Rising levels of CO2 from fossil fuel emissions contribute to a rise in earth’s temperature, and is often demonized because of this effect. What usually go unmentioned are the subtle effects that greater CO2 levels have on living organisms.
Plants require CO2 to perform photosynthesis. Photosynthesis is the process by which plants take light energy, water, and CO2, and convert it into sugars and oxygen – providing both the food we need to survive and the oxygen we need to breathe. In order for plants to perform photosynthesis, they need to get CO2 into their leaves. CO2 enters the leaves of plants through tiny holes called stomata. When CO2 enters, water is lost. In today’s atmosphere plants are usually limited by CO2 in their ability to grow. Thus we come to the first positive effect of increasing CO2 in our atmosphere: plants tend to grow better at higher CO2. This comes with a caveat however – this increased growth requires more nutrients, and plant growth is usually limited by the nutrients available in the soil (this is why farmers need to fertilize their crops). Next I will give an example of this effect.
Some scientists study the effects of elevated CO2 by using an equipment setup called a free-air CO2 enrichment (or FACE, for short). Without going into too much detail, the scientists take an experimental forest, and construct a series of pipes to deliver CO2 to the trees. To view and example of how these look when their setup, go to http://face.ornl.gov/expdes.html to view the Oak Ridge FACE site. Many of these experiments run for up to ten years, and one of the studies from the Oak Ridge FACE site found that, after 11 years of CO2 enrichment, the growth-enhancing effects of increased CO2 wore off (Norby, Warren, Iversen, Medlyn, and McMurtrie, 2010, Proceedings of the National Academy of Sciences 107:19368-19373). This study found that nutrients became limiting – essentially the plants that grew faster from having more CO2 depleted the soil of nutrients faster than they could be replenished. Without extra nutrients to support extra growth, the growth rate of the plants declined to a normal level without extra CO2.
From my example we learn that increasing CO2 can be a good thing, but its benefits might only be temporary, depending on whether the plants can meet the nutritional requirements needed for increased growth. On the other hand, if we look at plants that we fertilize for agriculture, increased CO2 will support greater yields and a greater food supply in the future. However, we need to remember that higher growth also requires more water, and weather patterns could change dramatically in the future. My take home message is that while a single study on a given environmental factor might show a positive effect (e.g. CO2 in the above study), other aspects of the environment can complicate the effect (as nutrient availability did in the study), making it difficult to predict how plants will respond. Our world is vastly more diverse and complex than we could ever imagine, and we are only just beginning to get an idea of how little we know and understand. So give scientists a break when they take back something they used to claim as true – often times it is because new evidence told them they were wrong, so they stepped up and changed their view of the world like a true scientist should.
Stay safe and stay informed,