Scientists at the Smithsonian Environmental Research Center have published a study, Evidence for a recent increase in forest growth, suggesting that climate change can quite literally be measured by treehuggers. Like the average American citizen, American trees look to have had increasingly bulging middles in recent decades. Having spent their careers quite literally hugging trees, SERC scientists Geoffrey Parker and Sean McMahon have written a study documenting
evidence that forests in the Eastern United States are growing faster than they have in the past 225 years. The study offers a rare look at how an ecosystem is responding to climate change.
For over 20 years, Parker has gone into a set of forests in the mid-Atlantic, tape measure in hand, 
and giving them a hug to measure their size. Parker’s own hugging has been extended with a robust group of volunteers conducting regular measurements of specified trees. (The boy scout to the right, while in a SERC forest, isn’t engaged in actual measurements for the study.) Some 250,000 hugs later, he has quite a database in hand.
The results of analyzing hugs surprised these researchers. Based on the data from these 100,000s of hugs, Parker’s and McMahon’s analysis documents
that the forest is packing on weight at a much faster rate than expected. … on average, the forest is growing an additional 2 tons per acre annually. That is the equivalent of a tree with a diameter of 2 feet sprouting up over a year.
Now, there are many things that contribute to plant growth, from soil quality to rainfall to temperatures to CO2 concentrations. Parker and McMahon have concluded that the driver for the bulging middles of the studied groves is best explained through human impacts: the rising levels of CO2 (a nutrition); and the warmer temperatures and extended growing season due to global warming (driven, in no small part, due to the rising CO2 levels).