How Global Warming Works

Global warming was once an uncommon term used by a few scientists who were growing concerned over the effects of decades of pollution on long-term weather patterns. Today, the idea of global warming is well known, if not well understood. It is not unusual to hear someone complaining about a hot day or a freak storm and remark, "It's global warming." 

Global warming is a significant increase in the Earth's climatic temperature over a relatively short period of time as a result of the activities of humans.

In specific terms, an increase of 1 or more degrees Celsius in a period of one hundred to two hundred years would be considered global warming. Over the course of a single century, an increase of even 0.4 degrees Celsius would be significant.

To understand what this means, let's start by reviewing the difference between weather and climate. 

Weather and Climate

Weather is local and short-term. If it snows in the town where you live next Tuesday, that's weather. Climate is long-term and doesn't relate to one small location. The climate of an area is the average weather conditions in a region over a long period of time. If the part of the world you live in has cold winters with lots of snow, that would be part of the climate for the region you live in. The winters there have been cold and snowy for as long as weather has been recorded, so we know generally what to expect.

It's important to understand that when we talk about climate being long-term, we mean really long-term. Even a few hundred years is pretty short-term when it comes to climate. In fact, changes in climate sometimes take tens of thousands of years. That means if you happen to have a winter that isn't as cold as usual, with not very much snow -- or even two or three such winters in a row -- that isn't a change in climate. That's just an anomaly -- an event that falls outside of the usual statistical range but doesn't represent any permanent, long-term change.

It's also important to understand that even small changes in climate can have major effects. When scientists talk about "the Ice Age," you probably envision the world frozen, covered with snow and suffering from frigid temperatures. In fact, during the last ice age (ice ages recur roughly every 50,000 to 100,000 years), the earth's average temperature was only 5 Celsius degrees cooler than modern temperature averages [Source: NASA].

Global warming is a significant increase in the Earth's climatic temperature over a relatively short period of time as a result of the activities of humans.

In specific terms, an increase of 1 or more Celsius degrees in a period of one hundred to two hundred years would be considered global warming. Over the course of a single century, an increase of even 0.4 degrees Celsius would be significant. The Intergovernmental Panel on Climate Change (IPCC), a group of over 2,500 scientists from countries across the world, convened in Paris in February, 2007 to compare and advance climate research. The scientists determined that the Earth has warmed .6 degrees Celsius between 1901 and 2000. When the timeframe is advanced by five years, from 1906 to 2006, the scientists found that the temperature increase was .74 degrees Celsius. 

The Greenhouse Effect

Courtesy U.S. Global Change Research Program

Global warming is caused by an increase in the greenhouse effect. The greenhouse effect is not a bad thing by itself -- it's what allows Earth to stay warm enough for life to survive.

Although it's not a perfect analogy, you can think of the Earth sort of like your car sitting out in a parking lot on a sunny day. You've probably noticed that your car is always much hotter inside than the outside temperature if it's been sitting there for a while. The sun's rays enter through your car's windows. Some of the heat from the sun is absorbed by the seats, the dashboard and the carpeting and floor mats. When those objects release this heat, it doesn't all get out through the windows. Some is reflected back in. The heat radiated by the seats is a different wavelength than the light of the sun that made it through the windows in the first place. So a certain amount of energy is going in, and less energy is going out. The result is a gradual increase in the temperature inside your car. 

The greenhouse effect is a little more complicated than your hot car. When the sun's rays hit the Earth's atmosphere and the surface of the Earth, approximately 70 percent of the energy stays on the planet, absorbed by land, oceans, plants and other things. The other 30 percent is reflected into space by clouds, snow fields and other reflective surfaces [Source: NASA]. But even the 70 percent that gets through doesn't stay on earth forever (otherwise the Earth would become a blazing fireball). The Earth's oceans and land masses eventually radiate heat back out. Some of this heat makes it into space. The rest of it ends up getting absorbed when it hits certain things in the atmosphere, such as carbon dioxide, methane gas and water vapor. After these components in our atmosphere absorb all this heat, they emit energy (also in the form of heat). The heat that doesn't make it out through Earth's atmosphere keeps the planet warmer than it is in outer space, because more energy is coming in through the atmosphere than is going out. This is all part of the greenhouse effect that keeps the Earth warm. 

Global Warming: What's Happening?

The greenhouse effect happens because of certain naturally occurring substances in the atmosphere. Unfortunately, since the Industrial Revolution, humans have been pouring huge amounts of those substances into the air. 

Carbon dioxide (CO2) is a colorless gas that is a by-product of the combustion of organic matter. It makes up less than 0.04 percent of Earth's atmosphere, most of which was put there by volcanic activity very early in the planet's life. Today, human activities are pumping huge amounts of CO2 into the atmosphere, resulting in an overall increase in carbon dioxide concentrations [Source: Keeling, C.D. and T.P. Whorf]. These increased concentrations are considered the primary factor in global warming, because carbon dioxide absorbs infrared radiation. Most of the energy that escapes Earth's atmosphere comes in this form, so extra CO2 means more energy absorption and an overall increase in the planet's temperature. 

The Worldwatch Institute reports that carbon emissions worldwide have increased from about 1 billion tons in 1900 to about 7 billion tons in 1995. The Institute also notes that the average surface temperature of Earth has gone from 14.5 degrees C in 1860 to 15.3 degrees C in 1980.
The IPCC says that the pre-industrial amount of CO2 in the Earth's atmosphere was about 280 parts per million (ppm), meaning that for every million molecules of dry air, 280 of them were CO2. In contrast, 2005 levels of CO2 were measured at 379 ppm [Source: IPCC].

Nitrous oxide (NO2) is another important greenhouse gas. Although the amounts being released by human activities are not as great as the amounts of CO2, nitrous oxide absorbs much more energy than CO2 (about 270 times as much). For this reason, efforts to curb greenhouse gas emissions have focused on NO2 as well [Source: Soil Conservation Council of Canada]. The use of large amounts of nitrogen fertilizer on crops releases nitrous oxide in great quantities, and it is also a by-product of combustion.

Methane is a combustible gas, and it is the main component of natural gas. Methane occurs naturally through the decomposition of organic material and is often encountered in the form of "swamp gas." Man-made processes produce methane in several ways:

• By extracting it from coal
• From large herds of livestock (i.e., digestive gases)
• From the bacteria in rice paddies
• Decomposition of garbage in landfills

Methane acts much like carbon dioxide in the atmosphere, absorbing infrared energy and keeping heat energy on Earth. The IPCC says that methane's concentration in the atmosphere in 2005 was 1,774 parts per billion (ppb) [Source: IPCC]. While there isn't as much methane as carbon dioxide in the atmosphere, methane can absorb and emit twenty times more heat than CO2 [Source: Hopwood, Nick and Cohen, Jordan]. Some scientists even speculate that a large-scale venting of methane into the atmosphere (such as from the release of huge chunks of methane ice locked under the oceans) could have created brief periods of intense global warming that led to some of the mass extinctions in the planet's distant past 

 

Effects of Global Warming: Sea Level

We have seen that an average drop of just 5 degrees Celsius over thousands of years can cause an ice age; so what will happen if the Earth's average temperature increases a few degrees in just a few hundred years? There is no clear answer. Even short-term weather predictions are never perfectly accurate because weather is a complex phenomenon. When it comes to long-term climate predictions, all we can manage are educated guesses based on our knowledge of climate patterns through history.

Glaciers and ice shelves around the world are melting [Source: Guardian Unlimited]. The loss of large areas of ice on the surface could accelerate global warming because less of the sun's energy would be reflected away from Earth to begin with (refer back to our discussion of the greenhouse effect). An immediate result of melting glaciers would be a rise in sea levels. Initially, the rise in sea level would only be an inch or two. Even a modest rise in sea levels could cause flooding problems for low-lying coastal areas. However, if the West Antarctic Ice Sheet were to melt and collapse into the sea, it would push sea levels up 10 meters (more than 32 feet), and many coastal areas would completely disappear beneath the ocean [Source: NASA].

The IPCC estimates that sea levels rose 17 centimeters (or about 6.7 inches) in the 20th century. Scientists project rising sea levels to continue through the 21st century, with levels increasing between 7 and 22 inches by 2100. The IPCC did not consider changes in ice flow in these projections due to a lack of scientific data. Sea levels will likely be greater than the range of projections, but we can't be sure by how much until more data can be gathered about the effect of global warming on ice flows.
With a rise in the overall temperature of the ocean, ocean-borne storms such as tropical storms and hurricanes, which get their fierce and destructive energy from the warm waters they pass over, could increase in force.

If the Polar Ice Caps Melt

Photo courtesy NOAA

Are the polar ice caps in danger of melting and causing the oceans to rise? This could happen, but no one knows when it might happen.
The Earth's main ice-covered landmass is Antarctica at the South Pole, with about 90 percent of the world's ice (and 70 percent of its fresh water). Antarctica is covered with ice an average of 2,133 meters (7,000 feet) thick. If all of the Antarctic ice melted, sea levels around the world would rise about 61 meters (200 feet). But the average temperature in Antarctica is -37°C, so the ice there is in no danger of melting. In fact, in most parts of the continent it never gets above freezing.
At the other end of the world, the North Pole, the ice is not nearly as thick as at the South Pole. The ice floats on the Arctic Ocean. If it melted, sea levels would not be affected.
There is a significant amount of ice covering Greenland, which would add another 7 meters (20 feet) to the oceans if it melted. Because Greenland is closer to the equator than Antarctica, the temperatures there are higher, so the ice is more likely to melt. Scientists from the Universities of London and Edinburgh say that ice loss in Antarctica and Greenland together contribute approximately 12 percent of the rise in sea levels [Source: Science Daily].
But there might be a less dramatic reason than polar ice melting for the higher ocean level -- the higher temperature of the water. Water is most dense at 4 degrees Celsius. Above and below this temperature, the density of water decreases (the same weight of water occupies a bigger space). So as the overall temperature of the water increases it naturally expands a little bit making the oceans rise