Volcanoes and Climate Change, Part 1
Nearly every time a new volcano erupts, we see a deluge in the blogosphere of mis-informed posts on the mechanics between vulcanism and the climate. Often times, they center on either the “Year without a Summer” or the Pinatubo eruption and the amazing climate anomalies which then ensued. We will likely see many new ones, as Kasatochi in the Aleutian Islands has now erupted.
Thus, this is the perfect opportunity to highlight the basics of vulcanism and its climatological impacts. I’ll write this post in two parts; the first (or one you’re currently reading) will focus on short-term impacts and what it takes to cause them. The second part will shift to long-term impacts. Often times, skeptics will confuse the two contradictory effects of vulcanism, prompting much unecessary derision of the sound scientific reasoning behind them. That said, let’s dive right in to the short-term impacts of volcanic eruptions.
In addition to magma, rocks, and other miscellaneous debris, volcanic eruptions can eject a great deal of gas into the atmosphere. In fact, the vast majority of those gases are comprised of H2O, CO2, and SO2. Note that these gasses are each important greenhouse gasses within our atmosphere. So, for our discussion, we’ll have to consider both the debris (which, through chemical processes, can result in aerosols – particularly sulfate ones), and the gases involved in the eruption.
Let’s focus first on those GHG’s. Interestingly, we’re not going to have to spend much time on them (at least, while we address the short-term scale of vulcanism impacts). Refer to a report on Volcanic Hazards by the USGS:
Scientists have calculated that volcanoes emit between about 130-230 million tonnes (145-255 million tons) of CO2 into the atmosphere every year (Gerlach, 1999, 1991). This estimate includes both subaerial and submarine volcanoes, about in equal amounts. Emissions of CO2 by human activities, including fossil fuel burning, cement production, and gas flaring, amount to about 27 billion tonnes per year (30 billion tons) [ ( Marland, et al., 2006) – The reference gives the amount of released carbon (C), rather than CO2, through 2003.]. Human activities release more than 130 times the amount of CO2 emitted by volcanoes–the equivalent of more than 8,000 additional volcanoes like Kilauea (Kilauea emits about 3.3 million tonnes/year)! (Gerlach et. al., 2002)
So, simply due to the magnitude of the quanity of carbon dioxide, specifically, that is emitted into the atmosphere during an eruption, we wouldn’t necessarily expect a major warming impact due to greenhouse gases. That means we need to look at soots and the aerosols which are emitted during an eruption. I found another good source at the USGS website which summarizes the overall effect which the aerosols have:
The most significant impacts from large explosive eruptions come from the conversion of sulfur dioxide (SO2) to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space and thus cool the Earth’s lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere.
I bolded the most important statement (which is supported by the references on the USGS website linked above). Aerosols alter the radiative balance of the atmosphere by blocking out wavelengths of incoming short-wave or outgoing long-wave radiation. Sulfate aerosols are adept at absorbing and reflecting short-wave radiation, which prevents it from reaching the surface and subsequently heating it. It’s very important to recognize that incoming and outgoing radiation in the Earth’s radiative budget are different wavelengths, and effects on incoming radiation do not always similarly occur on outgoing radiation. All too often I see skeptics conflate the two distinct areas of the radiative budget, which is an egregious mistake.
So, we’ve now established that the gases in volcanic eruptions can have a cooling effect on the atmosphere. We’re not done, though; we need to think about whether or not volcanoes can have a global effect rather than just a regional one. To do this, let’s do a thought experiment involving the recent Kasatochi eruption. Remember that in the three-cell model of global circulation, tropospheric air within the polar cell tends to stay near the poles; it doesn’t often spread to the tropics. What this means is that an eruption in the area of Kasatochi will tend to distribute its contents throughout the polar region. However, this is only the case of it can’t bust through the tropopause and into the stratosphere, where friction does not turn the winds and prevent geostrophic flow (where the coriolis force and the pressure gradient force are in balance, causing the wind to parallel isobars).
If an eruption plume can reach the stratosphere, it has the potential to cross out of whatever cell it is located in and have a spatially-larger impact. The tropopause is lower at the poles, which sometimes helps eruptions impact a wider area, but overall, what happens is that polar eruptions tend to have impacts isolated to whatever hemisphere they are located in. On the other hand, eruptions in the tropics tend to have a larger impacts, including global ones.
What this all boils down to is that there are three major factors which determine the spatial distribution of aerosols and GHG’s from a volcanic eruption and subsequently its effect:
- Composition of the volcanic plume and matter emitted
- Location of the eruption
- Intensity or vertical reach of the eruption
It should be clarified once more that the aerosols are the important short-term player from a volcanic eruption, and it should be common sense that the more aerosol particles emitted, the larger the effect. If a volcano erupts in the poles, it is less likely that it will have a global effect than if it had erupted in the tropics due to global circulation patterns. Finally, if the contents of the eruption fail to reach the stratosphere, it is much less likely that they can exhibit anything but a regional effect.
Let’s put this all togethor by comparing and contrasting Kasatochi and a more famous eruption, Pinatubo. Let’s start with location:
- Contents emitted into the atmosphere: The USGS estimates that nearly 20 million tons of sulfur dioxide (the player which can become sulfate aerosols) was injected into the stratosphere by the Pinatubo eruption. It is too early for an estimate of the quantity emitted by Kasatochi, but it would be a little bit more than reasonable to suggest that the number could rival Pinatubo. Regardless, we can assume for the sake of analysis that the composition is similar to Pinatubo
- Location: The Alaska Volcano Observatory lists Kasatochi at 52 degrees north; this places it in the polar cell. Pinatubo is located on Luzon, so that places it somehwere near 15 degrees north; it is in the tropical cell, close to the ITCZ.
- Vertical Reach: Pinatubo had a VEI of 6.1 (that’s high); it had a vertical reach of nearly 24 km for its largest volcanic plume. Kasatochi has reportedly reached a max height of 45,000 feet, which is about 13.7 kilometers. The tropopause slopes from about 8 km at the poles to 17 km at the equator, so both of these eruptions puncture the stratosphere.
So the big question remains: will Kasatochi have a global effect? The answer is: not likely, mainly due to its location. Pinatubo, on the other hand, had a major impact. To illustrate the behind-the-scenes effect of that impact, which resulted in global temperature decreases of about 1.0 degree Fahrenheit, we should take a look at the distribution of the aerosols from the eruption. We can do this by looking at optical depth maps generated from satellites. I found a good source from an EOS Volcanology page here.
Start by looking at the optical depth of the atmosphere before the eruption:
It’s pretty uniform, right? Look at how dramatically it changes in the month after the eruption:
As you can see, a heavy cloud of aerosols affected a great swath of the Earth after the eruption. Ultimately, check out how those particles settled over the next two years:
So, even for a significant period of time, the atmosphere equilibrated a bit, but there was still an elevated level of aerosols. Notice, though, how that level is nearly uniform for the entire globe. This is what skeptics miss; an eruption needs to ultimately effect the entire globe, not just a small portion of it. Eruptions that do that are rare.
I hope this is a clear, basic primer on volcanoes and their short-term effect on the climate. Please leave me feedback on how to improve this entry; I’ll get part 2 up – long-term impacts – in the near future. In the meantime though, enjoy the aftermath of the Pinatubo eruption in photograph form!