The Signal In The Atmosphere

Although its trend is always upwards, the actual amount by which the global C02 concentration increases tends to vary from one year to another. Years in which tropical forest regions are slightly hotter than usual tend to have a greater C02 increase. This suggests that in these warmer than usual years the tropical forests lose carbon through some sort of temperature-dependent process, perhaps increased respiration by the leaves, or increased rotting of dead wood and other litter in the forests. It seems that forests in South-East Asia and in the Amazon Basin particularly dominate this temperature response (Figure 7.17a, b*).

Could it be the effect of drought on the rainforest which is actually causing the big rise in global C02 in a hot year in the tropics? After all, the sunny, cloudless skies associated with high temperatures may also tend to be associated with lack of rain. However, when we look at the data in detail, drought in the tropical forests does not seem to have nearly as strong an effect as temperature itself; the statistical relationship of global C02 increment with rainfall is much weaker whereas if drought were so important we would expect it to be stronger. Nevertheless, there are exceptions—the big droughts that are associated with some El Nino events do seem to have at least some effect on carbon release in South-East Asia, where fires set by farmers often spread into tropical forests and burn across huge areas, sending smoke and haze across the region. For example, such extensive fires occurred across Indonesia and parts of Malaysia during droughts in 1982/1983, and also in 2003/2004, that they shut down airports hundreds of miles from the sources of the smoke.

Years with an El Nino event are generally strongly correlated with a large global C02 increment. However, except for the really extreme ones associated with strong droughts and fires, it looks like El Nino operates more through bringing about high temperatures that affect the carbon balance of the forests (El Ninos are generally associated with warmer conditions in the main tropical rainforest regions), rather than causing drought. Strangely, given the strength of the relationship between an El Nino event and the C02 increment that follows, it is not completely clear whether it is the opposite "La Nina" event (which often follows on from an El Nino) that is the major player in this release of carbon.

The mid-latitude forests of the USA, Scandinavia and northeastern Asia also seem to play a role in affecting variability in C02 increase each year, but their effect is weaker. It is also opposite to the trend in the tropics: in a warm year the mid-latitude forests tend to take up more carbon. The trend is also rather complex; a particularly cool year seems to shut down the decay of leaves and wood on the floor of the boreal forests of Siberia and Canada, and because there is so little decay, much less C02 is released from this forest litter to the atmosphere. This more than cancels out the smaller C02 uptake due to reduced photosynthesis in the tree leaves in a cooler year. A year like this occurred in 1991/1992 after the big volcano Pinatubo exploded in the Philippines and altered climate around the northern hemisphere with the cloud of sulfuric acid that it pushed into the stratosphere. The northern forest zones were cooled, and with less decay the C02 increment in the world's atmosphere during that year was unusually small.

Figure 7.17. (a) This map shows the strength of correlation between temperature and global C02 increment each year and that C02 increment in a given year is correlated with mean temperature in the tropics. When temperatures in South-East Asia and Amazonia are higher, there tends to be a big increase in global C02 in that year (NCEP/NCAR re-analysis, N0AA/ CIRES Climate Diagnostics Center). (b) A map showing the correlation between the amount of rainfall and the size of the global C02 increment around the world. The relationship to rainfall in forest regions of the tropics is much more scattered and weaker overall, suggesting that heat rather than lack of rainfall may be more important in producing a burst of carbon from the tropics is some years. This might be due to some combination of faster decay, poor photosynthesis and growth of trees under heat stress, or more rapid evaporation stressing trees and preventing photosynthesis. Source: Author, in collaboration with Gianluca Piovesan.

Figure 7.17. (a) This map shows the strength of correlation between temperature and global C02 increment each year and that C02 increment in a given year is correlated with mean temperature in the tropics. When temperatures in South-East Asia and Amazonia are higher, there tends to be a big increase in global C02 in that year (NCEP/NCAR re-analysis, N0AA/ CIRES Climate Diagnostics Center). (b) A map showing the correlation between the amount of rainfall and the size of the global C02 increment around the world. The relationship to rainfall in forest regions of the tropics is much more scattered and weaker overall, suggesting that heat rather than lack of rainfall may be more important in producing a burst of carbon from the tropics is some years. This might be due to some combination of faster decay, poor photosynthesis and growth of trees under heat stress, or more rapid evaporation stressing trees and preventing photosynthesis. Source: Author, in collaboration with Gianluca Piovesan.

In the tropics there is also a weaker and rather mysterious two-year delay between a blip in temperature and a blip in their contribution to the global CO2 increment. Compared with the "immediate" (same year) effect of temperature on CO2 release by the tropics, the two-year lagged effect is the opposite: it takes up rather than releases more CO2 in response to a warmer year. It is thought that this lagged response has something to do with the effect of increased temperature on recycling of nitrogen in forest ecosystems. In a warmer year more decay occurs, enabling nitrogen bound up in dead leaves and other material on the forest floor to be released as nitrates and ammonia that can then be used by the trees again. This produces a burst of growth of new leaves and wood about two years later when the trees have adjusted to the increased supply of nitrogen; and the addition of those new leaves takes up CO2 from the atmosphere as they begin to photosynthesize. The apparent role of the tropics, and to a lesser extent the northern forests, in controlling how the CO2 increment varies from year to year echoes their role as a consistent major sink of CO2 on a global scale. It is not surprising then that if the sink of carbon in the tropics is greater, variability in its behavior affects the CO2 increment the most.

As one would expect if plant and fossil fuel carbon is being burned to give CO2, the oxygen concentration in the atmosphere is slowly declining. The amount of the decline shows a seasonal wiggle that is the opposite of the CO2 wiggle: in the summer oxygen levels go up a bit as there is more photosynthesis producing oxygen and storing carbon in leaves. In autumn and winter the leaves decay back to CO2 taking up oxygen, and the oxygen level goes down. Although the amount of oxygen in the atmosphere is declining, there is plenty left for us to breathe. The total amount that has been lost in the last 200 years is much less than one-thousandth of the oxygen in the atmosphere.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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