The Berkeley Earth Project, an independent study of global warming, has found that the earth has become a degree warmer over the past half century. However, the statistical uncertainty surrounding pre-1920 estimates makes it very hard to say much about long-term trends - click here for graph. This is one of my concerns with the global warming debate - we simply don't have trustworthy long-run data which looks at temperature changes over the last millennium (or two).
My second concern with the global warming debate is that it is very hard to prove any sort of casual link between global warming and human activity. The scientists may be able to show correlation between global warming and our production of carbon dioxides etc., but correlation is not causation.
My third concern with the debate is that those who are sceptical or agnostic are stereotyped as flat-earthers or intellectually-challenged crackpots. This only stifles debate and the progress of science itself.
My final concern is with the incentive systems facing scientists. If it was conclusively demonstrated that global warming has not been occurring, many scientists would be out of work and many reputations would be in tatters. A lot of people have a stake in global warming being a real phenomenon. In addition, any scientist who speaks out against the majority global-warming position risks being ostracised from the profession, being unable to get research funding (the allocation of which will ultimately be determined by peers who are global-warming advocates) and their work published in peer-reviewed journals.
I ultimately don't know if global warming is a real phenomenon. However, the majority have been wrong before as typified by Galileo, who swam against the tide of his peers as well as the Vatican. By pejoratively describing some scientists as global-warming deniers, maybe the majority are attacking the next Galileo!