it)