The eruption of Mt.  Pinatubo in 1991 called attention to the climatic effects of explosive volcanic eruptions.  These effects can be important at all time scales.  Sulphate aerosols emitted in the Pinatubo emissions were sufficiently dense that they may have temporarily offset the warming trend due to carbon dioxide.  Aerosols act like dust or smoke to reduce the amount of solar radiation reaching the Earth's surface.  The biggest recent eruptions were El Chichon which released 7 – 20 Million Tons of sulphur dioxide (SO₂), and Pinatubo which released 15 – 30 Million Tons of SO₂.  Even these were relatively puny compared with eruptions of the historical past.  It has been a long time since the Earth witnessed a really big eruption, like Tambora in 1816. 

Recent claims of ancient rapid temperature changes of several degrees within time spans of only a few years raise alarms; such events could have major consequences to agricultural production.  Perhaps some of them can be attributed to volcanic events.  The obscuration of the Sun by volcanic dust and aerosols could lower temperatures over a vast region before settling out of the atmosphere within weeks to years.  The principal hazard is due to the sulphur dioxide and other gases, which form long-lived aerosols in the upper atmosphere.  These aerosols absorb solar radiation and can cause a global depression of temperatures.  The effects depend somewhat on the atmospheric circulation, and the latitude of the eruption.  Some volcanos may affect the climate in only one hemisphere.  Tambora, near the equator apparently caused a significant climatic deterioration lasting a year or more, that was felt all over the northern hemisphere.  The growing season in Europe and North America was significantly shortened, so much that the year 1916 has come to be known as "the year without a summer."

It may be significant that the twentieth century experienced a pronounced deficit of great volcanic eruptions, as compared with the past four centuries.  This coincidence could be responsible for at least part of the global warming since about 1912.  In previous centuries there was an average of about one large eruption, comparable or larger than Pinatubo, every decade.  There was a gap of more than 50 years with no big eruptions, from the eruption of Katmai in 1912 to the eruption of Agung in 1963.  Agung and Pinatubo were the largest of the century; and they were relatively modest eruptions compared with some in the past.  The eruption of Krakatao in the late 19th century is usually considered an exceptionally spectacular event; that century saw several eruptions greater than the eruption of Krakatao.

The tree rings of long-lived Bristlecone Pines, which live in the mountains of the western United States have yielded information on weather and growing conditions over several thousand years.  Many of the large eruptions of the past have been associated with poor growing conditions for bristlecone pines, which are in an extreme climatic zone where a slight deterioration can prevent growth for a year or more.  Some of those events are listed below, along the Dust Veil Index (DVI) which is a measure of the amount of junk injected into the atmosphere.  A DVI value of 200 or greater is sufficient to cause measurable world-wide cooling.  The eruption of Mt. Pinatubo held back the advance of global warming by perhaps several years; in the year following the eruption aerosols from Pinatubo reduced the global average temperature by about 0.4 C (0.7 F).  There is no simple relation between the DVI and the temperature decrease; one can imagine the effects of the monster eruptions of Toba and Yellowstone.

Fissure eruptions usually occur at places where the Earth's crust is relatively weak, such as on ocean ridges.  Iceland lies on the Mid-Atlantic Ridge, and experiences frequent fissure eruptions, where lava flows from a crack in the ground.  Those eruptions are relatively mild, with little of the explosive nature that forms the popular image of volcanoes.  Other, more vigorous fissure eruptions can occur at hot spots, where the crust has been weakened by volcanic activity over many thousands of years.  The vast expanses of lava on the Columbia Plateau and the Snake River Plain are the results of such giant fissure eruptions associated with hot spots.

It has been suggested that the eruption of vast lava plains, consisting of millions of cubic kilometers of lava could be associated with drastic climatic change.  The Deccan lava flows of India seem to have erupted about the same time as the extinction of the dinosaurs.  The Snake River plain is associated with the Yellowstone hot spot, and the total amount of lava there dwarfs the Yellowstone eruptions.  These lava flows, however consist mainly of relatively fluid basalt lavas, which can erupt quietly for years without ever producing an explosion violent enough to loft material into the upper atmosphere.

Cometary impacts have become a trendy topic since they were invoked to explain the extinction of the dinosaurs.  Impacts of large extraterrestrial bodies have happened throughout the Earth's history; fortunately they are so rare that man has not witnessed a really large event.  Nonetheless, the potential devastation due to a collision with a comet or small asteroid is a significant threat to the Earth.

A cometary impact was indicated when research workers found an enhanced abundance of the element iridium at the boundary between Cretaceous and Tertiary rocks (the KT boundary), laid down at the time the dinosaurs disappeared.  The iridium concentration is very low, but it is unusual because meteoritic material is practically the only known source of large amounts of iridium.  Iridium is also produced in some volcanic eruptions, and it was quickly pointed out that it might have come from the Deccan Traps of India, a huge outpouring of lava dated to about the KT boundary.  That issue has been laid to rest with the discovery of impact-shattered rock fragments throughout the iridium-rich layer.  Everywhere on Earth the KT boundary has been found to be enhanced in iridium and fragments of shattered rock.

The hunt for ancient impact craters has accelerated with the knowledge that they could be associated with major species extinctions.  Hundreds of ancient craters have been found in North America; the figure shows the locations of the largest of them.  The previously well known "Meteor Crater" in Arizona is a relatively puny example; many of them are more than 10 km across.  The green dots indicate several smaller, but more recent craters.  The impact crater from the event at the KT boundary, that supposedly killed off the dinosaurs has been located in North America, at the tip of the Yucatan Peninsula.  The buried Manson crater in Iowa (indicated by a red dot on the map) is of similar size and dates from about the same time, indicating that the Earth may have encountered a swarm of comets.

The Chicxulub crater in Yucatan lies partly under water, so the northern rim is difficult to determine precisely; but the crater appears to be as much as 100 km across; and is the right size to have been created by the collision of a comet or asteroid more than 5 miles in diameter.  The energy released in the collision was enormous, possibly as much as 100,000,000 1-Megaton bombs. 

A collision with a comet or asteroid would have both immediate consequences and long-term consequences.  At first there is a tremendous explosion, far greater than any of the nuclear explosions that have been detonated in the atmosphere.  The explosion vaporizes rock and water, and sends a plume of material upward.  In a really big event, due to a body a kilometer or more in size, the incandescant plume would rise far above the atmosphere.  The energy deposited in the Yucatan impact event is thought to have caused tremendous heating of the atmosphere, far from the influence of the blast waves.  Hot fragments rained down hundreds of miles from the impact and started catastrophic fires.  The dinosaurs who were far enough away to survive the initial blast would have first experienced a rapid heating of the atmosphere.  Then the smoke from the fires would have blanketed the the planet, causing winter-like conditions to set in everywhere.  Whatever creatures were not killed immediately would have had to survive long dark months with almost no food.

The extinction of the dinosaurs by a comet has been controversial, but most of the serious objections have been met.  That a great impact event could have caused a major extinction event raises a puzzle even more perplexing than the search for causes of the extinctions: many huge remnants of impact craters have been been identified, but none of the others have been associated with extinction events.  The two great remaining questions are

Some of the effects of impacts by comets or small asteroids are similar to the effects of huge volcanic eruptions.  The most important similarity is the atmospheric dust that might obscure the sun for as long as several years.  While at least one major extinction event has been attributed to an asteroid impact, volcanic eruptions are still suspect in others.  The most dangerous volcanic prospect is the eruption of a huge caldera, such as Toba or Yellowstone.  The principal effects—comparing an impact of a 5 km asteroid with a Tambora type eruption or the eruption of a large caldera like Yellowstone—can be summarized in a table (explosive energies are given in MegaTons of TNT):

Supernovae are cataclysmic explosions of aging stars.  Within a few minutes after the explosion begins the star is putting out more radiant energy than the entire rest of the galaxy, which comprises billions of stars.  What would a nearby supernova do to the Earth?

The most recent supernova visible to the naked eye on Earth was in a nearby galaxy.  For a brief period, Supernova 1987a was a bright star visible in the southern hemisphere, even though it was nearly 40,000 times as distant as the nearest star.  It faded perceptibly within days, and is now visible only to astronomers working with infrared and ultraviolet radiation.  If it had been as near as the closest star, a distance of about 4 light years, its brightness in visible radiation would have comparable to that of the Sun.  That itself is alarming, but the danger becomes acute when one realizes that most of the initial burst of radiation from a supernova is at very short wavelengths, along with tremendous numbers of energetic particles.  Most of the initial energy is carried by neutrinos, which can pass through the entire Earth without being stopped.  This seems harmless, but the intense neutrino fluxes from a nearby supernova could fry us all.  It has been estimated that every person on Earth stopped, on the average, one neutrino from 1987a.  If a supernova like 1987a had been at the distance of the nearest star, each of us would have absorbed about a calorie of energy from neutrinos alone; this is enough to do serious biological damage.  So, even though most of the damage would occur on the face of the Earth exposed directly to the initial radiation from a neargy supernova, there would be no hiding from the neutrinos.

A supernova explosion near enough to the Earth could cause a major extintion event, the magnitude of which would depend on the distance.  There are two potential damaging effects: the direct effects of exposure to intense radiation, and the indirect effects through heating of the the atmosphere and surroundings.  The effect on the Earth's atmosphere is difficult to estimate.  The nature of the radiation suggests that the direct effects on living beings would probably be more important than the indirect effects involving atmospheric heating.  There is an extremely small chance that it could wipe out life entirely.

While the dangers are great, the risk is almost infinitesimal.  There are no nearby stars that have been identified as likely candidates for a supernova explosion.  Indeed, supernova explosions are so rare that we expect to see only several each century in our entire galaxy.

The Earth's environment is extremely complex, not just within the atmosphere, but also in the domain where it interacts with the particles and radiation in space.  While the Earth appears to be well buffered by the upper atmosphere and its magnetic field, there exists a possibility that either the climate or species extinctions could be influenced by energetic particles and radiation that are normally stopped by the atmosphere.

Cosmic rays are energetic particles that pervade all space.  They usually carry an electrical charge; so, when they approach the Earth they are first deflected by the Earth's magnetic field.  The lowest energy particles, such as those in the Solar Wind, do not immediately penetrate the outermost part of the Magnetosphere.  When they do get in, they are usually accelerated by various processes in the Magnetosphere; and either become trapped or become part of the precipitation into the atmosphere that produces the Aurora.

The most energetic particles are only slightly deflected by the magnetic field.  At moderate energies most cosmic rays collide with atoms in the upper atmosphere, and their energy is diffused as a shower of lower energy particles, light, and ultraviolet radiation.  The ones that can penetrate a significant part of the atmosphere are very rare.  Very little of the energy reaches the ground.  That the cosmic ray intensity might increase is extremely unlikely, since many of them have been traveling for millions of years.  The Earth's magnetic shield might, however, be weakened, letting in lower energy particles.  The numbers of energetic particles in space increases rapidly with decreasing energy, so it is those lower energy particles that could pose the greatest threat.  Or the Sun's solar wind might increase—a possibility that is not predicted by any current theories.