In geological formations, it is more common to find fossil remains of plants than of animals, meaning that vegetation plays a key role in the reconstruction of ancient climates. If a specific species of vegetation is found in a geologic formation that is tens of million of years old, it gives scientists an idea of what the climate was like. If a palmlike tree fossil is found at a high northern latitude, it can be inferred that conditions at one time were much warmer than they are today.
When climatologists attempt to reconstruct more recent climate records, they often look at pollen deposits in sediments. Each plant species produces its own uniquely shaped pollen grains. Tiny grains of pollen are produced in huge amounts and are then distributed by wind and deposited in lakes, where they become part of the permanent sediment record. Oxygen-free environments, such as peat bogs, are the best places to find pollen because they do not support decay. Different levels of pollen signal climate change. Scientists first identify the pollen by major type, such as tree, grass, or shrub, and then subdivide it into individual species, which usually carry important climate implications. Pine trees, for example, indicate a cool climate, oak trees a warm climate, and palm trees a tropical climate. Larger remains from vegetation can also be studied to infer climate type. Often, larger remains—what scientists call macrofossils—exist in an area, such as cones, seeds, and leaves.
Fieldwork in the northern Great Plains of the United States has revealed climate change through evidence of drought and fire proxy data in the pollen record. Some pinecones release their seeds only during fire incidents. It is known that Florida was affected by the last ice age based on fossil pollen deposits there. In northeastern Illinois, evidence has been discovered that dates back 17,000 years and confirms the retreat of the immense Laurentide ice sheet. Fossil pollen evidence exists that indicates the area was once covered with tundralike vegetation, a drastically different ecosystem than the prairie vegetation that existed when Europeans first came to America, not to mention the belts of corn and soybean that thrive there today.
Many plant fossils have been found in areas where it is too cold for them to grow today. Palms have been found, for instance, in Wyoming and Utah that lived 45 million years ago. Today they would freeze. Red horn coral, a very rare fossilized form deposited during the mid- to late-Cretaceous period 65 to 135 million years ago, has been found on what are today the high mountaintops of the Uinta Mountains in Utah. Living 65 to 85 million years ago, it is a unique coral that was deposited in an ocean. At that time, the Earth's volcanic activity forced new ridge systems to rise high above the old ocean depths in the Pacific Ocean and lifted neighboring ocean floors with them.
Using proxy data in this area, scientists have been able to determine that the climate was not only once warm enough to support the growth of coral, but that temperatures increased significantly when massive volcanic activity released enormous amounts of CO2 into the air. The results were dramatic—the icecaps melted, and the oceans rose 656 feet (200 m) higher than they are today. The sea progressed inland through midwestern United States, almost into Canada, while much of Europe was under water. The sea covered a large portion of the Rocky Mountains, and because of the warming of the Earth's climate, it made an excellent habitat for coral to flourish.
According to the USGS, macrofossils (leaves, wood, cones, and seeds) have provided a wealth of information on how climate change affects Alaska's vegetation. Fossil records show that several significant changes have occurred in the vegetation at high (polar) latitudes during times of climate change. The fossils show that high latitudes are more sensitive to climate change than lower latitudes.
During the Miocene, a major global warming occurred about 17 to 14.5 million years ago. This warming drastically changed the vegeta tion composition. Instead of remaining forested with conifers, temperate species such as oak, hickory, beech, chestnut, and walnut moved into the area and dominated. Scientists at the USGS have calculated that during this period the temperature may have been 25-30°F (15-18°C) warmer than today.
Then the climate flip-flopped, and 14.5 million years ago, a global cooling effect began. The effect it had on Alaska's vegetation was not only abrupt but also dramatic. The temperate trees disappeared, and the conifers returned. Alaska experienced several of these heating-cooling cycles over the course of time, with similar results. USGS scientists believe temperatures during glaciations were 9-15°F (5.4-9°C) below today's temperatures.
By this switching off of cold-weather trees and mild-weather trees (and the shrubs associated with them), USGS scientists believe the fossil record is a valuable proxy that illustrates how past climate change has influenced Alaskan vegetation. They have discovered examples of climates both warmer and colder than the climate today, which has given them a better understanding of how various types of environments respond ecologically. Using this data allows them to test the results of new algorithms in computer modeling. Knowing that climate change and global warming affect species throughout an ecosystem, the USGS also sees its efforts at studying past climate as a way to predict the effects of future change on polar environments.
According to Thomas Ager at the USGS, "The study of past climates and ecological changes in Alaska are an important key to understanding the likely consequences of future climate changes in high latitude ecosystems. We can expect that future periods of cooler, drier climate will result in shrinkage of forest boundaries, lowering of altitudinal tree line, and expansion of tundra vegetation into lower elevations. A future change to warmer, moister climates will result in expansion of Alaska's forests into areas now occupied by tundra. The past record also shows that the magnitude of future global scale climate changes and ecological responses will be greater at high latitudes than at lower latitudes."
With all the proxy methods discussed in this and the previous two chapters, it is important to understand that there are limitations involved. Although technology has continued to progress and has made incredible advances over the past few years, there are still some major gaps in scientists' understanding of past and future climate behavior. Two very important components that affect weather and proxies that are not currently well understood are the properties of clouds and the composition of the atmosphere. As scientists continue to learn more about the complex system of climate, new windows, both to the past and future, will be opened.
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