Some people I respect think you should value our ecosystem in financial terms and then manage a portfolio of natural assets the way a fund manager might use portfolio theory to manage a diversified portfolio of financial assets. I could not disagree with them more profoundly.

Never mind the fact that this buys into the notion of a simple link between GDP per capita and well-being, which should have been dismissed by now in all high income countries, where there is demonstrably no such link.

Never mind that they habitually use a measure of ecological footprint which allows rich countries to 'export' their pollution to poor countries, allowing them to claim that ecological impact does not rise as GDP rises beyond a certain point.

My biggest problem with this approach is the idea that the financial economics of portfolio management implies the safe management of anything - let alone the natural environment

Portfolio theory requires measurable risks and know probability distributions, or in other words the absence of complexity, non-linearities, feedbacks and fundamental uncertainties.

The complexity and feedbacks and resulting uncertainties of financial systems is the reason we have so many endogenously driven financial crises in our history.

The thing about financial crises is you can recover from them.

Our natural environment is far more complex, has far more feedbacks and non-linearities and connections, and is as a result far more uncertain than our financial system.

So if you can't trust economists to manage the financial system so that it remains healthy and robust, why would you imagine that by financialising ecosystem services, you will be able to trust them to manage that far more complex 'portfolio'?

It is a profound mistake to financialise the natural world, in my opinion. Instead, we should identify where it is safe to be, add a big margin to allow for unavoidable uncertainty (if we can), and then set limits on what we can tolerate. Dollars shouldn't come into the limits.

Then we should take a step back and identify what we need to allow people to have the best possible chance of a good, secure, just, engaged life.

To an extent, this has been done in the UN Sustainable Development goals, but it is done better in Kate Raworth's doughnut, which can be and is being applied at national, regional and local levels.

What is someone who has spent a career training finance professionals doing saying we ought not to be applying the tools of financial management to our natural environment?

I am saying it is entirely inappropriate, misleading and liable to bias the narrative, policies and outcomes in potentially dangerous ways.