And the subduction process is quite complicated, giving rise to earthquakes and de-watering through chemical reactions. The water driven off contributes in ways still not completely understood to the generation of magmas that produce volcanoes, such as those in Japan.
This cycle has been going on through most of Earth's history. The oldest ocean is no more than 5 percent of the age of the Earth, and so this conveyer belt that moves oceanic plates from birth to death must have gone to full cycle more than twenty times, and probably a lot more than twenty times. I want to emphasize again the critical role of water in cooling, in modifying the chemistry of the crust, in contributing to the flux of materials off the subducting lithosphere, and in leading to the formation of volcanic centers above the subduction zone.
The second example of one of these coupled cycles I'll talk about is that between the solid planet and the atmosphere. And that coupling occurs in many ways, but one of them is through large volcanic eruptions. Because large volcanic eruptions throw gas and ash into the atmosphere (fig.20), and some of the gas interacts with water, creating aerosols, some of it rains down. But both the gas and the dust modify the radiative balance in different parts of the atmosphere.
And there's a transient that depends on the size of the eruption and the duration of the eruption. What you see on your left (fig.21) is an image of SO2 in the eruption plume from Mount Pinatubo which erupted in 1991 in the Philippines and was the largest explosive eruption of this century. The eruption has been well studied with respect to induced changes in climate and had a significant effect on the climate for more than a year.
We know that earlier in Earth's history there have been volcanic eruptions much more massive than Mount Pinatubo, and an important unanswered question is the effect of these actions of the solid Earth, like volcanic eruptions, on climatic variations. What are the extreme ranges of effects that volcanic eruptions can produce, and how would we study this in the records of climate ?
A third type of coupled system I will briefly mention is that between the oceans and life. On the left is an image that is labelled global biosphere (fig.22), which is a little misleading. It is a diagram produced by the National Aeronautics and Space Agency in the United States, and it represents a marriage of two kinds of remote sensing data： one is ocean colour, which is sensitive to the chlorophyll content of the surface waters, so all of the oceans are depicted in various colours. Purple corresponds to low chlorophyll, and as you move through the colour spectrum up to blue, green, yellow, red, you get higher and higher contents. And then on land, by using a combination of visible and infra-red imaging, the colours go from yellow to green and darker green, representing greater and greater biomass in forested areas.
So that's a kind of snapshot of the global biosphere. But what is dear, by looking at variations with time, is that there are very important interactions (beside seasonal changes) between the dynamic parts of the Earth and the biosphere. One famous example is shown here (fig.23), in which there is an interaction between wind and ocean currents in the eastern parts of the Atlantic shown on the fight, and the Pacific shown on the left, that affect the upwelling, the transport of nutrients to surface waters, and therefore, the high chlorophyll content depicted in these images here off Peru and off west Africa. And these, of course, are areas that are important for fishing grounds, because these are areas where the nutrients in the water are very rich.
Variations in upwelling, of course, affect the fishing industry, and the most famous area in which that affected industry contributes to our understanding of the coupling system is off Peru. All of you have heard of the phenomenon known as El Nino. El Nino is an event off western Peru, named by the Peruvian fishermen, when the surface waters are unusually warm, the nutrient levels are unusually low, and fishing is very poor. And it is named E1 Nino because that is the Spanish name for the Christ Child, and this event tends to occur around Christmas time.
