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It's the sun
Climate's changed before
There is no consensus
Surface temp is unreliable
Models are unreliable
It's cooling
Ice age predicted in the 70's
Al Gore got it wrong
We're heading into an ice age
CO2 lags temperature
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It's volcanoes (or lack thereof)

The skeptic argument...

There are two skeptic approachs to volcanoes: high volcanic activity causes global warming and/or low volcanic activity causes global warming (it's always a good idea to hedge your bets). Specifically, volcanoes emit CO2 so high volcanic activity will lead to more CO2 warming. Alternatively, volcanoes emit sulfate aerosols which cool the climate so a downturn in volcanic activity will similarly warm the Earth. Reid Bryson, the "father of scientific climatology", sums it up (source: The Capital Times):

"There is no question the Earth has been warming; it is coming out of the 'Little Ice Age'. However, there is no credible evidence that it is due to mankind and carbon dioxide. We've been coming out of a Little Ice Age for 300 years. We have not been making very much carbon dioxide for 300 years. It's been warming up for a long time. The Little Ice Age was driven by volcanic activity. That settled down, so it is getting warmer"

What the science says...

As far as CO2 goes, volcanoes emit only 0.3 Gigatonnes of CO2 per year - about 1% of human CO2 emissions which is around 26.4Gt per year.

What about the 'coming out of an ice age' theory? Volcanoes emit sulfate aerosols which reflect incoming sunlight, cooling the planet. A large volcanic eruption such as the Pinatubo eruption in 1991 can have a global cooling effect of 0.1°–0.3°C for several years (Robock 1994, Zielinski 2000).

However, mega-eruptions or a series of eruptions can have a cooling effect that take decades to wear off, giving a perceived warming effect. Zielinski 2000 studies past volcanoes, particularly over the past few centuries:

Zielinksi concluded "the lack of any climatically effective volcanism in the period 1920s to early 1950s undoubtedly contributed to the overall warm conditions during those decades." (Zielinski 2000).

This is confirmed by Hegerl 2003 who found that "early 20th century
warming is attributed to a composite of greenhouse warming, an uncertain contribution from solar forcing, and a recovery from a previous period of heavy volcanism".

Similarly, Bertrand 1999 found that "the lack of volcanism during the period 1925-1960 could account, at least partly, for the observed warming trend in this period". Bertrand was investigating the effect of solar and volcanic influence on climate and concluded "these are clearly not sufficient to explain the observed 20th century warming and more specifically the warming trend which started at the beginning of the 1970s".

In short, a lack of volcanic activity had some part in temperature rise over the first half of the 20th century. However, it has had little to no part in the modern global warming trend that began in the 1970's. On the contrary, relatively frequent volcanic activity in the late 20th century may have masked some of the warming caused by CO2.

Related Arguments

Further reading

If you're really into volcanoes, the Global Volcanism Program have a list of all volcanoes with a Volcanic Explosivity Index (VEI) greater than 4 over the past 10,000 years.

Comments

  1. greendirectionconsulting at 11:56 AM on 12 February, 2008
    A new sceptic argument that a "faith" based sceptic that I am arguing with currently has raised is that it is increased undersea volcanoes that are warming the oceans and therefore causing the current observed warming... I have not found too much to counter this apart from a few lines on Real Climate but those weren't too helpful - any suggestions?
  2. Undersea volcanos are indeed active but I don't think that is a big issue. The skeptical argument on volcanos has some merit but not in the way that they are explaining it. First, keep in mind that there are 2 and only 2 natural sources of heat on this planet. The first and more important is the sun. Block the sun and CO2 can have zero effect (it is a feedback). So you do have greenhouse conditions. Second is albedo, soot from volcanos and man-made sources collect over the poles, moreso over the north because that is where industry is largest. This lowers the albedo by darkening the ice. Less active volcanos in the northern hemisphere means that much less soot as well as other greenhouse gas emmitted from eruptions. Note that the south pole is not warming as much, nor south america or africa.
    The power of increased vulcanism is heat transfer to the ocean that causes upwelling currents and change the direction of air currents in doing so. The best known of these is El Nino/La Nina. See Johns "La Nina watch: March update" on this site.
  3. In rereading my comment I see that I misworded it. The entire first paragraph is the first source of heat and what increases it. The Second source of heat is the earth itself released by vulcanism or created by large forest fires. In both cases the soot while airborne will cool but once settled on the ice will warm as far as solar effects go. Both produce direct heat however and the second paragraph explains how internal heat controls weather.
  4. Recent articles indicate that AGW is not fully responsible for Greenlans glacial melts. von Frese explained. that "under a big place like Greenland or Antarctica, natural variations in the crust will makes some parts of the ice sheet warmer than others." from Magma May Be Melting Greenland Ice he also said "to effectively separate and quantify human impacts on climate change, we must understand the natural impacts too."

    "The researchers don't yet know how warm the hotspot is, but if it is warm enough to melt the ice above it even a little, it could enable the ice to slide more rapidly out to sea." from Volcano Deep Down Could Be Melting Greenland's Ice


    Since these articles were printed, I have seen none of the data from the stated conference that by now should be available.
  5. John
    Using a little circular logic I made a guess at where to place this comment. Sorry if I am incorrect.

    For starters Volcanoes Unleash El Niño.
    I had mentioned this in a couple of threads as there does not seem to be an EL Nino thread. But logically, since solar cycles stopped following warming in the late 70s (actually erratic weather conditions started then ie. the ice age scare of 1978-79) we need to take another look at the worst (hottest years) since:
    1997-1998 El Niño
    1982-1983 El Niño
    I would like to point out that the triggering and actual cause is volcanic/vulcanism and that these severe cycles must also include the La Nina events that follow each El Nino event. I would also like to point out that prior to the late 1970s there was very little mention of these climate cycles (in the US) other than California where they are very noticeable due to the lack of drastic seasonal changes.
  6. A graphic representation of this volcano induced phenomena showing both cooling and warming
    recent history clearly shows why 1998 was so hot and the cooling afterward.
    [ Response: I fixed your hyperlink (make sure it starts with http:// - you had the word at in the a href code. Interesting page - I especially like the little dig they have at NCEP to demonstrate how much better their forecast is. ]
  7. Some evidence:

    "Healy Researchers Make A Series Of Striking Discoveries About Arctic Ocean"
    — Contrary to their expectations, scientists on a research cruise to the Arctic Ocean have found evidence that the Gakkel Ridge, the world's slowest spreading mid-ocean ridge, may be very volcanically active. They also believe that conditions in a field of undersea vents, known as "black smokers," could support previously unknown species of marine life
    ScienceDaily (Nov. 29, 2001)


    "Fire Under Arctic Ice: Volcanoes Have Been Blowing Their Tops In The Deep Ocean"
    — A research team led by the Woods Hole Oceanographic Institution (WHOI) has uncovered evidence of explosive volcanic eruptions deep beneath the ice-covered surface of the Arctic Ocean. Such violent eruptions of splintered, fragmented rock--known as pyroclastic deposits -- were not thought possible at great ocean depths because of the intense weight and pressure of water and because of the composition of seafloor magma and rock.
    ScienceDaily (June 26, 2008)

    I would think that this is an indication of increased vulcanism.
  8. John
    A little more detail here.
    "A major part of Earth's volcanism happens at the so-called mid-ocean ridges and, therefore, completely undetected on the seafloor." Bathymetric chart of the Gakkel Ridge
    (Nice photo and charts)
  9. The current work on vulcanism in the south Pacific:

    Olsen et al.
    Rapidly changing flows in the Earth’s core.
    Nature Geoscience 1, 390 - 394 (2008) Published online: 18 May 2008 | doi:10.1038/ngeo203
    Subject Category: Geomagnetism, palaeomagnetism and core processes

    Rapidly changing flows in the Earth's core
    Nils Olsen & Mioara Mandea

    A large part of the Earth's magnetic field is generated by fluid motion in the molten outer core. As a result of continuous satellite measurements since 1999, the core magnetic field and its recent variations can now be described with a high resolution in space and time. These data have recently been used to investigate small-scale core flow, but no advantage has yet been taken of the improved temporal resolution, partly because the filtering effect of the electrically conducting mantle was assumed to mask short-period magnetic variations.. Here we show that changes in the magnetic field occurring over only a few months, indicative of fluid flow at the top of the core, can in fact be resolved. Using nine years of magnetic field data obtained by satellites as well as Earth-based observatories, we determine the temporal changes in the core magnetic field and flow in the core. We find that the core flow is spatially localized and involves rapid variations over a few months, with surprisingly large local accelerations. Our results suggest that short-term fluctuations of the core magnetic field are robust features of rapid core dynamics and should be considered in the development of future numerical models of the geodynamo.

    Danish National Space Center/DTU and Niels Bohr Institute at Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
    GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany
    Correspondence to: Nils Olsen1 e-mail: nio@space.dtu.dk
  10. Sorry the subject of comment 9 is the south Atlantic, although it also affects the south pacific.
  11. Buried Volcano Discovered in Antarctica By Dave Mosher, LiveScience Staff Writer
    20 January 2008:
    “This eruption occurred close to Pine Island Glacier on the West Antarctic Ice Sheet," Vaughan said. "The flow of this glacier towards the coast has speeded up in recent decades, and it may be possible that heat from the volcano has caused some of that acceleration."

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