Broadly speaking, the issue of climate change comes down to three questions.
Clarifying the Temperature Question
Is Earth getting warmer? (And, further, how much warmer, and will it continue to get warmer in the future?)
This seems like a straightforward question about observable, empirical matters that can easily be answered by looking at temperature records. But it's actually a bit more complicated than that.
The question itself needs clarification, because it is in search of a relational claim. Before we can answer the question, "Is Earth getting warmer?", we need to answer the question, "Getting warmer than what?" Warmer than 10 years ago, 50, 100, or 1,000 years ago? If Earth is warmer than it was 100 years ago, but colder than it was 1,000 years ago, does that count as global warming or global cooling? Our basis of comparison needs to be specified before we can answer the question, "Is Earth getting warmer?".
Of course, we don't have to pick a particular year to compare current temperatures to. Instead, we can ask about a counter-factual, hypothetical comparison, by asking, "Is Earth warmer than it WOULD HAVE BEEN had CO2 (carbon dioxide, a greenhouse gas) not been added to the atmosphere?" And this is the question most scientists seem to be asking, though it also calls for clarification (for instance, how much CO2, and added when?).
Another matter needs clarifying: When considering Earth's temperature, are we interested in the temperature of the land, the seas, or the atmosphere? It is not guaranteed that they will all change in the same way. Are we instead combining the temperatures of all three and considering them at once?
Once these issues are settled, then it makes sense to look at the temperature record for answers. This step also presents difficulties, as actual temperature records don't go back much further than a century or two. Beyond that, we have to look at things that exist today but were affected by temperature in the past - what's known as temperature proxies. These proxies - things such as tree rings and ice cores - are worth looking at, though getting information from them is tricky, because they are also affected by things OTHER than temperature.
Assuming that Earth is getting warmer, the next issue to be addressed is our causal relationship to that warming. That is, did we, human beings CAUSE the warming, and can we REVERSE it?
Discussion about global warming typically focuses on the question of whether or not humans caused it, in the belief that, if humans caused global warming, then humans can stop it. This is erroneous, however. Global warming might turn out to be like a boulder falling down a hillside: a person can set it in motion, but can't turn it around once it IS in motion. (A more morbid example is death: we're able to cause someone's death, but not reverse it.) Conversely, global warming might turn out to be something like polio: something that human beings didn't create, though they did find a way to counteract it.
In principle, there are four possible combinations regarding our causal relationship to global warming:
- Humans DID cause global warming and CAN reverse it
- Humans did NOT cause global warming and CANNOT reverse it
- Humans DID cause global warming but CANNOT reverse it
- Humans did NOT cause global warming but CAN reverse it
Costs and Benefits
Assuming that Earth is warming and that humans are in a position to reverse it, the final matter to consider is, what are the costs of global warming compared to the costs of reversing it? Is the cost of reversing it worthwhile?
This requires a great deal of predictive ability, in both the short term and the long term. Certainly, global warming will have costs, but there will also be benefits, as well, just as reversing global warming will have both costs and benefits. How do these costs and benefits balance out? Are there good effects of global warming - for instance, forestalling an ice age - that outweigh the costs? Or, do the costs of global warming - for instance, rising sea levels and increased desertification - outweigh any benefits?
These questions aren't merely empirical ones: they also involve judgments about which costs and benefits take moral priority over others, which further complicates things.
The issue of global warming is a mixture of several different questions: empirical, scientific questions regarding the reality and reversibility of climate change, and the question of assessing the moral costs and benefits of trying to influence climate change.