6.2.1 The degree of warming is uncertain.
The degree of warming is uncertain – it depends on the scale of GHG reductions achieved. Even within the IPCC scenarios, uncertainty exists
- IPCC reports identify likely outcomes for different temperature increases resulting from current GHG emission trajectories. A common assumption is that a safe global warming limit is a 2°C rise in average global temperature, although this is subject to debate and uncertainty.
- The 2015 climate summit (COP21) in Paris, resulted in reaffirmation by nations that the goal of 2°C remains, but with a strong signal of intent to limit the temperature increase to 1.5°C, in response to calls by vulnerable states (specifically low-lying island nations).
- The recent IPCC report calculates that GHG emissions reductions of between 40% and 70%, by 2050, will be needed for the 2°C scenario to be likely. Reducing emissions to zero by 2100 is likely needed to maintain temperatures within this limit.
The IPCC report calculates that in order to likely stay within the 1.5°C limit, GHG emissions will need to be reduced by 70-95% by 2050.
This is considered to be a significant technical and socio-economic challenge.
The IPCC reports use specific terminology to state its level of confidence in its conclusions and the likelihood of stated outcomes. These are referred to at times throughout this chapter, so a brief explanation is included.
Confidence is expressed using five qualifiers:
“very low,” “low,” “medium,” “high,” and “very high.”
Likelihood is expressed using probability qualifiers
Adapted from Mastrandrea, et al. (2010)
There is a degree of uncertainty involved in scenario projections, and this mechanism provides a transparent way of understanding the ‘firmness’ of judgements about outcomes.
6.2.2 Climate change can affect the food system as a whole.
Climatic change can impact all forms of agriculture and all stages of food systems
Measuring direct impacts of climate change on food systems can be difficult since changes in agricultural practices may happen in response to temperature rise (such as increased irrigation), so altering the impact, together with a lack of a consistent baseline against which to measure change.
6.2.3 How climate change will affect food systems is uncertain.
How climatic change will affect agriculture and food security is uncertain, and depends on many factors
There is considerable uncertainty, and impacts depend on the interplay between:
- Gradual temperature increase.
- The CO2 fertilisation effect (whereby higher ambient CO2 concentrations in the atmosphere can stimulate increased plant growth and boost productivity, although the strength of this effect is unclear).
- Extreme events such as drought and flooding; temperature peaks.
- Economics, demographics, infrastructure.
Impacts will also depend on the degree of climatic change – i.e. it will vary between a 2°C and 1.5°C temperature increase. Scenarios, largely based on models, are used to provide plausible predictions and reduce uncertainty.
Impacts on agriculture and food security are uncertain, and depend on the interplay between many physical factors such as temperature, CO₂ levels, precipitation and extreme events, as well as social, demographic and economic factors, that influence people’s ability to adapt to change and may also have an affect on future emissions.
Both positive and negative impacts are possible, depending on crop type and the interplay between variables
Can be positive :
- CO₂ fertilisation effect could stimulate plant growth, but crops differ in how they respond to CO₂ levels.
- Reduced frost (frost can damage or kill many crops).
- Precipitation (within limits, more rainfall is considered positive for agriculture).
Can be negative:
- Crops have different optimal temperatures for productivity; increases beyond this can have negative impacts, especially if there are peak temperature days at critical stages in a crop’s growth cycle.
- Crops which require vernalisation – a process in which cold temperatures trigger flowering – including wheat, may suffer from increased temperatures.
- Extreme changes in precipitation (drought or flood).
Climate impacts on the yields and nutritional profile of different crops, are not fully understood. There are many variables, such as regional temperature changes, precipitation levels, and seasonal variability. While predictions can be and are made, these involve high levels of uncertainty.
For example, in warm dry regions, increased CO₂ could boost productivity in C3 plants, but this could be offset by reduced rainfall, higher temperatures or less predictable precipitation, resulting in droughts or floods.
Plants have different pathways for assimilating CO₂ (referred to as either C3 or C4) reflecting differences in the number of carbon atom molecules formed when CO₂ is first absorbed. The majority of plants are C3 (such as wheat, rice, and soy) and are believed to respond more productively to higher CO₂ levels than C4 crops (such as maize and sugar cane). For more information about the difference between C3 and C4 plants, including efforts to introduce carbon-concentrating mechanisms such as C4 into C3 plants, see the resources available at the Cambridge CAPP website. This variation in response to climate change is just one factor that needs to be considered, alongside changes in precipitation, air temperature increases, and impacts of extreme events such as droughts and floods. The strength of the CO₂ fertilisation effect is therefore uncertain and depends on other factors.