Global Warming  Elmhurst College
Carbon Dioxide & Temperature Carbon Dioxide - Fossil Fuels  Chemistry Department
Carbon Dioxide - Atmosphere Greenhouse Gases    Virtual ChemBook

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Evidence for Global Warming

QUESTION: What is the evidence that temperature is currently increasing? After analyzing the following readings and graphs, you should give a general answer to the question.


This exercise investigates the variation in global temperatures over the past 150 years. The temperature values in the data set are provided as differences from a mean of 15 degrees C. These data have been analyzed by scientists to show a 0.5 degrees C increase in global temperatures. However, this finding is under dispute because some claim that the amount of error in the data is too large to justify the conclusion. This data set has been created using the following steps:
o Data was collected from land based stations, from ocean buoys, and from ships.
o For each year data has been averaged to come up with a yearly average.
o Data is smoothed to accommodate historical changes that skew the data (e.g., weather stations near cities record artificially high temperatures because create what is called an "urban heat island effect.").

QUES. a. State a generalization that incorporates time and temperature that interprets this data in the graph on the left.

Scientists extrude the core from its barrel with the utmost care.

 Wooden trays with ice cores stored at -15°C until measurements are ready to be taken.

Both Photos by Kendrick Taylor, Desert Research Institute, University and Community College System of Nevada.

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Antarctica is the coldest, windiest, highest and driest continent on Earth. The annual precipitation of snow, averaged across the continent, is about 30 centimeters. Because of the low temperatures, however, there is little or no melt. Thus the snow has accumulated year after year for thousands of years and, with time, is compressed to ice to form the Antarctic ice sheet, which is on average about 2,500 meters thick.
Since the ice sheet is formed by the accumulation of snow year after year, by drilling from the surface down through the ice sheet, we drill our way back in time. Ice drills are designed to collect a core as they cut through the ice, so samples are collected that are made up of ice deposited many thousands of years ago.

As the snow is deposited on top of the ice sheet each year, it traps different chemicals and impurities which are dissolved in the ice. The ice and impurities hold information about the Earth's environment and climate at the time of deposition. The Soviet Union drilling project at the Vostok Station in Antarctica has yielded over a mile of ice cores formed from the snows over the past 160,000 years.

Ice Core Dating

By sampling at very fine intervals down the ice core, and provided that each annual layer of snow is thick enough, several samples from each year may be measured for the different chemical properties. These include airborne continental dust and biological material, volcanic debris, sea salts which are deposited on the ice sheet surface along with the snow, thus mixing with the snow and also acting as a distinctive barrier between different ice layers.

The graphical correlation of ice core depth versus time is shown in the graph on the left.

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Historic Global Temperatures
The isotopic composition of water, and in particular the concentration of the heavy isotope of oxygen, O-18, relative to O-16, is indicative of the temperatures of the environment. During warm periods, the concentration of the slightly more nonvolatile O-18 in the ice is lower than during cold periods. This reflects the lower concentration of O-18 in the atmosphere available for condensation into snow during warm periods due to little evaporation of O-18 from the oceans.

A record of temperature variations over the last 160,000 years, shown in the graph on the left has recently been constructed by analyzing the O-18 to O-16 ratio in the Vostok ice core. The numbers in this data set are recorded as differences from the current average temperature (15 degrees C).

Starting on the left-hand side of the graph at about 140,000 years ago, the climate was about 6 C colder than it is today - an ice age period. Then at about 130,000 years ago, there was a quite rapid warming period until about 125,000 years ago, when the climate was, perhaps, 1 C or 2 C warmer than today - an inter-glacial period. From 120,000 to about 20,000 years ago, there was a long period of cooling temperatures, known as the last Great Ice Age. From about 18,000 or 19,000 years ago to about 15,000 years ago, the climate went through another warming period to the next inter-glacial, - the one we are now in.

Near Term Historical Temperature Records

Links to more Temperature Data Sets: Source: ChemConnections
Vostok CO2 and Temperature Data
Global Temperature Trends
Is it getting warmer?
Is there at temperature difference in trends for land vs. oceans?

How good are the computer models in predicting global temperature changes?

Northern Hemisphere
Southern Hemisphere

Final Answer to Question: What is the evidence that temperature is currently increasing?