Climate Dynamics I - ATOC 530
Summary of week 3
In the first half of the third week, we learned more about the global hydrological cycle on
Earth, with particular emphasis on the arctic hydrology. We concluded the week by talking about
the global carbon cycle of the Earth.
The global hydrological cycle budgets the flow of the water in all its possible forms. There is
evaporation over the oceans, which produces clouds that will precipitate back. A part of it finds its
way to the land through the atmospheric circulation and precipitates there. Evaporation takes also
place over the land, as well as transpiration, which will also precipitate back, but only over the land.
The last component of this cycle is the runoff that transports some land water to the oceans. The
mean rate of precipitation over the whole Earth is about 1 m/yr, which gives a total volume of water
exchanged of about 500 km3/yr. The biggest part of it comes from the oceans: 80% of all the evaporation and transpiration happen there, with 90% of it being precipitated back over them. However,
this represents only a tiny fraction of the total volume available in the oceans, which contain almost
all the water (the remaining is the ice (about 2%), lakes, rivers and atmosphere (traces)). On the
other side, a lot of the water precipitating over the land then comes from the recycling of its own
sources: the evaporation over the lakes and the rivers and the transpiration of the animals and plants.
That represents about 70% of the land budget, the remaining coming from (or going to) the oceans.
Examples of this budgeting can be found at smaller scales. For instance, we looked at the
arctic hydrology. Doing so, we learned that the ocean water is fresher there for two main reasons: 1st
10% of the total river runoff is drained in only 1.5% of the total ocean volume and 2nd the runoff is
lighter (fresher), which means that it stays on top of the ocean, with some mixing happening. This
last point explains also why there is ice in Arctic but not in Atlantic: the surface layer of fresh water
is the only one that need to get to its freezing point in order to get ice formation, since it does not
mix with the deeper layers. Therefore, the depth of water to cool down is reduced a lot and the
freezing point is higher, as salt decreases it. Finally, the presence of a cold halocline under the layer
of fresh surface water, created by the formation of ice and associated with the Eurasian shelves salt
rejection, buffers the surface waters from the warmer Atlantic water beneath the halocline.
The global carbon cycle is also a budget, for all forms of the carbon. It is a bit more complex
since it has six reservoirs (plants, atmosphere, surface soil, rock, surface oceanic layer and deep
ocean) and, therefore, more possible exchanges (through photosynthesis, oxidation, degradation,
fire, burial, chemical weathering, volcanoes, ...). The carbon transfers get smaller as the reservoir
gets bigger. For instance, the atmosphere and the plants (the smallest carbon reservoirs) exchange so
much together, that every 6 or 7 years, they have recycled all their carbon, while still exchanging
with other reservoirs. The cycle can also be seen as the sum of two subcycles: one organic and one
inorganic. The former is often thought to take a big part in the climate regulation. However, the two
subcycles are related through some exchanges made between the reservoirs. For example, the photosynthesis transforms some inorganic carbon of the atmosphere into its organic form and puts it in the
plants, with some nitrate or phosphate. Then, we saw that the atmospheric CO2 level varies with
time. This is mainly due to three causes: 1st an increasing trend of burning of fossil fuel (mostly
since the industrial period), 2nd a seasonal trend following the northern hemisphere cycle (since it is
mainly due to the photosynthesis and there are more green plants where there is more land) and 3rd
the el niño cycle with its period of 7 years (as warmer water can dissolve less gas). Finally, we
learned about two positive feedbacks linked with the vegetation and warming climate: one related to
the albedo of the vegetation (as warmer climate allows forests to replace tundra, which decreases the
planet albedo, implying more warming) and the other to the effect of the amount of precipitation on
the vegetation (as a warmer climate can imply increased precipitations, allowing forests to replace
grassland, which gives more evaporation and, then, even more precipitations).
Jasmine Rémillard