Dust particles in the vapor-rich impact plume ejected from the Chicxulub crater rose above the Earth's atmosphere, enveloped the Earth, and rained back through the atmosphere around the entire world.  The dust in the atmosphere blocked sunlight from reaching the surface around the entire globe.  Current estimates suggest that the dust made it too dark to see for 1 to 6 months and too dark for photosynthesis for 2 months to 1 year, seriously disrupting marine and continental food chains.  During this period, land surface temperatures decreased dramatically, possibly remaining below freezing in many areas.  The dust eventually fell back to Earth, forming part of the global clay layer that marks the end of the Cretaceous period.

     In addition to solid ejecta, toxic vapors were distributed worldwide.  Carbonate and evaporite rocks covered much of the impact site.  When the carbonate was vaporized it formed the greenhouse warming gas carbon dioxide.  Vaporized evaporite released sulfur oxides, which reacted with water to produce sulfate (or sulfuric acid) aerosols.  These atmospheric particles significantly reduced the amount of sunlight reaching Earth's surface and, thus, enhanced the effects of the ejected dust.  The darkness and cooler temperatures produced by these particles likely lasted for months to years.  Eventually, the sulfuric acid aerosols fell to ground as acid rain.

     When the solid ejecta reentered Earth's atmosphere, it shocked-heated the air and dramatically altered its chemistry.  The protective ozone layer was one possible casualty of this change.  The formation of nitric acid rain added to the consequences of the sulfuric acid rain.  This could have defoliated continental vegetation and even aquatic plants in shallow lakes or seas.  Asphyxiation of animals by nitrous oxides and toxic poisoning by metals leached from the ground may also have occurred.

Altered impact melt spherules from the K/T boundary sediments in Haiti. These objects form a bed nearly half a meter thick, which is covered by a second, iridium-rich bed. The scale along the bottom of the photograph has 1mm divisions.

        While some post-impact effects are still uncertain, one environmental catastrophe for which evidence exists is massive forest fires.  Charcoal and soot have been found in sediments around the world.  These fires were probably started when the impact debris came raining back down through the atmosphere. This debris quickly heated the air, producing a temperature spike so high that vegetation spontaneously ignited.  These fires apparently consumed large quantities of vegetation and killed countless animals, either by burns or starvation.

        Clearly, the Chicxulub impact caused severe environmental changes that could have led directly to the destruction of an entire species.  It is unlikely that any single process was responsible for all extinctions, but rather that they operated in concert.  The time scales for these processes were also very different.  The direct effects of winds and tsunamis lasted only a few hours, as did the temperature spike induced by falling ejecta.  Fires, darkness, and cold temperatures probably lasted a few weeks to months and certainly less than a couple of years.  The effect of the acid rain and toxic substances may have lasted for several years, while greenhouse warming persisted for centuries.