Climate Myths and Questions, Part II

In Part One of our series, we looked at polar bears, hockey sticks, Medieval Warm Periods and Little Ice Ages, among other topics. Today our list includes water vapor, volcanoes, and CO2. 

As with Part I, I offer the following caveat:

The topics tackled in this series by no means provide a comprehensive list; for such a list, I thoroughly recommend repeated visits to Skeptical Science, which addresses a great many more climate myths than we have room for here. Other excellent outlets include RealClimate and Climate Progress;  and New Scientist magazine also compiles an ongoing and regularly updated "guide for the perplexed." Each of the entries in this particular blog are, through necessity induced by the requirement for brevity, mere thumbnails; each contains links to background information that paints a broader picture.

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Earth's climate is always changing; it has been much hotter in the past, and humans had nothing to do with that.

This isn't actually a myth, of course; it has been much hotter in the past, and it is of course true to say that humans had nothing to do with it. Where the mythology comes in is the next step: That because humans had nothing to do with the changing climate in the past, they have nothing to do with it now.

Climate changes on scales from decades to eons, influenced at different times by a variety of factors: increases in solar output, changes in the distribution of continental land masses, volcanic eruptions above ground and beneath the sea, and "wobbles" in Earth's axis, tilt and orbit known as Milankovitch Cycles. But none of this is exactly news to the scientists who are warning about the impacts of modern-day global warming, who are of course fully aware of the planet's varied climatological history. In fact, one of the aspects that makes present warming particularly notable is that it comes at a time when the Sun is undergoing its deepest solar minimum for over a century, while this oft-cited study argues that, without anthropogenic impacts, Earth would be in a natural long-term cooling trend that should persist for the next 23,000 years.

Furthermore, while previous climatic changes inevitably were the result of natural processes, those processes that resulted in extreme warming almost invariably included the release of large amounts of greenhouse gases into the atmosphere - as was the case, for example, with this event roughly 56 million years ago - the same process we are repeating now. And it is worth noting that these previous very warm periods were hardly conducive to a comfortable environment for humans, natural or not.

Water vapor, not carbon dioxide, is the most important greenhouse gas

There is truth to this one, too, although as expressed by climate change skeptics it generally manifests  itself as a conflation of, or confusion between, the natural greenhouse effect - which is a good thing - and the enhanced greenhouse effect that is a consequence of anthropogenic climate change.

In the former, water vapor is indeed the most significant atmospheric gas, not least because of its relative atmospheric abundance. (It comprises approximately 3 percent of atmospheric gases by volume, whereas CO2 presently counts for about 0.39 percent by volume). It accounts for between 36 and 66 percent of the greenhouse effect by itself, and for between 66 percent and 85 percent when factoring in clouds.

So why is so much of the discussion about global warming focused on CO2 and not water vapor? Several reasons.

First, humanity is not adding much water vapor to the atmosphere, whereas the amount of CO2 being added is in the order of 90 million tons per day. Second, CO2 has a longer atmospheric lifetime (many years) than water vapor, which generally remains in the atmosphere for only 10 days or so. Third, the atmosphere can hold only a finite amount of water at a given temperature; add too much, and it exits in the form of precipitation.

However, as the atmosphere's temperature increases, so does its ability to hold greater amounts of water - and that, in turn, increases heating. So, water vapor is indeed an extremely important component in global warming; but whereas the addition of CO2 is referred to as a "forcing mechanism" - in that it directly contributes to a rise in temperature - water vapor is a "feedback": it requires the presence of a forcing mechanism - i.e. CO2 and other greenhouse gases - to warm the atmosphere to an extent that it is able to increase its atmospheric volume and thus prompt further warming. The extent of that further warming is considerable: recent research has suggested that the heat-amplifying effect of water vapor is sufficient to double the warming effects of carbon dioxide.

Increases in carbon dioxide levels have lagged behind previous temperature increases; therefore, greater CO2 levels are a response to, not a cause of, higher temperatures.

Studies of Antarctic ice cores (opens PDF) have shown that, when the planet has emerged from ice ages, temperature has increased before carbon dioxide levels. Skeptics assert that this shows rising carbon dioxide levels are a response to, and not a cause of, temperature increases.

But, however much skeptics may want to claim otherwise, CO2's properties as a greenhouse gas have been established now for almost 200 years. And the fact that atmospheric CO2 increases have initially (an important qualifier) lagged temperature increases as Earth has entered interglacial periods was predicted two decades ago (opens PDF).

As pointed out elsewhere, Earth's climate is affected on different time scales by a multiplicity of factors, including cyclical changes in Earth's orbit, axis and tilt known as Milankovitch Cycles.

These cycles are responsible for increases in the amount of sunlight reaching the planetary surface - or more precisely, the surface of one hemisphere or the other. Such an increase 18,000 years ago resulted in decreased Antarctic sea ice extent and retreating Southern hemisphere glaciers. This in turn reduced albedo, leading to further warming. It also led to a warming of the Southern Ocean; and, as seawater warms, its ability to absorb CO2 diminishes. As a result, the ocean released CO2 into the atmosphere, a process that generally takes about 800 to 100 years - which, lo and behold, appears to correspond with the lag observed in the ice cores.

Of course, once CO2 enters the atmosphere, it, too, causes warming - and this is important. The total period of post-ice age warming tends to be around 5,000 years, and while Milankovitch cycles explain that warming's initiation, the forcing is not strong enough to account for its full extent. CO2 and other greenhouse gases amplify the initial warming over 4,000 or so of the 5,000 years, with models suggesting they are responsible for approximately half of the full glacial-to-interglacial warming.

Volcanoes add far more carbon dioxide to the atmosphere than humans do.

Short answer: No they don't.

More detailed answer: OK, the detailed answer is pretty much the same. No they don't.

According to the United States Geological Survey, volcanoes presently contribute approximately 200 million tonnes of CO2 to the atmosphere annually; this British study (opens PDF) puts the figure a little higher, at around 300 million tons. Either way, that is a tiny fraction of the total anthropogenic contribution, which presently stands at about 26.4 billion tons annually.

Most of the time, of course, active volcanoes gurgle away happily, emitting some CO2 and various sulfates, but on occasion they erupt spectacularly. In recent years, the most significant such eruption was of Mt. Pinatubo in the Philippines in 1991; there have been some claims in the skeptical/denialist community that this eruption alone "put more CO2 into the atmosphere than did the whole human race during the most recent century of the industrial era." This is patently untrue - and demonstrably so, simply by looking at the famous "Keeling Curve", the graph that shows levels of atmospheric CO2 concentrations. Had Pinatubo blasted as much carbon dioxide into the atmosphere as some assert, then we would expect a truly significant spike in CO2 levels. No such spike exists. In fact, there was a slight drop in the immediate aftermath of Pinatubo's eruption. Some researchers have argued that this is because the brief global cooling induced by the huge amount of sulfate aerosols entering the atmosphere caused a reduction in plant respiration; however, others have proposed that the aerosols and clouds in the atmosphere created a diffuse light that stimulated photosynthesis, resulting in greater CO2 absorption by plant life.

The amount of carbon dioxide entering the atmosphere because of human activity is far less than the amount produced naturally.

This is also not a myth. As noted above, human emissions presently are responsible for approximately 26.4 billion tons of CO2 annually. (Closer to 29 billion tons when the results of changes in land use are factored into the equation). Of this, approximately 60 percent reaches the atmosphere; the rest is absorbed by carbon "sinks" on land and, especially, by the ocean. In contrast, natural sources contribute approximately 770 billion tons annually. So how much difference can human activities possibly be making?

The important point is that the 770 billion tons or so being deposited into the atmosphere by natural sources is approximately balanced by natural absorptions. The approximately 15 billion tons being deposited into the atmosphere by human activities each year are in addition to those natural sources and have caused atmospheric CO2 concentrations to increase from roughly 270 parts per million to approximately 390 parts per million over the last 120 years.

It's a little like having a rain barrel that is full almost to the brim after weeks of precipitation, and then pouring in another couple jugs of water until the contents spill over the top. Yes, there was a lot more water in the barrel to begin with, but it was the additional couple of jugs that caused that water to overflow.

That this additional atmospheric CO2 is the result of human activities can be demonstrated by the fact that the proportion of carbon-12 isotopes is increasing in the atmosphere relative to carbon-13 isotopes. This is significant because plants contain higher levels of C12 relative to C13 than does the atmosphere, and therefore a rise in the relative atmospheric abundance of the former is consistent with the burning of fossil fuels - which are ultimately derived from ancient plants.