Figure 1. Two main pathways for limiting global temperature rise to 1.5°C above pre-industrial levels are discussed in IPCC’s Special Report. The pathways are: stabilizing global temperature at, or just below, 1.5°C (left) and global temperature temporarily exceeding 1.5°C before coming back down later in the century (right). Temperatures shown are relative to pre-industrial but pathways are illustrative only, demonstrating conceptual not quantitative characteristics. Source: IPCC Special Report 15 (2014).
Introduction – defining pathways to the future
To determine what it takes to limit global temperature rise to 1.5°C above pre-industrial levels scientists have defined different pathways. The IPCC Special Report 15 (2014) identifies two main conceptual pathways: One where the global temperature is stabilized just below 1.5°C, and one where global temperature exceed 1.5°C for a while before coming back down, as illustrated above.
So how do we get to these two pathways?
The future is something we can only predict, therefore scientists use models to simulate the effects of greenhouse gas emissions on the future levels of warming. Simulations
with different amounts and intensities of greenhouse gas emissions result in different levels of warming. Each simulation describes a future possible pathway. There are many different pathways that can limit the warming to below 1.5°C.
The two pathways identified by IPCC have different implications on how much greenhouse gases we can emit, and how these emissions will impact the climate as well as sustainable development. The second pathway in the illustration above overshoots the target of 1.5°C for some time. The longer this overshoot is the more we have to rely on techniques that can actively remove CO2 from the atmosphere, in combination with reduced emissions. This is referred to as climate engineering, or geo-engineering, which is something we will address in a future post here at The Environmentalization.
What are we doing to meet the target to keep the warming under 1.5°C?
All countries that have formally accepted the Paris Agreement have to pledge how they will address climate change. At Climate Home News you can read more about what some countries have committed to do. Currently, however the combined effect of all the pledges that have been made are not enough to limit global warming to 1.5°C above preindustrial levels. This means that warming will exceed 1.5°C, for at least a period of time. As mentioned above, this pathway requires geo-engineering to remove CO2 from the atmosphere in combination with extensive reduction of greenhouse gas emissions to return warming to 1.5°C at a later stage.
So, which of the two pathways is the most likely one?
The answer to this question depends on how much each country manages to reduce its greenhouse gas emissions. We have learnt that the current pledges are not enough. Based on the level of commitment required (e.g. transition from fossil based energy system to one based on renewable energy, less flight travels, reduce consumption, less plastics, reduce meat production etc.) to reduce our greenhouse gas emissions, we can expect delayed action, limited international cooperation, and insufficient policies, leading to stagnating or increasing greenhouse gas emissions, preventing us from making the target of 1.5°C above pre-industrial levels. In other words: we should prepare for a situation when the global average temperature exceeds 1.5°C above the pre-industrial level.
- Many countries have made pledges to reduce CO2 emissions but it is not enough to keep the warming under 1.5°C.
- We will most likely experience an overshoot when the global warming exceeds 1.5°C for a time
- The impacts of a global warming above 1.5°C are uncertain and may even result in the climate exceeding a tipping point, i.e. a threshold that, when exceeded, can lead to large changes in the state of the system
- To reduce the global average temperature when it increased more than 1.5°C we have to apply practices and techniques of geo-engineering in combination with extensive reduction of our greenhouse gas emissions
- Geo-engineering at this scale has never been tested
This text is a reflection based on the Frequently Asked Questions (FAQ2.1) extracted from chapter 1 of IPCC’s fifth assessment report.
You can download the entire FAQ document here: IPCC Special Report 15, 2014
You can read more about what different countries have pledged to address climate change in the post: Which countries have a net zero carbon goal? published 14 June 2019 at Climate Change Home