Look at the graph above. Look at the red line. Look at its dramatic slope upwards, after about 2002. It's one of the scariest things in modern history.
It shows the way China's emissions of carbon dioxide, the key greenhouse gas, soared in a way no-one ever expected they would, as the Chinese economy expanded explosively after the Millennium, with incredible, double-digit growth – at one stage more than 11 per cent per annum. As a result, Chinese emissions of CO2 doubled in a decade, from 3 billion tonnes to more than 6 billion tonnes annually, and by 2007 China had overtaken the US as the world's biggest carbon emitter.
Although no one knows the current figures yet, it is likely that they are now running at well over 7 billion tonnes per year, with the annual increase alone greater than the entirety of Britain's CO2 output of about 580 million tonnes a year.
This abrupt, gigantic surge in the amount of carbon going into the atmosphere threw previous calculations about the future progress of global warming into confusion, and is the main reason why world CO2 emissions are now at the top of the very highest pathway previously imagined by the UN's Intergovernmental Panel on Climate Change. They are now putting the world on course, an international commission of climate scientists reported last week, for a catastrophic 6C temperature rise by 2100.
Anything China can do about its emissions, therefore, is to be given a fervent welcome, and this is the first, enormous significance of yesterday's announcement – the simple fact that China is taking internationally pledged action about them.
The Chinese have always insisted that, to bring their people out of poverty, unhindered growth is their natural right – the rich West did it, after all – and resented any idea of CO2 reduction targets. But the country's leadership has come to accept that the threat of climate change is so severe that they too must act.
They are not yet pledging an emissions cut, as the developed countries are doing; they want to carry on growing.
But the promised reduction in the economy's carbon intensity does mean that the rate of increase in emissions should slacken – i.e. that the slope of the red line above will become less steep, rather than steeper. And as 90 per cent of all future emissions growth will come from China and its fellow developing countries, that is very much worth having.
The other enormous significance of yesterday's announcement is that the 45 per cent figure is "a number on the table", something that is essential for a deal between the two key carbon players, China and the US (which account for 40 per cent of world emissions between them) at the forthcoming Copenhagen climate conference. Its announcement was brought forward and made immediately after the US announced its own number – a 17 per cent cut in US emissions from a 2005 baseline – on Wednesday.
To have any chance of holding global warming to the danger threshold of a C rise will require, of course, far, far more than the US and China are pledging. It will require far more than the European Union is pledging. But it is a beginning.