Someone has speculated that if all human beings were to be buried together, we would need a coffin roughly one kilometre in each dimension. This one cubic kilometre of humans has had a long and enduring impact on the Earth, whose atmosphere alone has a volume of 182 billion cubic kilometre (and 7,500 billion cubic kilometre as a whole). Yet, humans have had a more devastating effect on the planet than any other life form. So what would happen to the Earth if we all disappeared at once?
We have and continue to alter several aspects of the Earth's surface and environment. We have made extensive changes completely at variance with the way nature organises matter and life. Let us take stock of the kind of changes humans have forced on the planet, and try to estimate the time scale over which nature will be able to wipe out all signs of human existence on earth, and the way in which it will do so.
For the purpose of this estimation, we assume that humans have simply vanished from Earth. We do not assume that there has been any war or further devastation as humans cease to exist. We take the Earth as it exists today, where human presence and interference no longer exists, and is not replaced by any beings capable of continuing human tradition. However, we do assume that at time zero of human disappearance, all other life forms exist at the current levels, i.e. no other life form is affected by whatever makes humans disappear.
We have altered the structure of the Earth in many fundamental ways, isolating and mixing matter in a manner that would not be possible in a wholly entropy driven world. The longest lasting of these are:
* Structures made with cement concrete, including human habitation sites.
* Mining, oil extraction, and destabilisation of the Earth's crust.
* Plastics and other industrial products.
* Purified metals and artificial alloys.
* Garbage and pollution of the Earth.
* Artificially created molecules and organic compounds.
* Artificial sources of energy.
* Artificial satellites.
* Several systems that are maintained well away from thermal or environmental equilibrium.
* Plants and animals dependent on human intervention.
Nature will restore equilibrium through a variety of tools that have differing timescales of operation and different zones of effectiveness. These include:
* Heating by the Sun
* Environmental chemical and physical processes such as corrosion, erosion, winds etc.
* Water cycle:
a. rains, snow
b. rivers and seas including tides and meandering of rivers
c. effects of changing water (and ice) temperatures
* Living organisms
* Fault lines, earthquakes and terrestrial, convectional currents.
* Tsunamis as a combination of earthquake and large scale movement of water mass
* Meteors, comets and other heavenly collisions
* Cosmic rays
The effectiveness and timescales of each of these is different. We discuss the aspects of destruction timescales of human constructs against the background of these forces.
| |
Human construct |
Most effective natural forces |
Timescales of operation |
| 1 |
Structures made with cement concrete including human habitation sites |
A large fraction of human habitation sites are made with wood and other natural, material strengthened by metal or ingenious designing |
Most natural material will decay within a decade or so, while stones and metal additions will last for tens of thousands of years. |
| |
|
Some human creations of materials such as ceramics are made from processed but not altered natural material. |
Thermal effects and water in natural environment over a few thousand years will be needed to undo the impact of human induced artificial strengthening of material. |
| |
|
The structures that will take the longest to destroy are those made from cement concrete and they will have to be destroyed by atmospheric forces and solar heating. These are artificial boulders and are much weaker than natural boulders. |
Hundreds of years |
| 2 |
Mining oil and other underground extraction and destabilisation of the Earth's crust |
This artificial alteration of landscape will have to be wiped clean and redone by geological movements |
Few tens of thousands to millions of years depending on the geological environment |
| |
|
However, the hollowed out oil wells will either have to be filled with subterranean water or will cave in depending on local geography. |
A few decades |
| 3 |
Plastics and other industrial products |
Will need invention of new breeds of bacteria that can feed on plastic. |
Few tens of thousands of years |
| 4 |
Purified metals and artificial alloys |
Mixing with the earth's crust through convection, earth quakes and water cycle |
Few hundred million years. However, some re-homogenisation may never occur |
| 5 |
Garbage on land |
Various bacteria, plants and animal scavengers will be needed to eat the garbage |
Few thousand years |
| |
Pollution of air |
Environmental chemical reactions against CFC's and plant absorption of CO2 will decide a new equilibrium. Also, environmental chemistry, cosmic rays and solar radiation will all help in this. |
Few thousand years |
| |
Garbage in the sea |
Water and water borne life forms are probably more efficient at dispersal of garbage and the sea's rate of sedimentation ensures a quicker submersion of material. |
Few hundred years |
| 6 |
Artificially created molecules and organic compounds |
Humans have added a whole series of hydrocarbons and other gases into the atmosphere and in the biosphere which have, in some cases such as fertilizers improved fertility of the ground and in some cases such as tar destroyed the environment. |
The land artificially enriched by humans will take a few decades to reach a new equilibrium. Getting rid of tar will not be easy and will survive for thousands of years in disintegrated way. |
| 7 |
Artificial sources of energy |
The nuclear reactors will almost certainly explode once the cooling systems fail. |
A few years to explode and then few thousand years for the buildings to breakdown. |
| |
|
Other factories for metallurgy will cool down to local thermal equilibrium and the cement structures will collapse. |
Few thousand years for the buildings to breakdown. |
| |
|
Sterilised biological production units will be contaminated by natural life forms and will be thermalised with local environment |
Few decades to spoil the environment and then few thousand years to break the structure. |
| 8 |
Artificial satellites |
Solar radiation and atmospheric drag |
Hundred thousand years. |
| 9 |
Several systems that are maintained well away from thermal or environmental equilibrium |
All of these will reach their natural equilibrium state and then decay through winds, thermal expansions and water |
Roads will typically last for a few hundred years before in-growing plants and water or ice induced cracks destroy it. |
| 10 |
Plants and animals dependent on human intervention |
Many domesticated plants are almost completely dependent on human intervention. These include seedless plants and animals domesticated for food or support will vanish and be replaced by other more wild varieties. |
All these will face extinction within a few decades. With luck some of the plants may be able to readopt to the wild environment. |
In many ways, the Earth will also miss us. Some of the ways will be:
* There will be a sudden reduction in the CO2 production with the disappearance of more than 6 billion animals will produce difficulties for plants in some regions.
* Domesticated animals and plants will have a tough time adopting to the wild again. These will be devastated in the absence of human intervention.
* With the disappearance of the animal at the top of the food chain the imbalance between the predators and the hunted and, depending on the region it will take a few hundreds of years before a proper equilibrium is restored.
However, both these effects are minor in the global environment of nature and will require minor reorganisation of natural environment.
If the human race were to disappear from the Earth today, most of the signals of human activity would disappear over the timescale of a few hundred years to a few hundred million years. The impact will be severe on domesticated plants and animals, some of whom will not survive.
Cement structures are likely to endure for a few thousand years even if in dilapidated state. Artificial satellites orbiting the Earth may endure for a few tens of thousands of years before falling back on Earth. The disappearance time scale of plastics is uncertain since it will need an evolution of plastic digesting life forms.
However, the most long lasting impact of human existence, in the form of isolation of different materials and metals is likely to endure over millions of years until the normal geological activity absorbs these materials in the Earth's crust and sends it down to the magma level where it can be melted and re-mixed. Some of these manmade differentiations may well be permanent, visible to alien visitors even a few hundreds of millions of years later.
The humble staple pin in the Tibetan Plateau may well become a clinching evidence of the one-time existence of humans on this planet several hundred million years after they disappear.
Dr Mayank Vahia is a scientist working at the Tata Institute of Fundamental Research since 1979. His main fields of interest are high-energy astrophysics, mainly Cosmic Rays, X-rays and Gamma Rays. He is currently looking at the area of archaeo-astronomy and learning about the way our ancestors saw the stars, and thereby developed intellectually. He has, in particular, been working on the Indus Valley Civilisation and taking a deeper look at their script. He is also the author of the book 'Physical Sciences and The Future of India'.
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