Tuesday, December 8, 2009

Tree rings and Pollen helps in measuring global temperature.

For a scientist studying climate change, “eureka” moments are unusually rare.

by
Dr. Nitish Priyadarshi
photo credit:http:// boojum.as.arizona.edu

In 1938, British meteorologist Guy Stewart Callendar made the first proper measurement of global temperature. He gathered data from weather stations worldwide and found that the average temperature of the earth has increased between 1880 and 1930. Sadly, no one took his findings seriously. In 1957, two scientists set up instruments on the Mauna Kea Volcano in Hawaii to measure the level of carbon dioxide in the atmosphere. Its records show that the level of carbon dioxide has been increasing steadily since that time and that the earth is slowly warming up.

For a scientist studying climate change, “eureka” moments are unusually rare. Instead progress is generally made by a painstaking piecing together of evidence from every new temperature measurement, satellite sounding or climate-model experiment. Data get checked and rechecked, ideas tested over and over again.

Evidence for climatic change is taken from a variety of sources that can be used to reconstruct past climates. Reasonably complete global records of surface temperature are available beginning from the mid-late 1800s. For earlier periods, most of the evidence is indirect—climatic changes are inferred from changes in indicators that reflect climate, such as vegetation, pollen, ice cores, dendrochronology, sea level change, and glacial geology.

Pollen in fossils provides important clues to prehistoric global climates. Pollen falls from trees and plants into the soil and can be preserved for thousands of years. Each plant species produces a unique pollen type. By identifying the type of pollen found in a place, scientists can tell which trees grew there. In tropical regions like Indonesia, scientists have found pollen from plants, such as pine, that can grow in temperate climates. This means that the temperature in the world’s tropical areas may have been lower in the past, allowing temperate plants to grow there. Pollen is widely distributed, produced in vast quantities (one pine branch can produce 350 million pollen grains), easily recognisable and very resistant to decay (some pollen in East Africa goes back 3 million years). The fluctuating type and distribution of pollen assists in interpreting local and regional histories of vegetation and climate.

Scientists also study tree rings to learn about the earth climate in the past. Tree rings are a good place to start thinking about how climate researchers get information about past climates. In certain cases, trees can live for many hundreds of years and in an extraordinary case, like the bristlecone pine, thousands of years! Each year trees add growth rings, which can indicate what sort of growing season the tree experienced. Interestingly these rings are more than a temperature indicator, they also tell the researcher about moisture and cloudiness as well. Trees grow faster in warmer climates than in cooler environments. Each year, the trunk of a tree grows wider. If you can chop a tree down, you will see rings in the cross-section of the trunk. A tree produces a new ring each year. Scientists measure the width of individual rings to estimate climate conditions at the time each ring was formed. Tree rings grow during warmer years are thicker than those produced in colder years. In other words wide and thick rings indicate a fertile, well-watered growing period, whilst thin, narrow rings indicate a time of lower rainfall and less-than-ideal growing conditions.

In this way, scientists can calculate which were the warmer and colder years.

Tree-ring measurements can help to distinguish anthropogenic from natural environmental change. These data can be used to determine whether recent climatic changes are unusual and possibly due to anthropogenic effects (specifically, increasing CO2 and other trace gases) or are still within the range of natural climate variability.

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