Sustainability on a knife edge

Global warming is taking us by surprise. Scientists are regularly having to recalibrate their predictions for impacts in the face of the accelerating evidence of climate change. The most dramatic example of this occurred in 2005. Before this the definitive prognosis for global average temperature rise to 2100 was the IPCC Third Assessment Report of 2001. It estimated that CO2 emissions were likely to raise average global temperature between 1.4 and 5.8°C.

In January 2005 the results were published of the most extensive study ever conducted on a wide range of climate scenarios. They factored in such things as the extent and rate at which CO2 abatement was achieved on a world-wide scale and the uncertainties arising from positive feedback. To cope with the enormous computing power needed to manage the 2017 simulations of this programme 90,000 PCs across the world were recruited to use their spare capacity to run the general circulation model adapted from the Met Office Unified Model. The result was a range of 'climate sensitivities', that is, a range of potential temperature rises based on the assumption that CO2 concentration in the atmosphere would rise to over 500 ppm, or double the pre-industrial level.

The result was a new range of temperature increases from 1.9°C to 11.5°C. The scatter of probabilities formed a bell curve with the highest probability being 3.4°C, though many were much more severe at lower probability. Another multi-PC program is currently underway to enlist an even greater number of computers.

One of the events which jolted scientists was the breakaway of the Larson B ice shelf off north west Antarctica in 2002 amounting to 3250 square kilometres of floating ice. It will not contribute to sea level rise but, since ice shelves in this region buttress the West Antarctic ice sheet, there is now significantly less to prevent the ice sliding into the sea. In 2005 it was calculated that Antarctic glaciers are discharging 110 cubic kilometres to the sea each year.

Greenland is even more dramatic with its two main glaciers adding 120 cubic kilometres annually to the sea. The most recent study (2006) reported in the journal Science estimates that, overall, Greenland deposited 224 cubic kilometres into the sea in 2005.1 In total meltdown the combined effect of the Greenland and West Antarctic ice sheets would be to add 12 metres to the height of the oceans. Also in the last 18 months it has become apparent that mountain glaciers are rapidly contracting. Over the last century the Alps lost 50% of their ice, mostly in the late decades. The Himalayas are losing ice faster than anywhere else on Earth.

In September 2005 it was reported that the whole of the Siberian Sub-Arctic region is melting due to an overall temperature rise of 3°C in 40 years. The Siberian bogs could release 70 million tonnes of methane, a greenhouse gas 23 times more potent than CO2 described as an ecological landslide undoubtedly linked to global warming.2

Despite these signs, the international community is showing little collective will to take emergency action to stop the world crossing one of two critical thresholds, the so-called 'tipping points'. This is to do with concentrations of greenhouse gases in the atmosphere, especially CO2. At the moment the concentration is around 382 parts per million (ppm) and rising 2-3 ppm per year. The critical threshold is either 400 or 440 ppm depending on who you believe. Beyond this point the melting of ice becomes irreversible and positive feedback enters unknown territory. Unless something remarkable happens we shall cross the tipping point somewhere between 2012 and 2035. It is a sobering thought that when there was no ice on the planet sea level was 120 metres higher than during the last ice age - perhaps 80 m so higher than at present.

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Solar Stirling Engine Basics Explained

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