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In toxicology, there is a fundamental truism: "The dose is the poison." That means that any substance can be toxic, depending on how high the dose is. For example, aspirin is truly a wonder drug. But if you take a whole bottle of aspirin at one time, you can die.
This same truism holds for global warming. In order to figure out whether or not global warming is "bad," it's somewhat necessary to determine how much global warming is being anticipated. For example, the Intergovernmental Panel on Climate Change (IPCC) in their Third Assessment Report, has approximately 40 scenarios that produce predicted warming by the year 2100 that ranges from 1.5 to 5.8 degrees Celsius (2.7 to 10.4 degrees Fahrenheit). Now, pretty much everyone would agree that, if the warming were 5.8 degrees Celsius or more by 2100, that would be bad. And pretty much every one would agree that, if the warming were less than 1 degree Celsius by 2100, that would not be too bad. In between those two areas, there would be disagreement, with more people accepting that the results are bad as the temperatures get higher and higher.
In fact, there may be a certain (small) amount of global warming that's actually a "net" good. This gets into the discussion from Question #1: the world's temperature is always increasing or decreasing, and at virtually no time in the past has the temperature been exactly what it's been in the last few years. So if we're honest (some of us aren't...no need to discuss who that might be) we should acknowledge at least the possibilty that the world's "optimum" temperature is above (or below...but definitely not too far below!) the current temperature.
This concept of an "optimum" world temperature, and where we might stand at the present time, can be illustrated graphically. Figure 1 shows a trace of surface temperatures from the 1850's onward, with a projection of possible temperatures in the 21st century. The question is, "Which temperature is 'optimum'?" Was it better in the 1850's? The 1940's? The 1970's? The projected value for 2050 or 2100? It really might be hard to say. The only things we could probably say with a good deal of confidence are that temperatures, say, 5 degrees Celsius cooler or 10 degrees Celsius warmer than today would probably be bad. If it were 5 degrees Celsius cooler, we'd be in an ice age, which would make it hard to sustain the present human population (our agriculture would be so adversely affected). And if it were 10 degrees Celsius warmer, the Antarctic ice cap would be melting, raising sea levels by more than 100 meters. So, much colder or much warmer than present would clearly be bad. "The dose is the poison."
In order to try to come up with an "optimum" temperature between "obviously too cold" or "obviously too warm," we need to think about winners and losers in these temperature ranges. For example, in Shakespeare's time, the winter temperatures in London were routinely so cold that the Thames river froze over. The "winners" would be people that liked ice skating. But the poor people that were having trouble keeping their homes warm in the winter would be "losers." Similarly, farmers can be expected to be helped by both warmer temperatures (i.e., a longer growing season) and more CO2 in the air (since CO2 is a fertilizer for plants). This should be obvious, because when people want to grow plants year-round in cold climates, they put them in greenhouses, and frequently deliberately inject CO2 into the greenhouse. But the "losers" in a warmer world might be ski lift operators, if temperatures get so high that artificial snow can't be made.
The previous discussion omitted the philosophy that some people might have: that humankind causing even 1/10th of one degree change in the world is completely unacceptable. That is, some people might have a value system that says that, if humans cause even 1/10th of one degree heating (or cooling) that if a priori "bad." If that's you're value system, then even a fraction of a degree of warming (or cooling) caused by humans is "bad." I personally can't agree with that value system, since the earth's temperature is naturally changing by much more than 1/10th of a degree each year.
Whether or not global warming is "bad" also depends on where one is in the world. The conventional wisdom, based on computer models, is that the earth will warm the most at the poles, and that the warming will occur most at night, and when conditions are dry. In other words, places like the Arctic and Antarctic, Alaska, northern Canada, and Siberia will warm the most (if human emissions of carbon dioxide are causing the warming). Now, personally, I think this is pretty cool (i.e., good). I'd think that the Alaskans and Canadians and Siberians would actually WANT some additional warmth...especially during clear nights in the wintertime. (But, of course, maybe some folks in those places actually like the cold.) (They are insane, of course...but that doesn't make them bad people.) But if you're in Panama, or some place else in the tropics, and there is even a little more heat, that could certainly be bad. (Unless of course, you actually like the heat. Which means you're insane, of course. But that doesn't make you a bad person.)
Whether global warming is "bad" depends on how much will occur. A whole lot--say 7 degrees Celsius or more--is clearly bad. Whether global warming is considered "bad" also depends, to a certain extent, on one's value system. Some people may have a value system that says that even a small fraction of a degree warming in global temperature, if it's caused by humans, is bad. Finally, whether global warming is "bad" depends where one is located on the globe: certain parts of the world are likely to benefit from global warming, while other areas will suffer.
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