Values are now well above their pre-COVID levels, driven by the ultra-low interest rates and anticipation of the economy reopening.

Yet some experienced investors are concerned, knowing that when prices are high, unexpected surprises sometimes appear.

October is a common month for such surprises - the 1929 crash and the 1987 collapse were both in October.

What could go wrong?

One unanticipated possibility is the growing energy shortage worldwide.

It first appeared in Spain a few weeks ago with the government so concerned about electricity shortages that it passed a law preventing energy companies raising prices.

Shortages and unreliability have since become a problem in Britain, France, Greece and other European countries.

The Chinese Government is also concerned and has instructed energy companies to buy supplies at any price to ensure homes and factories don't run short.

Electricity, natural gas, petrol and coal are in short supply worldwide.

China, Europe and the US are all heading towards winter when demand for heating is very strong.

So the shortages seem likely to continue for another six months at least.

There hasn't been a statement from any global leader giving reasons for the shortages.

The most likely cause appears to be the policies in many countries to move quickly towards renewable sources of energy.

Politicians keen to capture the popular vote have embraced the idea that we can easily move to solar, wind, tidal, hydrogen and other renewable sources for all our energy needs to combat climate change.

They say we can quickly abandon coal, oil and gas as sources of reliable, base-load power.

Where is the impartial analysis from technical experts that says that?

Promoters are keen to explain how renewable sources can replace fossil fuels, and they probably can long term, but it seems the move is being made too rapidly, without proper planning for national and global needs.

Supply disruptions are demonstrating the unreliability of most renewables.

There has been little investment in coal mines and coal-fired power stations in recent years.

Investors prefer alternative energy opportunities.

Yet the output from renewables isn't enough to replace that of the coal fired stations being closed.

And coal fired power stations cannot ramp up production quickly. Fortunately, OPEC can increase oil output, which should help.

Some disruption during the energy transition was always likely but it may have arrived sooner than expected. Could it be a serious handbrake on the economic recovery and slow share market progress?