http://www.guardian.co.uk/commentisfree/2013/may/12/observer-editorial-carbon-dioxide-levels-high

The news that concentrations of carbon dioxide in our atmosphere reached a level of 400 parts per million last week might not appear to have immediate significance. The level is only a couple of units higher than last year, after all. Yet the development has undoubted importance.

With the realisation that carbon dioxide levels have achieved that symbolic 400ppm figure, it is now clear that two decades of warnings from scientists have fallen on deaf ears and that our leaders have failed completely to curtail rising outputs of greenhouse gases across the world. Indeed, they have allowed them to accelerate.

In the 1960s, levels rose at 0.7ppm a year. Today, they rise at 2.1ppm, as more and more nations become industrialised and increase outputs from their factories and power plants. As a result, the most conservative of scientific calculations suggest Earth now faces a 50-50 risk of a 2C rise in global temperatures by the end of the century. In fact, most researchers now believe the increase is more likely to reach 3 to 5C. Spreading deserts, rising sea levels and increasingly erratic violent storms look destined to blight our planet.

Earth has not had 400ppm of carbon dioxide in its atmosphere for millions of years. When it did, the Arctic was ice-free and sea levels were 40 metres higher. Our species has therefore never lived in a world that will be as hot as the one we are creating for our children and grandchildren. Civilisation rests on the happy fact that since the last ice age, the planet's climate has been cool and stable, giving ancient farmers a chance to experiment with the growing of grasses and plants and so create the crops that now sustain billions of humans. All that is set to change, as temperatures rise, deserts extend and life-sustaining weather patterns are disrupted. Hundreds of millions of people would then be rendered homeless.

It is an apocalyptic vision. Yet it is not quite our destiny. There are signs from China and the US - the two nations responsible for most of Earth's carbon dioxide output - that they are beginning to appreciate the dangers. American carbon dioxide outputs have dropped while China recently acknowledged it could not sustain its current levels of industrialisation without giving greater consideration to the environment. Equally, there are signs that low-carbon technology - new generation wind, tide and wave devices and carbon capture and storage systems - is beginning to be adopted in many nations. Given time, renewables could wean us from our oil, gas and coal dependency, cut carbon outputs and, in the process, give Britain an industrial boost. With our North Sea oil expertise and high winds and strong seas, we have the perfect credentials for developing a healthy, low-carbon power industry.

The problem is time. The government continues to send mixed signals about its commitment to renewable energy and so deters investment. A lull in outputs is now badly needed. If politicians can be made to understand that message and act at last, we have a chance. Hence the importance of highlighting the 400ppm figure reached last week.