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# 4.4 Your designs

In the following exercises, you might find it helpful to write down the results as you try different actions. Remember you can select ‘Reset’ at any time.

You wish to achieve the Paris temperature target of 1.5 °C, and quickly, so you test some designs for Solar Radiation Management (SRM).

- What values do you need to input to achieve a 1.5 °C temperature change using:

  - 1. solar shield only
    2. sulfate aerosol injection only.

- A solar sail area of 4.4 million km2 and sulfate aerosol injection rate of 1.7 Tg yr−1 achieve the closest temperature changes to the target of 1.5 °C.

- Which of the following impacts increase or decrease as a result of adjusting both a) and b) above?

- To achieve a 1.5 °C temperature change for both a) and b), the climate design tool predicts the following impacts:

  - number of human lives lost – decreases
  - food price increases – decrease
  - biodiversity loss – increases
  - economic costs – decrease
  - number of conflicts – increases
  - social unrest – increases.

  Sulfate aerosols have worse impacts than the solar shield because they lead to more human lives and biodiversity lost and greater incidences of conflict and social unrest. This is because both are predicted to have similar impacts on climate, but sulfate aerosols have additional physical risks of ozone loss, acid rain and resulting health impacts.

  The exception is food prices, which increase slightly less, due to sulfate aerosols diffusing sunlight which can increase plant growth (Session 3).

- Now use only marine cloud brightening to achieve the 1.5 °C target. Are the impacts better or worse than those from using a solar sail or sulfate aerosols?

- Overall, marine cloud brightening seems to reduce negative impacts relative to the solar shield and sulfate aerosols – slightly fewer lives lost, much less biodiversity lost, and less social unrest. The other impacts are similar.

  The reasons for the lower impacts from marine cloud brightening are the more local extent of negative impacts on climate such as reduced rainfall, and the lack of sulfate chemical interactions mentioned above.

Having considered the negative impacts of these SRM methods, you look at designing Carbon Dioxide Reduction (CDR) engineering instead. Reset the tool, then select the maximum possible options for all CDR methods (i.e. largest allowed actions for BECCS, ocean fertilisation and afforestation with biochar).

- Start of ITQWhat temperature change is achieved, and how does it compare with the Paris target of ‘well below’ 2 °C? End of Media Content

- Using maximum CDR achieves a temperature change of 2.6 ± 0.8 °C, somewhat higher than 2 °C (though with a small probability of achieving less than 2 °C due to the large uncertainty).

- What do you notice about the impacts of CDR, relative to those of taking no action? Why is this?

- All negative impacts are reduced, except social unrest which increases.

  Most negative impacts decrease because the root cause of climate change is reduced. Social unrest increases due to the large-scale conversion of cropland to biofuels and forests, and the toxic algal blooms and risks to ocean biodiversity from ocean fertilisation.

  It may be surprising, but food prices decrease overall. This is because – in this particular design tool – the reduction in global warming has a greater effect (smaller food price increases) than the land use changes by CDR (larger food price increases). Economic costs also decrease because the reduction in global warming lowers costs more than the cost of CDR.

Finally, you want to compare your geoengineering designs with mitigation or adaptation, or both.

- Try the following actions. What do you notice about the costs of these, compared with geoengineering or taking no action?

  - using only mitigation to limit warming to 2 °C (the least ambitious end of the Paris targets)
  - using only the maximum possible adaptation
  - using both.

- For mitigation, the main disadvantage is the cost. Using mitigation to reach 2 °C nearly doubles economic costs compared with taking no action, and nearly quadruples costs compared with geoengineering.

  Adaptation is much cheaper, costing around the same as taking no action: i.e. in this model, the extra costs of adaptation are balanced by the reduced costs of climate change.

  Setting aside cost, mitigation reduces all negative impacts relative to taking no action. Adaptation reduces all negative impacts except the temperature change and biodiversity losses.

So, the question is – what would you do? What future do you want? It is your turn to decide.

## Activity 1

Allow approximately 15 minutes

Try combinations of different actions and choose your own set that give the best, or least worst, future for the Earth according to your own priorities.

What is the temperature you achieve?

What costs or other negative consequences were you prepared to accept to achieve this?

How easy did you find it to make your choices?

(A text entry box would appear here, but your browser does not support it.)

### Answer

There is obviously no correct answer to this! One possible response is to be broadly in favour of CDR and local SRM, and not in favour of global SRM. But views may change depending on future political responses – or lack thereof – to climate change. Certainly, CDR through BECCS is a method we should approach with an awareness of its limitations and potential impacts on habitats and food security.

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