By growth I’m talking about the big two measures that tend to result in fewer gorillas, coral reefs, rainforests and icebergs: population and GDP. The first of these needs to stop growing (and go into reverse) and the second needs to plateau or at least stop being everyone’s go-to indicator for everything; ideally, it might become irrelevant.

Just by writing this, I feel that I may have placed myself in tinfoil-hat territory. But I don’t think either of these inevitabilities need to be bad things and I’m not nursing Hollywoodish post-apocalyptic fantasies populated by suspiciously good looking 18-25 year olds. However, I do believe that at some point the 20th century model of pretty much everything has to cease. As a leading indicator of this, you can already see that the model that worked so brilliantly for America in the 20th century is working a lot less well for China.

Let’s take population. You don’t need to be a genius or green (or even a greenius) to realise that our numbers are growing too fast for our planet. Virtually every environmental problem would be smaller if there were fewer of us. Rainforests would stay unfelled and beautiful coastlines undeveloped. The Med and the North Sea would be full of fish. China wouldn’t be buying chunks of malnourished Africa to farm in place of the land it has polluted or covered in megacities. In the UK, there would be no debate about paving over the few remaining green spaces in the South East (or for that matter ineffectual hand-wringing about London property prices).

In fact, if there were a lot less of us, we could behave as badly as many reactionary types would like us to. We’d be able to take monster trucks on rhino-hunting safaris. We could dine off roast panda and have tiger-skin wallpaper. We’d all be able to live like America’s fossil fuel lobbyists want us to. A lot less of us – say 60-90pc, would solve most of our environmental problems in one fell swoop. However, that’s not going to happen unless the hype around Ebola really delivers.

In the meantime, we have another, related problem. Not only are there more of us, but we’re all living larger – especially in developing countries where, in the past, living large was the preserve of a tiny elite. While rising living standards may be a good thing for individuals and societies, in terms of the planet, it would be much, much better if we all lived like Bangladeshis.

Quite what a problem this is is illustrated by the section of the Living Planet Report which calculates humanity’s “ecological footprint” or the amount of planet needed to support each person. The report concludes that at today’s average global rate of consumption, humanity would need 1.5 planet Earths to provide for its needs sustainably (i.e. without trashing the future). However, if we all lived like Brits, we’d need 2.5 earths and if we all lived like Americans we’d need four. It’s another indicator (as if one was needed) that points to us living beyond our means.

^{Lunar New Year travellers cram into West Railway Station in Beijing (AFP/Getty)}

When it comes to business, my dad taught me to do a few very basic checks on the financials. These are usually sums you can do in your head and, if they don’t stack up or are based on pie-in-the-sky predictions, you don’t go near whatever it is. But when it comes to humanity having a decent, comfortable, pleasant future on Earth, it strikes me that we’re prepared to accept a set of figures that would have had investors wetting their pants with laughter at the height of the dot.com boom.

The two mainstream are options are: pretend that the elephant doesn’t exist. A strategy that had some currency back in, say, 1970, when the end game was 60 years off, less so now. Or you just sit there and say that science will come up with a solution. Again, ask yourself, would you invest in a company that told you this? Worse still, the relatively low-ball population ceiling predictions that had lulled everyone into thinking we’d be OK are starting to look very conservative indeed. That reassuring nine billion figure is unlikely. What is now more likely is 11 billion plus, all of whom will, on average, be consuming more methane-generating cows than we do today.

So, again, why aren’t we talking about this? Really, we should be talking about how to solve this – to the exclusion of virtually everything else. In the medium term, nothing is more important.

The solutions do exist. Societies can and do successfully curb their growth. Japan and most of Europe are now below the replacement birth rate for their populations. Some growth in Europe comes from immigration but, in Japan, which restricts immigration, adult nappies famously now outsell infant nappies. This is usually portrayed as a bad thing, but I fail to see why. In many ways, Japan is just grasping the nettle and dealing with the problems we’ll all have to deal with soon. Perhaps we should start looking at what they’re doing right.

We might also recognise that many of our own values naturally lead to this end – and that, again this is a good thing. The biggest drivers of low birth rates are modern western values – the education of women, women in the workforce, easy availability of contraception and so on. The factors that drive population growth are largely their opposites – including all those religions which are more concerned about controlling women than the quality of life their children have.

We also need to look beyond GDP, because wealth is a kind of sufficient factor. In the US, there are innumerable studies which show that although GDP per capita has risen greatly since the 1970s, the average person is no happier. Of course, you can argue that, in America, much of this extra has gone to the top 1pc, but there are plenty of other examples that show the relationship between wealth and happiness is not linear. Indeed, the research tends to show that if you are dirt poor, a bit more money will make you happier, but if you are comfortable it makes no difference.

In the West many of us are already well beyond the GDP-satiety point, so perhaps it’s time we started devising different, more useful measures of national success. Serious, important types have tended to pooh-pooh alternative measures as hippy-dippy rubbish. But it’s worth remembering a few things here. One is that today’s politicians and economists are like generals who are always fighting the last war. Their failure to anticipate everything from the dot.com bubble to the banking crisis makes me wonder not why we still listen so closely to them, but why we listen at all.

The second is that much of the Western economy is essentially a pyramid scheme based on having ever more economically productive people. This is at its most clear with pensions – and, as we are never going replicate the post-war era, we need to think differently. Finally, there is the problem that the WWF points to and which China seems to be discovering: that, as every student’s favourite native American proverb has it, “…you can’t eat money.”

This doesn’t come easily to me. I’ve always been OK with my status as a carnivore who likes cars whose personal ecological footprint is probably 0.1 of a planet. But, I can do the maths – and all the maths and all the science is telling me that the sandal wearing, lentil-eating hippies I used to laugh at were right. We need to reduce our numbers and we need to tread more lightly on the earth. If we don’t, as lot of people have pointed out, we will eventually become part of the great mass extinction that we’ve already started.

I’m not calling for a Russell Brand style destruction of our entire economic system. But I am suggesting that there are good reasons to believe that the assumptions that have served us well in the past will not continue to do so in the future. We need to start talking about this. Not least because, if we don’t, the elephants in the room will be the only elephants left.