Based on an article by John Cook at 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 billion tonnes of carbon dioxide every year. In contrast, land and vegetation emit 439 billion  and, the ocean emits 332 billion tonnes of carbon dioxide per year. It is obvious that 29 billion tonnes of human emissions are small compared to 771 billion tonnes 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 billion tonnes per year and giving out 439. Similarly, the ocean absorbs around 338 billion tonnes per year and gives out 332. This creates a deficit of 17 billion tonnes, which is taken from the 29 billion tonnes 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 billion tonnes.

That amount of CO₂ that nature cannot absorb is accumulating in our atmosphere and contributing to our warming. This is sufficient to cause global warming.

The image below is an excellent example of using the big picture instead of the little picture. Skeptics are always failing to take everything into account.

Figure 2: The complete global carbon cycle in gigatonnes (billion tonnes).