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In the Middle Ages, the Catholic Church offered indulgences and let people exchange donations for notes promising a shorter time in purgatory after their death. Less controversial, today someone who is overly concerned with office tea supplies might feel compelled to pay for a replacement box. Are carbon offsets more like the former or the latter?
There are many reasons why you could try offsetting some of your carbon footprint to alleviate a general sense of guilt for your lifestyle, accurately cover a flight's estimated emissions, or just do something that is good for the environment. Regardless of motivation, all of these efforts are based on the belief that the money you actually paid will result in the promised amount of CO2 being removed from the atmosphere. Otherwise, you'll pay for a lie – or at least get a smaller box of tea than what you ordered.
It's really difficult to find out if you've been lied to. Here's what you need to know:
Downshifting the carbon cycle
Carbon dioxide in the atmosphere is part of a bio-geochemical dance called the carbon cycle. Carbon regularly changes shape and area code, moving between atmosphere, oceans, ecosystems and even bedrock. Plants take up carbon atoms from CO2 in the air in order to grow. Herbivores consume plants and breathe out the carbon again as CO2. Weathered rocks pull CO2 out of the air. Volcanoes release it into the atmosphere. These are just a few of the many exchanges in the carbon cycle.
Before the industrial revolution, these flows were in equilibrium, maintaining stable levels of CO2 in the atmosphere. But then we discovered fossil fuels and ways to set them on fire for fun and profit. This increasingly took up carbon that had been trapped deep underground for many millions of years and released it into the atmosphere. Some of this carbon (just over half, in fact) was taken up by the oceans and terrestrial ecosystems. The rest, however, accumulated in the atmosphere, causing greenhouse gas concentrations to rise.
This is the best estimate of the current carbon cycle flows in billions of tons of CO2 per year.
This is how much CO2 we've emitted since 1850, minus the amount that oceans and terrestrial ecosystems take up.
To stop this greenhouse gas from rising, our total net emissions of CO2 must reach zero. It's as simple as that. We can do this in part by eliminating the emissions caused by some of our activities. But we will probably also have to offset ongoing emissions in order to hit net zero.
Some carbon offset projects help with this goal by preventing emissions that would otherwise have occurred. Others promise to preserve what is known in the carbon cycle as a "sink" – a permanent form of carbon like forests or even mineralized carbonate in the soil. These offsets are not always identical to prevented emissions, as they can have climate effects separate from carbon. For example, forests can be darker than their surroundings, so that their expansion leads to a local warming effect as more sunlight is absorbed. However, reforestation also has the potential to provide other ecosystem services such as animal habitats. So there are many factors that go beyond carbon.
Make like a tree and a leaf
Planting trees is an alluringly simple response to climate change. (For evidence, see: Internet comments.) However, our ability to store carbon in this way is limited. We could reverse the emissions caused by past deforestation, but we can never plant enough to meet rampant fossil fuel consumption. But even if you accept this limitation, a lot can go wrong after your seedlings are stuck in the dirt.
Assuming you find a program you can trust to actually plant the trees that were promised, many things will affect their carbon cycle effects over time. The first is the way tree species grow. Some grow quickly and accumulate a lot of carbon per acre for the first ten years, which immediately helps our carbon emissions footprint. However, fast growing trees are typically less dense than slow growing species, so the final amount of carbon stored per acre is less.
Fast growing species tend to be shorter too. If trees in this plantation are cultivated for harvest, the end of life will come sooner. Then where does the carbon go? Some could be turned into sawn timber. At this point, its value as a carbon sink depends on how long things built with this lumber last. The leftover plant matter decays or burns and releases carbon back into the atmosphere, ending its time as a sink.
Even if the trees that are planted are never harvested, their carbon could be released in other ways. Old trees naturally die and rot. Insect infestation can rush through forests and kill trees en masse. Forest fires can do the same thing. Some of these risks are increasing due to climate change, which means that carbon storage in forests in many regions becomes less and less secure over time.