Blast from the past.

1982 to be exact.  In catastrophic blasts between March 28 and April 4 the El Chichon volcano in southeastern Mexico left 1,900 people dead and destroyed millions of dollars of crops.  Not the first time a volcano has rumbled awake and smacked humans up-side their heads.  Not the first time a volcano has done something else as well - affected the climate.  As much as 10 million tons of sulphur dioxide were blasted into the atmosphere.  The ash circled the globe quickly, dropping global temperatures as much as a degree F. Temperatures in the stratosphere dropped and remained at lower than average levels for 7 years after the eruption.  Fortunately for various living things, especially third world farmers, the greatest weather affects were ameliorated by the development of a powerful El Nino within weeks of the eruption.

Since the earth really doesn't care about environmentalists, it has been changing its own weather for millions of years without any contribution from humans.  Volcanoes are a prime example of that:

 "The sun was dark and its darkness lasted for eighteen months; each day it shone for about four hours; and still this light was only a feeble shadow; the fruits did not ripen and the wine tasted like sour grapes."  Michael the Syrian, 536 A.D.

"During this year a most dread portent took place. For the sun gave forth its light without brightness. and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear.  Procopius of Caesarea, 536 A.D.

Documents for the period of 535 - 536 A.D. speak of the sun being obscured by a "dry fog", summer frosts, widespread crop failures in Europe, drought and famine in China.   Tree rings reflect poor growth, and ice cores from Greenland and Antarctica contain highly elevated levels of sulfuric acid dust.  A single massive eruptive event, and the climate was drastically affected.

1816 was "The Year Without a Summer", the "Poverty Year", and "Eighteen Hundred and Froze to Death", created by the eruption of Mt. Tambora in Indonesia in 1815.  Killing frosts and snow storms in May and June in Eastern Canada and New England caused wide-spread crop failures, and Pennsylvania lakes and rivers were still iced in July and August.  Populations plummeted, with farm loss forcing the first major migration to the American mid-west from the north-east.  Artist J.M.W. Turner captured the atmospheric affects in his paintings:

And incessant rainfall during a nasty summer trapped a vacationing group inside during their Swiss holiday, leading to a contest to write the scariest story.  The winner, by a young Mary Shelley, was "Frankenstein, or the Modern Prometheus".

At least 6 volcanic eruptions are known to have caused summer anomalies in the last 2,000 years:

Kuwae - 1452
Huaynaputine - 1600
Krakatoa - 1883
Novarupta - 1912
Pinatubo - 1991

In reality, these were small eruptions compared to others that predated written history.  Roughly 75,000 years ago, the Toba Caldera in Sumatra erupted with a force 3500 times greater than that of Tambora.  An eruption of that magnitude can cool the globe 5 to 9 degrees F for years.  The sulphur aerosol is more than the atmosphere can absorb.  An earth that was already in a cooling cycle can be pushed into an Ice Age, or, at minimum, a long cold cycle, devastating for agriculture. 

It wasn't the first time Toba changed the climate.  788,000 years ago a Toba eruption coincided with the advent of a warm period.  And a Toba-level eruption would create major temperature anomalies today:

Volcanoes capable of these sorts of eruptions are very much still around and rumbling - Yellowstone National Park sits in the caldera of a still active mega-volcano.  Thousands of years are but a blink in the history of the earth.   And all "The sky is falling!" scurrying around of the "We must stop global climate change!" folks will not and never will matter a whit in the face of the processes that truly affect our climate.

This Isn't a Good Idea

So once upon a time - about 30,000 years ago - a squirrel buried seeds in what would become Siberia.   Scientists have been able to use tissue from plants found in that squirrel burrow to resurrect a viable plant, Sylene stenophylla.

Pretty.  Also still extant in the Russian tundra, so in this case it's not the rebirth of an extinct species.  But folks are all excited - this could be the path to bringing back Ice Age creatures like the wooly mammoth and mastodon.

They shouldn't.  Yes, it would have major, major WOW! factor.  But this isn't their world anymore.  Extinction is perfectly normal.  It always amazes me that the same folks who tout a nature-focused religion completely ignore some of the rules that are imbedded in nature.  Extinction is nature's way of saying that a creature doesn't fit in anymore.  That's true whether it's T. Rex or the dodo - they couldn't survive in the world that developed around them.  And any creature that has gone extinct that is brought back will immediately be non-native species.

We've had lots of experience with non-native species:

Rabbits were introduced to Australia as food in the 19th century - they are enormously destructive, creating "the grey blanket" over Australia with their numbers.

Kudzu, the "vine that ate the south", was introduced as an ornamental and for erosion control - now it suffocates the forests of the American South and has been found in Canada.

Nutria were brought into the country as fur animals - they are a major destruction force in American wetlands.

Brown tree snakes were accidentally introduced into Guam in the early 1950s - they have now wiped out at least 12 endemic bird species and are hitching rides into other islands.

The water hyacinth, an ornamental plant from South America, has choked waterways all over the globe, killing fish and other plants, and causing native peoples to starve to death because they can't get through the waterways that act as their roads to food.

I'd love to see a mastodon, a saber-toothed cat, a giant sloth.  But this isn't their world anymore, and no matter how well meaning the actions that would bring them back into it, that action would eventually have repercussions.  We've had enough examples of those sorts of repercussions that we should know better by now.