Climate feedback mechanisms

[If you possess a smartphone (and/or an electric car), then you might find the latest post by Ed Conway interesting. You can find it here. Indeed, if you are interested in the world’s resources, then his Substack is well worth subscribing to.

My next post will feature another A Level Geography Q&A - an essay question on immigration.]

The Earth’s climate is a dynamic system. It is impacted two significant external factors:

· variations in solar radiation (insolation)

· anthropogenic greenhouse gas emissions.

These factors cause changes to the climate by altering the Earth’s energy budget and the carbon cycle. Once these changes occur, they set in motion several additional processes which can further impact the climate. These secondary changes are climate feedback mechanisms and can either enhance the changes (positive feedback) or moderate them (negative feedback).

Positive feedback

Positive feedback occurs when an initial change triggers a series of processes that strengthen the initial change. So, a warming climate causes changes to the Earth system that then cause further warming.

One example of a positive climate feedback mechanism is the impact of warmer temperatures on permafrost, as shown in Figure 1. Permafrost is frozen ground which has a temperature at or below 0°C for two or more consecutive years. Much of this permafrost has a high organic content as it contains dead plant and animal matter, and so it acts as a significant carbon store.

Figure 1 Positive feedback from melting permafrost

Warming temperatures cause the permafrost to thaw. As the soil heats up, microbial action increases, breaking down the soil organic matter and releasing greenhouse gases as a biproduct of decomposition. When soils are waterlogged, the lack of oxygen means that decomposition is anaerobic, and methane (CH₄) is released into the atmosphere. When soils are drier, aerobic decomposition leads to carbon dioxide (CO₂) release. Through the release of these gases, carbon leaves the terrestrial store and enters the atmospheric store. This increase in atmospheric greenhouse gas concentration leads to temperatures increasing further, thereby amplifying the initial warming.

Positive feedback loops can lead to tipping points, which can be considered ‘points of no return’. Once the permafrost has melted and the stored carbon been released into the atmosphere, it will not be possible to sequester that carbon back into the soils.

Negative feedback

Negative feedback occurs when the initial change triggers processes that decrease the impacts of the initial disturbance.

One example of a negative feedback mechanism is the impact of higher atmospheric CO₂ concentration on vegetation growth. Vegetation uses CO₂ to photosynthesise, and through this process locks up carbon in its biomass. Hence, CO₂ is removed from the atmospheric carbon store and sequestered in the terrestrial store. Higher atmospheric CO₂ concentrations stimulate plant growth, leading to increased rates of photosynthesis, known as carbon fertilisation. As photosynthesis increases, more CO₂ is removed from the atmosphere, causing the planet to cool as the greenhouse effect is diminished.

In theory, if plants removed enough CO₂ from the atmosphere, this fertilisation effect would be reversed as low atmospheric CO₂ concentrations inhibit photosynthesis. As a result, plants would remove less CO₂ from the atmosphere, causing an increase in atmospheric concentrations (Figure 2).

Figure 2. Negative feedback through carbon fertilisation

When negative feedback loops dominate, systems tend to be in a state of dynamic equilibrium. There might be some small variations over time, but overall, there is no net change.

However, under current conditions, even if increased photosynthesis does remove some CO₂ from the atmosphere, inputs from the external factors given earlier and positive climate feedback mechanisms mean that overall, atmospheric CO₂ concentrations are still increasing. This prevents the feedback loop from closing, as illustrated in the lower part of Figure 2.

Climate futures

Uncertainties over climate feedback are one of the key reasons why there is some variation in possible future climate scenarios, like those produced by the IPCC. These uncertainties arise because many of the processes involved in the feedback loops are difficult to measure directly.

Another reason for these uncertainties is that it is difficult to predict how the relationships between the processes involved will change as the climate warms further. Warming might cause these feedback mechanisms to speed up, slow down or stay the same.

However, there is broad agreement among climate scientists that the combined impact of all feedback mechanisms will be an overall positive feedback effect – i.e. processes will significantly amplify changes to climate.

[Somebody please tell the MAGA crowd in the USA]