Potential impacts of climate change
It is hard for scientists to be precise about the impacts of a given temperature rise. We know what type of impacts to expect – melting sea ice, disappearing glaciers, rising sea levels, flooding of coastal areas, water shortages, crop failures, extreme weather events and species extinction. And we know these physical effects are likely to lead to social and economic impacts, such as famine, disease, mass migration, homelessness and tensions over water supplies. But we don’t know exactly what each degree, or fraction of a degree, of temperature rise will mean in terms of the level at which each of these impacts will be felt.
The IPCC, which draws on the work of thousands of scientists, was more specific in its 2007 report than it had been in its previous assessment in 2001. For example, it said that if increases in global average temperature exceed 1.5-2.5oC, around 20-30% of plant and animal species are likely to be at increased risk of extinction.
The Stern report included a table showing the potential impacts of different levels of warming. It suggested that a temperature rise of up to 2°C could mean glaciers melting and coral reefs being damaged, while a rise of over 2°C could mean the onset of the Amazon rainforest’s collapse and a rise of up to 4C could result in a 60% increase in people facing hunger and a billion people facing water shortages.
2°C has widely been regarded as the tolerable upper limit. The UK Government position is that “The UK Government and the EU consider that global warming must be limited to no more than 2°C temperature rise above pre-industrial times to avoid dangerous climate change.” The EU says limiting warming to 2°C is needed to avoid “irrevocable consequences” or “massive and irreversible disruption of the global climate system”.[1]
Writer Mark Lynas has pored over scores of scientific papers to produce his book Six Degrees, which looks at precisely the issue of what each extra degree of warming might mean. His findings highlight the second reason why urgency is growing, namely the possibility of one impact leading to another- or so-called ‘feedbacks’. His researches indicate that a rise of over two degrees would trigger the collapse of the Amazon forest and a release of carbon from the soil that will release a further 250ppm of CO2 and take us to a four degree rise. Once there, he forecasts, Siberian permafrost will thaw, releasing methane that has been stored for centuries, driving the rise up to five degrees, which in turn could trigger the release of more methane, leading to a six degree rise which brings humanity to the brink of extinction.