It is certainly not favourable as the Arctic melts, but that sea levels will rise as the Arctic melts is incorrect. The fact is that ice floats and the Arctic is a floating ice mass without land underneath it. A simple formula then shows that when a floating iceberg melts, the water from the molten iceberg has the same volume as the ice from the iceberg that was originally below sea:

mass = volume x density

Archimedes’ principle: A body immersed in a fluid experiences an upward force equal to the weight of the displaced fluid.

So the weight of the body is equal to the weight of the displaced fluid.

Then:

As ice melts, its mass is retained (Antoine Laurent Lavoisier: law of conservation of mass and energy), its density increases and its volume decreases.

The new volume of the ice after it has molten is then:

In other words, the volume of molten ice is as large as the volume of water displaced by the ice, i.e. the volume of the ice under water.

If you try to run the experiment with ice in a glass of water, the water will eventually run over the edge (and maybe lower at first). This is because water from 0 to 4 °C shrinks and then expands. The above reasoning applies only at constant temperature. Even then the melting of the ice in the Arctic do not affect the level of the sea. Before the ice melts, the total volume under the sea is as high as after it has melted. Only when the water temperature rises above 4 °C, sea levels will rise, but only due to thermal expansion and not because of melting ice. Salt water has no maximum density at 4 °C, but becomes denser as it cools to the freezing point of -1.8 °C. So thermal expansion always has a role in seawater and not only at temperatures above 4 °C. But since the total volume under the sea level remains the same anyway, only thermal expansion applies to the sea level rise, not the melting of the ice. Thermal expansion just occurs sooner in salt water than in fresh water (although the melting of the ice lowers the salt concentration, causing the freezing point to rise slightly).

However, the melting of the Arctic could still have an effect on the climate, since the melting of ice causes water with lower salinity to flow into the ocean. This has implications for the northern Gulf Stream (thermohaline circulation). Salt water is heavier than fresh water. Cold water is heavier than warm water. In the polar region cold and salt water sinks, cooled by the wind, resulting in water evaporation and an increase of salinity and hence weight (salinity is also enhanced by the formation of ice). This water then flows to the south. The water is reheated and mixes it with less salty water. Because the density of the cold and salty water at the poles is higher than in the south, the sea level at the poles is slightly lower than in the south. This causes warmer and less salty water from the south to flow towards the poles. Then the cycle repeats itself.

If the salinity of the Arctic water decreases due to the melting of the polar cap, the thermohaline circulation will slow down and perhaps even come to a standstill, since the pressure of the Arctic water decreases. So far, research shows that the amount of fresh water that can be released is not sufficient to have a significant effect on the Gulf Stream, however research is still ongoing. If the Gulf Stream does stop functioning, it can have adverse effects:

Less warm water will flow to northern Europe, possibly leading to a drop in temperature. Simultaneously however, there are indications that the temperature might rise. This is because the temperature in northern Europe is largely related to warm air currents. A temperature increase will increase the inflow of warmer air. There are also indications that the Gulf Stream affects the levels of CO₂ in the atmosphere. Warm water stores less CO₂ (or any other gases) than cold water. If the pump from the Gulf Stream comes to a still, less CO₂ is absorbed by the sea from the air since the cold water on the seabed is not pumped up. In addition, as the temperature rises, the sea temperature at the surface increases. This allows the CO₂ content in the air to rise and thus the temperature, causing a vicious circle (which could eventually result in a new equilibrium).

Besides temperature effects, the following effects may occur: increase in extreme weather events (floods, streams) and the mass death of plankton and algae (which are largely responsible for CO₂ uptake from the air and form the basis of marine life). On the other hand, a rise in sea temperature could also increase the amount of algae (by the warming itself and additional influx of nutrients by the extreme weather conditions), which results in an increased CO₂ uptake, resulting after some time in a new equilibrium. Other possible effects are changes in temperature and precipitation patterns in the tropics and/or polar regions, changes in air currents from the west and east of the great ocean by changes in air pressure between the two parts (El Nino effect) and complete depletion of oxygen in the sea.

In parts of the ocean where oxygen levels are extremely low photosynthetic purple and green sulphur bacteria thrive. They can survive without oxygen by burning sulphur. In addition they produce H₂S, which is a highly toxic gas. They do not like oxygen though, so they stay in deeper water, where sunlight however must still penetrate. If the entire ocean is virtually oxygen-free, these bacteria can also survive on the surface where they find more sunlight and can quickly spread across the ocean. This leads to a massive release of H₂S into the atmosphere. Research has shown that this can lead to a mass extinction of plant and animal species. In addition, the gas affects the ozone layer, increasing the effect even more.

Geologists have discovered that this process may have played a role in past mass extinctions. The trigger for this process shows a temperature rise of the ocean, caused by an increase of CO₂ at 1000 ppm. The current level (2010) is 387 ppm and over the past 20 years has increased by 39 ppm. According to climate models, the CO₂ levels in 2100 range from 592 ppm (in a scenario with a worldwide radical approach to decreasing CO₂ emissions and other environmental problems and global cooperation between countries) and 957 ppm (in a scenario with 15 billion people, no global approach to environmental problems, remaining differences between rich and poor countries, less technology diffusion and emphasis on productivity in the agricultural sector).

In short, we don’t have to worry that the sea level will rise directly if the Arctic melts. But there are possible side effects, which can have far reaching and radical consequences for the climate. However, different scientific climate models predict different outcomes, ranging from global warming to global cooling and from radical changes to new equilibriums.