Paleobiologists believe those prior extinction events variously each killed off about 65-95 percent of all prior living species.
In contrast, a conservative projection of the number of species anticipated to be destroyed in the present, or Holocene, extinction event is "only" 50%. Roughly 5000 mammal species survive today. If their numbers are halved over the next few generations, as is projected, of course many of the creatures we see now on hikes into wild areas, or in films, at zoos, and by visiting aquariums, just won't exist anymore. It remains an open question whether Homo sapiens can persist through the adverse conditions ahead or if we will join so many others in oblivion.
Contrary to popular wisdom, the previous mass die-offs apparently occurred over relatively longer periods, usually hundreds of thousands or even millions of years. Even the Cretaceous-Tertiary extinction event, likely catalyzed by earth's collision with a huge asteroid or comet 65 million years ago, was followed by a die-off that was not completed for a considerable period.
The special significance of the present extinction event is that it is believed to be occurring far faster (in terms of the total number of marine and land species being destroyed around the globe per unit of time) than ever before.
This seems to be the result of a combination of human related interactions with the environment, as through our agricultural practices, burning of forests, over-fishing and hunting, urban development, pollution, releases into new areas of highly destructive (to native species) animals and plants, and global warming.
The process is thought to have begun roughly 10,000-20,000 years ago and to have been accelerating as our kind of mammal has become more effectively the dominant one on the planet.
Many environmental scientists think that the threat of global warming is now the most severe of these hazards. While there is controversy over whether or not humans are causing this phenomenon, agreement is easier on the idea that increasing levels of carbon dioxide in the atmosphere tend to raise the global temperature.
Even if mankind has not contributed to global warming to date, then, it is reasonable to assume that, if we raise the level of CO2 in the air further, we must at some point be doing serious harm to our surroundings. This is all the more the case as carbon dioxide tends to stay in the atmosphere for centuries. Previously it was thought that trees would remove excess carbon dioxide from the air. However, it now appears that the planet's forests may have already done about as much as they can in this regard.
Rather than reducing adverse carbon dioxide levels, we are adding more carbon dioxide emissions all the time through the burning of carbon based fuels (wood, coal, petroleum products, and natural gas). As the ever increasing demands of a growing population here in the United States for more advanced, yet energy intensive lifestyles continue, but there is little serious effort at conservation, our contribution to the destruction of the environment and to added carbon dioxide in the atmosphere can only be larger. But as the remaining 75% of the world's carbon dioxide production, including from India, China, and others, grows even faster, because those nations' societies are modernizing or further advancing just as quickly as possible, the capacity of even our vast planetary atmosphere to absorb further extra CO2 without extremely altering the world's climate will be further tested.
Already, we pump 7 billion tons of carbon dioxide into the atmosphere annually. By 2030, it is anticipated the world's yearly output of carbon dioxide emissions will be 12 billion tons, 20 billion tons by 2100. Such levels of insult to the planet's natural recycling system are unsustainable and would almost certainly have disastrous consequences.
Not all global greenhouse effects have been bad for us. It is speculated that changes in human culture several thousand years ago, such as the introduction of rice farming and more widespread burning of forest or grassland areas, increased atmospheric carbon dioxide enough to hold off a new ice age, which otherwise might have precluded the rise of modern civilization.
But if fossil fuel usage is not soon dramatically curtailed, carbon dioxide in the atmosphere will be about doubled by mid-century, potentially raising worldwide average temperatures by about 5-8 degrees F. Most would agree this is too much of a good thing.
The last time temperatures were so high, in the Eocene, around 50,000,000 years ago, crocodiles thrived in what is now the western US and in the lands that would become northern Europe, there were no polar icecaps, sea levels were hundreds of feet higher, many places had little seasonal variation, palm trees were common in the London basin, and much of the world enjoyed virtually ceaseless sizzle.
Climate Change: Information and Suggestions. Debra Efroymson. Work for a Better Bangladesh Trust; June 2005.
Endless Summer. Jennifer Richler in Mother Earth News.com, Issue #182; October/November 2000.
Endless Summer No Picnic for Seniors. Carol Cruzan Morton in Sage Crossroads.org; July 6, 2004.
The Five Worst Extinctions in Earth's History. Lee Siegel in Space.com; September 7, 2000.