I recently read that the extreme drought in western North America during 2000-2004 actually resulted in more carbon dioxide being released into the atmosphere. In the article in Nature Geoscience, it explained that such droughts can further enhance global warming. When a drought occurs, the plants wither and die and no longer uptake carbon dioxide (normally living plants serve as a carbon sink in the global carbon cycle since they consume carbon dioxide). Furthermore, after plants die and start to decay they actually produce carbon dioxide, thereby serving as a carbon source. However, as our planet warms due to carbon dioxide being released at records levels into our atmosphere, climate extremes (such as droughts and floods) are expected to continue to become more frequent and severe (giving rise to the term non-stationarity).
A graphical representation of carbon dioxide variations. From Science Blogs
So I began to wonder, is global warming causing more droughts? Or are more droughts leading to more global warming? Which caused the other first (e.g., which came first—the chicken or the egg)?
While our understanding of the Earth System would imply that droughts alone have not caused global warming, it is now clear that they can further enhance it. This is an example of a positive feedback loop in the Earth System. A positive feedback means that one process occurs, causing a subsequent process to occur that results in an outcome that further enhances the first process, and the cycle amplifies and continues over time. A negative feedback, however, would cause the opposite to happen where the subsequent process results in an outcome that counteracts or weakens the first process.
There are also examples of negative feedbacks in our Earth System. Take for example when the Earth’s ocean surface temperature heats up, it causes more evaporation from the oceans. This additional source of moisture into the atmosphere over the oceans can lead to more low-level marine clouds. Low-level marine stratocumulus clouds are often very reflective of solar radiation, so more of these clouds can thus increase the Earth’s albedo (or solar radiation reflectivity) and thereby cool the ocean surface temperatures.
Another example of a positive feedback; the changing albedo when sea ice melts due to global warming. From Vancouver Observer
Don’t be fooled, however, by the terms positive and negative feedback, which may imply one is good and one is bad. It is actually often the opposite; that the negative feedbacks are what produce balance in the Earth System, whereas the positive feedback loops can act like a runaway train. Either way, most of these processes are completely natural; however, some can and are being influenced by human activity. As responsible residents of this planet, we need to do our best to understand how our actions are affecting our home and try to prevent any runaway trains from occurring on our watch.
Suggested activity: Investigate the albedo of various surfaces near you in the GLOBE Surface Temperature Field Campaign and try to estimate if the surface cover changed, would it act as a positive or negative feedback in your local community.
-Sarah Tessendorf
