We measure the magnitude of a volcanic eruption by the total mass that erupts. For example, an eruption, a super eruption may have a volume of 1000 km³. Now since the density of magma is about 2000 kg.m³, the total mass of one of these super eruptions is equal to the product of the density times the volume. If we use consistent SI units then that gives a mass of 2x10¹⁵ kg of magma. Now the gas content of a typical magma is circa 2 wt % ; most of that is H₂O and CO₂ and Sulfur gasses.

On the order of 0.1 wt %is CO, H₂S and other rather toxic gasses. Some of these gas components are oxidized when erupted. So if we say that 0.1 wt % of the total mass is toxic gas, then a super eruption expel on the order of 2x10¹² kg is noxious gas!

How it compares with the noxious gas of all the cars emit in Los Angeles (for example) over the years, I am not sure. Maybe you can look that up and compare.

Actually, the first statement is misleading and the second statement is not true.

While volcanoes emit poisonous fumes that originate from dissolved gases in the Earth's magma, these poisonous gases are only part of the concern associated with human activity. CO₂ and other "greenhouse gases" are what I would imagine are of greater concern. Every year, human activities emit around 100 times more CO₂ than volcanoes do in the same period of time (30 billion tons vs. 30 million tons -- to put it another way, 30,000,000,000 vs 300,000,000 tons -- that is A LOT of CO₂). The amount of CO₂ in the atmosphere is the highest it has been in about 15 million years. 15 million years ago, temperatures were about 5-10 degrees warmer on average than they are now, the sea levels were about 70-120 feet higher, and there was no permanent sea ice cap in the Arctic.

Regarding the second question, the average temperature on Earth has been getting warmer over the past couple of centuries. There are natural fluctuations that occur in the Earth's temperature, but the main point of concern is the levels of chemicals such as CO₂ and Non-Methane-Hydrocarbons ("NMHC's") in the atmosphere. There is a very delicate balance of reaction pathways in the atmosphere that in turn are coupled to chemical reaction pathways in the ocean and on land (i.e biogeochemical cycles). Upsetting this balance by emitting a large amount of these chemicals into the atmosphere can have lasting and far-reaching consequences. One example is that increased CO₂ in the atmosphere leads to increased CO₂ in the ocean, which in turn acidifies the ocean. Changing the pH of the ocean changes which lifeforms can live where or whether they can live at all. This affects many foodchains in the ocean, some of which humans are a part of. Furthermore, pollutants can be taken up by some of the organisms in the ocean and they can propagate up a food chain and back to human predators.

That is an interesting question. You are correct that volcanoes release gases such as carbon dioxide (CO₂), hydrogen sulfide (H₂S), and sulfur dioxide (SO₂) into the atmosphere. It is not clear to me which “deadly gases” you are referring to. Cars are known to be major sources of CO₂, which is a powerful greenhouse gas that has the affect of warming the global climate. Cars and other anthropogenic (caused by humans) sources of CO₂ release MUCH more CO₂ into the atmosphere than present day volcanoes.

Estimates for the amount of CO₂ released by present day volcanoes range from 0.13–0.44 billion tons per year, while the estimated anthropogenic CO₂ emitted in 2010 was 35 billion tons (Friedlingstein, 2010; Gerlach, 2011). That’s roughly 100 times more CO₂ coming from people than from volcanoes.

Additionally, a large proportion of the CO₂ released from volcanoes is recycled through the mantle of the earth (the layer below the crust). One study of the gases from an island arc volcano (like Japan or Sumatra) showed that 67% of the CO₂ was sourced from inorganic sedimentary carbon that was subducted beneath the volcano (carried down on a sinking ocean slab), and less than 12% was primordial (from the mantle; representing net addition to the atmosphere; Fisher et al., 1998).

There are indeed other gases (mentioned above) that are sourced from volcanoes. Sulfur dioxide is a gas that actually causes short-term (months to years) global cooling as it forms aerosols in the atmosphere. It forms sulfuric acid in the atmosphere, causing acid rain. To my knowledge, not much SO₂ is produced by cars, but huge amounts are generated from other anthropogenic sources like coal power plants.

In summary, volcanoes do emit gases that can have adverse affects on our climate, ecosystems, and health, but the scale is much smaller than that produced by humans. It is important to keep in mind (this is personal opinion, not scientific observation) that gas releases from volcanoes is a natural process! This process has been going on for billions of years and has reached a sort of long-term equilibrium; the concentration of different gases does indeed fluctuate up and down on scales of thousands and millions of years. The affect that humans have had on the climate (especially over the last couple hundred years) is much more rapid and causes conditions that are harmful to some of our societal infrastructures. As an example, check out the discussion on recent severe weather (hurricanes, tornadoes, etc.). I am not saying that recent intense storms are definitely caused by anthropogenic climate change, but it is an interesting debate. Be cautious about the sources that you use for research. Peer reviewed journal articles are those that have been checked by other experts in the field before being published. That does not mean that they are never biased, but it generally ensures that only the “good science” makes it to press.

And why is the average temperature of the earth getting colder?

Please read the answer #4 below

And why is the average temperature of the earth getting colder?

I am a little unclear about what this question is asking. The average temperature of the entire earth (the crust, mantle, and core) is getting colder, but the average temperature of the atmosphere is getting warmer. The solid part of the earth has steadily cooled down over billions of years as it loses heat generated during the formation of the planet and the decay of radioactive elements like potassium, uranium, and thorium.

The atmosphere, which is a major part of global climate, has been getting warmer at a very fast rate over the last hundred years or so. The figure below shows a global temperature anomaly curve for the time period between 1880–2005 (Hansen et al., 2006).

The “temperature anomaly” is relative to the temperature between 1951 and 1980. A positive anomaly indicates a time that when the temperature was higher than this period, a negative anomaly indicates a time that was colder. The graph shows that temperature has increased rapidly (up to 0.6 C more than between 1951 and 1980). You can see that there are little up and down jumps in this curve. There are changes in the mean global temperature (both up and down) that happen on a very short term (several years), and there is also some uncertainty around the values that were used to construct this curve, but the trend of increasing temperature is obvious.

I apologize if your question was about the cooling of the solid earth over geologic time and not the warming of the atmosphere caused by humans. I may have missed the mark.

References
Fisher, T.P. et al. (1998). Fluxes and sources of volatiles discharged from Kudryavy, a subduction zone volcano, Kurile Islands. Earth and Planetary Science Letters, 160, 81–96.

Friedlingstein, P.et al. (2010). Updated on CO₂ emissions. Nature Geoscience, 3, 811–812.

Gerlach, T. (2011). Volcanic versus anthropogenic carbon dioxide. EOS, Transactions, American Geophysical Union, 92, 201–208.

Hansen, J. et al. (2006). Global temperature change. Proceedings of the National Academy of Sciences, 103, 14288–14293.