The first and key role of science is to identify the problem. As those who have struggled to increase public awareness of global warming know only too well, identifying a problem is not the same thing as recognizing it. Science's role in this regard is that of watchdog, to sound an alarm when scientific data suggest a serious problem may have arisen. Let's look at our four cases of chemical pollution to see how this works:

OZONE DEPLETION. The impact of CFCs on ozone became great enough to begin the ozone hole in 1975, as judged by satellite photos. It was first noticed by atmospheric researchers in Antarctica four years later, in 1979. Within four years after that, by 1983, 180 countries had signed an international agreement to phase out the manufacture of CFCs. Not bad. Four years for scientists to identify the problem and four years to recognize and address it, NGOs and policy planners goading government to action.

ACID PRECIPITATION. The first tall stacks were built in the mid-1950s. The impact of acid rain and snow on forests and lakes was first reported in the early 1970s and became impossible to ignore by 1980. In the United States, the problem was addressed by the Clean Air Act revisions of 1990 mandating abatement (basically, scrubbers on the stacks to remove the SO2). So, eight years to identify the problem and 10 more years to recognize and address it.

GLOBAL WARMING. Starting in the 1980s, a sharp rise was noted by geoscientists in global mean temperature; looking at past records, it immediately became clear that the rise represented an acceleration of a general increase since the 1950s. It was not long before parallel increases in atmospheric carbon dioxide levels were noted. UCSD scientist Charles Keeling reported in 1955 that levels of COs were rising in air samples each year, and were now 30 percent higher than in pre-industrial times. The possibility of a relationship leaps out when you compare the two sets of data: if you plot CO2 concentration on a graph vs. global mean temperature, you get a nice line -- the two variables are highly correlated. Responding to this, many scientists argued on the grounds of precautionary principle that governments worldwide should immediately institute CO2 emission caps.

But -- and this is one of the most sticky points of science -- correlation is not causation. Because two things tend to occur at the same time does not prove that one causes the other. The proof came a decade ago, when a British statistician showed that the variance (a measure of the amount of random scatter in data) in annual global mean temperature is strongly correlated with the variance in atmospheric CO2 levels -- because scatter is random, this can only be true if the two variables are indeed entrained, one directly linked to the other. Any scientific controversy ceased with this report: Increasing CO2 is causing global warming.

So, 10 years for scientists to clearly identify the problem. As of today, 12 years after this identification, the world is still in the process of recognizing and addressing the problem, although with international protocols, last year's Nobel Prize, and the recent action in the U.S. Senate, we seem well on our way. Although China and India, with the U.S. the world's principle releasers of CO2, are not yet taking action, there is reason to hope they will soon begin to move in this direction.

BREAST CANCER. While bisphenol A was linked to estrogen levels in human breast cells in 1993, the link to breast cancer was not established until 2006, and is not yet firm. Much additional research will be required before scientists are sure. Until then, the identification of this problem must be considered preliminary, and its recognition by countries like Canada precautionary.

ways to solve the problem

The second role of science is to provide ways to solve the problem. So, as we confront planetary climate change, I think it's important to be optimistic. The general pattern we see in these four instances convinces me that global warming is a solvable problem -- while more complex, not intrinsically different from the other three. With ozone, the solution was relatively simple, a safe chemical substitute for CFCs and legislation to mandate the change. With BPA the solution can be anticipated to be much the same, also a straightforward chemical substitution. With acid rain, the solution was again direct, scrubbers to remove the SO2 from coal smoke before it is released into the atmosphere, and clean air legislation to mandate their use.

Global warming demands a more complex solution: Basically we need to shift to alternative energy sources that do not increase the atmosphere's carbon load. The mix will certainly include nuclear, solar and wind power. It must not include the burning of coal, no matter that coal may plentiful, as coal is simply a hard form of fossil fuel and its burning adds CO2 directly to the atmosphere just as burning petroleum does.

As automobiles are an important contributor to atmospheric CO2 pollution, it will be important to find a carbon-neutral substitute for gasoline as soon as possible. While solar, electric and hydrogen powered cars are all being explored, I feel strongly that a transitional solution will be required, and that biofuels like ethanol and methanol offer a very promising possibility. In this regard, it is important to understand two things:

1. Burning biofuels does not add extra carbon dioxide to the atmosphere. Burning ethanol obtained by fermenting plant tissue simply releases into the atmosphere CO2 just removed from it by photosynthesis; there is no net increase in CO2.
2. Biofuels have nothing to do with food. Or at least they certainly should not. Corn kernels, rich in starch, are being used in the United States as the principle source of ethanol. A starch molecule is a long chain of glucose (a 6-carbon sugar) molecules. To get ethanol, the chain is cleaved by enzymes into individual glucose units, which are then fermented by yeasts to make ethanol, just like making beer. But there is no reason, other than making money for corn producers, to use starch to make ethanol.

Starch is only 40 percent of a commercial corn plant's biomass, and less than 10 percent of the biomass of most plants. The bulk of a corn plant -- indeed of any plant -- is composed of cellulose (chains of 6-carbon sugars like starch, but linked together differently) and hemicellulose (the same kind of chains, but of 5-carbon sugars).

The reason you can't eat grass is that you don't have the enzyme necessary to break the chemical bonds linking the sugars of cellulose and hemicellulose. Termites do, which is why they can eat wood. Bioengineers have succeeded in transferring the necessary enzymes from termites into yeasts, which then can ferment all of the plant to ethanol. So, rather than using farmland the world need to produce food, biofuels can be made from plants like switchgrass, which grows with high productivity on poor soils not suited for agriculture. They can even be made from municipal dumps, which are 80 percent cardboard and paper!

The United States should be devoting major resources to develop the process engineering required to up-scale this cellulosic ethanol process from the labratory to the marketplace.

It is easy to become discouraged when considering the world's many environmental problems, but do not lose track of the single most important conclusion that emerges from our examination of these problems -- the fact that each is solvable.

What is required is a clear understanding of the problem in each instance and a commitment to doing something about it. Because these problems are global in nature, we cannot simply assume they are too complex for solution, or wait for other countries to solve them. All countries will have to work effectively together as we set out designing and implementing workable solutions. The consideration of approaches to combat global warming by the U.S. Senate is a welcome first step. It cannot be the last.

George Johnson, Ph.D., professor emeritus of Biology at Washington University, has taught biology and genetics to undergraduates for more than 30 years. Also professor of genetics at Washington University’s School of Medicine, Johnson is a student of population genetics and evolution, renowned for his pioneering studies of genetic variability.

He has authored more than 50 scientific publications and seven texts, including "BIOLOGY" (with botanist Peter Raven), "THE LIVING WORLD" and a widely used high school biology textbook, "HOLT BIOLOGY."