The Earth's natural carbon cycle
The Earth’s natural carbon cycle

By Claire

I’m not certain where this myth originated, but I have a hunch that Alan Jones might have had something to do with it. Unfortunately, people have picked up on this and are continuing to spread it around the internet, particularly in the comments section of climate change articles.

Are human emissions too small to have an effect on climate?

So, to begin tackling this myth, we first need to find out exactly how much CO2 is naturally emitted by the Earth. Carbon naturally moves throughout the various Earth systems in what’s known as the “carbon cycle“. This is a completely natural process and has been occurring for millions of years, long before humans came along.

Let’s start looking at some simple numbers:

+  ~220 Gt CO2 each year released from rotting plants

+ ~220 Gt CO2 each year released by vegetation respiration

– ~440 Gt CO2 each year is absorbed by vegetation

+ ~330 Gt CO2 is released by the oceans each year

– ~335 Gt CO2 is absorbed by the oceans each year

So far, we can see that naturally, what is released approximately equals what is absorbed (of course there is some variability in these numbers, but generally the natural carbon cycle is relatively stable over short time scales).

So, those numbers roughly add up to 750 Gt of carbon moving naturally through the carbon cycle each year.

Here is where it gets concerning…

Humans release approximately 30 Gt of CO2 each year. Now this seems like a really small number, and it is when you look at the whole carbon cycle – it accounts for only 4% of all the carbon moving around.

BUT, these are not naturally balanced by carbon uptake. At the moment, approximately 40% of these emissions are taken up by the land and the ocean, which leaves the rest to build up in the atmosphere.

CO2 concentrations measured at Mauna Loa in Hawaii
CO2 concentrations measured at Mauna Loa in Hawaii

And that’s where the problem arises. The fact that CO2 is left to build up in the atmosphere is why current levels of CO2 are higher than they have been for at least 400,000 years.

We can see this gradual build up of CO2 when we look at measurements taken from Mauna Loa in Hawaii. The annual up/down signal seen in these measurements is essentially the Earth “breathing”. In Northern Hemisphere summer and spring, vegetation in the northern hemisphere grows, taking up CO2, causing the little dip you see. In autumn and winter, these trees lose their leaves, releasing CO2 back into the atmosphere. This is the natural carbon cycle.

Underlaying that natural cycle, you can see a gradual increase in CO2 emissions, and that’s the impact of our ~30 Gt. By burning fossil fuels we are releasing carbon that has been naturally locked up underground, returning it to the active carbon cycle.

So although humans only contribute a relatively small amount to the Earth’s total carbon cycle, it is out of balance with natural processes, allowing it to build up in the atmosphere and gradually warm the planet.

For more information, see the great post on the skeptical science page.