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The effect of natural forces

Can solar variability, cosmic rays or volcanic eruptions explain global warming? Read myth five in this Jackson School of Geosciences climate change series.

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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.