Read here. In their concerted effort to prove current temperatures are warmer than Medieval temperatures, IPCC paleo-climate scientists scoured tree-ring research to identify tree populations (even single trees) that exhibited a sharp uptick in growth during the 2nd half of the 20th century. If they found a tree-ring study that exhibited a hockey-stick blade, they would include it in their published research as an indication that the trees were responding to unprecedented warming.
But was this really the case? Although some species of trees respond with faster growth from higher temperatures, there are other more important factors that contribute to all trees' growth. The most important factor is availability of atmospheric CO2 - vegetation responds to higher levels of CO2 with accelerated growth. More often than not, it is CO2 levels that primarily create a hockey-stick "blade" effect, not temperatures.
The chart below is research from a study of Greek fir trees, which clearly shows the post-1980 accelerated growth of tree-rings versus earlier periods, as compared to concurrent levels of CO2, temperatures and precipitation. Without doubt, the research reveals that CO2 levels are driving growth. Interesting update on trees and temperatures here.
"....there was a "strong acceleration of growth over the second half of the 20th century," and he notes that "the sustained increase in growth since 1990 in particular is unprecedented over the full length of the data set." He also correctly notes that these positive growth trends "bear no relationship to regional temperature or precipitation variations and therefore are unlikely to be climatically induced....About the only rational explanation for the late 20th-century growth acceleration seen in the ring-width data, therefore, is Koutavas' suggestion that "the enhanced growth reflects a fertilization effect due to rising CO2 in the global atmosphere." We agree. Although the correlation between the increasing trends of both atmospheric CO2 content and ring-width growth over this period does not prove causation, such is certainly a logical conclusion when (1) no other environmental parameter can explain the growth enhancement and (2) the results of hundreds of experiments have shown that tree growth is indeed enhanced in CO2-enriched air."
Other CO2 charts
