Why doesn't carbon dioxide suffocate us all?

CO2 is indeed heavier than air and when it exists at high concentrations it does displace the air at ground level, sometimes with disastrous consequences.

Within the atmosphere there is a constant mixing of the air caused by convection currents and the winds As a result, heavy gases mix with the lighter ones.

At STP* the heaviest gas is tungsten hexaflouride, this exists only in minute quantities. If there was no mixing in the air then there would be a very thin layer of this gas at ground level, it would accumulate in depressions and valleys.

In fact, without any mixing, the atmosphere would be layered with the densest gases at the bottom. Every time you went up or down a hill you’d pass through various layers of gas. Close to sea-level would be various layers of poisonous and flammable gases including chlorine, hydrogen sulphide, sulphur dioxide, xenon, argon, krypton, propane, benzene, butane and toluene.

The first couple of hundred metres of the atmosphere would kill anyone who entered it without breathing apparatus.

Because carbon dioxide is heavier than air, if there is a significant release of this gas, it initially acts like a fluid by spreading out and running downhill. This is where it can become deadly.

CO2 can become trapped in solution on the bed of lakes, the downward pressure of the water compresses it and very significant deposits can built up. If something happens to disturb the water, such as an earthquake or landslide, or the pressure simply gets too high, the CO2 can be dramatically released in what’s known as a limnic eruption or a lake overturning.

The most significant such event in recent times occurred when Lake Nyos in Cameroon overturned in 1986. The CO2 erupted from the lake and silently flowed into the surrounding valleys. The gas cloud killed about 1,700 people as they slept in their beds.

Over a billion cubic metres of CO2, weighing more than 1,500,000 tonnes was released. It caused such violent turbulence in the lake that the shoreline was scoured of vegetation to a height of about 25 metres above the lake. The cloud moved through the valleys at about 40km/h and suffocated people as far as 25km from the Lake.

This was the first major limnic eruption in modern times and as a consequence many lakes in which carbon dioxide can accumulate are being carefully monitored. Some now have degassing columns that allow the CO2 to escape to the surface before it can built up to dangerous levels.

STP = Standard Temperature and Pressure, 25°C and 1 atmopshere.

It's well mixed throughout the atmosphere (troposphere and stratosphere), primarily due to convection and diffusion. Think of it this way, a severe thunderstorm may have updrafts of 80 miles per hour or more, that can lift hailstones lifting more than a pound, it's not going to have any trouble lifting a CO2 molecule.

That does not mean that it's impossible for carbon dioxide to suffocate us. At nighttime, when there are light winds, no convection and a stable atmosphere, there is very little mixing. Conditions like that were responsible for the large number of deaths (>1000) when Lake Nyos "burped" a large amount of carbon dioxide in 1986.

EDIT: Carbon dioxide DOES accumulate in unventilated mines and can be deadly. It can be given off by the slow oxidation of wood timbers in the mine.

CO2 is taken up by plants and converted to oxygen. Plus the atmosphere near earth is well mixed due to convection, winds, etc.

A nice question.You should consider CO2 as a water that entering a sinking boat and trees as a motor that pumps out the water.Thus level of O2 in the lower atmosphere retains and that of CO2 decreases!

The gases in the air mix with Carbon Dioxide.

In minesn they have shafts where oxygen goes down through. And without doubt in a basement, your doorway or your window allows oxygen to enter.

well we also have other gases, nitrogen which make most of our surroundings,and inert gases. if they really accumulate, the trees and planktons are sufficient to turn them into O2. too much O2 will also kill us just so u know. the world works just nice i think

Don't worry, CO2 is working hard on this issue!

I'm just guessing but probably because of diffusion.