[SOUND] [MUSIC] The mean equilibrium line on a glacier is A, the altitude at which the glacier starts. B, the altitude at which the glacier terminates. Or C, the altitude at which the losses from the glacier by melt equal the gains from snow fall. The answer is C, the altitude at which the losses from the glacier by melt equal the gains from snow fall. This next section deals with how the chemistry and the physical properties of the land and the atmosphere interact. And the importance of this is that the land provides something called feedback to the climate system. These feedbacks are processes that are amplified or retarded, dampened, or even returned into an opposite direction by processes on the land. So of course what happens on the Arctic landscapes is extremely important for how the climates of the planet may be affected in the future. So for this section of the course, we will look at some of the feedbacks and the processes behind them. Of course, we all know that carbon dioxide concentrations are increasing in the atmosphere. And that hits our newspapers all the time. And we know that most of these are from burning fossil fuels. But in fact the CO2 concentrations in the atmosphere above the Arctic are far greater than elsewhere on earth. And if we look at the graphic, you can see that if we start in the Antarctic, the CO2 emissions are very small. As we move through the red band, which is the equator, where our reference sites are measuring CO2 in the atmosphere, we can see a little bit of seasonality coming in. But by the time we get to the high latitudes in the Arctic, we see really high concentrations of CO2, really big variations between summer and winter, when we have CO2 draw down by the tundra in summer and massive releases of CO2 in the winter. These releases of carbon dioxide are sometimes the result of respiration of carbon in soils in the Arctic. And we touched on this in an earlier section, the imbalance between decomposition and photosynthesis, which leads to accumulations of carbon in soil. Scientists have now mapped the amount of carbon in the soils throughout the Arctic down to a depth of one meter. And this map shows the concentrations of carbon. And what we find, is there are vast stores. These vast stores contribute about 1,700 petagrams. A petagram is a very big number. So the amount of carbon in the soils, in the top meter of soils in the Arctic is more than twice the whole of the carbon dioxide we have in our atmosphere. And it's also almost twice the amount released by the burning of fossil fuels which is about nine petagram. At the moment, that carbon in the ground is safe. It's not acting as a greenhouse gas. But we are worried about what happens in the future with warming and how that will that will affect the stores of carbon. And whether carbon will be mobilized from where it's safe now, and be emitted back into the atmosphere. If we look at these processes slowly, one-by-one, climate can convert relatively inactive carbon, matter from dead plant material in the soil into two greenhouse gases, one carbon dioxide, and one methane. And methane over a period of about 100 years is about 30 times stronger than carbon dioxide as a greenhouse gas. So we are particularly worried about releases of methane from the soil into the atmosphere. If we look at the current Arctic, or what's happened over the past, and start with looking at the summer processes, then in summer, carbon dioxide is taken out of the atmosphere in photosynthesis by green plant. Some of the CO2 that's been captured over the past periods is released in what we call heterotrophic respiration. That means the microbes that are working in the soil and are respiring, breaking down dead plant material, release carbon dioxide back to the atmosphere, but in smaller quantities than the capture of carbon by green plants. Also, those green plants themselves release carbon dioxide from the respiration of roots and other plant tissues like stems and leaves. But together the release of carbon by respiration is smaller than the drawdown of carbon dioxide by photosynthesis. And that leads to these big carbon accumulations that we see in the soils. If you have a dry system of permafrost, then a little bit of carbon in the form of methane can be taken up in the soils. If we now move from summer to winter, of course, the green leaves are now either gone or they're covered by snow. And the system is relatively inactive, but we can still have some releases of carbon from respiration, both types, into the atmosphere. The big thing in winter is something slightly different, particularly in late winter when we have the sun above the horizon. And that's not an exchange of greenhouse gases, but a reflection of the energy coming from the sun back into the atmosphere. And that is due to something we call albedo, reflectivity. And that reduces the temperature of the area, and in fact planet Earth. So the ice and snow in the Arctic is extremely important for cooling planet Earth. If we now move forward from the current period into a warming climate of the future, what happens? Well the first thing that we can see is the permafrost now has started to thaw. And it's led to an accumulation of water where we didn't have water before. And we also see the plants have grown. The plants have grown resulting from a warmer period. So what happens then is that the amount of carbon captured by increased growth of green plants, it increases. I have a bigger green arrow. But the amount of carbon released from warming soils is even bigger. So we have the plants working faster, the respiration working faster. And most importantly, the carbon in the soil is very often old carbon, previously stored in permafrost, going into the atmosphere. So the release of greenhouse gases is greater than the capture. If we now go into late winter in a warming climate, we can see two things have changed. One is that more CO2 is being given off during the winter period than before, because the winter period is shorter and because we have spells in winter where there is no snow. That's one effect. The second effect is that its albedo, the reflectivity is now broken up because it's not just a pure snow surface. It's snow with vegetation above it. And the vegetation absorbs radiation rather than just reflecting it back into space. One important thing that I should say is that methane is released from these wet areas. And you see that particularly under the summer warming regime. The concentrations of methane are so great, sometimes, in the lake bottoms or pond bottoms, that you can actually ignite the methane. And you can see bubbles, as on the right-hand side, trapped in the ice as ice formed on the lake. And importantly, that amount of methane, I think or scientists think depends on the amount of organic material in the ponds, and this is determined by the size of the water body. So if we go through from the bottom here, which is just a very small, less than a meter area of water in a seepage area through to tundra ponds through to a big lake. Then we see that even for the same area of open water, the emissions of methane increase substantially. You've seen the big map of the whole Arctic and the carbon and the soils. You've seen that the amount of carbon varies from wetlands to dry lands. And this is just a graphic from Northern Siberia that shows that even in an area of 1 square kilometer, we have big differences in the concentrations in carbon and soil moisture, which will determine the release of carbon to the atmosphere. I mentioned the word albedo, and albedo is of major concern in an understanding of how the Arctic will change the planet's future climate. In this particular project from Northern Greenland, equipment have been set up, as you can see the mast here, to measure energy, various types of energy exchange between the land surface and the atmosphere. And here you can see a measurement of the reflectivity. And if we look along the bottom axis of the graph, you can see that snow is reflecting between 80 and 90% of all the energy falling on it. That cools planet Earth. But then if you look at what happens if that snow is replaced by heath, by meadow, by wetlands, by shrubs and by bare soil without vegetation, then only 10% of the energy is reflected. 90% is captured, and that increases the heating of the area. Another feedback to the Arctic is the release of very tiny particles, and they're called biogenic volatile organic compounds. They're very chemically reactive with the atmosphere so they interact with ozone, for example. And most importantly, they can help in the formation of clouds, and these clouds can actually cool the surface. What controls them? Well, we don't know an awful lot about biogenic volatile organic compound, but we do know that different types of plants emit different substances in different quantities. And if they're damaged, even more can be released. So at the moment we have some rather simple experiments where we simulate grazing by reindeer to see how many, what effect that has on the release of biogenic compounds. And we also use little shelters to increase the temperature, to see what effect temperature has. What are we learning? Well we learned that damage affects the volatile organic compounds by increasing their release, and we see that temperature also affects their release. If we look at the top graph with the red dots, we see that as temperature increases, so does the release of isoprene, an important volatile organic compound. If you look at the graph underneath, you can see that both methanol and isoprene vary in their release day-by-day, hour-by-hour. Which is true? A, methane is a more powerful greenhouse gas than carbon dioxide. B, carbon dioxide is a more powerful greenhouse gas than methane. Or C, methane and carbon dioxide are similar in their strengths as greenhouse gasses.
