Some economists worry the world economy isn't growing fast enough. It's slowing down and reaching the point of "secular stagnation".
On a very different wavelength, however, environmentalists worry that if the world economy keeps growing the way it is, it won't be long before we run out of the natural resources on which that growth depends. Whoops.
But if all that's a bit heavy for the holiday season, here's something lighter. Remember all that crazy talk a few years back about the paperless office? What a joke.
Then there was peak oil. Whatever happened to that looming disaster?
If any of those possibilities piques your interest, I have news - courtesy of an essay by Professor John Quiggin, of the University of Queensland.
Quiggin thinks the paperless office is on the way, especially because the world has already reached "peak paper".
Despite continuing economic growth, peak paper was reached in 2013. "Global paper production and consumption reached its maximum, flattened out, and is now falling," he says.
Until relatively recently, the growth and spread of information was directly linked to the growth in paper, books and newspapers.
The closely related information revolution and digital revolution have broken that link. Businesses and governments don't print reports, they just put them on their website. We read e-books and online newspapers.
Banks and businesses want to stop sending us statements and bills through the post. If we hold out too long, they impose a fee for continued paper statements.
As for peak oil, Quiggin says that, in terms of oil consumption per person, the world reached it in 1979.
"In the developed countries, the decline in oil consumption per person has outpaced population growth, with the result that total consumption is declining. The average person in a developed country now uses less oil than her parents did 40 years ago," he says.
Why has this remarkable change attracted so little notice? Partly because much of the reduction in energy use has taken the virtually invisible form of improvements in energy efficiency. Both industrial processes and household appliances use far less energy than they used to.
But also because, until fairly recently, the main substitutes for oil have been other fossil fuels, such as coal and gas. Only in the past 10 years have renewable energy sources, especially wind and solar, begun to play a significant role, he says.
Peak coal has already arrived in the developed world. Coal consumption has fallen substantially in the US and Europe, and is set to fall further.
Until recently, the decline in fossil-fuel use in the developed world has been more than offset by rapid growth in the developing countries.
But even China - the planet's largest coal consumer by far - has changed course. Beginning with Beijing, it has begun closing down all the coal-fired power stations near major cities.
"In fact, China reached peak coal in 2013, at the same time as it reached peak paper," Quiggin says.
As for peak steel, it's different. Steel lasts a long time and can be recycled almost endlessly, but demand for it is finite.
In developed countries, the stock of steel reached about eight tonnes a person decades ago, he says, and has remained stable or slowly declining since then.
"With the stock of steel on a gently sloping plateau, the need for more can be met almost entirely by recycling scrap, rather than by burning coal to smelt iron ore in blast furnaces.
"The result has been described as a 'circular economy'. When this arrives, peak steel will have been reached."
All this has happened while economic growth has continued and living standards have risen.
Economists have been saying for years, particularly in the developed world, that growth is becoming "weightless". The part of the economy that's growing isn't goods - things you can drop on your toe - but services: people doing things for people, whether fixing their health, teaching them nuclear physics or waiting on their table.
With an ever greater proportion of gross domestic product - the quantity of goods and services produced in a period - accounted for by services, economic growth becomes ever less dependent on the increased use of natural resources.
Over the long term, growth in real GDP comes less from the use of more raw materials, human labour and man-made machines and structures and more from improved "productivity" - greater efficiency with which those inputs are transformed into outputs of goods and services.
What drives productivity improvement? Advances in technology and accretion of human capital. That is, the growth and spread of knowledge and information.
But an information-driven economy is very different from the one we've become used to since the industrial revolution, one driven by the use of natural resources to produce goods plus a few conventional services.
Natural resources are finite. If you want to use my coal or paper you must pay me (they're "excludable"). Any coal or paper you use can't be used by someone else (they're "rivalrous").
This makes economic growth relatively easy to measure. But knowledge and information are opposite to natural resources: they're often freely available (non-excludable) and my knowing something doesn't stop you knowing it, too (non-rivalrous).
What's the difference between a taxi and Uber? Information. What's the difference between renting a hotel room or self-catering accommodation and Airbnb? Information.
A knowledge and information-driven economy is one whose continuing growth makes less demands on the natural environment than many scientists and environmentalists imagine. That's particularly true as we move to renewable energy.
But a knowledge and information-driven economy is harder to measure, especially using the metrics (GDP) we developed to measure a raw materials and goods-based economy.
We're now in a world where GDP is going one way and raw-materials use is starting to go the other way.
That's why Quiggin doubts that world economic growth is grinding to a halt.
