Here it is:
the satellite record indicates the globe as a whole warmed in the 80's and 90's.
Also, sea level rose in the 90's and early 00's. Arguably, this
indicates thermal expansion of the oceans.
CO2 has been rising (and isotope measurements show people are likely to blame).
For reliable data, that's about it. (Actually, some have challenged these data
as well, but I'll stipulate them to be a reasonable guy.)
There's also a surface record that shows some warming over the 20th century, but as discussed here that record is so corrupted no-one should credit the data. Also, it doesn't show a remarkable trend anyway. Even if you credit it, it would by no means be conclusive that people had anything to do with it.
There's also the claim that the arctic ice has been melting. But at the same time as arctic sea ice reached historic lows the summer before last, (where historic means, historic for the satellite era ie the last 30 years!) antarctic sea ice reached historic highs and global sea ice anomaly at the time everybody made a fuss about arctic ice was actually + 1 million sq Km (higher than its average over the satellite era!) (Right now, its -.5 million sq km.) Global Sea Ice Anomaly.
Lets put this in perspective. You have 20 years of global warming, followed by 10 years of flat or cooling. You have seas rising slowly-- but they've been rising for 20 thousand years-- and in fact there's a hint in the short term trend over the last few years that sea level may be flattening or falling.
If you look at proxy data to get historic estimates of the temperature (tree ring, ice core, etc), none of this warming is remarkable. The planet has been warmer many times in history.
So why believe it is anthropogenic? The IPCC says, they have Global Circulation Models, which are huge computer programs, that can only explain the rise if greenhouse gas warming is included. Therefor they conclude human cause, and predict big warming in the future. That is their argument, if you read the IPCC report and look for one.
But as discussed, no independent and reasonable person should believe their models, I'm aware of no very good reason to believe they are even debugged, past the point where they showed the desired behavior. They fail any sort of predictive test-- predictively they've been falsified. They ignore climate effects like cosmic rays, for which the evidence is excellent. etc etc. The technology just doesn't exist, and won't any time soon, to simulate the globe's climate in a reliable fashion.
There are also simple, pencil and paper arguments that CO2 should cause warming. But these arguments do not say it should cause warming by an alarming amount. And they also say the warming should be logarithmic-- each new CO2 molecule contributes less than the last, because the CO2 can only interfere with radiation in certain frequencies, and after you've got some CO2 in the air, those frequencies are mostly blocked anyway. So unless you believe the GCM's, future CO2 emissions are just not important for climate.
On this basis we are supposed to believe the planet will warm many times more than it has been observed to, and to spend trillions of dollars curbing CO2 emissions?
