The 350 movement is a group concerned about climate change that has adopted an upper limit of 350 parts per million (ppm) of carbon dioxide equivalent in the atmosphere as their target. The target is a good and extremely ambitious one, and the group is doing well in the media. That said, I worry that some of the 350 proponents don’t understand what they are arguing for.
The carbon cycle
To understand climate change, you need to understand the carbon cycle. In a normal situation, this refers to carbon in sugars being released as CO2 when animals, bacteria, and fungi metabolize them. This adds CO2 to the atmosphere. In turn, green plants use sunlight to make sugars out of CO2, releasing oxygen. These processes happen in a balanced way, with more CO2 emission in the winter (when plants are inactive) and more CO2 absorption in the summer.
Alongside the biological processes are geological ones. Two are key. Volcanoes emit greenhouse gases, and the erosion of certain kinds of rock locks up CO2 underground. The latter process happens very slowly. It is very important to understand that this is the only long-term phenomenon that keeps on drawing CO2 out of the atmosphere. The oceans will suck it up when CO2 accumulates in the air, but only until the seas become more acidic and come into balance. CO2 likewise accumulates in the biomass of living things, but there can only be so many forests and so much plankton on Earth.
When we burn fossil fuels, we add to the concentration of greenhouse gases in the atmosphere. Before the Industrial Revolution, it was around 280 ppm. Now, it is about 383 ppm and rising by 2 ppm per year.
What 350 means
It isn’t impossible to get back to 350 ppm. This is because the oceans haven’t caught up with the atmosphere yet. If we suddenly stopped burning coal, oil, and gas the quantity of CO2 in the atmosphere would start to fall as more of it went into the sea. That being said, if we keep burning these fuels in the way we are now, getting back to 350 ppm will become impossible.
When you argue to cap the atmospheric concentration at 350, you are arguing to cut the net human emissions of the entire planet to zero – and to do so before we cross the point where the oceans can’t draw us back under the number. The same is true if you argue for stabilizing at a higher level, such as 450 ppm or 550 ppm; those scenarios just give us more time to keep emitting before we reach zero net emissions. When you support 350 ppm, you are committing to keeping the great majority of the carbon bound up in remaining fossil fuels underground and unused by human beings. ‘Net’ human emissions means everything that goes up into the air from burning fossil fuels, minus the trickle of CO2 into rocks (as described above) and possibly minus whatever CO2 we can suck out of the air and bury (a costly and energy-intensive procedure).
Cutting net human emissions to zero is a laudable aim. Indeed, it is the only way concentrations (and global temperatures) can ever be kept stable in perpetuity. I just hope that more 350 supporters will come to understand and accept that, and realize that achieving that ambition requires massive societal change, not just marches and savvy media campaigns.
P.S. If all that isn’t enough of a challenge, remember that there are also positive feedback effects within the climate system where, once we kick off a bit of warming, CO2 concentrations rise on their own in response. These feedbacks include melting permafrost and burning rainforests. Keeping below 350 ppm requires cutting net human emissions to zero before these positive feedbacks commit us to crossing the threshold.
Why Bury Coal? explains this in more detail.
- What it means to stabilize climate
- How much carbon dioxide can we release?
- Climate sensitivity and stabilization concentrations
[Update: 24 March 2011] Some of what I just added to the bottom of my Earth Hour post is also relevant here, in that symbolic acts can help environmental groups achieve attention, even if the acts capturing the attention are dubious in some ways. 350.org should be commended for attracting so much general public attention.