Many people use the words weather and climate as if they mean the same thing. They do not. It is important to understand the difference in order to understand the idea of climate change.
Everyone knows what weather is. It's what is going on in the atmosphere at a particular place and time. Weather is measured in terms of wind speed, temperature, humidity, atmospheric pressure, cloudiness, and precipitation. In most places, weather changes from hour-to-hour, day-to-day, and season-to-season. The word climate refers to the average pattern of weather in a region.
An example may help. If on a day in July, you are asked, "What's the weather been like in New Orleans?" you might answer, "Today it's clear and cool, but yesterday was hot and muggy." On the other hand, if you are asked, "What's the climate like during the summer in New Orleans?" it would be correct to answer, "In the summer it's hot and muggy." The fact that on a particular July day New Orleans happens to be clear and cool doesn't mean its climate has changed.
When climate does change, it usually changes slowly. For example, the climate or average weather of Iowa involves cold snowy winters and hot summers. While the specific weather in Iowa varies from year-to-year, the average pattern is pretty much the same today as it was back in our grandparents' days.
Because the specific weather we experience may be a bit different from one year to the next, a couple of very hot summers, or a couple of very rainy winters, may lead people to conclude that the climate is changing. Of course, rapid climate change might cause such shifts, but it is far more likely that these differences are just natural year- to-year variability. However, because everyone notices a really hot summer, and television and newspapers sometimes talk about climate change in the same story as they talk about recent unusual weather, it is easy to get the two confused.
What is the "greenhouse effect" and how does it keep temperatures on most of the earth moderate?
The greenhouse effect is what keeps the earth a good deal warmer than our neighbor planet, Mars. Unlike Mars, the earth has a thick atmosphere that can trap and redistribute energy from the sun. Perhaps the easiest way to understand how the trapping works is by thinking about what happens when you park your car in the sun with its windows closed. The inside of the car gets warm because light energy can easily pass through the glass of the car windows and enter the car. Some of the light energy bounces off the lighter colored surfaces of the car's interior and is reflected back out through the windows, but much of it is absorbed by the darker seats and other things inside the car. If that was all that was going on, the inside of the car would just keep getting hotter and hotter. However, as the seats and other things in the interior warm up, they give off heat in the form of infrared energy. Unlike light, this infrared heat energy cannot pass easily through the glass of the car's windows, so only a little of it gets back outside. However, as the temperature rises, more energy gets through the window glass. Finally, a balance point is reached at which the amount of sunlight energy that is being absorbed is just balanced by the amount of heat energy that escapes in the form of infrared. At this point, the inside of the car reaches a stable temperature.1
The same kind of balancing goes on with the earth. You can think of the earth's atmosphere as playing roughly the same role as the glass in the car window. Sunlight can easily pass through the atmosphere. About a third of the sunlight is immediately reflected back out into space by light colored surfaces such as clouds, sand, snow, and ice. Most of the rest is absorbed by darker surfaces. If this were the whole story, the earth's surface would keep heating up and soon we would all fry. To reach a balance, the energy that is absorbed must somehow get back out into space as infrared heat energy.
Water vapor, ozone, carbon dioxide, and several other natural and man-made gases in the atmosphere, all absorb infrared energy, so infrared energy has trouble getting out to space. Just as with the car (or a glass-covered greenhouse), the earth's system reaches a temperature level at which the amount of light coming in is just balanced by the amount of infrared heat energy that is escaping. This process is what is commonly known as the "greenhouse effect." It is a natural process that has gone on ever since the earth first had an atmosphere.
Because of this natural greenhouse effect, on average, the earth is about 59°F (33°C) warmer than it would be without its atmosphere. About 57°F (32°C) of this extra warmth is due to water vapor, the rest is due to ozone, carbon dioxide, and several other naturally occurring "greenhouse gases."
What is global warming?
The phrase "Global warming" or "Greenhouse warming" refers to the fact that as more carbon dioxide or other greenhouse gases are added to the atmosphere, the temperature of the earth will rise, assuming nothing else changes. For the past few hundred years, people have been burning fossil fuels such as coal and oil in ever increasing quantities. While some of the carbon dioxide released is absorbed into the ocean or taken up by plant life, in the short-term about half of it remains in the atmosphere. Industrial activities also have been releasing several other greenhouse gases into the atmosphere. The best estimates today are that these gases should have already increased the average temperature of the earth by about 2.3°F (1°C). Since it appears that the average temperature of the earth has only increased by between 1 and 2°F (.6 to 1°C), it is likely that some other things have also changed. In Part 1 of the main brochure, we discussed one change - small particles that are created from sulfur air pollution and that cool the earth by reflecting sunlight. There are others. Some of these changes may be directly or indirectly caused by the increase in carbon dioxide or other greenhouse gases. These changes are called "feedbacks." It is largely the uncertainties about these feedbacks that makes the science of climate change so uncertain and controversial.
How is heat redistributed by the atmosphere and the oceans?
The largest amount of energy coming in from the sun strikes the equatorial region of the earth. Only a modest amount strikes the area around the poles. While the poles are cooler than the equator, the difference in temperature is much smaller than you would expect on the basis of the amount of sunlight each region receives. This is because the earth's atmosphere and ocean constantly move heat from the equator toward the poles. This giant "heat engine" drives the earth's climate and the weather.
When light from the sun is absorbed at the equator it warms the air and evaporates water from the surface. The warm moist air rises. As this air rises, the water condenses to form clouds and rain. This produces the heavy rain that is characteristic of the tropics. As water condenses, it releases heat. Since it is now high up, much of the heat energy that is released can be radiated back into space as infrared energy. That leaves dryer cooler air which is pushed north from the equator and ultimately settles back toward the surface in a region about 30°From the equator.2 From there it once again makes its way to the equator (in what are known as the trade winds).3 This circulation pattern, of air rising near the equator and settling about 30° toward the pole, is referred to as the "Hadley cell" after the meteorologist who first suggested it in 1735.
Planetary circulation and the "Coriolis effect."
When Hadley originally suggested the idea of large scale circulation patterns in the earth's atmosphere in 1735, his idea was that warm air would rise at the equator and then travel all the way to the poles, settling as it went. While this was a plausible first theory, several things prevent it from happening on the real earth. One important complication results from the fact that the earth is rotating. Due to rotation, the surface of the earth moves more rapidly at the equator than it does at the pole. When a parcel of air moves towards the pole it tends to get ahead of the surface below it. Similarly, when a parcel starts to move towards the equator, it tends to lag behind the surface below it. The result is that flows of air that start out in a north-south direction curve around in what is known as the "Coriolis effect." One result is that the very large-scale circulation, that Hadley proposed, breaks up into several smaller-scale patterns. You can see one practical consequence of the Coriolis effect in the direction of the trade winds. Rather than flowing straight down the lines of longitude toward the equator, these winds, which are created by the air motion of the tropical Hadley cells, curve toward the west.
The circulation of the Hadley cell moves energy from the equator up to the mid-latitudes. The atmosphere carries energy further north through a different set of mechanisms which include the large circulating high and low pressure weather systems that characterize weather in middle latitudes such as in the United States.4 The drawing on this page shows a simplified sketch of the general circulation of the earth's atmosphere. It is best to think of this picture as showing the average flow over time. At any given moment, the details may be quite different.5
About half the energy carried from the equator toward the poles is carried by the atmosphere. The remainder is carried by currents flowing in the ocean. Some of these currents are directly created by the wind. Others are driven by the different concentrations of salt in different parts of the ocean. Water naturally flows from regions of more salt to regions of less salt in order to try to balance these differences. The drawing on the bottom of the next page shows one very large circulation pattern, often called the "ocean conveyer belt." One important consequence of this ocean circulation is that it carries warm tropical surface water north through the Atlantic ocean so that Europe ends up being much warmer than it would otherwise be. If this "ocean conveyer belt" were suddenly to shut down (as paleontological and other data suggest it has on occasion in the past), very rapid climate change could occur in northern Europe and perhaps in several other regions.
The "Ocean Conveyor Belt."|
Dense, cold, salt-laden water sinks in the North Atlantic. This drives the "Ocean Conveyor Belt" that carries it under the Atlantic and Indian oceans to re-surface in the northern Pacific. The North Atlantic remains more salty because, the general circulation of the atmosphere carries fresh water (as vapor) from the North Atlantic to fall as rain in the North Pacific. Geological records suggest that on several occasions in the past this large circulation has suddenly shut down, bringing regional climate changes, such as much colder temperatures, to northern Europe.
To learn more see: W. S. Broecker, "The Great Ocean Conveyor," Oceanography, Vol. 4, No. 2, pp. 79-89, 1991.
What are "feedbacks?"
How might they effect the way the climate changes?
As more carbon dioxide or other greenhouse gases are added to the atmosphere, the temperature of the earth will rise, assuming nothing else changes. As we noted earlier, perhaps the most important set of changes that can occur are called "feedbacks."
Feedbacks come in two kinds: negative feedbacks that work to slow down or offset the warming and positive feedbacks that work to speed up or amplify the warming.
There are many feedbacks in the climate system. For example, carbon dioxide acts as a fertilizer that makes some plants grow faster. As the concentration of carbon dioxide in the atmosphere increases, these plants may grow faster and as a consequence take more carbon dioxide out of the atmosphere. This would result in a negative feedback, slowing the rate at which carbon dioxide increases, and hence slowing the rate of warming.
As the earth warms, some ice and snow are likely to melt. Ice and snow are good reflectors of sunlight. The dark ground that is exposed when the snow and ice melts is not as good a reflector. When the ice and snow melt less light energy from the sun will be reflected and more will be absorbed by the earth. This would result in a positive feedback that would tend to speed up the rate at which the earth warms.
Climate scientists have identified a number of positive and negative feedbacks in the climate system. Some of them are well understood. Others are still only partly understood. For example, water vapor is not typically considered part of the climate change problem. However, the greatest uncertainty in predictions of future climate are related to different views on how water vapor and clouds will respond to changed greenhouse gas concentrations.
Aren't there large computer models of the climate? How good are they?
Yes, there are a number of large computer models called General Circulation Models, or GCMs, that have been built to study and predict climate. These models use the basic laws of science (conservation of mass, conservation of momentum, etc.) to represent the large-scale circulations and interactions of the atmosphere. Recently scientists have begun to connect some of these models to similar models of the oceans and the biosphere.
These models all predict roughly the same amount of warming when the amount of carbon dioxide in the earth's atmosphere is doubled. Some people see these similar predictions as a source of confidence that we can make reliable predictions. However, there are several awkward details. To get them to produce these results the models first must be carefully "tuned up" to get them to reproduce the existing climate. This is largely because many of the important processes of the atmosphere and the oceans take place over dimensions of space and time that are too small to be included in the models, so they have to be estimated. The best way to do this is by comparing with past and present climates. Perhaps more troubling, while the GCMs all give roughly the same overall answer, if you look at what is going on in the detailed physical process of each model, things are very different from one model to the next. The same answers come out of the models, but for somewhat different reasons. Finally, while the models all produce about the same result for global averages, the predictions for specific locations are quite variable.
In order to understand and predict the climate system better, we will need a more complete understanding of the basic science of climate process. Many ongoing research programs, both in the United States, and elsewhere around the world, are dedicated to producing this better basic knowledge.
How much will the climate change? How fast will it change?
The answer depends in part on how people behave over the next several decades. If we carry on pretty much with "business as usual," expanding the amount of energy each person uses, the developed world will continue to add large amounts of carbon dioxide to the atmosphere. As developing countries like China and India continue to add population, and become more economically developed, they will begin to add even larger amounts of carbon dioxide.
Recently, a group of leading scientists from around the world were gathered in a special study group called the "Intergovernmental Panel on Climate Change" or IPCC. The estimates made by this group are widely viewed as the best consensus judgment about the science of climate change now available. They are summarized in the drawings on the next page.
The uncertainty and controversy about climate change arise because we are trying to make predictions about how a complex dynamic system will respond when we do things to it which are not quite like anything that has ever been done to it in the past. Most scientists believe that small changes in the "inputs" to the climate system will result in small changes to the resulting climate. But the atmosphere and the ocean, and the interactions they have with living things make up a very complicated system that has many interconnections and feedbacks. Because of these feedbacks, the response of the climate to changes such as more greenhouse gases could be very "nonlinear." While many scientists think it is unlikely, there is some chance that a small change in an input might produce a big change in the resulting climate.
In 1988 The World Meteorological Organization and the United Nations Environment Program jointly established the Intergovernmental Panel on Climate Change or IPCC. The IPCC consists of a set of committees of leading scientists from all around the world whose task it is to periodically review and report on the state of understanding of the climate problem. The panel operates under the general leadership of Prof. Bert Bolin of Sweden. In 1990, IPCC Working Group 1 on climate change science issued its first "consensus report." This was followed in 1992 with an update. The drawings show three scenarios which IPCC developed of how the earth's temperature might change over the next century. IPCC projects that "the land surfaces warm more rapidly than the ocean, and high latitudes warm more than the global mean in winter." Night time low temperatures are found to be increasing more than day time high temperatures. They estimate a "global mean sea level rise of about 6 cm [2.4 inches] per decade over the next century (with an uncertainty range of 3-10 cm [1.2-3.9 inches] per decade), mainly due to thermal expansion of the oceans..."
For references to the IPCC reports, see "For further reading" in Details Booklet Part 3.
Will more research about climate get us all the answers?
Probably not. More research will certainly improve our understanding and help us to make better predictions. However, the climate system is extraordinarily complex and we may never be able to predict fully how it will respond. Indeed, within limits it may even be a "chaotic system," which would mean that precise predictions would never be possible.
Thus, while our knowledge and our ability to make predictions should get better in the next few decades, we will probably always be stuck with a great deal of uncertainty when we face policy choices about the climate.