<p id="d15004252e172">A mass extinction occurred at the Cretaceous−Paleogene boundary
coincident with the
impact of a 10-km asteroid in the Yucatán peninsula. A worldwide layer of soot found
at the boundary is consistent with global fires. Using a modern climate model, we
explore the effects of this soot and find that it causes near-total darkness that
shuts down photosynthesis, produces severe cooling at the surface and in the oceans,
and leads to moistening and warming of the stratosphere that drives extreme ozone
destruction. These conditions last for several years, would have caused a collapse
of the global food chain, and would have contributed to the extinction of species
that survived the immediate effects of the asteroid impact.
</p><p class="first" id="d15004252e175">Climate simulations that consider injection
into the atmosphere of 15,000 Tg of soot,
the amount estimated to be present at the Cretaceous−Paleogene boundary, produce what
might have been one of the largest episodes of transient climate change in Earth history.
The observed soot is believed to originate from global wildfires ignited after the
impact of a 10-km-diameter asteroid on the Yucatán Peninsula 66 million y ago. Following
injection into the atmosphere, the soot is heated by sunlight and lofted to great
heights, resulting in a worldwide soot aerosol layer that lasts several years. As
a result, little or no sunlight reaches the surface for over a year, such that photosynthesis
is impossible and continents and oceans cool by as much as 28 °C and 11 °C, respectively.
The absorption of light by the soot heats the upper atmosphere by hundreds of degrees.
These high temperatures, together with a massive injection of water, which is a source
of odd-hydrogen radicals, destroy the stratospheric ozone layer, such that Earth’s
surface receives high doses of UV radiation for about a year once the soot clears,
five years after the impact. Temperatures remain above freezing in the oceans, coastal
areas, and parts of the Tropics, but photosynthesis is severely inhibited for the
first 1 y to 2 y, and freezing temperatures persist at middle latitudes for 3 y to
4 y. Refugia from these effects would have been very limited. The transient climate
perturbation ends abruptly as the stratosphere cools and becomes supersaturated, causing
rapid dehydration that removes all remaining soot via wet deposition.
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