KEY POINTS:
Tomorrow night the world gets an updated consensus view from the climate scientists on the pace of global warming and how much of it is our fault.
The news is unlikely to be reassuring.
The United Nations set up a process in 1988 to produce, every five years or so, for the benefit of policymakers and the rest of us, a summary on the state of climate science, on the climate itself, on the environmental and socio-economic impacts of climate change and on what can be done about it. Tomorrow's release is the summary of the science part.
These reports from the Inter-governmental Panel on Climate Change (IPCC) are not just tossed off. Each chapter is written by a team of lead authors, hundreds in all, who trawl through the published and peer-reviewed work of their fellow scientists, write draft reports which are then put out to the relevant expert community for feedback before the final document, which usually runs to more than 1000 pages, is compiled.
It takes years, but then the stakes are high.
If this year's report continues the trend of the previous three it will be more emphatic that climate change is happening, that human activities are driving it, that the impacts overall will be destructive, but that there are things we can do to avert or at least mitigate them.
The first report in 1990 concluded there had been about half a degree Celsius of warming over the preceding century, which might be a result of greenhouse gas increases, but which fell within the range of what was then known about natural variation.
But as more data has accumulated and the sophistication of the computer models of the climate has improved, the conclusions have got firmer.
By 1996 they were prepared to say "the balance of evidence suggests a discernible human influence on global climate".
And by the last report in 2001 they were saying that it was likely most of the observed warming over the past 50 years was due to the increases in greenhouse gas concentrations, and that we might see between 1.4 and 5.8 degrees of further warming by the end of the 21st century.
To calibrate the scale of that assertion, the high end of the range is equivalent to the increase in the global average temperature since the depths of the last ice age, when the world was a very different place.
The idea that children already born might see as much again is arresting.
There are nay-sayers, of course, who point to data apparently at odds with the prevailing view and who maintain it is difficult for dissenting views to get published in the peer-reviewed journals, which are the starting point of the IPCC process.
There may be instances of that, but it is not plausible to suggest that the normal standards of intellectual honesty and professional ethics are absent or widely abandoned in this branch of science.
What would be dishonest is for those of us who are not climate scientists to go shopping around for a more convenient alternative view and ignore the conclusions of the majority of experts - especially when the IPCC's processes and conclusions have been reviewed and endorsed by the national science academies, guardians of the scientific method.
The sceptic's role is an honourable one. Science progresses by the orderly overthrow of the established wisdom from time to time. But that process requires the successor theory to be able to explain everything the displaced orthodoxy could, plus what it could not.
In this case the sceptics cannot just point to things that do not fit the prevailing view. They also have to be able to explain away all the things that have persuaded the majority of their colleagues to take the view they do.
This is not an exact or mature science and the world is a big and complicated place, so there is a limit to how much certainty or precision we can expect from models of the climate.
It gets worse when we ask them to predict the next 100 years, because that involves making assumptions about the future track of man-made emissions.
That depends in turn on assumptions about population growth, rates of economic growth and technological change. Would anyone in 1907 attempting such predictions for the 20th century have got it right?
The economic scenarios plugged into the IPCC's 2001 report have drawn fire.
They assume for example that incomes in developed and developing countries will have converged by the end of the century - a plausible assumption if you look at China, less so if you look at Africa.
And in calculating how much catching up poor countries have to do, it turns out it makes quite a difference whether you use market exchange rates or purchasing power parities in determining the relative starting points.
Some of the IPCC scenarios assume an average world economic growth rate of 3 per cent a year - high by the standards of the past century, but not of the past 30 years.
The question is how much difference this makes.
When such concerns were raised by a House of Lords select committee in a cogent 2005 report on the economics of climate change, the British Government's response was this:
"Most commentators have reached the conclusion that any changes in the emissions projected in the scenarios - which reflect uncertainties in future projections of economic growth - is likely to translate into a very small effect on projected temperature.
"Overall the IPCC's emissions scenarios cover a very wide range and the most reasonable expectation about global emissions, in the absence of further mitigation action, is likely to be included within the envelope."
It would be easier to join Her Majesty's Government in that sanguine conclusion if the IPCC this time provided some sensitivity analysis, to indicate how much difference varying the economic parameters makes to the temperature projections.
Inevitably we will be left having to make decisions in the face of considerable uncertainty.
The temptation is to throw up your hands and leave it to the next generation, which will be richer and technologically better armed to deal with it.
The problem with that is the long time lags between emissions and their environmental impacts.
Although some of the CO2 we emit today will quickly be dissolved in the oceans or taken up by growing plants, much of it will hang around in the atmosphere for generations adding to the greenhouse effect.
The thermal inertia of the oceans is awesome: the full effects of today's emissions' increases, or tomorrow's reductions, will take centuries to play out.
One thing we can be sure of is that the extra energy we are pumping into these systems like the oceans and the atmosphere, upon which we depend, is not going to spontaneously vanish.
The first law of thermodynamics is not "Ignore it and it will go away".
Economists can argue about what discount rate should be applied when weighing present costs against future benefits, or how robust the economic underpinnings of future emissions scenarios are.
But when we are messing about with such things as how much rain falls where, where you can grow what crops, the geographical ranges of pests and diseases, and doing all that with the prospect of billions more mouths to feed, it becomes a moral question, not an economic one:
What risks are we entitled to take with the only world we have to leave our children?
