Nonsense. Based the temperature records twenty years ago, it was not obvious that the 20th century had a warming trend. You think moving on and accepting new data is being opportunistic? Sorry, that's wrong. Its being scientific. Keep in mind that the period from 1980 to 2000 accounts for most of the 20th century warming.
Meanwhile, the prime highlight of the IPCCs AR3 was to "forget" the existence of climate change prior to the 19th century. Natural variation over the past thousand years was reduced to quiet gradual downtrend with an abrupt surge upward in the 1800s. In so doing they discarded thousands of studies and work of thousands who previously carefully documented periods of great warming and cooling throughout the history of man. That work is precisely the work implicated by these inquiries, and which the climategate archives now shows that the literature was manipulated to fit a desired narrative.
I suggest comparing comparing the IPCC-1990 report, which concisely shows the consensus of an old guard (now largely dead) with AR3. A very warm, much warmer period during the middle ages (shown in red) is visible. The IPCC AR3 and AR4 replaced this with the blue curve. Shown a flat-changeless temperature history with a slight downtrend, suddenly accelerating upward.
But their claim was bespectacled from the start by way of special pleading they had explained away each interruption in warming that occurred during the 20th century, but then after the report was published, yet another unexpected cooling period emerged.
Suddenly the meme switched from being about "Global Warming" to being "Climate Change". The focus shifted from temperatures to sea-levels and hurricanes. Yet this turned out to be an even more tenuous footing. Its already no longer considered reputable among intellectual circles to discuss such extravagances. Eventually the talking point was settled upon: weather is not climate. The recent cooling is just weather.
Indeed, weather is not climate. Climate is the expectation of weather--and so yes, it surely does matter when year after year goes by somewhat cooler than had been predicted by the IPCCs latest report.
Meanwhile, the very people who had steadfastly refused to deny climate-change are now labeled the climate change deniers. This stemmed from an Orwellian campaign to redefine terminology. Suddenly believing in climate-change meant believing in anthropogenic climate change. The language literally twisted to be an embodiment of the "one true belief"--no need for that pesky modifier anthropogenic, and all the better to co-opt what everyone knows: climate changes.
Several very cogent critiques of the AR3 temperature series have been published which eviscerated that graph (Mann's work) as a product of flawed statistical methods and bad data. Yet a loud cadre continues to deny any problem exists, and banks on the lack of specialized knowledge among the public and other scientists to trade on their word alone.
And, no, the claim is no not that there is no contribution from Man. CO2 is a greenhouse gas, but its effect on temperature depends on poorly understood feedback effects. These effects are in part also responsible for the long history of natural temperature variation that the IPCC otherwise ignores. Ultimately, what it comes down to is this: The IPCC claims a temperature rise of 2C/century. To arrive at this number they assume almost all strong feedbacks are amplifying rather moderating the C02 driven warming. Why does this matter? Much of the impetus for "ACTION NOW!" stems from the notion of a climate tipping point, but if the feedback effects are more moderating than the IPCC claims, this is highly unlikely.