The principles of the terrestrial climate were outlined in the first handout, and I gave the four main reasons how climate could change. The main factors with anthropogenic influences are albedo and 'greenhouse' variations. The orbital and solar fluctuations are not (yet) influenced by people. The notion of an anthropogenic greenhouse enhancement goes back to Arrhenius at the end of last century, and it has a strong scientific rationale. More greenhouse gases mean that more heat that would be radiated back into space is trapped in the atmosphere, and the temperature will have to go up slightly so the overall heat flux to outer space will be back to 239 W/m2. The observations from ice cores and the record of atmospheric composition from Hawaii since 1958 both show convincingly that the amounts of CO2, methane (CH4) and nitrous oxide (N2O) have increased dramatically since the middle of last century. For CO2, that increase is most likely linked with the burning of fossil fuels. Remember, that fossil fuels formed over periods of millions of years, and they are now blown back into the air in a few decades. This leads to a strong disruption of the global carbon cycle. The methane and N2O fluxes are related to decay of organic matter and changes in land use patterns may catalyze these flux enhancements: e.g., extensive water-logged soil areas may emit large amounts of methane and N2O. Methane and N2O absorb infra-red light at a wavelength where very little other gases absorb. As a result, the impact of even small amounts of methane and N2O have a measurable effect on the radiation balance. The CO2 window of absorption is already very "dirty" and so small added amounts of CO2 have little impact and large amounts of CO2 added have moderate impacts.

The amount of carbon in the atmosphere is part of the global carbon cycle, which is strongly influenced by the activity of plant life. The amount of CO2 in the atmosphere is very small compared to that dissolved in the oceans and in rocks. As a result, many different processes can influence the atmospheric CO2 levels. The rate of photosynthesis is a function of sunlight and the availability of nutrients and trace metals. The essential nutrients are P (phosphorus) and N (nitrogen), the latter only usable by most plants as nitrate (NO3-). The P is weathered from rocks, but the N comes from the N2 in air and is first converted to nitrate. Bacteria in soils convert the air N2 into nitrate and make it available to plants. It is interesting to note that the marine biosphere is totally dependent on the landsoils for its share of nutrients: rivers bring the fixed N and weathered/dissolved P to the oceans where it is used by marine plants. Much of the nutrients in the oceans are rapidly recycled in endless cycles of growth and decay.

When we add the fossil fuel emissions of the last century up, we could predict the increase in atmospheric CO2 if it all remained there. It turns out that there is about 40 - 50 % less than expected, so part of that carbon has gone "somewhere". We can think of the upper layer of the oceans, where the CO2 willl dissolve in the water. The surface waters are dragged to greater depth by the thermo-haline ocean circulation, so part makes it way to the deep sea. The enhanced atmospheric CO2 levels may promote a faster growth of the terrestrial biosphere, and as P and N are available in abundance, the biomass may increase in size. So there are various pathways that "buffer" a strong increase in atmospheric CO₂, but still, the levels are rising. The strong influence of the biosphere is also noticeable through the seasonal oscillation of CO2 levels, which is largely driven by the N-hemispheric terrestrial biomass seasonality.

It will take many decades to hundreds of years before this CO2 wave in the atmosphere will dissipate, but we can help to avoid further build up by limiting CO2 emissions. Sell that V8-car and use public transportation! The IPCC has developed various scenarios that relate population growth and per capita energy use to CO2 emissions. It turns out that our most abundant fossil fuel (coal) has the most kg CO2 / energy unit (low caloric content), so coal is for many reasons an environmental unfriendly energy source. In reducing CO2 emissions we can look at the developing world, and try to limit their energy consumption, or we can also attack the energy hunger of the western world. WE can think of various approaches:

1. limit population growth

2. limit per capita energy growth - public transportation, efficient heating and cooling systems and buildings

3. store the waste CO2 somewhere else then in the atmosphere

4. switch to non-fossil fuel based energy systems (wind, solar, nuclear)

The third option is actively being researched in Holland, where there are many drill holes into old gas fields that have been used for natural gas exploitation. Since we know that the fields contained the natural gas very well for many millions of years, we can pump the CO2 back back to where it came from, so to say through the holes. This may be an attractive and cost-efficient way to limit CO2 emissions, which can only be applied to large central power-generating facilities, not your average family coal stove. Other options that were explored are storage of CO2 in the deep sea (dissolution in seawater at 3 km depth) or conversion into a solid, like CaCO3 (calcite). The latter would be expensive since the process needs Ca(OH)2 or Portland cement-type material.