So what do we really know?
We’re deep in an Ice Age, the third that has occurred in the past half billion years. In between Ice Ages, the Earth has been very warm, in its ‘hot-house’ phase, with high sea levels and free of glaciers and ice caps.
Our Ice Age began 2.6 million years ago, caused at least in part by the drift of Antarctica to cover the southern polar region, and the northern continents closing off much of the Arctic Ocean. Without cold polar water easily mixing with warm tropical water, polar ice caps and glaciers formed and grew, and the global average temperature dropped from about 22 degrees C to about 12 degrees C.
During our Ice Age glacial and interglacial periods have cycled regularly. We’re in an interglacial period now, but even the interglacial periods are cool – the ice caps just retreat, they don’t completely melt, for cold polar water is still trapped in the Arctic and around Antarctica.
Our interglacial period, which we’ve named the Holocene Interglacial, started about 12,000 years ago. Probably not merely coincidentally, while we as a species have evolved over the whole time of this Ice Age, our explosion into whatever it is we are now began with the onset of the Holocene Interglacial.
We have also learned much from the analysis the last interglacial, the Eemian Interglacial, which began 130,000 years ago and lasted for about 20 thousand years. An ice core drilled into the northern Greenland ice sheet reached down to bedrock through 2.5km of ice, and back 250,000 years. The drilling took three years, the analysis another year, and the results were published in Nature last January.
In the Eemian Interglacial, global average temperatures were about 4 degrees C warmer than our current global average; CO2 levels rose to about 320 parts per million, sea levels rose about 6-8 meters higher than present, and the Greenland ice sheet melted from about 200m higher than present to about 130 lower than it is now. Since the Greenland ice sheet didn’t all melt during the Eemian Interglacial, the rest of the sea level increase must have come from the melting of the West Antarctic ice sheet.
Knowing all this, what can we truly predict about our future? We have clear models from the recent and from the more distant past of what the possible outcomes actually would be.
With atmospheric CO2 levels as high as they are at present, 393 ppm as of October, we can expect the global average temperature to become at least a few degrees warmer, and sea levels to rise at least a few more meters.
If CO2 levels continue to rise, as they are likely to do, we may force the Earth prematurely out of this Ice Age and back to its hothouse phase, over-riding the impact of continental drift in keeping the poles cold.
Obviously this is an ever changing planet, whether or not we are here to ride it out, and any sense we have that it is stable, benign or in any kind of equilibrium is shear delusion on our part.
But we have increased the pace of change, and this is going to be quite a trip. There are an awful lot of us on the planet, and if the changes happen as quickly as all the graphs from the past indicate they will, the human cost of the upheaval is going to be huge.
Of course we can adapt, but we need more time to do so without excessive misery.
We still do have the potential to limit both the extent and the pace of global warming.