Climate Change: It’s Really Happening

I was privileged to hear Dr. Philip Mote speak at last month’s International Society of Arboriculture conference in Portland, Oregon on climate change. Among other things, Dr. Mote is a professor in the College of Oceanic and Atmospheric Sciences at Oregon State University. He is not an arborist, but he translated the complexities of climate science for us. Here are a few of the main points:

  • Global warming is really happening and has been for 50 years – the evidence is unequivocal.
  • Global warming is closely linked to human causes.
  • Scientists have considered possible natural causes of global warming, but have ruled them out.

This may come as no surprise to those who realize that Michael Crichton’s book “State of Fear” is a work of fiction with a faulty scientific premise.  But several politicians may still need convincing.

If we continue with business as usual – polluting, creating more greenhouse gases, reproducing and consuming at the current rate – the Arctic could warm up by 7 to 7.5 degrees and temperate zones (where most of us live) by 4 to 5 degrees. Wet places will get wetter and dry places drier. Impacts on wild forests can already be seen and will continue – an increase in wildfires, bark beetles, invasive plants, and diseases. We’ll see a change in the growth rate of trees, and the distribution of tree species.  Already, the USDA Plant Hardiness Zone map, which is based on annual minimum winter temperature, has been redrawn to reflect warmer winters. What zone are you now? What zone might you become?

Peter Raven, the keynote speaker at the same conference and president emeritus of the Missouri Botanical Garden, reminded us that world population is growing by 200,00 per day. According to the Global Footprint Network, we are currently consuming 135% of all that the world produces. Demand is exceeding supply, and any economist can tell you that leads to high prices.

These are scary numbers, and it may feel like there is nothing we can do. But trees can make a difference, and we should continue to grow them and advocate for them. All the good things about trees are still true: Trees reduce energy consumption by shading buildings and intercepting wind. They reduce the formation of ozone and other greenhouse gases by shading pavement and parked cars. They slow down and absorb stormwater, conserving this ever more precious resource. They sequester carbon, but they don’t make it go away: when you cut a tree down, the carbon is released again.

Trees start providing maximum benefits about 30 years after planting.  As arborists, horticulturalists and urban foresters, we need to think about how to grow that tree to 30 years and beyond. Not just plant it, but establish it – give it enough soil, structural pruning – so it will get big enough to provide the benefits we need in future.

(Thanks to Ramie Pierce from the City of Tacoma, who suggested in her recent presentation that we stop using the words “plant trees” and instead say “grow trees“- it implies more of a long-term commitment.)

Choosing the right tree for the spot will become challenging – in 30 years, perhaps the trees doing well in the warmer USDA zones will be right for our current zone. It might be a good idea to start trying some species from those warmer places now. The definition of “native plant” is also likely to evolve.

Growing more trees won’t mean we don’t have to reduce consumption and stabilize our population. But it might take a little of the heat off things.



  1. john p

    Ramie Pierce is absolutely right- we need to change the notion that simply planting a tree is enough…

    • Leda Marritz

      Absolutely. I also like the Ramie Pierce’s distinction between planting trees and actually GROWING them. A small tweak that makes a great impact.

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