Myth: Compared to natural carbon dioxide emissions, manmade emissions are insignificant.

Fact: The argument is occasionally made by global warming skeptics that manmade carbon dioxide emissions are much smaller than natural emissions. Some skeptics will even quote figures in decimal or percent form, which makes it look even more true. If this is the case, according to their logic, our attempts to stop global warming are futile because we can't make a difference. This argument is blatantly false and is based on a complete lack of understanding of our environment.

On our planet, there is an ongoing process called the carbon cycle, in which carbon dioxide is both emitted and absorbed naturally. The absorption part of the cycle is what is missing from this myth. It is true that very large amounts of CO2 are emitted naturally, but there is also a carbon dioxide sink, which is the name for the absorption process. The sink removes slightly more carbon dioxide from the air than is emitted naturally. Therefore, any natural CO2 emissions are a wash.

Carbon Cycle

Actually, the natural sink is also absorbing much of the emissions resulting from human activity. In fact, the only reason any of our emissions end up in the atmosphere is because we're emitting CO2 faster than the planet can absorb it:

The roughly 500 billion metric tons of carbon we have produced is enough to have raised the atmospheric concentration of CO2 to nearly 500 ppm. The concentrations have not reached that level because the ocean and the terrestrial biosphere have the capacity to absorb some of the CO2 we produce. However, it is the fact that we produce CO2 faster than the ocean and biosphere can absorb it that explains the observed increase. - RealClimate

This has resulted in an increase in CO2 concentrations over the last century and a half of about 100 parts per million, from 280 ppm to 380 ppm. Nearly all of this increase has been due to human activity. Another way we know this, besides a simple look at the increase in human activity over the past two centuries, is by measurements of different isotopes of the carbon atom:

Isotopes are simply different atoms with the same chemical behavior (isotope means “same type”) but with different masses. Carbon is composed of three different isotopes, 14C, 13C and 12C. 12C is the most common. 13C is about 1% of the total. 14C accounts for only about 1 in 1 trillion carbon atoms. CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs. 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases. - RealClimate

So, not only are manmade CO2 emissions significant, they actually make up an overwhelming majority of the recent increase in CO2 concentrations in the atmosphere. Looking into the future, manmade CO2 emissions are predicted to continue increasing, which will no doubt result in higher concentrations of the gas in the atmosphere. Therefore, our efforts to cut back on emissions will indeed make a difference for the future of our planet.