In other words, it's perhaps the biggest and most rigorous process of peer review conducted in any scientific field, at any point in human history.
There are no radical departures in this report from the previous assessment, published in 2007; just more evidence demonstrating the extent of global temperature rises, the melting of ice sheets and sea ice, the retreat of the glaciers, the rising and acidification of the oceans and the changes in weather patterns. The message is familiar and shattering: "It's as bad as we thought it was."
What the report describes, in its dry, meticulous language, is the collapse of the benign climate in which humans evolved and have prospered, and the loss of the conditions upon which many other lifeforms depend. Climate change and global warming are inadequate terms for what it reveals. The story it tells is of climate breakdown.
This is a catastrophe we are capable of foreseeing but incapable of imagining. It's a catastrophe we are singularly ill-equipped to prevent.
The IPCC's reports attract denial in all its forms: from a quiet turning away – the response of most people – to shrill disavowal. Despite – or perhaps because of – their rigours, the IPCC's reports attract a magnificent collection of conspiracy theories: the panel is trying to tax us back to the stone age or establish a Nazi/communist dictatorship in which we are herded into camps and forced to crochet our own bicycles. (And they call the scientists scaremongers …)
In the Mail, the Telegraph and the dusty basements of the internet, Friday's report (or a draft leaked a few weeks ago) has been trawled for any uncertainties that could be used to discredit. The panel reports that on every continent except Antarctica, man-made warming is likely to have made a substantial contribution to the surface temperature. So those who feel threatened by the evidence ignore the other continents and concentrate on Antarctica, as proof that climate change caused by fossil fuels can't be happening.
They make great play of the IPCC's acknowledgement that there
has been a "reduction in surface warming trend over the period
1998–2012", but somehow ignore the fact that the past decade
is still the warmest in the instrumental record.
They manage to overlook the panel's conclusion that this slowing of the trend is likely to have been caused by volcanic eruptions, fluctuations in solar radiation and natural variability in the planetary cycle.
Were it not for man-made global warming, these factors could have made the world significantly cooler over this period. That there has been a slight increase in temperature shows the power of the human contribution.
But denial is only part of the problem. More significant is the behaviour of powerful people who claim to accept the evidence. This week the former Irish president Mary Robinson added her voice to a call that some of us have been making for years: the only effective means of preventing climate breakdown is to leave fossil fuels in the ground. Press any minister on this matter in private and, in one way or another, they will concede the point. Yet no government will act on it.
As if to mark the publication of the new report, the Department for Business, Innovation and Skills has now plastered a giant poster across its ground-floor windows: "UK oil and gas: Energising Britain. £13.5bn is being invested in recovering UK oil and gas this year, more than any other industrial sector."
The message couldn't have been clearer if it had said "up yours". It is an example of the way in which all governments collaborate in the disaster they publicly bemoan. They sagely agree with the need to do something to avert the catastrophe the panel foresees, while promoting the industries that cause it.
It doesn't matter how many windmills or solar panels or nuclear plants you build if you are not simultaneously retiring fossil fuel production. We need a global programme whose purpose is to leave most coal and oil and gas reserves in the ground, while developing new sources of power and reducing the amazing amount of energy we waste.
But, far from doing so, governments everywhere are still seeking to squeeze every drop out of their own reserves, while trying to secure access to other people's. As more accessible reservoirs are emptied, energy companies exploit the remotest parts of the planet, bribing and bullying governments to allow them to break open unexploited places: from the deep ocean to the melting Arctic.
And the governments who let them do it weep sticky black tears over the state of the planet. More
