Environment Health Science

Correlation does not equal causation: tree planting episode

Replied to The association between tree planting and mortality: A natural experiment and cost-benefit analysis (

Tree planting in Portland, Oregon is associated with decreases in non-accidental and cardiovascular mortality, and the magnitude of this association increased as trees aged and grew.

Look, I’m a huge proponent of planting trees but you gotta be careful about correlation and causation. The article itself admits that “It is an observational study, so it cannot establish a causal relationship between trees and mortality.” Yet you’d believe the study established causation based on the monetary claims about the value of tree planting if you only read the abstract, which claims, “Using US EPA estimates of a value of a statistical life, we estimated that planting a tree in each of Portland’s 140 Census tracts would generate $14.2 million in annual benefits (95 % CI: $8.0 million to $20.4 million).” You cannot make this estimate if you do not know it to be a causal relationship.

I get really pissed when environmentalists mislead the public towards their preferred outcome using faulty or over-interpreted data*. I wrote a report about single-use items a few years ago, and the deeper I dug into statements about straws and such that were listed on environmental advocacy sites, the more of a sham they turned out to be. I wound up doing a literature review to find valid, empirical evidence in support of our paper, and uncovered that compostable packaging actually has greater impacts when it is not composted — which is often!

Tell people to plant trees because they clean the air, provide shade, and reduce flooding, but don’t lie and tell them planting a tree will make them live longer.

*There are many reasons for this in the scientific literature, but at the core comes down to two challenges: science is insufficiently funded and researchers are rewarded for significant findings. See also: Imagining a better way — for everything


Watched How Trees Bend the Laws of Physics

Watched How Trees Bend the Laws of Physics by Veritasium from

Hope this was worth the wait! So many people helped with this video: Prof John Sperry, Hank Green, Henry Reich, CGP Grey, Prof Poliakoff, my mum filmed for me in beautiful Stanley Park and Jen S helped with the fourth version of the script.

Prof John Sperry
Hank Green (SciShow)
Henry Reich (minutephysics)
CGP Grey
Prof Poliakoff (Periodic Videos)

Also thanks to the Palais de la Decouverte – they helped me with the whole vacuum pump setup in Paris. No, I could not actually suck water up 10m – I did about 4m, but the vacuum pump was easily able to do it and I saw spontaneous boiling on all of our various trials. Footage from this may end up on 2Veritasium.

Trees create immense negative pressures of 10’s of atmospheres by evaporating water from nanoscale pores, sucking water up 100m in a state where it should be boiling but can’t because the perfect xylem tubes contain no air bubbles, just so that most of it can evaporate in the process of absorbing a couple molecules of carbon dioxide. Now I didn’t mention the cohesion of water (that it sticks to itself well) but this is implicit in the description of negative pressure, strong surface tension etc.

Interesting, didn’t realize the mechanism behind being able to grow so tall and transport water so high.