Based on an article by John Cook in skepticalscience.com.
Some Skeptics argue that human CO2 emissions are small compared to natural emissions therefore human emissions are of no consequence. Humans emit 29 gigatonnes (Billion tonnes) of carbon dioxide every year. In contrast, land and vegetation emit 439 gigatonnes and the ocean emits 332 gigatonnes of carbon dioxide per year. It is obvious that 29 gigatonnes of human emissions is small compared to 771 gigatonnes of natural emissions. But is this the full picture?
Figure 1: Human CO2 emissions compared to natural CO2 emissions in gigatonnes
What this argument fails to disclose is that nature both emits and absorbs CO2. As can be seen in the image below, land and vegetation are a strong carbon sink, absorbing 450 gigatonnes per year. Similarly, the ocean absorbs around 338 gigatonnes per year. This creates a deficit of 17 gigatonnes which is taken from the 29 gigatonnes that humans emit. In other words, nature is trying to take away from what humans are releasing but there is still an amount that is left over – about 12 gigatonnes.
That amount of CO2 that nature cannot absorb is accumulating in our atmosphere and contributing to our warming.
Figure 2: The complete global carbon cycle in gigatonnes
The figures above are excellent examples of using the big picture instead of the little picture. Skeptics are always failing to take everything into account.