SOMETIMES, the world seems more like science fiction than science fiction itself.

Pat Murphy had that experience recently when she worked on a project for the Exploratorium¹, the science museum where she and Paul Doherty work. With a grant from the National Science Foundation, the Exploratorium created a Web site designed to let members of the general public gain access to some of the tools that scientific researchers use to understand how the Earth's climate is changing.

As a science fiction writer and someone with an interest in the environment, Pat had a general awareness of the increase in atmospheric carbon dioxide and the effects of that increase on the climate of our planet. But pulling together the Exploratorium's Web site (www.exploratorium.edu/climate) gave her a whole new view of the problem.

At the Exploratorium's Global Climate Change Research Explorer, you can monitor today's sea surface temperatures, taken from satellite measurements of microwave energy emitted by ocean waters. You can find out today's coral hot spots—where coral reefs are experiencing stress and possibly dying from elevated water temperatures. You can see, in near real time, the extent of sea ice around the Antarctic continent. You can tap into a satellite view of wildfires in Central America.

Each item, on its own, is interesting. Put them all together, and you get something that is more than interesting—it's terrifying.

The very best science fiction has a way of changing our view of the world around us. As science fiction readers, we are accustomed to contemplating situations where life as we know it may cease to exist. Excellent science fiction has been written about drastic changes in the Earth's climate. (Fritz Leiber's story, "A Pail of Air," comes to mind, and Bruce Sterling's novel, Heavy Weather.)

It's interesting to read fiction about drastic changes on our planet. Unfortunately, the changing state of the Earth's climate is not fictional. We'll warn you up front: this won't be one of our cheery, upbeat, isn't-science-fun sort of columns. Science is fun, but not all the things that science makes possible or reveals are fun.

Analyzing and understanding the Earth's climate is complex. It involves multiple disciplines. In this column, we're going to talk about oak trees and cherry blossoms, ocean currents and the concerns of the folks who live on the Maldive Islands, tornadoes in the Midwest, and drought in Australia.

We're also going to point out up front that assessing a global climate change is not a simple thing to do. Paul likes to tell people "counting is difficult." Children are always fed easy problems like "how many apples are in this picture?" Real world counting problems are much messier. Ask someone to count how many trees are there on a particular acre of land, and you'll get different answers. (Is that a tree or a bush? Does this tree on the border count as a tree, half a tree, or no tree at all?)

Measuring things like temperature can be even tougher than counting things—particularly when you are looking for small changes over a long time taking place over the area of a planet in a system as chaotic as weather. Local changes—the growth of a nearby tree that provides shade and evaporative cooling, the paving over of an area that used to be lawn—can have a significant effect on the measured temperature at a particular weather station. It's very hard to extract the global change from the local change.

Even so, there are ways to measure trends. They are unorthodox ways to be sure, but we are confident that you, as science fiction readers, can handle an unorthodox approach.

SPRING IS IN THE AIR

Let's start with a British landowner named Robert Marsham. Back in 1736, Marsham began recording when certain indications of spring occurred on his family estate in Norfolk County, England. He noted when the first wood anemones flowered, when the oaks came into leaf, when the rooks began nesting, when certain birds returned. For the next 211 years, members of the Marsham family kept on recording the dates of these and twenty or so other natural events each year.

In 1947, Jean Combes, an observer in Surrey, England, started noting the timing of certain natural events—including the date when oaks came into leaf. She kept her records through to the present day—and is still keeping them.

Meanwhile, several thousand miles away, the people of the town of Nenana, Alaska, were celebrating spring in their own special way. Starting in 1917, the inhabitants of Nenana would raise a wooden tripod on the frozen Tanana River each year and bet on the exact minute in spring that the tripod would fall through the melting ice. Records of the contest (which is still going on and currently offers prize money of over $300,000) provide a very precisely recorded and consistent record of the time of ice melt each year.

What do we learn from this (other than that some Brits have too much time on their hands and many Alaskans like to bet)? Well, analysis of the leafing times of trees and the melt of the Alaskan ice indicates a trend: spring is arriving earlier than it used to.

How much earlier is hard to say. It depends on what indicators you're using and where you are. Horse chestnut trees get their leaves twelve days earlier than they did back when Jean Combes started keeping records; oaks, ten days earlier; ash, six days earlier. Analysis by ecologists at Stanford University shows that ice melt on Alaska's Tanana River has, on average, advanced by five and a half days relative to the time of spring equinox since 1917.

Ecologists using a variety of indicators have come to the same conclusion: Washington, D.C.'s famed cherry trees are blooming earlier; migrating birds are returning to the Midwest earlier; hibernating animals in the Rockies are emerging earlier. Recording the timing of natural events is known as phenology. Records kept by beekeepers and birdwatchers and gardeners and many other amateur naturalists are proving valuable to ecologists interested in tracking climate change. (In fact, if you find your great grandmother's gardening journal in the attic, don't toss it. Let us know and we'll try to track down an ecologist who wants the data.)

Ecologists are continuing to gather phenological data, making use of school groups and volunteers all over the world. If you are interested in helping, check the Phenology Networks Home Page (http://www.uwm.edu/~mds/markph.html) to see if there's a project that includes your area.

Changes like the ones noted above confirm the assertion in a 2001 report by the Intergovernmental Panel on Climate Change (IPCC), a group established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). The IPCC reports that the average surface temperature of the Earth has increased during the twentieth century by about 0.6° ± 0.2°C. (That ± 0.2°C means that the increase might be as small as 0.4°C or as great as 0.8°C.) (The science teacher in Paul loves the inclusion of the error estimate, the scientist in him cries out for even more information on how the number was arrived at, but we don't have space for that here. You can find out more—much much more—by reading the IPCC report (www.ipcc.ch/))

Error estimate or no, that temperature increase may not sound like much. Hang on, we'll get back to that. First, we'll tell you about what scientists learned at the top of a volcano in Hawaii.

THAT PESKY CARBON DIOXIDE

Take a look at the graph below. This is the Keeling Curve, which shows changes in the atmospheric concentration of carbon dioxide from 1958 to 2000, measured in a remote lava field near Hawaii's lofty Mauna Loa volcano.

Charles D. Keeling, of the Scripps Institute of Oceanography, says that when he started making these measurements, he expected carbon dioxide concentrations to be constant—but he learned otherwise. First he found an annual cycle. Carbon dioxide concentrations decrease in the Northern Hemisphere in the summer and rise in the winter, reflecting the activity of plants in the Northern Hemisphere, which absorb carbon dioxide during their growing period, then release it in the wintertime.

Monthly Average Carbon Dioxide Concentration (click for a really big version)

The other thing that's obvious in the graph is the steady rise in carbon dioxide—a seventeen percent increase in carbon dioxide concentrations from 1959 (about 316 parts per million by volume) to 2000 (about 369 ppmv). And carbon dioxide levels are continuing to rise—as we continue to burn fossil fuels in our gas tanks and our factories. And there's every reason to believe that carbon dioxide levels will continue to rise. President George W. Bush has refused to agree to the Kyoto Protocol, a United Nations effort to reduce the amount of greenhouse gases emitted by developed countries. Since the U.S. contributes about one-fourth of the world's total greenhouse gas emissions annually, an emissions reduction effort can't really succeed without U.S. participation.

Why does the concentration of carbon dioxide in the atmosphere matter? Well, carbon dioxide helps keep our planet warm—in this case, too warm.

You've probably heard the term "greenhouse effect" and read descriptions that say the carbon dioxide acts like the glass in a greenhouse, letting in light and trapping the heat. That's the general idea—but the greenhouse effect in the Earth's atmosphere is not what happens in a greenhouse (or a car that's parked in the Sun).

In a greenhouse, sunlight enters and is absorbed by the ground, which warms up. The warm ground then heats the nearby air. The roof of the greenhouse prevents this warm air from rising and leaving the greenhouse. So the air inside the greenhouse becomes hotter than the air outside.

In the Earth's atmosphere, it's not quite that simple. In the atmospheric greenhouse effect, visible, infrared, and ultraviolet light from the Sun penetrate the transparent atmosphere and are absorbed by the ground or by the ocean. The ground or water then radiates energy back into space in the form of longer wavelength infrared light. Certain molecules—like carbon dioxide—absorb this infrared light. These molecules then reradiate this energy—again as infrared light. The molecules emit infrared in random directions. Some of the absorbed radiation is radiated out toward space, but some is reradiated back toward the ground. The effect of all this is to make the ground under an atmosphere full of carbon dioxide warmer than the ground beneath an atmosphere with less carbon dioxide.

It's a good thing for us that the atmospheric greenhouse effect exists. If there were no greenhouse effect, the Earth would lose more heat, and the Earth's average temperature would stabilize at about minus 18°C.

Unfortunately, as the concentration of carbon dioxide increases, the greenhouse effect increases as well—which has an effect on the Earth's climate. This effect is usually summed up as "global warming," but it's not as simple as that. Shifts in the climate mean warming in some areas—and other consequences elsewhere.

WHY THE VIKINGS LEFT GREENLAND AND OTHER STORIES

Pat has heard folks dismiss global warming. "Who cares if the temperature goes up a degree or two—or even four or five?" these people say. "I like warm weather."

But turning up the heat on the planet Earth is not like turning up the thermostat in your house by a degree or two. The Earth's climate is an amazingly complex system. Being a savvy science fiction reader, you've probably heard of the butterfly effect. A hot topic in discussions of chaos theory, the popular description of the butterfly effect suggests that a butterfly flapping its wings in China can set processes in motion that lead to a tornado in Kansas. There's a lot more to it than that—but the takeaway idea is this: in a chaotic system (like the Earth's climate), tiny changes can have enormous consequences.

Global warming isn't a tidy, uniform sort of thing. Just because the planet as a whole warms up, doesn't mean your particular area will be warmer. And the consequences of small temperature increases in certain areas may well be catastrophic.

Consider, for example, the ocean, which plays a vital role in regulating climate. Water has a great capacity to absorb and store heat energy. Because of this, ocean currents can transport heat energy from one part of the planet to the other.

The Gulf Stream is a current that takes warm water from the tropics and brings it north to the east coast of North America and then on to Europe. Water from this warm current evaporates and warms the air, giving northwestern Europe a milder climate than Canada at the same latitude.

You might think that warming up the tropics would just make the Gulf Stream warmer—but it's not as simple as that. (Of course not!) The action that propels the Gulf Stream and other ocean currents comes from simple physics: when warm Gulf Stream water evaporates up by Europe, the remaining water becomes colder and saltier—which makes the water denser. Because it's denser, this water sinks—and warmer surface water flows in to replace it. That simple action keeps the current flowing.

How would climate change mess up this nice process—which has been carrying on placidly at least a few centuries? Well, the extra heat is melting ice in the Arctic Ocean (and incidentally threatening the life style of polar bears that hunt on the pack ice (www.newscientist.com/hottopics/climate)). When the Arctic ice melts, it becomes fresh water, which flows into the salty North Atlantic. And here's the problem: that fresh water may dilute the salty current of the Gulf Stream so much that it stops sinking and stops the flow of the Gulf Stream. If that happens, Europe would freeze as a result of global warming.

Some researchers blame Europe's "Little Ice Age," the cold snap that lasted from 1300 to 1800, on just such a slowdown in the Gulf Stream. Incidentally, scholars studying the rise and fall of Viking civilization link the abandonment of settlements in Greenland and Iceland to that climate shift. But we digress.

So while you're thinking about the ocean, think about sea level. Members of the Alliance of Small Island States, a coalition of small island and low-lying coastal countries, are more than a little upset about climate change. That's because increasing global temperatures cause glaciers and polar ice to melt and sea water to expand. (Warm water takes up significantly more volume than cold water.) And so, as the planet's temperature increases, the sea level rises. How much does it rise? Geological evidence indicates that sea levels have risen by ten to fifteen centimeters (about the width of your fist) over the past one hundred years.

How much more is it likely to rise? Hard to say exactly. One recent estimate in a report from the IPCC says that sea level may rise between .09 and .88 meters over the next 100 years. Bad news for the Maldive Islands in the Indian Ocean, which have a mean height of one meter above sea level. If the sea level rise is at the high end of the estimate, their whole country is pretty much gone.

And there's more to say about those melting glaciers and polar ice caps. The Earth warms up by absorbing heat energy from sunlight. Ice and snow are particularly good at reflecting light. When they melt, they expose dark underlying surfaces—dirt and rock—which absorb more heat. So melting ice leads to more absorption of heat, which leads to more melting, and so on in a positive feedback loop that boosts the warming trend even further. Some researchers surmise that such an effect was at work in the Cretaceous Period (that's 120–65 million years ago—think dinosaurs), when there was little or no snow and ice cover and global temperatures then were at least eight to ten degrees C higher than they are now.

For those of you who are still imagining basking on balmy beaches on the new coastline (wherever that may be), we'll mention another predicted consequence of global warming: an increase in what meteorologists call "severe weather events." That means hurricanes, tornadoes, extreme heat waves or cold snaps, and the like.

Like the circulation of the Earth's oceans, circulation of the atmosphere is strongly influenced by temperature difference around the globe. Shifts in temperature can change patterns of atmospheric circulation—and modify patterns of rainfall. Higher temperatures mean that the air can hold more water vapor—and changes in atmospheric circulation mean that water may fall as rain and snow in places it usually doesn't, so that some areas experience flooding and others drought.

In July 2003, the World Meteorological Organization, an organization that normally produces detailed scientific reports and staid statistics at the year's end, reported that the world is experiencing record numbers of extreme weather events such as droughts and tornadoes.

The WMO noted that the U.S. experienced 562 tornadoes in May 2003, a record for any month. (The previous record was 399 in June 1992.) In 2002, much of Australia was hit by the longest drought in recorded history, which devastated crop yields and sparked continual bushfires. At the same time, many parts of China and East Asia were hit by severe flooding. The year 2003 is a hot contender for the title of the hottest year ever recorded. The ten hottest years in the 143-year-old global temperature record have now all been since 1990, with the three hottest being 1998, 2002 and 2001.

No one example cited by the WMO is remarkable, taken on its own. But considered together, the WMO notes, these events and records represent a trend toward weather extremes.

TRENDS, UNCERTAINTIES, AND WHAT TO DO NOW

The issue of global warming has received some attention from the news media—but not as much as it deserves. There are a couple of reasons for this. Bruce Sterling, author of the aforementioned Heavy Weather and founder of the Viridian movement, has compared our dependency on fossil fuels and the "chronic, creeping" change of global warming to alcoholism: "It isn't one moment or one single drink that does you in. Can there be a single 'ah-ha moment' when you realize that civilization has moved from social drinking (of oil and coal) to a substance-dependent blackout situation?" The very slow nature of the change makes the calamity a difficult story to cover.

The other thing that makes global warming a difficult news story is the very complexity of the Earth's climate system. Some of the effects of climate change don't seem to fit with a rise in temperature.

Take, for example, the changes in the world's glaciers. Last time Paul was in Ecuador, he decided to climb Chimborazo to get to the point on the Earth's surface furthest from the center of the Earth (a physicist's high point versus the geographer's Mt. Everest measured from sea level.) But Paul couldn't complete the climb: he had to turn back. The route commonly used by climbers was impassible, partly because Chimborazo's glaciers were crumbling. Across Europe, Asia, and North and South America, almost every glacier is retreating. Some, like Maclure Glacier (the first discovered in California) have disappeared altogether.

So that's simple enough, you say. But hold on one minute. Back in 2001, Paul visited Antarctica's Dry Valley. (Hey, the guy gets around.) There, the glaciers are advancing. Researchers say this is another effect of global warming. The ice has warmed (though it's still minus 17 °C). The warmer ice is more fluid than colder ice and so flows more easily downhill, expanding the area of the Antarctic glaciers.

We told you it was complicated! And in a complicated system, when trying to figure out what's likely to happen, scientists (and science fiction writers) look for patterns and trends. You can't predict what will happen, but you can indicate that one event is more likely than another. In this particular system, the trend looks very bad indeed. We don't know for certain that human-generated carbon dioxide is contributing to this trouble, but it sure looks likely.

What can you do about it? Let's see—on the political front, you can lobby the Bush administration to ratify the Kyoto Protocol. On the personal front, you can cut back on your own energy use—drive less; use a more fuel-efficient car; buy energy-saving appliances. You can help out directly by downloading a screensaver that makes your computer part of a distributed computing network that runs climate prediction models (www.climateprediction.net). You can plant a tree or tear up your driveway and put in a natural garden. (Plants absorb carbon dioxide, removing it from the atmosphere.) You can join Bruce Sterling's Viridian movement. You can support alternate energy sources, like wind power or solar power or even nuclear power. But more than anything else, you can become aware of the problem, probably the defining problem of our century.

To learn more about Pat Murphy's science fiction writing, visit her web site at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.