By Bill McGuire

More evidence has come to light to support the thesis that we are on a warming trajectory that will leave our planet unrecognisable from the one upon which human civilisation developed and thrived. Scientists congregated, this week, at Imperial College London for a Royal Meteorological Society meeting, which focused on the Earth’s climate during the Pliocene era, around three million years ago (the last time the Earth had >400ppm of atmospheric CO₂).This is the last time that carbon dioxide levels were above 400ppm (parts per million). They are currently 412ppm; up from around 280ppm during pre-industrial times and climbing at two or three ppm a year.

The
papers presented at the meeting transport us to a world that is
similar to our own in the sense that carbon dioxide levels are
comparable, but there the similarity ends. During the Pliocene,
global average temperatures were 3°C – 4°C higher than they are
today; a seemingly small difference, but big enough to drive changes
that make Pliocene Earth dramatically different from that of the
21^st
century.

Evidence
from plant fossils found in Antarctica show that during this part of
the Pliocene, beech trees – and maybe conifers – grew on the
continent, which had a drastically reduced ice cover surrounded by
scrubby tundra. Summertime temperatures were around 5°C compared
with minus 15°C – 20°C today. At the other end of the world,
Greenland was completely ice-free. The vanished ice at both poles
translates into global sea levels that are around 20m higher than
they are today.

So,
forget the widely touted idea that we can keep the global average
temperature rise below 1.5°C or 2°C. It is now clear that the
atmospheric carbon levels we have already are compatible – in the
longer term – with temperatures 3°C – 4°C higher than they are
now. The climate system is relatively slow to respond to change, so
it will take some time for temperatures to catch up with greenhouse
gas levels. But catch up they will, almost certainly by the end of
the century. In other words, whatever actions we take to reduce
emissions, we cannot avoid a hothouse planet scenario that will see
our society torn apart by a combination of extreme heat, wild weather
and catastrophically rising sea levels. And remember, there is still
no sign of rising greenhouse levels being brought under control.
Given current inaction, carbon dioxide concentrations in the
atmosphere could quite feasibly top 500ppm and keep going, dragging
global temperatures ever higher.

I hope to God that it never happens, but it is worth keeping in mind that burning most fossil fuel reserves is projected to lead – ultimately – to a global average temperature rise of a staggering 16°C. This would bring the mean temperature of our world to a furnace-like 30°C. This would likely – to all intents and purposes – be an extinction-level event as far as the human race is concerned. Some might say, quite justifiably in view of our appallingly bad stewardship of the planet, good riddance too!

Bill
McGuire is Professor Emeritus of Geophysical & Climate Hazards at
UCL and author of Waking the Giant: How a Changing Climate Triggers
Earthquakes, Tsunamis and Volcanic Eruptions. He was a contributor to
the IPCC 2012 report on Climate Change & Extreme Events and
Disasters.