1. What is meant by ‘stock’ and ‘flow’?

Economists often describe climate change as a ‘stock-flow’ problem. This refers to the fact that the greenhouse effect, and in turn climate change, depends on the concentration of greenhouse gases such as carbon dioxide in the atmosphere – the ‘stock’ –while the thing humans can control as a response is the rate at which additional greenhouse gases are emitted – the ‘flow’. The total stock of atmospheric greenhouse gases depends on the difference between man-made emissions and the natural removal of greenhouse gases from the air, particularly by oceans and plant life.

Imagine a bathtub. The tub itself represents the atmosphere and the water it contains represents the stock of greenhouse gases. The taps represent man-made emissions, from burning fossil fuels for example, while a slow-draining plug represents the natural removal of greenhouse gases, by trees for example. If the taps are left running and the plug can only drain the liquid away slowly, the volume of water in the tub will increase. That’s what is happening at present: humans are adding greenhouse gases to the air faster than natural processes can remove them. The result is an increasing stock of greenhouse gases and, in turn, rising temperatures.

2. What should be done about this problem?

To arrest man-made global warming, the world needs to stop the atmospheric stock of greenhouse gases from increasing. This requires emissions to be reduced until they balance with the rate of natural removal from the atmosphere. At this point the atmospheric stock is stabilised, which is why many economists and scientists talk about targets for ‘stabilisation’.

The lower the level at which the atmospheric stock of greenhouse gases is stabilised, the greater the chance the world will have of meeting the target of the Paris Agreement, adopted in 2015, of keeping temperature rise this century well below 2°C above pre-industrial levels.

3. How is the world doing in meeting the target?

To meet the 2°C target, scientists recommend that annual emissions should reach their highest level by 2020. While researchers find that emissions have peaked in many developed countries, the rate at which this is happening is not currently consistent with meeting the target. In 2017 global carbon dioxide emissions from fossil fuels and industry were projected to grow by 2%following nearly no growth from 2014–16. 

Therefore accelerated action will be needed if the Paris Agreement target is to be met. The United Nations Environment Programme’s most recent assessment of what countries are currently doing to reduce emissions and their stated plans for meeting the target show that not enough is being done. However, UNEP says that the gap between the reality and the target could be closed before 2030 if the world increases its adoption of technologies such as solar and wind power, more efficient cars and appliances, plants more trees and stops deforestation. 

This FAQ was updated in January 2018. The original FAQ was a reproduction of the following article: Why does climate change get described as a stock-flow problem? © The Guardian, 2012, used under a Creative Commons No Derivative Works licence.

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