Human Footprint: Human Carbon Emissions Quiz

Flux is measured in Gigatonnes of carbon per year (GtC/yr). While natural fluxes between the ocean and atmosphere are large, they are mostly balanced. Anthropogenic flux is a one-way addition that causes atmospheric accumulation.

The Keeling Curve shows a steady long-term rise due to human activity, but it also features a zigzag pattern. This "seasonal breathing" is caused by the Northern Hemisphere's forests growing in summer and decaying in winter.

Fossil fuels are depleted in Carbon-13 because plants prefer Carbon-12. As we burn fossil fuels, the ratio of C-13 to C-12 in the atmosphere drops (the Suess Effect), providing a chemical "smoking gun" for human impact.

While fossil fuels are the largest source, cement production releases CO2 through a chemical reaction called calcination. Deforestation removes "sinks" and releases the carbon stored in tree biomass back into the air.

Human activities currently release roughly 9 to 10 Gigatonnes of carbon (equivalent to about 36 billion tons of CO2) into the atmosphere every year, far exceeding the natural geological removal rate.

This is known as the Airborne Fraction. Roughly 25% of our emissions go into the ocean and 25% into land plants, leaving about 50% to accumulate in the atmosphere and drive warming.

Sinks like forests and the ocean are currently working overtime to absorb human emissions, but their ability to do so may decrease as the planet warms.

When the ocean absorbs CO2, it forms carbonic acid. This lowers the pH and consumes carbonate ions, making it difficult for calcifying organisms like corals and shellfish to maintain their structures.

Because the residence time of a portion of CO2 is so long, even if we stopped all emissions today, the warming effect from the carbon we have already released would continue for many generations.

Deforestation and soil degradation release carbon that was previously sequestered in biomass and organic matter. This remains a significant, though smaller, contributor compared to fossil fuel combustion.

Radiative forcing is expressed in Watts per square meter ($W/m^2$). Positive forcing, primarily from CO2, means the Earth is receiving more energy from the sun than it is radiating back into space.

The geological cycle moves carbon at a rate of about 0.1 GtC/yr. Human flux is 100 times faster. Expecting the geosphere to fix human emissions is like trying to drain a swimming pool with a thimble while a fire hose is filling it.

If warming or fire causes forests to release more carbon than they absorb, the percentage of human emissions staying in the air will rise, causing global temperatures to increase even faster.

Carbon-14 has a half-life of about 5,730 years. Because fossil fuels are millions of years old, all their C-14 has decayed away. Adding fossil carbon to the air dilutes the atmospheric C-14 concentration.

The carbon budget is a finite limit. To stay below 1.5°C or 2°C of warming, we can only add a specific total mass of carbon to the atmosphere before net-zero emissions must be achieved.