First of all, The Guardian is hardly a neutral media source; it's rather radical left leaning with a clear agenda.
Now, let me remind everybody that global warming is a theory like everything else in science. Even the laws of nature we use every day are basically just theories, usually backed up by a lot of evidence sure, but still just theories. Plenty of laws have been struck down or modified as new evidence comes to light.
Same thing with global warming. Right now lots of evidence seems to confirm this theory so that's what we chose to believe in.
But lots of evidence also indicate that we are not seeing the whole picture. Geological data points to much more radical temperature changes in the distant past and while we have theories that can explain cooling (nuclear winter effects from volcanic eruptions or meteor impacts etc.) we still have no causes except "humans" for warming, and yet it is clear that we've had periods with a much higher average temperature than we do now, even with the global warming in effect, at times when mammals were barely invented yet, let alone anything remotely like humans.
So - I'm inclined to invoke Occam's Razor here. Given the still unknown factor at play in the distant past, I'd rather believe that the same factor is a play again than something new and complex is doing the same thing.
Note that I'm not saying that humans are not causing some warming. I'm just saying that it might not be the complete picture.