Day 2 Part 4
In Day 1 of this lab, we focused on the carbon cycle in terms of sources and sinks. In the first few parts of this lab, the emphasis has been on human activity – mostly the burning of fossil fuels – as an additional source of carbon that gets emitted into the atmosphere. As with any system in nature, the other parts of the system can respond to such a stress on the system’s equilibrium. One response from the parts of the carbon cycle system to the increased CO2 levels from fossil fuel combustion could be increased absorption of CO2 by green plants to drive photosynthesis. There is evidence that more CO2 is being absorbed by these plants than scientists had thought (e.g. http://www.nytimes.com/1993/06/08/science/a-forest-absorbs-more-carbon-dioxide-than-was-predicted.html).
Given the importance of this process, let’s turn our attention to the terrestrial biosphere as a carbon sink. As shown in the picture on the left below, the terrestrial biosphere absorbs approximately three petagrams of carbon annually. When you see the two words “terrestrial biosphere” you should think of forests. There are three general types of forests across the globe: (1) tropical forests, which are predominantly evergreen broadleaf forests, (2) temperate forests, which have varying types of trees as a result of yearly variations in the temperature, and (3) boreal forests, which are predominantly evergreen needleleaf forests. Click MODIS Land Cover to open the file in Google™ Earth and including location of these different types of forests. [The image was derived from data from the MODIS (MODerate resolution Imaging Spectroradiometer) instrument on NASA’s Terra satellite (see the picture on the right below).]
Q16: Identify one place across the globe where there is a (1) tropical forest, (2) temperate forest, and (3) boreal forest?
Q17: Which of the three forests do you think is the largest carbon sink and why?
On Day 1 of this lab, you learned that green plants within the terrestrial biosphere can act both as a carbon sink – as they turn CO2 and water into carbohydrates through photosynthesis – and as a carbon source – as they respire CO2 produced by the metabolism of carbohydrates when the plants need energy. What is really important to know is how much more carbon is taken up by photosynthesis than released through respiration. This is known as net primary production (NPP). Positive values of NPP indicate that more carbon was absorbed by photosynthesis than was released by respiration (i.e., that the terrestrial biosphere was acting as a carbon sink); negative values indicate the opposite situation.
NPP changes over time and space. We can explore these changes for one year by clicking on NPP_2010 to open this file in Google™ Earth and watching the animation of the variation of the NPP values for different places over that year. The data come from NASA’s MODIS instrument. Values range from near 0 grams of carbon per square meter per day to 6.5 grams per square meter per day.
Q18: Which forests have large positive values of NPP during each month (i.e. what forests have a green color year-round)?
Q19: Why do these forests have little seasonal differences in NPP?
Focus on North America by typing “North America” in the “Fly to” box.
Q20: Which of the three types of forests in North America had the highest NPP values during the summer months? Why did this occur during the summer and not during the winter?
After viewing NPP values for each month of 2010, you should now revisit your response to Question 17.
Q21: Which of the three forests do you think is the largest carbon sink and why?
Scientists are still trying to figure out which of the three forest types is the largest carbon sink. The three images below provide recent information on the terrestrial biosphere carbon sink. The first two images are the first pages from two articles from the journal Science, while the third image is a portion of Table 1 from the second Science article.
Q22:Which of the three forests is in fact the largest carbon sink?
