For a system as complex as the earth’s climate, small changes in one part of the system can lead to big changes in another part. That means the relationship between the global temperature and the climate change impacts we know about isn’t linear. And that means that once we pass the global temperature rise scientists recommend to avoid the worst impacts of climate change, that small change in temperature could lead to big changes in our earth system. The possibility of exceeding the 1.5°C global temperature rise and then attempting to reduce global temperature afterward is what is known as an “overshoot.” Think of a free throw shot that arcs up and over the basket before it falls into the net. That was the subject of a panel discussion I attended today at COP28, called “Exceeding and Returning to 1.5, What are the Knowledge Gaps.”
The discussion was held at the Science for Climate Action Pavilion at COP28 in Dubai, UAE. The global stock take indicates that 1.5°C is still within reach, but will require significant cooperation and effort on behalf of all parties to be successful. It is generally accepted that it is not possible to reach 1.5°C without major progress in the transition away from fossil fuels on the part of the global community.
Given the challenge of meeting the 1.5°C target, there has been some conversation about whether an overshoot of the target, followed by a return back to 1.5°C with some future measures to reduce carbon, would be acceptable.
According to the panelists, this is a dangerous proposition.
The problem is that although we might be able to bring temperature back down to 1.5°C, other impacts of the global temperature increase operate on different timescales, and may even be irreversible. For example, once global temperature increases past 1.5°C, melting land-based ice causes sea level rise, which is dangerous to coastal locations and an existential threat to small island nations. Once the process of melting land-based ice begins, there is no way to reverse it on a human timescale, even if the global temperature is reduced so that it is less than 1.5°C above preindustrial levels.
Exceeding 1.5°C also comes with the loss of ecosystems we depend on, especially in our oceans. Once our massive coral reefs die, reducing global temperature rise back below the 1.5°C target won’t bring them back. Whole ecosystems that depend on coral reefs will be gone. It also means that many of the nature-based climate adaptation solutions, like building mangroves, will no longer help protect coastal communities.
Even if you can find a way to slow and reverse global temperature rise after exceeding 1.5°C, it is not possible to return to the climate you had before. This understanding is especially important when considering some of the geoengineering strategies which have been proposed to slow climate change. Even if you were able to lower global temperature through a geoengineering technology, the climate would not return to “normal.”
The best way to ensure the climate stays stable and safe for human life is to do the old-fashioned work of reducing carbon emissions by transitioning away from fossil fuel use to renewable forms of energy.
As one member of the audience commented, “why would we wait to reduce our carbon emissions until after we exceed 1.5 °C when we could do it now instead and avoid the irreversible impacts?”
