Global warming refers to the average increase in the global mean surface temperature of the Earth. The Earth has warmed by about 1.0 degree F since the late 19th century. See the EPA website.
What is the greenhouse effect?
The greenhouse effect is an elevation in the surface temperature that occurs when certain heat-absorbing "greenhouse gases," particularly CO2 and water vapor, are present in the atmosphere. It is a natural phenomenon -- the Earth has had a greenhouse effect for billions of years. Without it, the oceans would be totally frozen and life would not exist on Earth!
Are global warming and the greenhouse effect the same thing?
In a word, no! The greenhouse effect is a natural process whereby certain gases in our atmosphere (water vapor, carbon dioxide and methane for example) retain some of the infrared radiation that is emitted by the Earth. This process maintains the Earth's average surface temperature at about 60 F, making life possible. Without it, the Earth would be too cold for life to have developed here. Global warming occurs when the greenhouse effect increases in strength, causing the surface temperature to rise beyond our "comfort zone."
Scientists believe that global warming is caused by an increase in the concentration of greenhouse gases within our atmosphere. It is well known that we have drastically altered the composition our atmosphere - we have increased the carbon dioxide level by 30% and increased the methane level by more than 200% from pre-industrial revolution levels (See the EPA website.) These increases are thought to be largely due to human effects -- burning fossil fuels, decreasing vegetation (a natural reservoir for carbon) and landfills all release significant amounts of greenhouse gases. More greenhouse gases results in a strong greenhouse effect and a warmer planet.
How does the greenhouse effect work?
It is commonly stated that the greenhouse effect causes warming because greenhouse gases "trap" the heat, preventing it from escaping. This is a misconception! If all of the absorbed sunlight were trapped as heat, the temperature would be rising by 26 deg F per week, and the oceans would boil away in only 1500 years! This is totally contradicted by everyday experience as well as detailed observations of the climate system.
In reality, the Earth resides near an equilibrium where, under normal conditions, the rate of heat loss by infrared radiation to space almost exactly cancels the energy absorbed as sunlight. The temperature can thus remain stable over time. The surface temperature at this equilibrium depends on whether the atmosphere acts as an insulator. Without a greenhouse effect, the atmosphere has no insulation, so Earth can easily lose its heat to space while remaining cool. With a greenhouse effect, the atmosphere acts like an insulating blanket, so Earth can only lose its heat if the surface becomes warmer. Either way, the Earth loses almost exactly the same amount of heat -- it's just that, to do so, the surface temperature must be higher when greenhouse gases are present in the atmosphere. This is analogous to putting on a sweater on a cold night -- you feel yourself warming up at first, but then your temperature stabilizes: you have reached a new, warmer equilibrium than before you put on the sweater. In a similar way, the greenhouse effects acts as a blanket that insulates the Earth and makes it warmer -- while still allowing heat to escape.
A more technical, yet physically illuminating, description of the greenhouse effect can be seen here.
Is global warming controversial?
No. An abundance of evidence proves that, in the past century, the Earth has experienced about 1oF of warming and many other climate changes, including melting glaciers, changes in sea-ice thickness, increases in sea level, early arrivals of spring, late arrivals of autumn, ocean chemistry changes, unusually hot summers, and a variety of adverse effects among wildlife. There is also a consensus among scientists that this warming is not a "natural" climate fluctuation but instead is driven primarily by the release of greenhouse gases by human activities.
There is a perception among the general population that scientists are having a raging debate about whether global warming is real and/or caused by people. This is a misconception! Scientists are in widespread agreement that global warming is real and that the climate changes of the past 50 years have been caused primarily by human activities. Unfortunately, however, a number of powerful companies and government officials have, for political reasons, attempted to portray global warming as controversial. Well-meaning journalists have also contributed substantially to this problem by giving equal airtime to global warming critics (many of whom are not even scientists). However, although the views of these critics may sound convincing to a layperson, the scientific issues that they raise have generally been discredited. As climate expert Michael Mann states in realclimate.org, "In the case of climate change, a clear consensus exists among mainstream researchers that human influences on climate are already detectable, and that potentially far more substantial changes are likely to take place in the future if we continue to burn fossil fuels at current rates. There are only a handful of 'contrarian' climate scientists who continue to dispute that consensus. To give these contrarians equal time or space in public discourse on climate change out of a sense of need for journalistic 'balance' is as indefensible as, say, granting the Flat Earth Society an equal say with NASA in the design of a new space satellite."
The existence of an overwhelming consensus among climate scientists about global warming was demonstrated scientifically in a 2004 article published in Science by Professor Naomi Oreskes of U.C. San Diego. She randomly selected 928 articles published in the peer-reviewed scientific literature that contained the words "global climate change." 75% of these articles dealt either directly or indirectly with recent climate changes and their causes, and all of these -- 100% -- supported the view that recent climate changes are caused primarily by human activities. None of the 928 articles disagreed with this consensus position. Dr. Oreskes has an Op-Ed piece in the Washington Post discussing this issue.
Is global warming a natural process?
We know that there have been many variations in global temperatures over the history of the Earth. For example, we know that the Earth has experienced ice ages. However, a wide variety of evidence indicates that the increase in global temperatures since 1800 are the result of human activities, primarily the release of CO2 from burning oil, coal, natural gas, and forests. The carbon dioxide levels today are higher than they have been at any time in the past 450,000 years. This suggests that this is more than just a natural process.
Isn't carbon dioxide produced naturally? Why are humans to blame?
Carbon dioxide is a by-product of many natural processes on Earth. However, these processes cannot explain the rise in carbon dioxide levels that we have measured over the past several decades. For example, natural decay of plant matter releases CO2 into the atmosphere, but averaged over a year this is almost exactly cancelled by removal of CO2 by plants as they photosynthesize and grow. In fact, estimates of deforestation and oil, coal, and natural gas consumption allow the yearly release of CO2 by humans to be calculated. The actual atmospheric CO2 abundance is currently rising at only half this rate. Therefore, not only is the current CO2 increase caused by humans, but much of the CO2 released by humans is going somwhere other than the atmosphere. See here for a more detailed summary.
Where does all of the carbon dioxide go?
Only so much carbon dioxide can be absorbed by vegetation. The rest generally ends up in one of two places -- the atmosphere or the oceans. In the atmosphere it contributes to the greenhouse effect described above. In our oceans, it reacts with water to produce carbonic acid. Thus not only does increasing our carbon dioxide levels affect the atmosphere, it also affects global water quality and aquatic ecosystems. At present, only about half of the carbon dioxide released by human activities is entering the atmosphere; increases in vegetation and especially absorption by the ocean are taking up the rest.
How will global warming affect the Earth?
Scientists predict that, over the next century, the average surface temperature of the Earth will rise by 5-9 deg F (3-5 deg C). (?) More importantly, it is likely that a wide range of climate changes will accompany this temperature increase, although the complexities of the climate system make precise predictions difficult. The continued melting of glaciers and the expected increase in ocean temperature should lead to a sea-level rise of at least several feet (1 m). Increased drought would occur in some areas and increased rainfall in others. Reduction of mountain glaciers and winter snowpack would decrease the stability of the water supply in many parts of the world. Sea ice in the Arctic would continue to decrease and the polar permafrost will continue to thaw, leading to major changes in polar ecosystems. As the tropical ocean surface warms, tropical storms and hurricanes may increase in frequency and/or intensity, potentially leading to more frequent Katrina-like disasters.
There is a small chance that the warming will cause a runaway melting of the Greenland or West Antarctic ice sheets, in which case a 50-foot (15-20 m) increase in sea level would result. This increase would be sufficient to flood many coastal cities around the world and cause a mass migration of humans on an unprecedented scale.
A cautionary lesson is provided by the last ice age, when a naturally occurring 30% increase in CO2 caused a 9-deg-F warming that led to melting of mile-thick ice sheets that covered much of North America, Scandinavia, and Siberia. This in turn caused a 300-foot increase in the global sea level as the glacial meltwater flooded the oceans. This record demonstrates that very-large-magnitude climate changes can result from modest changes in the CO2 abundance and mean surface temperature.
How would sea level rise affect coastlines?
In many parts of the world, the coastal land is very flat, so even modest sea level rise can lead to substantial changes in coastlines. The images below, courtesy of Jeremy Weiss and Jonathan Overpeck, show how sea level rise would affect coastlines in susceptible areas (see their website). In some areas, a sea level rise of only 3 feet (1 meter) would cause inward retreat of coastlines by tens of miles or more. Many of the the world's largest cities, such as New York (population xx), Shanghai (population xx), and xx are in shallow near-shore areas that would be
 |
 |
 |
 |
What must be done to stop global warming?
The abundances of CO2, methane, and other important greenhouse gases, which have been increasing steadily over the past 200 years, must become constant. This will require zero emissions of CO2 from fossil fuels -- in other words, to prevent global warming from getting worse, the world's countries must stop burning oil, coal, natural gas, and other fossil fuels. A complete switch to alternative energy sources is necessary.
In the meantime, a variety of solutions can decrease our CO2 emissions and hence slow -- though not stop -- global warming. Increasing vehicle fuel-efficiency standards, adopting energy-efficient building codes, placing more reliance on alternative energy sources, and urban planning that encourages mass transit (e.g., subways and buses) can all help reduce our CO2 emissions. Scientists are also evaluating stop-gap solutions such as capturing CO2 at power plants and pumping it underground (rather than releasing it into the air).
What is the Kyoto Protocol? Can it stop global warming?
Is it possible to reverse global warming?
How will global warming affect natural ecosystems?
How do human-induced climate changes compare to natural ones that have occurred in the past?
Are recent weather events like Hurricane Katrina caused by global warming?
How do scientists study the climate?
What can I do?
There are several possibilities for reducing our own personal carbon dioxide emissions. Five examples of simple changes you can make in your everyday life to reduce your carbon dioxide emissions are:
* Drive less and drive smarter: try to walk, ride or use public transport whenever possible. If you do need to drive, try to combine as many different errands into one trip. Maintain your car so that it runs as cleanly as possible. Consider a hybrid car the next time you're buying a new car.
* Use energy efficient lighting: common incandescent light bulbs are very inefficient with approximately 10% of the energy used going to light. The rest ends up as heat. Compact fluorescent light bulbs are much more efficient making them cooler and less expensive to run and use less energy than common light bulbs. Using less energy reduces the carbon dioxide emissions.
* Change to using green power: solar and wind power both emit significantly less carbon dioxide than traditional fossil fuel powered electricity generators. Install your own photovoltaic cell at your house or buy green power from your local provider.
* Heat and cool wisely: on average, almost half of a households energy usage goes to heating and cooling. Raising your thermostat by just 2 degrees during summer and lowering it by 2 degrees during winter can produce big reduction in carbon dioxide emissions.
* Recycle: Production of paper, plastic, and metals from raw materials requires substantial energy. Recycling these materials reduces energy usage and leads to lower CO2 emissions than producing these materials from scratch.
