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We found that unforced variability is large enough so that it could have accounted for multidecadal changes in the rate-of-increase of global average surface temperature over the 20th century. However, our estimate of unforced variability was NOT large enough to account for the total warming observed over the 20th century. Therefore, our results confirm that positive radiative forcings (e.g., from human-caused increases in greenhouse gas concentrations) are necessary in order for the Earth to have warmed as much as it did over the 20th century.

Having looked at the actual paper, I would claim they can conclude nothing of the sort: they assume that the excess "forced variability" is due to human greenhouse gas emissions on the basis of considering only volcanic aerosols, solar irradiance and human greenhouse gas emissions as possible sources of external forcing. They have omitted variation in cosmic ray flux (as modulated by solar magnetism) as a driver of cloud formation, soot emissions changing the albedo of the Arctic (but not Antarctic) ice pack, and (of necessity) all the unknown unknowns that bear on the earth's climate.

We were still coming out of the Little Ice Age (caused by changes in cosmic ray flux -- the most important cause they omit consideration of) in the early part of the 20th century.

The school of climatology based on astrophysics and planetary motion can easily make models that match data back millions of year. The school of climatology based on general circulation models of the earth's atmosphere struggles to make models that work on the scale of centuries or even decades.

The sun is blank. No spots. Winter is coming.

Please see my post#11. realclimate.org is one of the most evil sites on the web.

It is run by very smart leftists who use their intellect in the pursuit of pure evil.

They publish papers filled with high-sounding research. It is all lies and distortions.

They use their sophisticated sounding research to smear EVERY SINGLE paper and author who does "true" science.

realclimate.org is pure propaganda.

David; Thank you for your analysis. I do not have the training in statistics to read the paper. I have heard of comic ray effect on cloud formation. (Smaller the particle, the greater the number of cosmic rays, the greater the cloud development I think?)

Your analysis coincides with the grade school general Science that I was taught in the 1960s, that we were in an interglacial warming period and at some point it would start to cool and we could expect new continental ice sheets to form and start racing down from the north! (Oh Canada!) I do not recall if they discussed causation, we did discuss sun spots and 11 year cycles.

I have a lot of respect now for the science taught in my grade school and high school.