Even the conservative estimates of global climate change mean serious changes ahead, writes John Gibbons
THIS COLUMN last week offered the injunction that humanity would permit our planet to heat by two degrees Celsius at our peril. Given that on a typical Irish summer's day, temperatures may lurch by more than 10 degrees, you would be entitled to wonder, what can possibly be so serious about a mere two-degree average rise?
This is the difference between weather, which changes constantly, and climate, which is calculated over long periods. If you threw a dice once, you could get any result, from one to six. That's like today's weather. If you threw the same dice thousands of times, you would get an average of around three. That's climate.
Right now, human-driven activity is in a sense changing the spots on the dice; this in turn is shifting the averages upwards. Natural systems are highly sensitive to climate averages, and these are now rising a lot faster than nature's ability to adapt.
The Intergovernmental Panel on Climate Change (IPCC), in its fourth assessment report, set out possible temperature rise scenarios for the 21st century. These range from 1.8C to 6.4C. The lower figures assume urgent steps are taken to cut global emissions, and the higher reflect the business-as-usual path.
In Charles Dickens's classic, A Christmas Carol, Ebenezer Scrooge, the bitter old miser, is visited by The Ghost of Christmas Yet To Come, and shown the fearful future that awaits him. Happily, old Ebenezer changes his ways - and thus his fate. A contemporary reworking of this morality story might be renamed A Carbon Carol, with the Ghost of Celsius Yet To Comethe spectre sweeping us on a terrifying journey across coming decades. Gentle reader, come join us now, if you dare.
One degree:The first stop on our journey is almost upon us, as the world has already warmed by 0.8C from its pre-industrial average. In a one-degree world, the melting of the Arctic sea ice continues apace. The top of the world will most likely be free of summer ice in less than a decade, while mountain glacier systems continue their headlong retreat.
A sustained one degree rise eliminates fresh water from up to a third of the world's land surfaces by the end of this century. Species extinction, already in full flow, intensifies.
Two degrees:This is the line the EU has been warning us about. The world's oceans turn acidic due to high levels of dissolved CO2. This further weakens the marine food chain. Corals also bleach and die on a massive scale.
Weather extremes, such as the 2003 heatwave in Europe that left so many thousands dead, are routine. High temperatures cause serious crop losses, leading to food shortages.
Severely stressed trees and plants begin to emit, instead of absorb, CO2. This is a "positive feedback" leading to still more warming.
The Arctic region is already heating much faster than global averages. At two degrees the massive Greenland ice sheet may be doomed. Greenland melting will, over time (it could be as little as a century), push up sea levels by seven metres and literally redraw the map of the world.
The massive Himalayan glaciers that supply fresh water for most of Asia buckle in a two-degree world. The river systems on which China, Pakistan and India depend are all but doomed. Meanwhile, between a third and a half of all species on Earth face extinction.
Three degrees:Mass starvation, due to worldwide crop failures related to heat and water shortages. The seven million square kilometres of the Amazon dries out and burns; the world has lost its lungs. Stressed soils as well as vegetation spew out CO2. The carbon cycle effectively collapses.
Four degrees:Millions of refugees flee inland to escape coastal inundation. Food and water crisis approaches critical level. Massive fatalities. Political chaos and violence; almost total economic collapse. Thawing of permafrost injects billions of tons of methane (a potent greenhouse gas) into the atmosphere, fuelling runaway heating.
Five to six degrees: hell on Earth. Total societal collapse, billions of deaths. Oceans lifeless. Massive sea level rises and super storms add to the misery for survivors. Most species approach extinction. Deep ocean warming destabilises deadly methane hydrates, causing destruction on a par with a nuclear holocaust.
As Mark Lynas, author of Six Degrees- a book which synthesises findings from hundreds of major scientific studies - warns, at the end of the Permian era some 251 million years ago a global temperature rise of six degrees caused the most severe extinction ever, with 95 per cent of species wiped out.
It bears repeating: the IPCC represents conservative scientific consensus, yet within its spectrum of scenarios for this century alone lie the six clear steps to climate hell. How long these scenarios might take to play out, nobody knows.
We share Ebenezer Scrooge's good fortune in being shown the future in time to save ourselves, if we choose. We can still keep that needle below two degrees - but only just. It won't be easy, but considering the alternatives, it has to be worth our best shot.
John Gibbons is founder of Climatechange.ie and the blog, ThinkOrSwim.ie