The advent of life on Earth had a considerable impact on the planet. More than 4 billion years ago, Earthís atmosphere was unbreathable: it contained no oxygen but a significant quantity of greenhouse gas, being 85% water vapor and 10 to 15% carbon dioxide (as opposed to 0.03% today). In time, a primitive form of algae known as cyanobacteria appeared in the waters of the ocean, and photosyn thesis was born. Oxygen was released into the atmosphere as a byproduct of photosynthesis and the concentration of oxygen gradually increased until it reached todayís level of 21%, remaining stable for around 550 million years.
The phytoplankton (tiny algae) in our oceans plays the same role today, renew?ing the oxygen in the atmosphere and storing carbon dioxide. Its biomass is considerably greater than that of the continental forests. Sixty to 70% of the oxygen that we breathe comes from our oceans, so it is the oceans, not the forests, that are the planetís “lungs.“
For some decades now, another life form has been considerably modifying the Earthís atmosphere: mankind itself. Our industrial activities have been releasing compounds that destroy ozone-a gas that forms naturally from oxygen and is therefore also a product of photosynthesis. Ozone is responsible for filtering the Sun’s damaging ultraviolet rays. In southern Chile and southern Australia, the “hole“ that has been opened up in the ozone layer exposes the population directly to these rays, with the result that cases of retinal damage and skin cancer are increasing in theses regions. The Montreal Protocol, signed in 1987, banned the use of gases that damage ozoneófortunately for us, it because they could have wiped out all trace of life on Earth.
Today, it is greenhouse gases that we are releasing into the atmosphere in excessive quantities. These gases are responsible for climate change, increasing the mean temperature of the planet and threatening its ecosystems. The composition of the atmosphere is not fixed, therefore. Human beings and all living organisms, in particular chlorophyll-bearing plants, are part of this process. We need them if we are to pre?serve an atmosphere that we can continue to breathe in.