Below is yet another notable installment of the XKCD web comic that is very much worth a look. It addresses the far-too-common misconception that since the world's temperature has changed before, global warming is no big deal and possibly just natural variation. (That argument is parodied in the title text that you see if you mouse over the comic: "[After setting your car on fire.] Listen, your car's temperature has changed before.")
As the graph shows, the best data we have indicate that the annual average surface temperature Earth is higher today than at any time in at least the past 22,000 years. Even more concerning, the rate of increase is also higher, by a lot.
The graph includes three projections of future global temperature rise for the remainder of this century depending on how urgently the world addresses the problem. Some people dismiss such projections. After all, they say, we can't forecast the weather even a month in advance. That misses a basic point: Forecasting weather is a lot different from forecasting climate. We can't predict the weather for a particular date in Toronto next year, but we can be pretty confident the monthly average temperature will be higher in August than in February.
For a detailed explanation of the comic see http://explainxkcd.com.
An earlier, shorter XKCD comic on the same topic can be found here.
A few points in passing:
The comic addresses mainly the average air temperature near the surface. The oceans are warming as well, which accounts for the sea level rise we're observing. A good source of sound information on all the consequences is a website created by the U.S. National Academy of Sciences at this link.
The term "ice age" is ambiguous. In modern geology it refers to any extended period of time cold enough for glaciers and polar caps on the Earth's surface, including today. The older meaning, still often used in other fields, in popular articles, and in this comic is a time of really extensive glaciation, with most of Europe, norther North America, and comparable latitudes around the world covered with deep glaciers. In geology that's a "glacial period," and the last one ended roughly 12,000 years ago.
Anyone still doubting climate change should pay attention to Richard Muller. A respected physicist long doubtful about climate change, Muller assembled a collaboration of scientists to re-examine the whole question from scratch, avoiding what Muller and others suggested might be sources of inaccuracy and bias in mainstream climate science.
The co-chair of the panel was Muller's daughter, a scientist working as a consultant in the fossil fuel industry, and some of the funding came from conservative political activists and multibillionaires Charles and David Koch, known for promoting doubts about climate change. This led to concerns about whether Muller and his team would be able to treat the subject objectively. But Muller and his colleagues insisted that they would go wherever the data led them, no matter what their preconceptions. And as it turned out, the hard data convinced Muller that he had been wrong. Not only were climate scientists correct in their conclusions about recent climate history, but the Intergovernmental Panel on Climate Change had if anything understated how much change had been caused by human activity. The IPCC has attributed some warming in the first half of the 20th century to increased solar activity, but Muller et al argue that humans are responsible for almost all of it.
For whatever they're worth, my other posts on climate change can be found here and my other references to the XKCD web comic are here. Of course, the main sources for XKCD are its main website http://www.xkcd.com and the independent http://www.explainxkcd.com, which is often very helpful to those of us who don't always get the subtleties or have a clue about the sometimes-esoteric references.
Finally, for anyone who still believes that "climate change" is a new term (which has nothing directly to do with this comic but just pisses me off), see this earlier post. Short version: Scientists and even politicians have been using the terms "global warming" and "climate change" pretty much interchangeably for human-caused heating of the Earth for decades. See for example this scientific paper from the 1950s. Also recall that the largest scientific collaboration studying global warming, the Intergovernmental Panel on Climate Change, was established under that name during the Reagan administration.
(Updated 2016 September 14 in an effort to improve some wording.)