modern global warming

Jacinta: The Eocene epoch lasted from about 56 million years ago to about 34 million years ago, and it obviously can teach us much about climate change, and we will return to it, and maybe try to summarise earth's climatic and atmospheric history, but let's return to the current situation, and the human contribution to global warming.
Canto: Well, first, we need to combat those critics who claim that global warming, anthropogenic or not, isn't even occurring. I mean, there are those who argue that it's much more clear that carbon dioxide levels are increasing than that global warming is happening. Which of course also means that they don't accept the connection between carbon dioxide levels and temperature. So what's the evidence that global warming is occurring, and that it's connected to carbon dioxide levels? Can you explain the nature of that connection?
Jacinta: Okay, we have a number of questions to address here. First, is the planet warming? Second, is the warming due to human activity? Third, what is the nature of the relationship between carbon dioxide levels and global warming? Fourth, what is the nature of the human contribution to global warming? Fifth, how can this contribution be minimized?
Canto: Pretty well summarized. So is the planet warming? After all, as far as I know we only have detailed data about global temperature for, what, about a century and a half, and that is nothing at all in planetary time.
Jacinta: You're talking about direct data, from measuring surface temperature presumably. Of course surface temperature vary considerably and would have to be averaged out, but we can make indirect measurements by checking the volume of greenhouse gases [carbon dioxide, methane, water vapour and CFCs] in the atmosphere. Satellites can measure changes in the infrared radiation spectrum, and in so doing can quantify changing levels of each of the greenhouse gases in turn.
Canto: Yes, so they measure greenhouse gas levels and can compare them with earlier satellite measurements of, say, thirty years ago, but that's not much of a time differential. What about levels 50 thousand years ago, or 50 million years ago?
Jacinta: Let's not complicate matters too much. What they are finding is that these levels are rising quite rapidly in the reasonably short term.
Canto: And what about surface temperatures?
Jacinta: Global surface temperatures rose by around three quarters of a degree celcius over the period of the twentieth century, according to the IPCC.
Canto: That's not much, surely.
Jacinta: The IPCC and most, if not all, climatologists predict that the increase in temperature will accelerate over the twenty-first century, with predictions ranging from one to six degrees celsius. It's not an exact science, the variables are enormous.
Canto: Yes, I heard that the hottest years on record have been the most recent - though that's no indication of an accelerated increase, of course. It's just what you'd expect to find with an increase, even a decelerating increase.
Jacinta: The records show an accelerated increase in fact. This is one of the obvious reasons why the experts predict faster warming in the near future. It has a snowballing effect, with the melting of ice sheets, glaciers, permafrost and so on, and carbon dioxide tends to linger a long time in the atmosphere. So hopefully we've established global warming as a fact.
Canto: Okay, so next we examine the evidence as to causes.