Scientists, environmentalists and politicians have got the message about global warming out loud and clear. We know that if we pollute less, temperatures will fall; glaciers and polar ice will then stop melting, sea levels will drop and there will be fewer severe climate patterns. But not so widely known is what we have to aim for to achieve this.
The 1997 Kyoto Protocol on climate change generally puts this target at a reduction of 10 per cent of 1990 carbon dioxide emission levels. Each country has a different figure. Few in the world have an inkling of just exactly what this might be. With greenhouse gas emissions ever-rising, the number is constantly changing.
Nasa climate scientist James Hansen, who tuned us in to global warming and the need to do something about it 20 years ago, has been looking for an answer for some time. He revealed one that we can all relate to and aim for, speaking at the American Geophysical Union meeting in San Francisco last December. It is 350.
This, specifically, is 350 parts per million of carbon dioxide in the atmosphere. Dr Hansen says this is the absolute upper boundary at which the Earth can be safe from global warming. If we can keep it lower, then all the better.
We should be alarmed. The foremost measurement station for atmospheric readings of carbon dioxide, the Mauna Loa Observatory, 3,397 metres up a volcano in Hawaii, puts the latest figure at 387 parts per million - 2 ppm more than when Dr Hansen gave his presentation. Based on his assertion at the time, this means that our climate patterns are fast unravelling and will continue to do so until the number is back down to 350.
Our leaders are telling us that global warming is a problem and advising what to do about it. The big picture is not so clear, though: Do we cut by a certain percentage, or reduce by half, or decrease to previous levels, the amount of carbon dioxide we pour into the atmosphere? We would do much better to follow the preaching of American environmentalist and author Bill McKibben and express how much over the minimum level we are, as with temperatures.
