Editor's note: This Q&A occurred in October of 2005.

As one study after another has pointed to carbon dioxide and other man-made emissions as the most plausible explanation, the cautious community of science has embraced an idea initially dismissed as far-fetched.

The result is a convergence of opinion rarely seen in a profession where attacking each other's work is part of the process.

Every major scientific body to examine the evidence has come to the same conclusion: The planet is getting hotter; man is to blame; and it's going to get worse.

Climate researcher John M. Wallace of the University of Washington, UW research scientist Dr. Amy Snover and Ph.D. candidate in atmospheric sciences Justin Wettstein fielded your questions about global warming in a live Q&A session Tuesday. Thanks to everyone who participated.

Read the Oct. 9 Times story "The truth about global warming"

I very much appreciated Sunday's article about global warming. A friend believes that this trend may in fact be good for us and our planet. Others argue that little can be done to slow the warming, and nothing significant would come of modifying our buying habits or creating green production requirements except to disrupt our economy. I believe that our society - nationally and worldwide - may break down, which could involve war as we compete for severely limited resources, including food and energy. But I don't have facts to respond to the statements of these friends. Would you describe some potential scenarios for how life could change and how I might respond to these people?
— Suzanne Krom, Seattle

Snover: Societies around the world have developed with anticipation of certain climate patterns and depend on climate-dependent natural resources. (In the Pacific Northwest, we rely on winter snow pack to store water that arrives in the winter for delivery in the summer when our demands are highest.) Because global warming would alter accustomed climate patterns and regional natural resources (some natural systems will be irreversibly damaged by global warming) it could pose disruptions to socioeconomic systems around the world. These disruptions would be worse where global warming worsens existing conflicts over scarce resources and where the funding or capacity for preparing for or adapting to these changes is lacking. Many people agree that global warming is likely to have worse consequences for those with the least resources and therefore least able to adapt - the economically or politically vulnerable, for example.

And as for being able to "stop" global warming, your friends are right - that it's a hard challenge, but wrong to say that there's nothing we can do. If humans can cause the climate to change (and we can!) we can certainly stop doing so. See other posts for ideas about what to do and check out the Apollo Alliance (http://www.apolloalliance.org/), a "broad coalition within the labor, environmental, business, urban, and faith communities in support of good jobs and energy independence." The Apollo Alliance is pursuing a $300 billion, public-private program to create three million new, clean energy jobs to free America from foreign oil dependence in ten years.

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What is the evidence that the global climate changes that are in question are the result of human behavior rather than a natural cycle? Hasn't the earth seen fluctuations in climate temperatures during periods where humans weren't burning fossil fuels?
— Doug Kilishek, Seattle

Wallace: Yes, there have been dramatic climate swings in the past, but with very few exceptions, they have occurred gradually, on time scales of thousands of years.

Some of the most dramatic climate change in the past was the alternation between glacial epochs, or ice ages, like the one that ended 15,000-20,000 years ago, and non-glacial conditions, like we're experiencing now.

The cause of these swings was the subtle variations in the Earth's orbit around the sun due to the pull of the other planets. These orbital changes don't change the total incoming solar energy significantly, but they dramatically change the strength of the sunlight over high northern latitudes, like Northern Canada, during the summer season.

The conditions that favor weak summer radiation in these areas are:

1. A large tilt to the Earth's axis

2. A large departure of the Earth's orbit from a circular shape, so that the Earth is much closer to the sun in part of its orbit than in the other part.

3. The Earth is farthest from the sun during the Northern Hemisphere's summer.

When these three things occur, solar radiation is weak in the high northern latitudes in the summer. Under these conditions, snow and ice that accumulate during the winter don't melt during the summer, so the ice builds up and an ice age results.

The end of the ice age comes when the orbit arranges itself so that solar radiation during summer is very strong. These changes take place very slowly - over thousands of years. In contrast, greenhouse warming is taking place much more rapdily - on a time scale of a century.

Hence ecosystems have much less time to adjust to the changes.

Human civilizations have experienced climate change before, but nothing as dramatic worldwide as we're seeing now.

Why are we convinced the current warming is not due to natural processes, like these orbital changes? First: Because we know greenhouse gases are accumlating in the atmosphere in levels that haven't been experienced in the past few tens of millions of years.

Second: Because the pattern of the observed warming fits the pattern we would expect from warming caused by the buildup of greenhouse gases. (ie- almost all areas of the planet are warming; the Earth's surface and lower atmosphere are warming; the upper atmosphere is cooling; the temperature changes are greatest in the Arctic during winter.)

Third: The warming is much more rapid than most of the natural variations we've seen in the past.

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Paint a picture of what the landscape and lifestyle of the Northwest will look like in 10-20-30 years under the current global warming conditions.
— Tim Cummins, Black Diamond

Snover: During the next 20-40 years, the climate of the Pacific Northwest is projected to change significantly. Global climate models project mid-21st century temperatures in the PNW warmer than most temperatures observed in the 20th century.

As a result, the PNW is likely to see:

Changes in water resources:

• Decreased mountain snowpack

• Earlier snowmelt

• Higher winter streamflow in rivers that depend on snowmelt

• Lower summer streamflow in rivers that depend on snowmelt (most rivers in the PNW)

• Earlier peak (spring) streamflow in rivers that depend on snowmelt (most rivers in the PNW)

• Decreased water for irrigation, fish, and summertime hydropower production

• Increased conflict over water

• Increased urban demand for water

Changes in salmon:

• Increased difficulties due to increased winter floods, decreased summer streamflow, and increased water temperature

Changes in forests:

• Seedling regeneration increased in high snow forests, decreased in dry forests

Tree growth:

• Increased in high snow forests

• Decreased in dry (east-side) forests

• Potential increases in forest fires

• Potential for extinction of local populations and loss of biological diversity if environmental shifts outpace species migration rates and interact negatively with population dynamics.

Changes along the coasts:

• Increased coastal erosion and beach loss due to rising sea levels

• Permanent inundation, especially in south Puget Sound around Olympia

• Increased coastal flooding due to sea level rise and increased winter streamflow from interior and coastal watersheds

For more information about these and other impacts of climate change, see http://www.cses.washington.edu/cig. You might also be interested in a report coming out next week from the Puget Sound Action Team summarizing climate impacts on the Puget Sound marine ecosystem and in a conference being held by King County later this month on climate impacts on Washington state (http://dnr.metrokc.gov/dnrp/climate-change/conference-2005.htm)

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What is the relationship between population rise in Third World countries and global warming? What about the rumor that HIV-AIDS and other diseases are man-made to reduce population and then to stop global warming?
— Zela, Atlanta

Wettstein: There is an historical relationship between population, affluence and the amount of greenhouse gas emissions released by a country. This is part of the reason policy makers are concerned about the continued development of China and India, countries with large population and growing affluence. One should consider that the U.S., however, is still the largest single emitter in terms of greenhouse gases on the planet, despite the fact that our population is much smaller than either India or China. The hope in terms of climate change policy is to increase our efficiency of energy generation and use through new technologies, thereby breaking eventually (and bending initially) this historical relationship.

As to your question on creating disease to eliminate population, obviously that's morally bankrupt.

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I heard a while ago that warming was 2 degrees Celsius, with a +/- of 2 degrees worldwide. Has that changed to a point where there is statistical significance?
— Mat Chavez, Seattle

Wettstein: I don't think this statement is entirely accurate, though you are right that there are significant uncertainties (about 1/2 of which emanate from uncertainty in the physics of climate science and about 1/2 of which come from the different assumptions you might make about future greenhouse gas emissions around the world). attached is a figure of the different output for climate models of the past 150 years or so, through the present and up to 2100:

http://www.grida.no/climate/ipcc_tar/wg1/350.htm

Note that essentially no climate models show cooling and also that the temperature scale used in the IPCC reports is in Celsius, so multiply by 1.8 to get change in degrees Fahrenheit.

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This is really out there, but millions of people smoke cigarettes and it all adds up. How much does smoking contribute to global warming?
— Linda Neilsen, Kent

Wallace: There are plenty of reasons for people not to smoke, but fear that smoke will contribute to global warming isn't one of them. In fact, if the smoke is white enough, it might even counteract global warming by a minuscule amount.

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Isn't it true that scientists in the 1970s said the earth was cooling?
— (several readers)

Wallace: I remember the brief flurry of excitement about global cooling during the late 1970s. I think that most of it was prompted by the media hype about the 1976-77 and 1977-78 winters, which produced some remarkably cold weather over the United States. There were a few scientists at that time who were promoting the idea that these cold winters were harbingers of a trend towards a new ice age. They pointed out that the temperatures of the Northern Hemisphere had been cooling slightly since the 1950s, and they postulated that this was the beginning of a long-term cooling trend that could culminate in an ice age.

Most of us were very skeptical of these claims and largely ignored them. Within a few years, the pendulum had swung the other way and scientists were beginning to recognize that the worldwide climate had actually been warming quite dramatically since the early 1970s.

The cooling scare involved a handful of scientists and lasted only a few years, until evidence proved it wrong. In contrast, nearly the entire scientific community is concerned about global warming and this concern has been steadily getting stronger as the evidence accumulates.

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When we moved here in 1950s I noted to my mother the snow capped mountains to the west of our home. The view was of the mountain tops only. My mother answered that what she saw were only white clouds. Today, from the same viewpoint, she would not see white clouds but mountains. Has anyone documented the decreasing snow and glaciers of the Olympic Mountains in Washington state on film over the past years?
— Linda Neilsen, Kent

Snover: The United States Geological Survey has interesting documentation of glacial changes around the Northwest, some of it put together by Jim O'Connor, a research hydrologist with the U.S. Geological Survey in Portland, Oregon. There are also many monitoring stations where snow pack has been measured all around the western U.S. (including Washington) throughout the past century. These measurements have recently been put together in an analysis by Philip Mote at the University of Washington to show dramatic decreases in Cascade Mountain snow pack (-30 percent to -60 percent at many locations over the past 50 years). You can read an article about this summary here: http://ams.allenpress.com/amsonline/?request=get-toc&issn=1520-0477&volume=86&issue=1.

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When crude oil is removed from the ground is it replaced with any type of filler?
— Linda Neilsen, Kent

Wallace: Some oil reservoirs remain empty, and it has been proposed that one way of eliminating some of the carbon dioxide gas that we are releasing would be to pump it into those reservoirs under high pressure.

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What are references for world temperatures and sea levels over the past 75,000 years. Can you recommmend good books?
— Bob Moyer, Bellevue

Wettstein: Much of the world temperature and sea level rise information over various time scales of the past is contained in a figure I just located from Wikipedia, that contains two records of temperatures from Antarctica and a record of sea level for the past 400,000 years. As ice volume goes up, sea level goes down. http://en.wikipedia.org/wiki/Image:Ice_Age_Temperature.png

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What is the major argument used by the "debunkers" of global warming? What can be done by the average citizen to help stop global warming?
— Kate Yamamoto, Bellingham

Snover: It's interesting to look at how the major argument used by the skeptics has shifted over time. From "the greenhouse effect is a myth" to "there's no evidence of warming" to "there's no way humans can influence climate" to "the climate will probably warm, but it will be a good thing," the argument has shifted over the years.

In an earlier post [below] I described some things that an "ordinary" person (is there any such thing?) can do about global warming. These could be broken down as follows:

• Individual action (reduce actions that rely on fossil fuels, e.g., in transportation, home heating and lighting, consumption.)

• Collective action (encourage friends and neighbors to do the same, elect politicians who will make the needed policy changes, work for broad institutional change.)

Shifting the world's economy away from its dependence on fossil fuels is a huge challenge that won't be accomplished by any one change. It will require actions big and small by individuals, corporations and governments around the world. Sometimes people feel like nothing they do matters, but I like to say that the only thing that matters is what people do.

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Is there any credence to the concept that tropospheric warming may be partly due to oceanic warming and not the reverse? For instance is atmospheric reradiation in the IR (infrared) deeply penetrating or as deeply penetrating as UVB (ultraviolet)?
— N. Finley

Wallace: The temperature of the atmosphere and ocean are so strongly coupled that it's not strictly correct to say that one of them is causing the other to warm. Both are responding to changes in the Earth's energy balance due to the buildup of greenhouse gases in the atmosphere. I am not aware of any processes taking place within the ocean that could alter the global energy balance in a significant way. The ocean can cause atmospheric temperatures to change locally, as they do during El Nino in the equatorial Pacific.

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What if just more human beings are causing global warming? I remember going to a cold unheated dance hall and by the time all the people arrived the temperature was nice and warm. Also, my computer room is 5 degrees warmer then my living room, approximately the same square footage.
— Gloria Anderson, Lynnwood

Wallace: The waste heat put into the atmosphere by humans is negligible compared to the large amount of the sun's energy absorbed by the earth and emitted back to space as infrared (or heat) radiation. Our greenhouse gases are much more important than our waste heat, because by blocking the outgoing radiation they cause the Earth to warm.

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Brazil by law requires the manufacture and use of biofuels, and cars built to burn these fuels are common, since 1979. GM actually builds these cars in Brazil. Why does the U.S., the largest consumer of fossil fuels, still refuse to change? This will not solve all our problems, but its a large step in the right direction.
— Al, Seattle

Snover: Biofuels are a promising alternative to gasoline for powering cars. Although the greenhouse gas emissions associated with using biofuels would be less than the emissions from using gasoline in your car, biofuels still come with a greenhouse gas "cost." This is because it takes energy to produce them. The pesticides and fertilizers used to grow soybeans (often used as a source for biodiesel), for example, require significant amounts of energy (now almost all fossil fuel-derived energy) to produce and transport. As we make choices about the types of alternative energies we want to promote (or require, as you suggest), it will be important to think through the whole process associated with each alternative to make sure we make good decisions. In any case, options that are available today, like biofuels, provide good opportunities for lessening dependence on fossil fuels.

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Why are the concerned scientists obsessing on carbon dioxide and ignoring other aspects of the problem? The upward trend on the hockey stick curve starts also at about the time of the introduction of steam and internal combustion engines. While these do emit carbon dioxide, they also dump billions of BTUs of waste thermal energy into the atmosphere and the oceans. Why not some support for finding non-entropic heat engines? In an isentropic expansion half of the "expended" thermal energy produces mechanical energy and the other half expands the working fluid and is then dumped either directly into the atmosphere or through a condenser. Work obtained through an isentropic compression would not involve ending the process with an expanded fluid. I have been trying to tell people this for over thirty years but no one will listen.
— Patrick McNenny

Wallace: The waste heat that humans and their machinery dump into the atmosphere is very small in comparison to the huge amounts of solar radiation absorbed by the earth and infrared radiation emitted by the Earth. Hence our waste heat is a negligible part of the Earth's energy balance. The small amount of heat that we do put into the climate system is quickly emitted away as infra-red radiation. Greenhouse gases are much more important because they block the infrared radiation emitted from the Earth's surface.

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Is global warming reversible?
— Tammy, Seattle

Wettstein: Global warming is indeed reversible, at least in the sense that we could, eventually, bring global greenhouse gas emissions (the "human" part of global warming/climate change) back down. The issue here is a matter of how long you are willing to wait. By way of example, every molecule of carbon dioxide (the primary human-caused greenhouse gas in terms of its influence on climate) released today will be in the air for several hundred years. So, if we are already having a demonstrable impact on climate as the science suggests, slowing greenhouse gas growth and eventually reducing greenhouse gas emissions (and thereby turning around the human forces of global warming) is a pretty major challenge for every world economy. This and other information is summarized in the IPCC summary for policymakers: http://www.ipcc.ch/pub/spm22-01.pdf

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So, just what can an ordinary person do to help stop this problem?
— Marla, Seattle

Snover: Because global warming is caused by emissions of greenhouse gases, anything a person does that reduces those emissions will help counteract global warming. In your personal life, driving less, carpooling and switching to a vehicle that gets more miles to the gallon are all ways you can reduce the emissions resulting from burning gas in your car. Buying food grown locally would reduce the emissions that result from transporting food long distances. Decreasing energy use at home could help too. Beyond changing your own consumption and energy-use habits, you could encourage others to do so, too. Friends, family, elected officials. Transportation, energy use and other policies at the local, state and federal levels will all influence how much greenhouse gases are emitted. Some corporations are beginning to act to "offset" (or reduce) their contribution to global warming. This is voluntary in the U.S., but mandatory in Europe. You might think about considering the way a corporation is addressing global warming when making investment choices or electing politicians who favor making such limits mandatory in the U.S.

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What about the possibility that the migration of people from the continental heartlands to the coastlines contributes to global warming as the balance between coastline and inland heat is thrown off and in some unknown way increases ocean warming? I do understand that warming is the same in both urban and non-urban areas. Specifically, as populations have moved to the coastlines dramatically in recent years, could this shift in the location of the populations affect global warming? Pollution has long been with us but what is different today than just a few years ago is where the populations around the world are congregating, namely at the coasts. Urban and rural warming and the location of that warming are not the same. Thank you.
— Linda Neilsen, Kent

Wallace: Scientists have been looking carefully at the pattern of the warming, but they haven't seen any evidence that the warming is any greater on the coasts than it is in the interior of the continents. In fact, the interior of continents is expected to heat up more rapidly than the coasts, just as they do with the daily temperature cycle between day and night.

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Why does The Times ignore the fact that Greenland, Iceland and the Antarctic Continent are cooling? The land-based glacial ice in these 3 areas constitute 96 percent of the total land-based ice on the planet. Why does the Times insist that "consensus" automatically means "truth"? In science, those who refer to "consensus" usually have a hidden agenda. Since there is no technology that exists today to reduce the amount of atmospheric aerosols, what do you propose we do about it?
— J. R. Leicester, P.E., Shoreline

Wallace: When people use the term "global warming" they mean the temperature averaged over the surface of the earth is getting warmer. That doesn't mean to imply that there is warming everywhere on Earth.

Greenland, Iceland and Antarctica have cooled very slightly over the past few decades, in contrast to most areas of the Earth, which have experienced warming. Temperature trends over the Earth's surface are not uniform because, in addition to the warming, there have also been changes in circulation patterns, like El Nino and the Arctic Oscillation and the Pacific Decadal Oscillation. Changes in these patterns accelerate the warming in some areas and retard it in others.

Two areas where the warming has been accelerated are Siberia and the Antarctic Peninsula, which extends from the Antarctic continent toward South America. The Antarctic Peninsula is the place where warming is causing the collapse of some of the ice shelves.

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I live where the weather is cold, wet and dreary. Might global warming actually benefit us by bringing warmer, sunnier, and drier days?
— Kurtis Jones, Seattle

Wallace: Most cold, dreary places will benefit from global warming. However it's conceivable that places like Western Washington, which are cool during summer because of the sea breeze, will get even cooler if the seabreeze gets stronger. If cool, dreary places depend on winter snow pack for their water supply, as we do in Seattle, there could be some problems.

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Are any scientists looking at ways to absorb carbon dioxide from the air as an alternative to reducing carbon dioxide emissions from human activity?
— Don Schaechtel, Seattle

Wallace: Two ways of absorbing (or sequestering) carbon dioxide that would otherwise build up in the air are to incorporate it into logs by planting forests and to pipe it deep into the oceans or geological reservoirs, under high pressure. Planting forests can take up only a limited amount of CO2, and it's only a temporary solution, because the carbon in logs will eventually return to the atmosphere when the logs decay or burn. Deeper burial is much more expensive, but it's the only long term solution.

Personally I favor reducing our dependence on fossil fuels not only to slow the rate of greenhouse warming, but also to preserve what's left of the world's precious petroleum reserves for our grandchildren and their grandchildren and to limit the environmentally destructive mining of coal.

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I learned that water expands (fills up more space) when it freezes. Conversely, doesn't water contract when it melts? Aren't scientists forgetting this when they preach impending doom about the coming Waterworld?
— J. Adam Pang, Ellensburg

Wallace: The melting of sea ice has no appreciable effect on sea level. However when the continental ice sheets melt, water flows into the ocean and sea level rises. If the Greenland ice sheet were to melt entirely, sea-level would rise by about 15 feet and if the Antarctic ice sheet were to melt completely sea level would rise by about 150 feet. (Scientists are not predicting that this will actually happen.) The warming of ocean water also causes sea level to rise because sea water expands when it warms.

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What are some ways an average person can decrease their contribution to global warming?
— Andrea Liggett, Redmond

Wallace: You can reduce your contribution to global warming by eliminating unnecessary driving and using an energy efficient vehicle when you do drive, by insulating your home and installing double pane glass windows, by setting your home thermostat as low as comfort allows, by exploiting solar and/or wind energy for home use, by buying energy-efficient appliances and lighting, and by eating locally grown, organic food products which require less energy to grow and transport.

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Recent imaging from the Mars Global Surveyor shows a rapid depletion of glaciers and ice fields on the Martian surface. Analysis of this data shows this to have taken place over the last several hundred years. (Aviation Week and Space Technology Oct. 3, 2005) What is the likelihood of a common cause for the warming of the Earth and Mars?
— Pat, Federal Way

Wallace: I wasn't able to find the article that you're referring to, but here's my understanding of the status of what we know about Mars that relates to your question. The polar ice caps on Mars are composed of frozen carbon dioxide (dry ice). They are shallow features that expand and contract with the seasons. There are no glaciers or ice fields on the surface of Mars, at least at present. Whether there might be water hidden below the surface of Mars is a matter of speculation. There are gullies in the rocks on the surface of Mars that look like they might have been caused by liquid water running downhill. However, it is my understanding that these are ancient features whose origin is a matter of debate. The Mars Global Surveyor has been orbiting Mars for 8 years. That's a long time, but not long enough to document changes on Mars over hundreds of years.

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I understand that grapes were growing in parts Norway during the Medieval Warm Period where it is currently too cold to grow grapes today. If it is warmer today than in the past why is it currently too cold to grow grapes in those same areas of Norway?
— Janice Mehringer

Wallace: It's possible that the Vikings were making wine from Concord-like grapes, which can grow in relatively cold climates. I've tried it back in my wine-making days, and it was drinkable. (They would have had to use honey as a sweetener.) But, like you, I'm open to the idea that northern Europe might have been as warm at the peak of the Medieval Warm Period as it is today. Does that make me a global warming skeptic? Definitely not. Whether this is or isn't the warmest century in human history is an interesting question, but it has little bearing on the way I think about greenhouse warming. At the heart of my concerns are the rapid changes that we can see taking place right now and the highly credible estimates that the Earth's mean surface temperature will rise by something like another 5 degrees F by the end of this century if we don't find a way of substantially reducing greenhouse gas emissions soon.

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I have been under the impression that the MIT Global Change Joint Program is the primary organization gathering and disseminating information on weather change. http://web.mit.edu/globalchange/www/. Yet, I rarely see it mentioned as an information source by those scientists forecasting a disastrous future-world due to human enabled weather change. Is it a primary information source for serious scientists? Is there a split between the MIT group and others studying global warming? What's your opinion of the scientists who run that program?
— Steve Ginn, Kirkland

Wallace: I know Ron Prinn, director of the MIT project, pretty well and have a lot of respect for him. I'm of the opinion that he shares the views expressed in Sandi Doughton's articles. However, his project is only a small part of the U.S. research effort on greenhouse warming. That may be why you haven't heard more about it in the newspapers. The person at MIT who is the skeptic about greenhouse warming is Richard Lindzen. Type his name into Google and you can find out a lot more about his views.

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Why does the Law Dome Antarctica Ice Core show that atmospheric CO2 began a significant increase around 1750, a century before significant fossil fuel burning began during the Industrial Revolution?
— Ken Schlichte, Tumwater

Wallace: You can see the ice core curves for yourself by typing "co2 ice core" into Google and choose "images". Most ice cores show an increase beginning around the middle of the 1800s. A few may show slight wiggles before then, but I wouldn't pay much attention to such small fluctuations. It isn't really until the middle of the 1800s that you see things getting beyond the levels where you can really say there's a change.

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I'm curious if you share the view of reporter Sandi Doughton inherent in her Times article, "The truth about global warming," that scientific fact is determined by majority vote? It seems to be the only consistent criterion considered.
— Doug Parris, Shoreline

Wallace: Truth is what it is, regardless of what scientists think. But if we want to know the truth about something, doesn't it make sense to ask the people who study it, and to see whether they agree among themselves? Wouldn't you be more convinced that a certain medicine was going to help you if all the doctors that you asked about it agreed that it was good than if some thought it was good and others thought it was a waste of money?

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How can scientists be sure that the warming that has occurred during this century isn't a "normal" occurrence, especially since the last ice age ended only 12,000 years ago? How do they know that such a warming did not occur 5,000 years ago or 100,000 years ago? Or a million years ago?
— Robin Heflin, Camano Island

Wallace: Cores drilled in the Greenland and Antarctic ice caps and in sediments at many sites in the bottom of the ocean provide a detailed and consistent history of the temperature and carbon dioxide levels over the past 500,000 years. These records document a remarkable series of ice ages, the last of which was at its peak 20,000 years ago. There was a brief return to colder conditions in Europe about 12,000 years ago. This event is called the "Younger Dryas". Since around 11,000 years ago the ups and downs in the temperature curve have been much smaller. There's also geological evidence and evidence based on carbon dating that supports this scenario.

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In what ways has global warming affected the Pacific Northwest?
— Josh, Lynnwood

Wallace: The warming of the past few decades has raised the elevation at which snow falls in the mountains, reducing winter snow pack, which supplies water for many cities and towns in the Pacific Northwest. If you're interested in learning more about this, type "philip mote snow pack" into Google and you'll find lots of news stories. Other effects are more subtle and controversial. Some scientists think that the recent warming has made forests more prone to fires and insect infestations and that it has reduced the rate of survival of salmon returning to spawn in streams.

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I am compiling research on global warming for a geography project and was wondering if you thought that global warming was the main catalyst behind Katrina? Do you think this may have dramatically changed people's views about signing the Kyoto Protocol?
— Deborah Sneddon, Glasgow, United Kingdom

Wallace: I don't think we can say that an individual storm like Katrina would have not developed had it not been for greenhouse warming. The most we can say is that storms as powerful as Katrina are likely to occur a bit more frequently in a warmer world. I think you're right that witnessing what the damage these Category 4 and 5 storms can do may cause some people to rethink their views about the Kyoto protocol. It is also likely to cause the insurance companies to raise their premiums.

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I heat with wood. I once read that the same gases are given off, be it high temperature oxidation (in my high efficiency wood stove) or slow oxidation (rotting out in my woods). True statement or not?
— Dennis Coons, Snohomish

Wallace: True, and burning wood for fuel in your wood stove does not contribute to greenhouse warming because trees grow back on a time scale of a few decades regardless of whether they rot or you burn them in your stove. It's only the burning of non-renewable fuels like oil and coal that contributes to greenhouse warming.

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The natural environment changes over time. There have been ice ages and warmings. Why would 50 years of activity make any difference to me? Isn't the global warming argument mainly created as a vehicle to forward radical environmental laws?
— Dale Smith, Sammamish

Wallace: There weren't many people around to experience the coolings and warmings of the ice ages and those who were living at the time didn't leave any written records. However, the proxy evidence, like isotopes in sediments, indicate that the major warming and cooling events, like the recovery from the most recent ice age which peaked 20,000 years ago, took place over thousands of years. In contrast, the greenhouse warming story is unfolding on a time scale of decades to hundreds of years. My belief in greenhouse warming has nothing to do with my political persuasion. Believe me, if I thought I had the evidence to disprove it, I'd be racing to be the first one to get it published!

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Has there been a study on the effects of the reduction in "green" — trees/plants — in relationship to increase in carbon dioxide? What correlation is there between this reduction in green and global warming - i.e. less green increases global warming?
— Alan, Seattle

Wallace: Yes, deforestation (for example, in the tropics) has accelerated the buildup of atmospheric CO2, and, by the same token, reforestation, as has gone on in parts of the United States, tends to retard it. However, compared with fossil fuel burning, the exchange of carbon between the atmosphere and plants from one decade to the next is pretty small. Another way to think about it is that there's a lot less carbon stored in plants than there is in fossil fuels.

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We have heard the negative arguments as to global warming. Now, in reading the N.Y. Times this morning a series of articles advocating some of the positive points of global warming are coming forth. I am well aware of the negative points and agree with them. However, what are the positive points and how will areas as Seattle, Boston, N.Y., etc. deal with rising oceans?
— Ed Jenkins, Seattle

Wallace: Some positive points are longer growing seasons and greater agricultural productivity in high latitude countries like Canada and Russia, smaller home heating bills, fewer hassles with icy roads.

Barring a precipitous disintegration of the continental ice sheets, the buildup of sea-level will be so gradual that cities like Seattle and New York will be able to adjust without a great deal of hardship. It's the low lying coastlines like those in Louisiana and Bangladesh and on coral atolls that will bear the brunt of the damage.

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If, by some miracle, every CO2-producing machine in the world (not including biological) were to suddenly stop working, how long would it theoretically take to reduce the rate of change in global warming?
— Liora Van Natta, Seattle

Wallace: Good question. If we were to stop fossil fuel burning altogether, atmospheric CO2 concentrations would start to drop off right away, but it would take much longer for them to return to normal (preindustrial) levels than they did to build up. Model calculations indicate that if all fossil fuels on earth were burned, it would take a few thousand years for atmospheric concentrations to drop halfway back to normal, and much longer than that to get nearly all the way back to normal. The uptake of CO2 from the atmosphere is slow because it depends on geologic processes.

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Mike, since infrared greenhouse radiation cannot penetrate into the oceans (beyond a "skin" of thickness 10 microns)**, just how does the greenhouse effect warm the ocean?

(**from physical optics theory and actual measurements)
— Fred Singer, Arlington, Va

Wallace: Fred, I knew you'd ask a hard question. I'm not accustomed to thinking of the radiative fluxes in isolation. I think of the atmosphere and ocean as a coupled system in which the radiative fluxes act in combination with the latent and sensible heat fluxes. To determine whether your question really has any merit I would need to consult with someone with more of a modeling orientation.

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