Deniers usual try to deflect this by talking about the numbers for natural biomass emissions which are indeed very high rotting vegetation and plant respiration emit around 440 billion tons per year, of course what deniers leave out (they just love leaving stuff out) is that respiration is by it's nature a two way process and for all plants put out they take in as much in a continual cycle, which is why natures is not really much of an issue and why our small but continual addition is quite important. Because ours has no second part absorption, fossil fuels, in there numerous forms are Co2 that has not been in the atmosphere for hundreds of millions of years, it is a direct increase, which is why over ~100 years, Co2 has risen almost 40%.
That's this is all "caused by volcanoes" is one theory (of many) deniers try push, sadly it's not one that has any real evidence to support it. Certainly in the past we have had far stronger volcanic activity and that certainly would have affected climate and did through a number of events like the PETM (Paleocene–Eocene Thermal Maximum) or the unimaginable large Deccan Traps 60 million years ago or the far more recent but much much smaller Loki event in the 1783.
note: there are several theories on what caused the PETM but volcanic activity is the most widely accepted.
By historical standards we have had a rather quite last 100 years when it comes to volcanoes, with Krakatoa being the largest of perhaps the last 150 and Tambora in the last 200, we will at some point have another large, even super eruption, but so far we have not.
USGS have posted the figures on relative emissions here
http://volcanoes.usgs.gov/hazards/gas/cl...
It would take something like 700 Pinatubo eruptions to equal current human emissions, one Pinatubo event (1991) does not even show up on the Co2 record.
http://www.esrl.noaa.gov/gmd/webdata/ccg...
yet another thing deniers just wont talk about, as they try and peddle their pseudo science.
Human Sources
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The latest year for which complete data are available is 2011 and in that year we released 34.1 billion tonnes of CO2 into the atmosphere, this was an increase of 0.6 billion tonnes on the previous year.
We also release many other greenhouse gases such as methane and nitrous oxide. The quantities released are much less than for CO2 but these gases are a lot more effective at causing global warming. If we compare them to CO2 (something called carbon dioxide equivalence or CO2e) then it’s the same as adding a further 14.3 billion tonnes of CO2.
In total then the greenhouse gas emissions in 2011 were the equivalent of adding 48.4 billion tonnes of CO2, or 1,530 tonnes per second.
Natural Sources
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There’s a natural carbon cycle in which carbon dioxide is exchanged between the oceans and the atmosphere and between biomass and the atmosphere. This system is more or less balanced.
Each year 121.2 billion tonnes of CO2 enters the atmosphere from biomass (including soils) and 122.6 billion tonnes is removed from the atmosphere by biomass. The seas and oceans release 90.6 billion tonnes and absorb 92.2 billion tonnes of CO2.
In total, natural sources release 211.8 billion tonnes of CO2 and absorb 214.4 billion tonnes.
In my point of view CO2 in the atmosphere is rising by only 15 gigatonnes per year.
Humans are adding 30 billion tons each year by burning fossil fuels.
This is the answer from the IPCC:
http://www.ipcc.ch/publications_and_data...
I'll accept 30 odd billion tons, but so what?, it still only a small fraction of our atmosphere, and is doing good not harm.
I don't know exactly, but I do know the plants are loving it.
