Carbon dioxide, or CO₂, is a greenhouse gas.

The global carbon cycle

Greenhouse gases are constantly cycling between the land, ocean, and atmosphere.

There are different "stocks" of carbon, such as in the atmosphere or permafrost. They are measured as "gigatons of carbon", or GtC. 1 Gt is 1 x 10⁹ metric tons. "Fluxes" are the movement of carbon from one stock to another. These are generally measured using GtC/year. Currently we see a net positive flux of 5.1GtC/year.

Carbon sinks

A "sink" draws carbon out of the atmosphere. The main carbon sinks are vegetation (plants using CO₂ for photosynthesis) and the ocean, which dissolves massive amounts of CO₂ from the atmosphere.

Carbon sources

A "source" is a stock that emits carbon to the atmosphere. The largest source is the burning of fossil fuel.

While sinks have grown somewhat over time to absorb the additional carbon in the atmosphere, both sinks and store may become less reliable over time. Wildfires are one example of this. As wildfires become more frequent and intense, more trees that had been absorbing and storing carbon are burned, releasing that carbon straight back into the atmosphere.

It is now likely that the Amazon is a net source of greenhouse gases, with more than half of Amazonia impacted by humans. In California, emissions from the massive wildfires of 2020 were more than enough to cancel out the state's years-long efforts to reduce GHGs.

Another issue is ocean acidification. As the ocean absorbs carbon, it becomes more acidic. This is already affecting coral reefs and broader food chains, and it may lead to new feedbacks and complications that are hard to predict. Permafrost melting is another concern, as it releases vast amounts of methane.

Carbon dioxide emissions

Calculating CO₂ emissions

Carbon dioxide emissions are created when carbon is burned. Carbon emissions can be calculate as follows (this equation is simplified):

Plugging in the atomic weights, we get:

Therefore:

But approximately 55% is removed by sinks.

Measuring CO₂ emissions

Global CO₂ emission levels have essentially stayed flat for the last decade (Global CO2 emissions have been flat for a decade, new data reveals - Carbon Brief). However even a declining rate of emissions will still contribute to warming, just at a slower rate, as CO₂ is so long lived in the atmosphere. Therefore a more important metrics is the concentration of greenhouse gases, and especially CO2, in the atmosphere.

Concentrations are measured in ppm, or parts per million.

Impact of atmospheric carbon

CO₂ emissions can impact the climate for a very long time.

When you drive your car and pump out CO₂ through the tailpipe, roughly 60 percent of that CO₂ dissolves into the ocean, or is absorbed by other sinks, over a period of 100 years. The rest stays in the atmosphere for 300 to 1,000+ years, according to NASA.

In 2021 we reached 414.7 ppm CO₂ average concentration. The last time it was this high was more than 3 million years ago, when the temperature was 2–3°C hotter than during the pre-industrial era, and sea levels were 5–25 meters higher.

High atmospheric carbon dioxide can impact food quality as it lessens the amount of nutrients.

Global carbon budget

Scientists have calculated the total amount of CO₂ we can add to the atmosphere and still have a reasonable chance of staying within 1.5°C of warming globally. This is known as the global carbon budget.

The IPCC's AR6 report, published in 2021, estimated that to have a 67 percent chance of staying within 1.5 C, our global carbon budget was 400 GtCO2 starting in 2020. This equates to about eight more years of current emissions. ¹

Carbon pricing

Carbon pricing is one tool that governments have in the fight against climate change. As of 2022 68 carbon pricing initiatives exist globally. These cover 23 percent of global emissions.

While carbon prices have hit record highs in places from the EU to California to New Zealand, "prices must rise considerably more to meet the Paris Agreement temperature goals, as less than 4% of global emissions are currently covered by a direct carbon price within the range needed by 2030." Updated estimates of the “right” price for carbon range from $185 per tonne of CO2e to much, much higher amounts. ¹

While carbon pricing mechanisms are very popular with oil companies, the evidence suggests that they haven't been especially successful at reducing emissions so far. ¹

One major issue with carbon markets is the concept of "leakage". This refers to the ability for an emissions-intensive industry to relocate production to areas where no carbon tax or cap exists.

The IPCC 1.5 C Report estimates that to be effective—that is, to send the right signal to polluters to incentivize the types of deep emissions reductions we need—a carbon price would need to range from 5,500 per ton of carbon dioxide pollution in 2030, and from 27,000 per ton by 2100. ¹

As of 2022 the highest carbon prices in the world was Uruguay's new carbon tax, at about $137, just scraping the low end of the IPCC's range. ²

The social cost of carbon

The social cost of carbon is economists' attempt to add up all the quantifiable costs and benefits of emitting one additional tonne of CO2, in monetary terms. The idea behind this is that folks designing climate programs can compare the benefits of reduced warming against the costs of cutting emissions. ¹

References

1. Terra.do coursework ↩ ↩² ↩³ ↩⁴ ↩⁵

2. 2022 World Bank report ↩