Engineering Rain Harvesting Systems

The science background section gives teachers more in-depth information on the phenomena students explore in this unit on climate and human activity. 

Arctic Ice

Data show that the thickness and extent of Arctic summer sea ice have both decreased dramatically in the last 30 years. Scientists are measuring these changes in the Arctic to better understand their causes, how the melting ice is affecting ocean currents, and how these changes affect global climate.

Determining cause-and-effect relationships is not easy when it comes to climate. To understand this, it’s important to understand the difference between causation and correlation. Correlation means that there is a relationship between two variables, but it does not necessarily mean that a change to one variable is caused by the other variable. For example, scientists measuring changes in sea ice might also notice a change in the speed at which a particular ocean current is moving. These two variables—changing sea ice thickness and changing ocean current speed— are correlated. However, scientists would need to do more research to prove that the melting sea ice actually caused the ocean current’s speed to change.

There are so many factors that influence climate, as well as complicated feedback loops, that it can be difficult to establish which changes are causing other changes. In addition, scientists know that there are natural fluctuations in Earth’s climate over time. For example, scientists know there have been several ice ages— global climates marked by long periods of cold temperatures that cause glacial formation and expansion.

However, there is broad scientific consensus that human activities are causing Earth’s climate to change more quickly than it has in the past. Scientists believe that in recent decades, the most dramatic influence on Earth’s climate has been human activities, primarily the burning of fossil fuels for energy. As fuels are burned, carbon dioxide is released back into the environment. Carbon dioxide is a molecule made of one carbon atom and two oxygen atoms (CO2).

Greenhouse Gasses

The reason that carbon dioxide affects Earth’s climate is that carbon dioxide is a greenhouse gas. Greenhouse gasses are molecules in the atmosphere that absorb thermal energy from the sun and warm Earth’s surface and atmosphere. Most of Earth’s atmosphere is made up of just two gasses: nitrogen (78 percent) and oxygen (21 percent). A third gas, argon, makes up about 1 percent of the atmosphere. These gasses are not greenhouse gasses because they don’t absorb thermal energy.

Carbon dioxide, water vapor, and methane are greenhouse gasses, and they are found in the atmosphere in such small quantities they are called trace gasses. Even though trace gasses make up such a small percentage of the atmosphere, they have a major impact on warming Earth because they act in a similar way to the glass walls of a greenhouse, trapping thermal energy from the sun. The trapped thermal energy from the greenhouse gasses is what makes the Earth a livable 14 degrees Celsius (57 degrees Fahrenheit). Without greenhouse gasses, scientists estimate that Earth’s surface would be too cold for life at -19 degrees Celsius (-2 degrees Fahrenheit).

There are natural causes that influence the amount of carbon dioxide in the atmosphere. For example, water in clouds holds in some of the heat from Earth's surface because water vapor is the most common greenhouse gas, accounting for more than half of all greenhouse gasses. But the bright white tops of clouds also reflect some of the sunlight back to space, helping to cool Earth's surface. Scientists are still trying to figure out how much clouds affect the warming or cooling of Earth's surface.

Sudden events, such as a volcanic eruption or a forest fire, can also impact the climate. Volcanic eruptions send ash particles into the environment, blocking sunlight from reaching Earth’s surface. This contributes to Earth’s cooling. However, volcanoes also release carbon dioxide, which over millions of years causes warming.

Relationship Between Carbon Dioxide and Warming

Even though carbon dioxide is not the only greenhouse gas, many scientists are concerned about it because humans release so much carbon dioxide into the atmosphere. There are many factors that influence how much carbon dioxide people put into the atmosphere. One such factor is geography. Developed countries such as the United States tend to burn more fossil fuels than less-developed countries such as India. Other factors include population growth, economic growth, new technologies such as renewable fuels, changing behaviors, and seasonal temperatures.

Scientists know that Earth has warmed by 0.6 degrees Celsius (1 degree Fahrenheit) in the last hundred years. They don’t know exactly why, but many believe that the increasing amounts of carbon dioxide in the atmosphere are largely responsible. The more carbon dioxide molecules there are in the atmosphere, the more energy from the sun is trapped, warming Earth’s surface and atmosphere.

Rising global temperatures, called global warming, lead to other changes, such as stronger hurricanes, melting glaciers, and the loss of wildlife habitats. These changes are all part of climate change, and they happen because the Earth's air, water, and land are all related to one another and to the climate. A change in one place can lead to other changes in other places. For example, as Earth gets warmer, more water evaporates from the surface, becoming water vapor, a greenhouse gas. More water vapor in the atmosphere leads to even more warming, which will lead to more evaporation, which will lead to more warming. Scientists are still trying to understand how this feedback loop impacts global warming.