This source argues that there are other factors besides temperature that can affect ring growth (although it would seem to me that temperature must be one of the main factors..)
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That's why it's important to use other sources. This is a list of most of the other proxies besides tree rings. The source isn't an environmentalist group or the Al Gore site. It's your National Oceanographic and Atmospheric Administration

http://www.ncdc.noaa.gov/paleo/proxies.html

I suppose they could be blowing smoke. But they do seem to think that trees rings and all the other proxies combined have a compelling story to tell.