The Earth’s climate system is very complex. It is affected not only by what happens in the atmosphere (gaseous area surrounding Earth), but also in the oceans, the cryosphere (glaciers and ice caps), the geosphere (land areas), and the biosphere (area with living organisms). The interaction of these systems is difficult to predict. Understanding the changes in the Earth’s climate is one of the biggest questions facing scientists today.
Over the 4.5 billion years of Earth’s existence, its climate has changed many different times. These changes are evident in the fossil record and in other records, such as ice cores. Several factors have contributed to climate change in the past. These include wobble in the Earth’s axis; variations in the Earth’s orbit and tilt; solar output; volcanic gases and ash in the atmosphere, which affect the sun’s reflection off the Earth’s surface (called the albedo effect); and changes in land masses due to plate tectonics. As a result of these factors, Earth has had five major ice ages in the past (see Figure 1), as well as times when there were no frozen poles. For more information about each of these factors, see NASA’s Climate Science Investigations: Causes of Climate Change.
Carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and water vapor (H2O) are sometimes referred to as greenhouse gases. It is important to remember that these compounds are a natural part of the atmosphere. In fact, they are necessary to support Earth’s biosphere. These gases (along with others) let in ultraviolet radiation given off by the sun but prevent some of the infrared radiation that is emitted by the Earth from leaving the atmosphere, similar to the way that the glass panes of a greenhouse trap heat. Without this heat-trapping mechanism—called the greenhouse effect—the planet would be too cold to support life.
The concentrations of many greenhouse gases have fluctuated during the history of the planet. However, air samples from ice cores representing the past several thousand years indicate that CO2 levels in the earth’s atmosphere have been fairly stable at about 280 parts per million (ppm). Those levels started to rise at the start of the Industrial Revolution, around 1860. By 1958, atmospheric CO2 levels reached 315 ppm. In 2014, CO2 levels topped 400 ppm for the first time in over 800,000 years.
Carbon dioxide levels go through an annual cycle. Every spring, when plants start growing aggressively, they take up CO2 and the levels drop until the fall. In the fall, the levels of atmospheric carbon dioxide begin to rise again as leaves die and decay.
According to National Oceanic and Atmospheric Administration (NOAA) scientists, there are other natural fluctuations in climate that influence the carbon balance. Oceans and soils take up and release CO2. Other events like volcanic eruptions and El Niños can impact CO2 levels. The recent increases are beyond what is historically considered normal (or natural) levels. Many believe that this increase is already changing the Earth’s climate.
Most increases in carbon dioxide emissions come from burning fossil fuels to run automobiles, power plants, and industries. The second largest source is land-use change, mainly the clearing of forests for agriculture or development. In the 19th and early part of the 20th century, land-use change actually contributed more to greenhouse gas emissions than fossil fuel combustion. Today, this order has flipped dramatically, with approximately 80% of carbon dioxide emissions attributed to fossil fuel combustion. This switch is mostly because our fossil fuel use has increased dramatically, but also because in some areas such as the northeastern United States, abandoned agriculture fields have reverted to forests, re-absorbing some of the CO2 that was emitted 100 years before.
In 2013, total global CO2 emissions reached approximately 40 million metric tons, 61% higher than 1990 (the Kyoto Protocol reference year) and 2.3% higher than 20121. The most CO2 emissions were from China (28%), the United States (14%), the European Union (10%), and India (7%) – with China, the United States, and India all having increased emissions over the previous year. (The 28 member countries of the European Union showed a 1.8% decline in the years between 2012 and 2013.)
Although industrialized nations have traditionally produced the greatest percentage of excess greenhouse gases, developing nations will likely contribute more of these gases as they acquire more cars and other fossil fuel-burning technologies. Carbon dioxide is of primary concern, increasing by estimates of up to 30 percent since the beginning of the Industrial Revolution. But there are other greenhouse gases causing concern, including methane and nitrous oxides, which result from fertilizers used in agriculture, livestock farming, landfills, and energy production. Methane and nitrous oxide —while smaller in quantities than CO2—actually trap heat even more effectively than it does (methane over 20 times more and nitrous oxide almost 300 times more).
Some researchers have noted that the rise in greenhouse gases parallels rises in overall global temperatures, which have increased over the last century. According to the U.S. Environmental Protection Agency (EPA), average temperatures have risen across the contiguous 48 states since 1901, with an increased rate of warming over the past 30 years. Average global temperatures show a similar trend, and the top 10 warmest years on record worldwide have all occurred since 19982. Most scientists are convinced that the average temperature levels on Earth will rise by several degrees Celsius if greenhouse gases are not significantly reduced.
While these scientists agree that an increase in atmospheric CO2 and other greenhouse gas concentrations will raise average world temperatures, they do not agree on the effect of such global warming. Many researchers believe that global warming could disrupt weather patterns worldwide, causing coastal flooding, an increased range of disease-causing organisms, an altering of natural habitats, and the potential extinction of some plants and animals. Others believe that some of the changes could be beneficial—increasing food production, lengthening growing seasons for crops and decreasing the need for heating in many areas.
Some people believe either that global warming is not happening at all or that it could not be caused by human activities, pointing to natural warming and cooling trends that have occurred over the eons. Predicting future warming is difficult because the natural processes that contribute to climate are complex and have been difficult to simulate. However, recent advances in computer modeling technologies have strengthened the case for global warming.
Scientific Argumentation in the Classroom
Most people think of argumentation as a confrontation or quarrel. Scientific argumentation, however, is a process by which scientists and researchers present their ideas, results, and conclusions. Unlike a confrontation, scientific argumentation has the goal of refining and build consensus for scientific ideas—based on evidence—to come as close as possible to understanding the reality of the natural world.
In this process, scientists and researchers use their data to make a claim supported with evidence. Other scientists can then critique the work, evaluating whether the claim is adequately supported, the evidence sufficient, the data appropriately analyzed, and the reasoning sound.
In scientific argumentation:
The claim is the conclusion made after analyzing the data or an answer to the question being tested. This should be no more than two sentences. The claim is a statement, not an explanation that delves into the reasons why it is so. The claim should sum up the conclusion of an investigation or answer a question. This is where less is more.
The evidence is the data collected that support the claim. The data should be appropriate for the problem and the amount provided should be sufficient to make the case. A quantitative summary of patterns and trends can be used if it clearly reflects the data, but data tables and graphs should not be included here.
Reasoning explains how the evidence supports the claim. What inferences can be made? This is where valid data examples are given to substantiate the claim. Data results should be quoted with explanations that tie them to the claim. This will be the longest part of the process and should include vocabulary terms and other information pertinent to the topic.
- In the past, what factors contributed to changes in Earth’s climate?
- What are the major sources of increased levels of carbon dioxide in the atmosphere since the mid-1800s?
- What is some of the evidence scientists have presented to support or refute the claim that Earth’s climate is changing due to more atmospheric carbon?
- CO2.earth. CO2Now.org ↩
- Climate Change Indicators in the United States: Weather and Climate. U.S. Environmental Protection Agency (EPA) ↩