For many people tipping points are the most frightening thing about climate change (and that's up there for me) but what really scares me are stocks. Or to be more precise, what really scares me is knowing something about stocks, and knowing that tipping points are out there.

In #systemsthinking a stock is anything that accumulates. The vehicle fleet, the stock of housing, all the iPhones in the world, etc.

The thing about stocks is that they change SLOWLY. The average fuel efficiency of vehicles changes as older cars are scrapped and new ones come online. Even if the new vehicles have much better fuel efficiency than the old ones, the average for the whole fleet changes slowly.

The strongest carbon tax in the world can't change the properties of the stock as a whole (say the stock's average emissions) faster than those turnover dynamics, unless the tax forces people to stop using stuff that still has functionality, or forces early retirement.

Picture a giant lake with an inflow (the new efficient stuff) and an outflow (the retiring older stuff). Picture the new water red and the "old" water blue. (And imagine a slightly weird lake, more like a conveyor belt, where the older water leaves first).

What does the lake look like at first? Pure blue, right? Then bluish purple, then reddish purple, and finally red. And the longer the residence time of water in the lake the slower and more gradual the transition.

The building stock is like a lake where the water hangs around for a hundred years. Cars are a somewhat faster turning over lake, phones faster than that and so on.

The slow-to-change nature of stocks limits how fast we can change over to a climate-safe economy, once we are united in the need to do that.

On top of that, not all the stocks that matter are physical. The 'number of people who are alarmed' is a stock. It grows as people become newly alarmed. 'Political will to act' is stock. 'Credibility of the fossil fuel industry' is a stock.

Put all that together and you start to see why emissions are still going up, why policy is out of step with science, why impacts are rising and yet politicians are dithering why breakthrough technologies haven't taken over the world.

Now add a tipping points to the mix. We know there are some level(s)of warming, not knowable with certainty in advance, where we loose some control of the ability to steer the system. We know there are cliffs off in the distance on a dark road. And our breaking mechanism?

Our breaking mechanism is the only thing we can change, the stocks, the oh so slow to change stocks.....first the knowledge stocks, then the political power stocks, then the crafting of policy stocks, then finally the physical stocks.

Since 1972 the field that I come out of, system dynamics has been trying to explain this simple idea: in a system made of slow-to-change stocks (whose momentum is growing, not slowing) you have to push the brakes long before you hope to stop.

A pledge to reach zero emissions is saying 'yeah, I'd like to stop before I reach the cliffs' but to do that you have to actually push on the brakes. All the new stuff going into the lake has to be as perfect as you can get it to be, because it will be there for so long.

All to say....when people demand 'no new fossil fuel infrastructure' they are trying to press on the brakes in time. And we should listen to them, and help elevate their voices.