My twopenn’orth on whether climate change is natural or manmade - drawing on climate models from geological history remembered from a Geology degree thirty years ago.... (OK – it won’t convince someone who believes the earth was created just a few thousand years ago – but you’ve got to start somewhere)
Earth seems to develop negative feedback systems to return its surface conditions to a steady state when disturbed – this is the (non-mystical) basis of the Gaia hypothesis.
Forget carbon dioxide levels varying over the past thousand, ten thousand or hundred thousand years (and try to ignore the suggestion that we are about 40,000 years overdue for an ice age on past indications), we are talking about processes taking tens if not hundreds of millions of years. I’m thinking of the Carboniferous Period (354-290 million years ago) and the Cretaceous Period (144-65 million years ago). From what we can glean from the geological record (it’s called ‘palaeo-climatology’) both were marked by small or non-existent ice caps, extensive shallow seas and higher average temperatures than now.
And both Periods are named for mechanisms which absorbed carbon dioxide from the atmosphere and trapped it in the earth’s crust.
In the Carboniferous, 'trees' from mangrove swamps were buried over tens of millions of years forming coal (oil and gas were formed in a similar way - though not from trees).
In the Cretaceous, micro-organisms flourishing in the warm seas deposited chalk and limestone trapping the carbon as calcium carbonate – again over tens of millions of years. I’m interpreting this in each case as periods with high levels of carbon dioxide in the atmosphere and a negative biofeedback mechanism which trapped it in the crust and – eventually – reduced CO2 levels, the earth cooled and the sea levels dropped as ice caps grew again.
So – it’s a natural process?
Trouble is – what we’re doing, by burning fossil fuels, is releasing carbon dioxide that took tens if not hundreds of millions of years to deposit in a matter of a one or two centuries (and that’s not counting carbon dioxide released when we make cement or concrete from limestone). This is simply too fast for the earth to develop new feedback systems!
We're getting the 'greenhouse effect' – global warming, ice caps melting, sea levels rising – including dramatic weather during the (geologically brief) transition period.
And this may well be enough to put paid to what we call ‘civilisation’ – and maybe also to the human race.
But unless the process is so extreme that the earth loses a lot of its water into space (in which case Earth will become like Venus), I think we can rely on negative biofeedback mechanisms developing to remove the carbon dioxide naturally, though it make take a few hundred million years!
To summarise – even though we are releasing carbon dioxide in one-millionth of the time it took to deposit it, it’s likely that biofeedback mechanisms will develop to re-trap the carbon dioxide naturally – but over rather a long time. It is not really the earth that is at risk from climate change – it is humans and human civilisation.
Oh – and a thought about offsetting carbon emissions by planting trees. It took the mangrove swamps of the Carboniferous – hundreds of millions of years, thousands of millions of generations of trees growing over most of the planet – to trap enough carbon as coal to make a difference to CO2 levels. Planting trees will trap a little CO2 while the tree is actually growing, though not when it reaches maturity – but unless the ‘offsetters’ have plans to turn their trees into coal – its nothing like a longterm solution.