Questions & Answers

Taking Our Temperature

Do you have questions? Our experts have answers to some of your most frequently asked questions.

How do scientists know that global temperature has been rising for the past several centuries?

Actual temperature records dating back about 250 years for many spots on Earth show a general trend upward. The temperature—averaged across all locations over many years—shows an increase, even though each individual location has not warmed. And even though actual temperature readings were scarce before the 1700s, nature has a way of preserving temperature records in the bodies of living things. Trees, for example, tend to grow faster in warmer weather. By looking at the growth rings for different species over time, scientists can estimate which years were cooler or warmer.

Another example comes from the ocean, where tiny, shelled creatures called "forams" live. These creatures take oxygen out of the seawater and combine it with calcium and carbon to make their shells. How can the oxygen in these shells tell us about Earth's temperatures long ago? Even though oxygen seems all the same to us, it has several different forms, or "isotopes." Normal oxygen is called oxygen 16, but there is a heavier isotope called oxygen 18. More 18O in seawater indicates more glacier ice in the world (and, therefore, a colder global climate). By comparing the ratio of 18O to 16O in the seashells of creatures who lived hundreds or even thousands of years ago, scientists can tell how Earth's surface temperature has changed.

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How does a greenhouse trap heat?

Visible light is part of a broad band of radiation called the "electromagnetic spectrum." Visible light can easily pass through the glass of a greenhouse, but when it strikes soil, plants, water, and the ground, some of the light is absorbed and changed to a different type of radiation called infrared. You can't see infrared energy, but you can feel it—it's also known as heat! Although visible light can easily pass through glass, infrared radiation gets blocked by it. As a result, the heat gets trapped inside the greenhouse, and, as long as new light keeps shining in, the temperature keeps rising. This is why on a bright, sunny day, the temperature inside a greenhouse can get close to 100 degrees F even when the temperature outside is freezing.

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Why do concrete and asphalt create an "urban heat island"?

Concrete and asphalt create an urban heat island because of several factors:

  • They conduct heat energy better than grass, trees and water do. When sunlight shines on asphalt and concrete roads, parking lots, and sidewalks, much of the visible light is absorbed and changed to infrared or heat energy. (This is especially true of the dark-colored asphalt, which absorbs heat better then light-colored materials do.)
  • Because buildings and roads are massive, they can store heat for a long time, even after it gets dark.
  • Not only do the leaves of plants and trees help to shade the underlying ground from direct sunlight, but they also "transpire" (give off) water vapor, which cools the air around them.

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How can forests help prevent droughts?

Trees help "buffer" moisture conditions in the atmosphere and safeguard against droughts in a number of important ways.

  • Their leaves help shade the ground below, reducing the overall temperature and slowing the rate of moisture evaporation from the surrounding soil.
  • Leaf litter on the ground and tree roots slow the rate of runoff from the land surface after it rains, giving the water more of a chance to seep into the ground and "recharge" the ground water system.
  • When trees grow, they transpire, releasing moisture into the air, which not only cools the atmosphere but also provides a source of new rainwater when it condenses back into clouds.
  • Tree roots tap into moisture found deep in the soil, which helps to add more water to the local cycle.
  • Tree roots also hold the soil in place, keeping it from eroding away and becoming compacted. Once soil has been damaged, it cannot hold moisture and tends to dry out faster, making drought conditions worse.

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What can a person do to avoid getting a vector-borne disease?

Most vector-borne diseases are carried by blood-sucking arthropods such as ticks, fleas, and mosquitoes. The easiest way to protect yourself from getting the diseases that these creatures carry is by preventing them from biting you in the first place. Wearing long pants and shirts when you go into wooded and grassy areas helps. So does wearing insect repellent. Reduce breeding grounds for mosquitoes by making sure that there are no stagnant pools of water near where you live. Remove or empty tires, garden pots or any outdoor containers. It's a good idea to try to eliminate rats and mice, which often play host to the fleas and ticks that carry the diseases.

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Doesn't adding chlorine to water kill disease-causing organisms?

While it is true that chlorine kills many disease-causing organisms found in drinking water, it does not kill all of them. The parasite that causes cryptosporidiosis("crypto" for short), for example, is extremely resistant to chlorine and is sometimes found living in public water supply systems even after the water has been treated with chemicals.

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Do all water filters remove disease-causing organisms from drinking water?

The answer is a resounding no! In order to be effective against many disease-causing organisms, including cryptosporidiosis, a filter must be extremely fine. The openings have to be 1 micron (that's 1 millionth of a meter) or smaller. When purchasing filters for your home, read the package carefully. It will usually tell you which diseases it protects against.

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What's the most effective way to treat drinking water if you think it's been contaminated?

High-grade filters and chemical water treatment kits sold for hikers and campers are effective against most contaminants, but the only sure way to kill off all the living parasites found in drinking water is to boil the water for at least five minutes before drinking it. It is important to note that although boiling kills germs, it does nothing to remove chemical contaminants that may be in the water. The only way to remove them is by distilling the water.

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Doesn't using anti-bacterial soap and detergent kill germs found in water?

Anti-bacterial soap kills some bacteria, but it does very little to stop the spread of viruses and other microbes, especially if these "germs" are found in the water you are washing with. Washing your hands or food in water contaminated with parasites such as "crypto" can actually help spread them to your food. It is also important to note that because of the excessive use of antibiotics, many strains of bacteria have become resistant to standard anti-bacterial soap.

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How can eating fish give you cholera?

Scientists now know that the bacteria that cause the disease cholera can live inside a tiny creature called a copepod. These marine invertebrates are especially plentiful in warm coastal waters, where fresh water from streams and rivers mixes with ocean water. Copepods and other forms of "zooplankton" are the main food source for small fish, crustaceans (such as crabs and shrimp), and especially shellfish (such as clams, mussels and oysters, which filter them out of the water. By eating raw or undercooked fish and shellfish that have eaten these copepods, you may also be eating the cholera bacteria that they carried. Making sure that all fish is well cooked is the best way to safeguard against getting the disease in the first place. Cooking at a high temperature usually kills the cholera bacteria.

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How can you avoid getting cholera from drinking water?

The easiest way of safeguarding your drinking water if you are in an area where there is a cholera outbreak: Boil the water for five minutes before drinking it. Boiling water kills the cholera bacteria. Most sanitation systems that use chlorination to kill bacteria are quite effective in eliminating cholera. Although not completely safe, when infection levels are high and no other methods are available, filtering water through sari cloth (as is done in parts of Bangladesh) can reduce the transmission of cholera.

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Can you get water-borne diseases from recreational water?

Absolutely! Swimming or bathing in water contaminated with bacteria or disease-causing parasites can have the same negative effects as drinking the water. Disease-causing organisms can enter the body through cuts in the skin, and even through your eyes and nose. In addition, if you accidentally swallow any water while swimming, it's as bad as drinking it.

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Is the melting of glaciers the only thing responsible for the rising sea level?

Only a small part of sea-level rise is a result of melting glaciers and polar ice caps. A far more significant problem is caused by "thermal expansion" of the water itself as it gets warmer. Most forms of matter (solids, liquids and gases) expand or get bigger as they heat up—and seawater is no exception. Because the volume of the ocean is so enormous, even a small rise in temperature results in a measurable rise in sea level.

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Could global warming affect our food supply?

Yes, in surprising ways that no one can anticipate. For example, the Gulf Stream, a powerful ocean current that runs from the southeastern U.S. to northern Europe, keeps huge areas of Europe warm enough to support agriculture. Melting ice in the north Atlantic could cool the Gulf Stream, which would, in turn, bring colder weather to much of Europe. While this would not occur overnight, it is possible. Indeed, fossil evidence indicates that it may have happened several times in the past.

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How can you "desalinate" water once it's been contaminated with salt?

Though bacteria and other "living" contaminants can be filtered from water fairly easily, it's extremely difficult to filter the salt from water. That's because once the salt dissolves in water, it goes into solution (changing the actual substance, so there's no going back). The most effective way of getting the salt out is to distill the water, which means evaporating the liquid, capturing the vapor and then recondensing the vapor back into liquid water again. When the water turns into a vapor, the salt is left behind. In order to evaporate large quantities of water, you need to heat it to high temperatures, which requires a great deal of energy. In some areas, water is distilled using solar stills, which get their energy directly from the sun. Stills that get energy from fossil fuels are expensive to operate and add new greenhouse gases to the air.

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How does turning off a light help reduce global warming?

Most of the electricity used for running computers, lights, television sets, and video game systems is generated by power plants that burn fossil fuels to make steam to turn a turbine that spins a generator. Since the burning of fossil fuels adds greenhouse gases to the air, any reduction in their use will help turn the problem around. The bottom line is that the less electricity we use, the less we need to burn fossil fuels.

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