When strangers meet at a bus stop or in a coffee shop, weather is the universal icebreaker. Yesterday's sweltering heat, the storm predicted for this weekend: it's all fair game. Even longer-term climate shifts find their way into chit-chat. "It used to snow harder when I was a kid" is a classic example - and one explicable in part by the fact that any amount of snow looks more impressive from a child's height.
Today, however, such clichés have an edge to them, because we know that humans play a role in determining the course of climate. When we hear about Arctic tundra melting or a devastating hurricane, we're now forced to consider the fingerprints of humanity - and that's going well beyond small talk. Indeed, climate change is as much a divider as weather has traditonally been a unifier. Weather has always seemd to transcend politics, but human-induced climate change is wedded to politics: it's an outgrowth of countless decisions made by local, regional and national governments, as well as individuals and corporations. Sadly, it's also become - particularly in the US - a polarised subject, linked to other issues so frequently that it often serves as shorthand for one's entire world view.
It might come as a surprise then, how much of the basic science behind global climate change is rock-solid and accepted by virtually all parties. A handful of sceptics aside, the debate between experts these days revolves around interpretation. Just how warm will our planet get? Which computer projections for the year 2100 are likely to be the most accurate? How should we go about trying to reduce the blanket of greenhouse gases that's getting thicker each year? How can we best adapt to unavoidable changes? These are difficult questions - but they're about the nature of global climate change, not it's mere existence.
Climate refers to the average weather experienced over a long period. This includes temperature, wind and rainfall patterns. The Earth has warmed by 0.74°C over the last hundred years, and over 50% of this warming has occurred since the 1970s. Global temperatures are likely to rise between 1.1 and 6.4°C (with a best estimate of 1.8 to 4°C) above 1990 levels by the end of the 21st century, depending on our emissions. This will result in a further rise in global sea levels of between 20cm and 60cm, continued melting of ice caps, glaciers and sea ice, changes in rainfall patterns and intensification of tropical cyclones. Food shortages and the spread of disease are commonly predicted.
All recent scientific studies on Climate Change leave us in no doubt that human activity is the primary driver of the observed changes in climate. The main human influence on global climate is emissions of the key greenhouse gases - carbon dioxide (CO2), methane and nitrous oxide. The accumulation of these gases in the atmosphere strengthens the greenhouse effect. At present, just over 7 billion tonnes of CO2 is emitted globally each year through fossil fuel use, and an additional 1.6 billion tonnes are emitted by land use change, largely by deforestation. The concentrations of these gases in the atmosphere have now reached levels unprecedented for tens of thousands of years.
Climate Change presents a significant challenge to the international community. There are also enormous opportunities if we are willing to take action. Government, business and individuals all have a part to play, and all of us will benefit from rising to the challenge of climate change.