Typhoon Jangmi a few hours before
hitting Taiwan. Image: Jeff Schmaltz
MODIS Land Rapid Response Team,
Typhoon Jangmi struck Taiwan on September 28th, 2008. This was the strongest storm to hit Taiwan in 2008, and it killed two people and injured 30 more. Along with the devastation, the storm did something else: Jangmi was responsible for burying many tons of carbon on the ocean floor.
Geologic processes are often perceived as occurring very slowly, over decades to eons. However, more and more often, we are recognizing that short and intense events, such as typhoon Jangmi, may really drive geologic systems. A recent study in Nature Geosciences showed that the carbon eroded during a typhoon may be 100 times greater than the norm. This is important, in part because predictions for future climate change include an increase in these carbon-burying storms.
Let's look at the process as a whole first. The primary geologic mechanism of long term carbon storage involves burying that carbon in deep ocean sediments, where it will likely stay for millions of years (at least until it turns into oil). This carbon typically comes from one of three places: organisms living in the ocean that died and dropped to the sea floor, carbon that has bonded to minerals as part of natural weathering, and organic carbon eroded from the land surface and carried out to sea. Typhoons cause massive erosion on land, and this eroded material is carried far out to sea.
In the past, we have thought that, in large storms, the eroded material is primarily sediment. Even though carbon is part of that sediment, it was believed to be old carbon that came from the rocks themselves, thus it was not considered a net carbon sink. The thinking was as follows: in small rainstorms, water carried organic material from the hillsides and river banks to the ocean. But, in large storms, erosion is dominated by landslides which primarily carry rocky material.
The current studies measured not only the carbon load in the LiWu river in Taiwan, but also the age of that carbon. By using carbon 14 dating methods, they were able to distinguish ancient carbon from modern carbon, and what they found was surprising. More than 40 percent of the carbon transported by river during typhoon Mindulle in 2004 was modern carbon, while less than five percent was modern during the rest of the year. When they estimated modern carbon transport in the LiWu since 1970, they determined that 77-92 percent of it occurred during large storms.
Of course, not only do you have to get the carbon from the land to the sea, it has to stay there too. It has been estimated that 90 percent of the carbon transported to the ocean is returned to the atmosphere but, if that carbon is transported along with copious amounts of sediment, it can be buried fast enough to stay put.
Because typhoons cause large amounts of inorganic sediment to be transported along with the carbon, it is more likely that this carbon will be sequestered for the long term. Unfortunately, they do not have any estimate of just how much of this carbon does end up trapped in the deep ocean.
Finally, it should be noted that global carbon fluxes are measured in gigatons of carbon per year, while estimates of carbon transported by rivers is on the order of tens of megatons of carbon per year. Thus, the net effect on the global carbon cycle is small, but every little bit helps.
Nature Geoscience, 2008. DOI: 10.1038/ngeo333