The University of Texas at Austin
  • The effect of natural forces

    By Marc Airhart
    Marc Airhart
    Published: Nov. 12, 2010
    The Nevado del Ruiz volcano in Colombia as viewed from space.Photo: NASA on Flickr/CC

    Climate scientists at the Jackson School of Geosciences address common myths about climate change in this eight part series.

    Myth No. 5: Natural forces such as solar variability, cosmic rays or volcanic eruptions can explain the observed warming.

    Nearly all of the heat at the surface of Earth comes from radiation from the sun. Perhaps, as one hypothesis goes, that radiation has become more intense in recent decades and is making the planet warmer. A second, more complicated hypothesis involving the sun proposes that higher solar activity tends to suppress the levels of cosmic rays, high energy particles from space, hitting our atmosphere. Cosmic rays help form water droplets and clouds. Clouds are thought to have an overall cooling effect on the planet. Still with us? So in this view, if the sun is more active, then there are fewer cosmic rays, less cloud cover, and a warmer Earth.

    Cosmic rays graph

    Cosmic ray intensity has stayed essentially flat since the 1960s, at the same time that global temperatures have risen most rapidly in the past century. View a larger version of this graph. Source: Climax Neutron Monitor

    Rong Fu, an expert in climate observations, noted that solar irradiance and cosmic rays have stayed essentially flat since the 1970s, at the same time that global temperatures have risen most rapidly in the past century. Laboratory experiments and paleoclimate records have failed to convince the climate community that the cosmic ray hypothesis is valid. Also, the cosmic ray hypothesis fails to explain why Earth is warming more at night than during the daytime, a fact which is consistent with the warming effects of human produced greenhouse gas emissions.

    Image of a solar irradiance graph

    Apart from the regular 11-year solar activity cycle, Total Solar Irradiance (TSI) has stayed essentially flat since the 1970s, at the same time that global temperatures have risen most rapidly in the past century. TSI is the sun’s brightness (as measured daily by earth orbiting satellites) summed across all the wavelengths of the electromagnetic spectrum. View a larger version of this graph.Source: C. Frohlich

    Ginny Catania, an expert on polar ice sheets and climate observations, added that sunspot activity — another way of measuring solar activity based on counting dark spots on the sun — does vary in a regular 11-year cycle, but that since at least 1950, average sunspot activity has remained flat. According to the Intergovernmental Panel on Climate Change, from 1950 to 2005, it is “exceptionally unlikely (<1 percent chance) that the natural variability in the Sun spot cycle has had a warming influence comparable to that from anthropogenic greenhouse gases.”

    There’s a third hypothesis here about the effect of volcanoes. Volcanoes produce aerosols that tend to cool the atmosphere, so if there were less aerosols the planet would actually warm. Perhaps volcanoes are less active now than they were 50 or 100 years ago.

    Volcanic aerosols have actually increased in the atmosphere since the 1960s, noted Fu, which would tend to lead to global cooling, not warming.

    Read the rest of the myths in this series …

    You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.

    • Quote 2
      Marc Airhart said on March 14, 2011 at 10:39 a.m.
      Joseph, thanks for writing. As I understand it -- and I have to stress I'm not an expert on volcanic effects on climate -- following the Mount Pinatubo eruption in the early 1990s, global temperatures dipped for a year or two due to increased sulfate aerosols in the atmosphere reflecting more solar radiation back to space. Sure, some heat was released into the air from the volcano, but climatologists I've talked to say the cooling effect far outweighed any release of heat.
    • Quote 2
      Joseph Oruoch said on Feb. 4, 2011 at 6:17 a.m.
      Volcanism is the only credible cause of climate change. The heat that warms the temperatures around the earth comes from within the earth and not the sun. When pressure builds within the earth then more heat diffuses from the mantle through the crust into the surrounding atmosphere causing global warming. But sometimes the pressures inside the earth becomes too much thereby cracking the crust and causing volcanic eruption. During a volcanic eruption the earth looses some of its heat rapidly and directly into the atmosphere meaning that the gradual heat that comes out through diffusion reduces and allows the global temperatures to cool. This regulates global temperatures causing both global warming and cooling. For further details on how this happens please go to
    • Quote 2
      Erl Happ said on Nov. 16, 2010 at 7:23 a.m.
      David,there is background to my comments at Change in atmospheric pressure and the resulting flux in the strength of the world wind systems is at the heart of natural climate change. In the comment above I simply wished to point out that the assumption that Cosmic Rays or Total Solar Irradiance are the only mechanisms whereby the sun can influence the climate of the Earth is incorrect and that our debunkers need to look a little harder. The proposition that carbon dioxide is driving change is derived as a residual when people say that they can not imagine that anything else could be responsible.The reasoning is weak, and particularly weak if people are not diligent in examining known sources of natural variation. The connection between the Arctic Oscillation and the climate of the northern hemisphere is well documented. If our 'climate scientists' want some peer reviewed literature to look at there is much to choose from. Here is a recent instance. So far as volcanoes are concerned the effect of an eruption is to cut light transmission reducing surface temperature.....not much use invoking this if you want to explain warming.
    • Quote 2
      David said on Nov. 15, 2010 at 10:41 a.m.
      These are very interesting questions, in my view; though I wonder what you mean when you say that pressure in one area of the globe "dominates" pressure in another area of the globe. I suppose you just mean that it's higher in one region. Indeed, that seems to be the main question that the previous commenter is asking: what causes these pressure differences? But the commenter also mentions relevance. Maybe the Arctic Oscillation bears further investigation, but I do not find it to be relevant to the article, which aims to debunk climate change myths. Even if there is a correlation here (I would want to see where the data came from before I ceded even that much) that does not mean that this oscillation is tied to the cause of climate change. Neither do I believe that the volcanic and solar aspects of the natural forces myth are straw men, if that was the implication. In the straw man fallacy, one party in the debate attributes a postion to the opposing party which that opponent does not really hold. In fact, I know many people who hold to the volcanic theory of climate change, which is shown in the main article to be shaky.
    • Quote 2
      Erl Happ said on Nov. 12, 2010 at 7:24 p.m.
      There is yet more to think about before one dismisses the notion that natural processes can account for the variation in climate that has been with us since time began. There are plenty of straw men that can be thrown up and dismissed. But the responsible thing to do is to be relevant. It has been observed for thirty years or more that northern hemisphere climate is subject to swings that are in tune with the oscillation in the balance between mid latitude sea level pressure and atmospheric pressure in the Arctic. When the balance tips towards domination by the latter, as is apparent over the last couple of northern hemisphere winters, colder conditions are experienced. When the mid latitude pressure increases in relation to that at high latitudes, Westerly winds bring warm moist air to high latitudes and winters are warmer, the growing season longer and the incidence of damaging frost is less. That is the essence of the change that we have seen since the mid nineteen seventies. Now,tell me why the Arctic Oscillation suffered a long term swing to weaker polar pressure since the phenomenon was first recognized and why we see a reversal since 1997. And tell me why the long term decline in polar pressure (both poles) has been accompanied by warming of the tropical ocean by successive El Nino events and why we see the reverse, La Nina dominance, as Arctic sea level pressure has built up since 1997. Is this not a natural variation of consequence? What is driving it?
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