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HomeCOP27 InsightsOvershoot: what does it mean to exceed 1.5°C?

Overshoot: what does it mean to exceed 1.5°C?

At an Intergovernmental Panel on Climate Change (IPCC) side event of COP27, we learnt about what overshoot implies, that is, what it means in different sectors of our daily life to exceed 1.5°C.

Overshoot

Overshoot is the period of time in which warming increases past the 1.5°C mark and then cools back. The Earth’s temperature is rising. As we learnt from the State of the Global Climate Report released by the WMO, the global mean temperature in 2022 is currently estimated to be about 1.15 [1.02 to 1.28]°C above the 1850-1900 pre-industrial average. So far, international climate negotiations have focused on limiting the temperature increase to 1.5°C, as exceeding it would have irreversible impacts on people, species and ecosystems of our planet. Currently, it is more likely that before stabilisation the climate will overshoot 1.5°C for years if not for decades.

The first and most important message of the side event was that the journey is as important as the destination. The solutions should not be placed on our shelves but mobilised and deployed for humanity and the Earth’s ecosystems. The different damaging outcomes of an overshoot would be increased coastal flooding, forced human migration, loss of biodiversity and greater devastation and frequency of forest fires, which affect a wide variety of ecosystems. Later when temperatures level off and cool, animals might start to migrate in search of habitats that no longer exist because of the former conversion of new land for agricultural use.

When it comes to the implications of overshoot on adaptation, there is one thing to remark, that adaptation is local: in many regions, overshoot pathways will require adaptation to higher risks that persist far beyond the period of global overshoot (for example, sea level rise and species extinction). Some ecosystems will reach hard limits of adaptation above 1.5°C. So overshoot may mean more expensive grey infrastructure options having to be deployed.

Overshoot
Diana Urge-Vorsatz. Photo: Dr Barbara Botos.

The consequences of temperature overshoot for biodiversity are the worst for islands. Endemic species have the highest risks. There will be a longer duration of biodiversity exposure to overshoot in many locations compared to the duration of overshoot. Not to mention indirect risks from overshoot. For example, reliance on large-scale land-based carbon dioxide removal can compound risks to biodiversity, water and food security, through afforestation and the establishment of monocultures.

Diana Urge-Vorsatz, vice-chair of Working Group III of IPCC contributed to the debate by asking what the maximum temperature was by which we can return after an overshoot. Do humans hold the thermostat to the climate of the Earth? The good news is that in theory, we can return the temperature by any amount. Nevertheless, biophysical, physical and political constraints suggest that we should not treat the global climate as our home thermostat: turning it up and down.

Photo: Dr Barbara Botos.

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