Was the last time.
••
Hermit - C02 is a GHG - there are others but C02 is the global moderator that along with water vapour magnifies changes in the atmosphere created by orbital drivers.
The role of C02 has been understood longer than evolution. It's rather simply physics and keeps our planet habitable.
A primary driver of climate is something which imposed on the atmosphere - the sun is a primary driver and as the planet moves to different configurations in it's orbit and inclination against how far or near we are to the sun ( Milankovitch cycles ) - then ice ages wax and wane with that.
C02 is the accelerator - it accelerates the warming AND the cooling.
Assuming you accept the the physics of C02 which is unassailable
Background/history
http://www.aip.org/history/climate/summary.htm
The ocean releases C02 as it warms and absorbs C02 as it cools. In this role C02 is a feedback.
This is within the carbon cycle
http://wufs.wustl.edu/pathfinder/path20 ... _13_07.htm
The planet to some degree breathes and small changes are magnified by C02 and other shorter duration GHG gases such as methane.
These gases are also sequestered over time - that's where our coal and fossil fuels come from. Plant matter picks up C02 from the atmosphere as part of it's food cycle and in many cases dies without releasing to the atmosphere ( think peat bogs, diatoms in the ocean etc ) .
So the taiga and tundra and all fossil fuels contain millions of years of carbon that has been sequestered. and the atmosphere plods along varying between 150 and 280 ppm in the recent cycles.
had we not come along the planet was drifting slowly on orbitals towards another ice age 1500 or so years out.
You can see the drift downward here from the Holocene Optimum

until about 350 years ago when the industrial revolution started and we began to mine those millions of years of fossil carbon and release them into the atmosphere.
Now that slow drift down in temperatures ( which would cool the oceans which would absorb more C02 so cooling the atmosphere which cools the ocean and around we go in the normal course of feedback into another ice age. )
has been interrupted by us mining and releasing more and more fossil carbon

and

and true to it's nature carbon is acting as a feedback as well so the ocean can hold less and less - we release more, the ocean releases more and the ice age is cancelled and we are proceeding towards a disaster at a rate faster than occurred leading up to the Permian extinction.
What happened then to release fossil carbon??
The mother of all continuous volcanic eruptions in Siberia ( due to global drift what is now Siberia ).
Over 10,000 years continuous eruptions intruded into carboniferous strata and released massive amounts of fossil carbon - just as we are doing now.
The planet warmed to the point where most of life was extinguished. Scientists can see this in the record and the physics of C02 in both the atmosphere and the ocean are consistent with this.
Now we are never going to get to the levels then all by ourselves, the risk is the positive feedbacks of methane and carbon still store in the tundra and taiga and along the continental plateaus as clathrates - frozen methane.
Our nudge sets off a release of carbon many times what we do.
If we burned ALL the fossil fuel available we would raise the temp some 10 degrees C and effectively alter the world as we know it - coastlines, climate bands and extremes of weather.
But that's just us....there is way more sequestered that can and is being released by forest fires ( turning sequestering plants into carbon sources ) and by direct release from melting permafrost and methane bubbles.
This is not speculation - it is happening now - not wide spread yet but it has started.
http://thinkprogress.org/climate/2012/0 ... tic-ocean/
and more and more amplification is occurring as we lose the Arctic ice and alter the albedo which also is a climate driver. ( more ice more albedo more reflection less heat retained ) - less ice, less albedo, more heat retained - around we go adding on to what we are doing.
The ocean and the cryosphere ( ice ) are huge moderators but it just slows the progress.
At this point if we stopped emitting cold turkey the climate would perhaps get to a new balance in about 3,000 years somewhat higher than now as the bulk of our carbon release gets sequestered - but there would still be some effects 100k years out.
Methane is 20 x more potent a heat trap but it drops out in 6 years or so.
Now this is a very simple explanation but it is real and there is good reason the scientists are dire in their warnings as the edges of climate change are here now and yet we are only .6 degrees C into the change.
Every single estimate of the rate of change has been conservative - in other words the real change has exceeded what was expected with each report.
Even back in 1981 the predictions were there
http://www.universetoday.com/94468/1981 ... -accurate/
and they were accurate but on the lower end.

The rate of change is accelerating and the window to mute it closing rapidly.
There is simply no greater risk to the biome and human industrial civilization - the insurers know it - they are charging accordingly.
Better wake up to the reality as how to go about blunting it is mired in sowed confusion and timid leadership.
Everyone can reduce their carbon footprint. Many do and both companies and individuals are doing good things along these lines.
Carbon needs to be taxed to help the transition to a lower carbon emission world. Sweden is committed to carbon neutral by 2050 and is well on it's way.
It can be done and every study has shown it is less costly to reduce carbon now than deal with the consequences of a warmer planet with more extreme weather.
Of course the dinosaurs like Koch and Exxon love the likes of M&M.
The science is clear and has been for two decades or more.
The sowing of confusion is also clear to the point where the Royal Science Academy in Britain called Exxon on the carpet - the first time in it's long history it has done so - underlining the severity of the situation.
If you just grasp how carbon dioxide works in the atmosphere Hermit you'll see the risk we face.
There are consequences
