First of all lets be clear there is no debate about climate change. 97 % of scientists working in the field are in agreement and there has been not been one single peer reviewed scientific article refuting the basic science of climate change. Adding additional carbon dioxide (and other greenhouse cases) to the atmosphere will cause the earth to warm. Fact. This has been know since 1896. This warming will then interact with other highly complex processes and create additional warming. It is the speed and magnitude of this additional warming that is in dispute.
There is now overwhelming evidence that that this warming has started. The most recent assessment of the UN’s Intergovernmental Panel on Climate Change (IPCC) asserted that “Warming of the climate system is unequivocal” and that “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” They indicate that very likely means “the assessed likelihood, using expert judgement” is 90%. In real speak that means it’s 90% certain that humans are responsible for this warming. And these are scientists here doing the talking, an inherently conservative bunch not prone to exaggeration. If your doctor told you that there was a 90% chance you would get an ailment you might do something about it. The same report suggested that average temperatures were likely to increase by 2.4 to 4.6 °C by 2100 if no action was taken to reduce emissions. To avoid dangerous climate change concentrations of atmospheric carbon dioxide would need to be stabilised at 450 parts per million (ppm) resulting in global temperatures of around 2 to 2.4°C above pre industrial levels.
However recent research suggests that even limiting warming to 2 to degrees will pass tipping points at which positive feedback loops kick in, creating catastrophic climate change. Essentially what this means is that a slight warming will trigger events which will create further warming. These so called positive feedback are numerous. For example white ice currently reflects around 70% of the suns radiation back into space, as the ice melts it is replaced by darker sea which absorbs 90% of the sun’s heat creating further warming. Another positive feedback is the melting of the permafrost in Siberia and Canada which will release thousands of tonnes of methane (a powerful greenhouse gas) into the atmosphere creating even greater warming. Greater warming could cause the Amazon rainforest which currently absorbs thousands of tonnes of carbon dioxide to burn down releasing more CO2. Warmer air can hold more water vapour which is a greenhouse gas, and so on it goes. Once the temperature rises enough for these events to kick in it may unleash a cycle of even greater warming which becomes impossible to stop no matter how deeply emissions are cut subsequently. James Hansen the NASA climate scientist suggests that “if humanity wishes to preserve a planet similar to that on which civilisation developed ….carbon dioxide will need to be reduced from its current 385 ppm to at most 350 ppm”. Ie we have already released too much CO2 so we don’t just have to stabilise greenhouse gases we have to actually remove them. Such analysis leaves Clive Hamilton to conclude “To have any hope of avoiding catastrophes, emissions must peak within the next few years and certainly no later than 2020, then begin a rapid decline to the point where all energy generation and industrial processes are completely carbon free”