I believe the IPCC accepts the hockey stick temperature record (temperature is flat over the last 1000 years until about 1900) and ignores the medieval warm period and the little ice age as supposedly not being representative of the globe as a whole. If they recognized these earlier temperature excursions, they would have to admit that things other than greenhouse gases affect temperature, namely variation in solar radiation.
Prof. Lindzen of MIT, who participated in the IPCC study, described the IPCC process as follows,
"The preparation of the report, itself, was subject to pressure. There were usually several people working on every few pages. Naturally there were disagreements, but these were usually hammered out in a civilized manner. However, throughout the drafting sessions, IPCC 'coordinators' would go around insisting that criticisms of models be toned down, and that 'motherhood' statements be inserted to the effect that models might still be correct despite the cited faults. Refusals were usually met with ad hominem attacks. I personally witnessed coauthors forced to assert their 'green' credentials in defense of their statements."
This sounds like a debate on evolution chaired by a fundamentalist church.
Your link to the IPCC discussion of the correlation between solar activity and global temperature actually referred to discussions they had about how the sun might influence cloud cover. Cloud cover may indeed be one of the mechanisms at work here, but perhaps not the only one. Nowhere did I see where the IPCC refuted the correlation between solar activity and global temperature. They are just finding fault with theories about how the sun could cause this to happen. That is not the same as refuting that it happens.
Energy balance calculations show that solar radiation explains most of the earth's temperature level. When variations in the sun's behavior correlate with global temperatures for hundreds of years without any need to invoke soot, aerosol particles, volcanic eruptions, el Ninos, etc., it is a strong argument in favor of the sun being the main cause of global warming.
Some wags have suggested that human activity on earth must be causing the corresponding variations in solar behavior. That makes as much sense as attributing the bulk of global temperature variations to greenhouse gas.
This might constitute a refutation:
On the length of the solar cycle and the Earth's climate
This appears to be from Ralph Cicerone, who was the chair of the National Academy of Sciences panel that evaluated the "state of the science" regarding global warming at the behest of the Bush Administration.
It also appears that the 1991 paper showed the correlation without proposing a mechanism. The 1997 paper proposed a mechanism. The IPCC section that I provided examined (by looking at quite a few research papers) whether the proposed mechanism was viable, and they concluded that the evidence supporting it was weak.
Note that there have been other papers recently (notably one by Bond of the Lamont-Doherty Earth Observatory) that strengthened the case for solar forcing of global temperature, particularly for such events as the Medieval Warm Period and the Little Ice Age. Here's the paper reference:
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.
Here's the abstract of that paper:
"Surface winds and surface ocean hydrography in the subpolar North Atlantic appear to have been influenced by variations in solar output through the entire Holocene. The evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon-14 and beryllium-10 and centennial to millennial time scale changes in proxies of drift ice measured in deep-sea sediment cores. A solar forcing mechanism therefore may underlie at least the Holocene segment of the North Atlantic's "1500-year" cycle. The surface hydrographic changes may have affected production of North Atlantic Deep Water, potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally."
So he's got a mechanism and data that shows it works. So the main "bone of contention" (and one that I don't think has been entirely resolved) is whether the currently observed warming is still primarily a consequence of emegence from the Little Ice Age solar minimum period or whether it is a combination of solar and human effects. The main factor weighing against the "emergence from the LIA" scenario is that the Sun has been active and hasn't had a sunspot minimum similar to the Maunder Minimum, a 400-year period in which there were virtually no sunspots.
Also, the Bond paper cites a paper by Shindell et al. in which model results indicate that small changes in solar output can affect the stratosphere and influence global climate.
So... where does that leave us? Essentially this: there's NO doubt that the Earth is warming. Solar variability could be part of that. So also could greenhouse gas-induced warming. If both are important (which I suspect is the case), then we would be making any warming caused by the Sun (which we can't control) worse due to our activities (which we can probably to some extent control). The difference between what we can control and what we choose to control might be the difference between the survival or demise of many of Earth's ecosystems. And that brings us back to the subject of the paper that kicked off this thread: whatever the cause of the warming, it is being felt, and responded to, by many components of major ecoysystems around the world.