July 3, 2006
Clarion News
A great deal of recent attention has been placed on the impact of human activities on climate change.
However, very little attention has been placed on natural causes of climatic variations. In reality, there are many natural influences on climate change, with most (variations in solar output comes immediately to mind) dwarfing the contribution by humans.
One natural influence that garners considerable attention is volcanic activity.
The surface of the earth is comprised of about twenty major crustal plates, and many minor ones.
These crustal plates move very slowly relative to one another. The rate of movement is about the same rate at which your fingernails are growing. While this seems inconsequential, over very long periods of time, the rate of movement changes the face of earth. For instance, there was no Atlantic Ocean only 200 million years ago (the blink of an eye in earth time).
One of the consequences of crustal motion is volcanic activity.
Volcanic processes have always been important to the atmosphere. The primordial atmospheric composition resulted from the out-gassing of molten rock as the earth cooled and the molten material hardened.
This led to an atmosphere rich in the rather inert gas nitrogen, and CO2. Over time, the composition of the atmosphere changed, primarily though the proliferation of plant life extracting CO2 and respirating oxygen.
Long-term climate changes are not initiated by single volcanic events. Instead, volcanoes lead to long-term climate changes only during long periods of above normal activity.
Individual volcanoes are, however, capable of changing hemispheric and global climates over relatively short time periods.
When volcanoes erupt, gases and solid aerosols (particulate matter) are ejected into the atmosphere. Most of the aerosols fall back to the surface over short periods of time but smaller aerosols are capable of being suspended in the atmosphere for much longer periods. The suspended aerosols are capable of cooling the surface by reflecting away incoming solar radiation.
The most dramatic example of a recent volcanic eruption that led to surface cooling was Mount Pinatubo which erupted in the Philippines on 15 June 1991 . The eruption blasted over 1.8 x 1010 kg (20 million tons) of sulfur dioxide (SO2) and ash into the atmosphere.
The vertical column of ejected material was measured at 19 km (12 miles) high during the eruption. The resulting stratospheric SO2 plume spread rather evenly across the globe over time, leading to an estimated global surface temperature decline of 0.5ºC (~1ºF) for the two years following the eruption.
Perhaps the most notable recent eruption was the 1815 Tambora eruption in Sumbawa , Indonesia . This eruption is regarded as the largest in modern history as it ejected an estimated 50 cubic kilometers (12 mi3) of magma and over 1.8 x 1011 kg (200 million tons) of SO2 into the atmosphere.
The eruption led to the widely-regarded “year without a summer” in 1816. Snow fell in July in Boston , Mass.
Northern hemisphere surface air temperatures are believed to have decreased by 0.4-0.7º C (~1-2º F) over the year following the eruption.
Another notable eruption was the Laki fissure eruption in Iceland in 1783.
This eruption lasted for eight months and was responsible for devastating most of the human and animal population of Iceland .
It is estimated that 14 km3 (3.4 mi3) of basalt was ejected. In addition, over 9.1 x 1010 kg (100 million tons) of SO2 was emitted, leading to an estimated 1ºC (1.8ºF) temperature drop for the northern hemisphere.
The most catastrophic eruption known occurred on Mount Toba in Sumatra approximately 71,000 years ago. This eruption was so cataclysmic that it is thought to have accelerated the onset of the last glacial period.
While these are only recent examples of natural causes of short-term climate change, they cannot be ignored. Such examples demonstrate that climate is indeed a very fragile condition that is impacted by a wide variety of short-term and long-term influences.