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It's an unusually well-structured week that begins with cause and ends with effect.

But that is one way of looking at what we've just had.

On Monday, the UK's Royal Society announced a study into the science of human population growth.

What they mean by "the science" is pretty broad, and the panel they've convened includes experts on law and theology; so clearly we're well into the social sciences at one end of the spectrum.

Many of you have suggested in comments on these pages and that unless population growth is addressed, all the other problems I burden you with here are not going to be solved; and it's a view that garners a lot of sympathy in sustainability circles.

This is where the Royal Society comes in.

How scientifically valid is the view? How many people can the Earth sustain, and at what level of development? What would be the societal, economic and technological shifts that might bring the human footprint down to the size of a single Earth without trading away hard-won prosperity?

Answers to some of the questions might come only as value judgements; but where there is something more substantial, the society hopes to find it and distil it so that policymakers can decide whether they should attempt to do something about it.

That, of course, is the hard part. People want to have children - of course, it's the most wonderful thing on the planet and most would say how we get to make them isn't too bad either.

In principle, money set aside for general environmental "good things" could be spent on family planning projects, which in the eyes of some of the aid agencies running them are win-win-win-win options.

They'll tell you that when women in poorer societies are given the power to take their own decisions regarding fertility, they choose to space out their babies.

That leads to healthier mothers, better-fed children, more prosperous families and - on a societal level - fewer people on the planet.

But should Western governments spend money this way? Is it morally right?

The Royal Society won't be able to answer that one - but it might be able to say whether addressing population size in this way would be more or less effective in environmental and human terms than other options for aid.

At the other end of the week comes the latest attempt to analyse the impacts that humanity's emissions of greenhouse gases may have in the future.

The National Research Council of the has pulled together what's in the scientific literature and tried to relate impacts to degrees of warming, and then back to "stabilisation levels" for carbon dioxide in the atmosphere.

In some ways, it covers the same ground as .

But conceptually, the report - - differs a bit.

The IPCC's top-line projections focus on time - how much sea level rise this century, and such like - whereas the National Academies have less to say about when impacts might arise, and more to say about the temperature levels at which they might arise.

To make the analysis, chair Susan Solomon told me, the team had at times to go back into the raw model outcomes to get the projections in the form they needed.

Part of the rationale, she said, is that:

"The range of model projections is broad when you plot them as a function of time; but they're much more robust as a function of warming."

Being a US publication, it also has more to say about specifically US impacts: 5-10% less streamflow per degree Celsius in some southern river basins, 5-15% less yield per degree Celsius from corn, and so on.

In the longer-term:

"The report concludes that the world is entering a new geologic epoch, sometimes called the Anthropocene, in which human activities will largely control the evolution of Earth's environment.
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"Carbon emissions during this century will essentially determine the magnitude of eventual impacts and whether the Anthropocene is a short-term, relatively minor change from the current climate or an extreme deviation that lasts thousands of years."

The idea of an "Anthropogene epoch" isn't limited to climate change, but to the notion that most of the big changes taking place on Earth now can be laid at the hand of humanity.

Whether that really counts as geological change is another matter.

Whatever the merits of the concept, and whatever the scale of the changes, it's a matter of simple mathematics to suppose they'd be considerably smaller if the human population was not growing so rapidly...which is roughly where we came in.

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