Well, everyone *should* be a skeptic, but skepticism should diminish with evidence. In terms of anthropogenic global warming (AGW), there are basically three different, but connected, issues.
Firstly, there is the physical science basis on which AGW is based. We go around the world digging and pumping up fossil...
Best answer: Well, everyone *should* be a skeptic, but skepticism should diminish with evidence. In terms of anthropogenic global warming (AGW), there are basically three different, but connected, issues.
Firstly, there is the physical science basis on which AGW is based. We go around the world digging and pumping up fossil fuels. We burn them in an atmosphere containing oxygen. We release additional CO2 into the atmosphere that wouldn't be there had we not burnt the fuels. We've seen an increase in CO2 levels in the atmosphere of 100 ppm in 120 years. Based on analysis of bubbles of air trapped in ice cores, we can have a very high confidence that this rate of increase is 42 to 167 times faster than anything the planet has experienced in the last million years. We know that CO2 levels are more than 400 ppm today, and that the planet hasn't experienced this level in at least 3 million years. We have multiple lines of evidence that link this CO2 rise to our burning of fossil fuels, such as isotope studies and so on. We have predicted that this CO2 rise should result in a 0.2 Watts per square meter per decade increase in radiative forcing. We have measured the impact experimentally and found it to match the theoretical value. So, to me, it is hard to justify skepticism about the fact that our CO2 emissions are warming the planet.
There is, however, a question remaining over the feedback between the warming caused by our CO2 emissions and an increase in water vapor, which is itself a greenhouse gas. There are very well tested theories that allow us to calculate that impact, and doing these calculation and combining them with the impact of CO2 provides good fits to the observed warming over the past century. This is now being examined by satellites so we should be able to refine that over time. But this shouldn't be a source of great skepticism since we have multiple models that use the upper and lower limits we think apply based on the laws of physics and chemistry.
So, to me, skepticism about the basic physics (the planet is warming and we're responsible and the impact is amplified by the increase in water vapor) isn't justified. I no longer accept any view to the contrary, just as I no longer accept any view that the earth is flat or that evolution is wrong. We've moved past that point in the discussion no matter how many people insist otherwise.
Where skepticism is partially justified is in terms of the impacts of global warming. I think there are certain things, however, that I can't accept as an argument. Sea-levels, for example, are globally rising by 3.1 mm per year. Anyone who says that sea levels aren't rising or isn't a problem - well, I'm sorry but that's just wrong. However, there are still questions about 'how bad or good will things be'. But if you accept that there are significant unknowns at the moment, then arguing the only impacts will be beneficial is not something I can believe. So I think some of the skepticism is justified, I think some of the media hype up natural events (which may have nothing to do with global warming - for example, big pieces of ice breaking off Antarctica), and that people tend to jump on papers as if stating 'fact' when the role of science is to study something, draw a conclusion, and then use peer-review to test those conclusions.
Finally there is the political issue of what, if anything we do about it. Skepticism on policies or strategies is fully warranted and public debate necessary.
So, in terms of climate change, skepticism is justified and healthy in terms of the potential impacts and political strategies to deal with it. Where it's denial rather than skepticism is largely in terms of the physics of what's happening to our planet and why.