Scientists estimate Earth's total carbon store.

There are 1.85 billion, billion tonnes of carbon on Earth, with more than 99% of it resident beneath our feet.

Scientists from the Deep Carbon Observatory (DCO) project have spent 10 years assessing the "reservoirs and fluxes" of the chemical element.

In other words, they worked out where carbon is held and in what form, and how it moves through the Earth system.

The findings could help understand the limits of life on our planet and in the forecasting of volcanic eruptions.

"This work really came out of the realisation that much of the carbon that we are concerned about for climate change is only a tiny fraction of our planet's carbon. More than 90% of it is actually in the interior of the Earth - in the crust, in the mantle and the core," said Prof Marie Edmonds from Cambridge University, UK.

"Very little was known about its form, how much there was, and how mobile it is. And, obviously, this all has huge importance for both the climate of the Earth, but also the habitability of our surface environment," the DCO collaborator told BBC News.

*LISTEN HERE : "Prof Marie Edmonds: "10 years of human CO2 emissions is roughly equivalent to an asteroid impact"

The accounting was a painstaking process that included monitoring gas emissions from major volcanoes and examining the deep-sea muds that are drawn, or subducted, into the Earth's interior at tectonic plate boundaries.

Through the use of lab experiments and models, the team was then able to simulate the likely stores and flows of carbon.

Just two-tenths of 1% of Earth's total carbon - about 43,500 billion tonnes - is judged to be above the planet's surface, in the oceans, on land, and in the atmosphere. Everything else is in the deep reservoir, with two-thirds of the total contained within the core.

In a fascinating exercise, the DCO attempted to describe how this inventory has changed through time. Working with the University of Sydney, it has reconstructed the history of plate tectonics - to in effect replay the movie of the Earth's inner workings.

This revealed that the planet's carbon budget through much of the last billion years has been in a relatively steady state. Put another way, the carbon that has been drawn down into the Earth's interior is roughly equal to what has been outgassed to the atmosphere through the likes of volcanoes.

Every now and then, however, there have been major catastrophic perturbations in this cycle.

These disturbances were the result of asteroid impacts or prolonged, large-scale volcanism that put substantial volumes of carbon dioxide into the atmosphere - leading to warming, acidified oceans, and even mass extinctions.

The suspicion, of course, is that we are now in another of these great perturbations.

Over the past 100 years, carbon emissions from human activities such as through the burning of fossil fuels have been 40 to 100 times greater than our planet's geologic carbon emissions.

"It's really revealing that the amount of carbon dioxide we're emitting in a short time period is very close to the magnitude of those pervious catastrophic carbon events," said Dr Celina Suarez from the University of Arkansas

"A lot of those ended in mass extinctions, so there are good reasons why there is discussion now that we might be in a sixth mass extinction."

While this might sound depressing, there is some hopeful news in the new study.

In trying to gauge the amount of carbon emitted from volcanoes, the DCO scientists have discovered that eruptions are very often preceded by surges in gas discharge.

"Putting very high-resolution sensors on crater rims allowed us to see very short time-scale changes in CO2 flux," explained Prof Edmonds.

"The flux increased dramatically in the days and weeks before eruptions. We think this holds great promise for forecasting in the future, when used in tandem with things like volcano seismicity, and how the ground is moving."

The DCO has published its findings in several papers in the journal Elements.