Climate Clock Ticking: Can We Turn Back Now?

In June, a stark warning was issued by over 60 climate scientists: at the current rate of emissions, the remaining “carbon budget” to avoid exceeding a critical warming threshold could be depleted in a mere three years. This raises a crucial question: if we surpass the 1.5-degree-Celsius (2.7 degrees Fahrenheit) warming threshold, is a climate catastrophe unavoidable? And, crucially, can we reverse this temperature rise?

While exceeding the 1.5 C threshold presents significant challenges, particularly for vulnerable island nations and ecosystems, it doesn’t necessarily signal an immediate apocalypse. Experts suggest that once emissions are brought under control, strategies exist to gradually reduce temperatures, even after crossing the 1.5 C mark.

However, this doesn’t diminish the urgency of immediate action. A leading climate scientist and director of the Center for Science, Sustainability and the Media at the University of Pennsylvania, has emphasised that preventing even a fraction of a degree of warming is beneficial, as it is far cheaper, easier and more effective than attempting to reverse a temperature rise that has already occurred.

The Urgency of Now: The Carbon Budget

A recent report revealed the concerning reality that the world has approximately 143 billion tons (130 billion metric tons) of carbon dioxide (CO2) remaining to emit before we are likely to breach the 1.5 C target outlined in the Paris Agreement, a landmark accord signed by 195 countries to combat climate change. Currently, the world emits around 46 billion tons (42 billion metric tons) of CO2 annually, according to the World Meteorological Organization.

Presently, the world is already 1.2 C (2.2 F) warmer than the pre-industrial average, with human activities being almost entirely responsible for this increase. However, the actual warming impact of our emissions may be even more significant, partially masked by the ocean’s absorption of excess heat.

The Ocean’s Role and Delayed Warming

The National Oceanic and Atmospheric Administration (NOAA) has indicated that the ocean will release this accumulated heat over the coming decades through evaporation and direct heat transfer, regardless of emission reduction efforts.

This implies that even if carbon emissions were to cease immediately, global temperatures would continue to rise for several decades, with experts predicting an additional 0.5 C (0.9 F) of warming solely from the oceans.

Eventually, temperatures would stabilise as heat radiates into space. Over thousands of years, Earth would naturally regulate temperatures back to pre-industrial levels through natural carbon sinks, such as trees and soils absorbing CO2.

Why 1.5 C Matters: A Critical Threshold

Climate scientists view 1.5 C as a critical threshold, as exceeding this limit poses significant risks, particularly for people living in economically developing countries and island nations.

One expert, a professor of climate science at Simon Fraser University in Canada, describes the 1.5 C limit as “an indicator of a state of the climate system where we feel we can still manage the consequences.”

Exceeding 1.5 C could lead to the release of a substantial amount of additional heat stored within the ocean. Furthermore, surpassing this threshold increases the risk of triggering climate tipping points, which are elements of the Earth system that can rapidly transition into dramatically different states. Examples include the potential collapse of the Greenland Ice Sheet into the ocean or the transformation of the Amazon rainforest into a dry savanna.

Reversing Temperature Rise: A Potential Path Forward

While prioritising emission reduction is paramount, reversing a temperature rise of 1.5 C or more may still be possible, even if the critical threshold is breached. However, the necessary technologies are still under development, introducing considerable uncertainty regarding feasibility.

It’s crucial to note that even if temperatures are successfully reduced, the effects of passing climate tipping points would be irreversible. For example, refreezing ice sheets or reversing sea-level rise after they have already occurred would not be possible. However, it would significantly mitigate risks for ecosystems that respond more rapidly to temperature changes, such as permafrost-covered tundras.

Net-Zero vs. Net-Negative Emissions

Reversing temperature rise necessitates not only achieving net-zero emissions but also attaining net-negative emissions. Net-zero emissions imply sequestering as much CO2 through natural carbon sinks and negative emissions technologies as is emitted. Negative emissions, on the other hand, require systems that extract carbon from the atmosphere and store it underground – a process commonly known as carbon capture and storage.

While net-zero emissions may halt warming, reversing warming demands the removal of more carbon from the atmosphere than is emitted.

Scientists estimate that 0.1 C (0.2 F) of warming corresponds to 243 billion tons (220 billion metric tons) of CO2, a “massive amount.” Consequently, reducing temperatures by 0.1 C would require removing approximately 220 billion metric tons of carbon dioxide.

Currently, nature-based carbon-removal techniques, such as planting trees, sequester around 2.2 billion tons (2 billion metric tons) of CO2 each year. A significant scale-up, by a factor of 100, would be needed to reduce temperatures by 0.1 C in a single year.

However, due to competing demands for land, it is highly improbable that sufficient forests or peatland could be restored to meaningfully reverse temperature change.

This underscores the need for negative emissions technologies. However, most of these technologies are still in the testing phase, making it challenging to assess their effectiveness.

These technologies are also extremely expensive and are likely to remain so for the foreseeable future.

In practice, ensuring the rollout of these technologies achieves more than simply bringing us to net-zero would be a significant accomplishment. There remains some uncertainty regarding the Earth’s response to net-zero, and it is possible that the planet may cool at that point. However, any cooling would occur very slowly, with an optimistic scenario projecting a decrease of 0.3 C (0.5 F) in 50 years.

The Paris Agreement and Future Action

The Paris Agreement does not mandate the implementation of negative emissions technologies. However, the agreement’s objective to remain well below 2 C (3.6 F) may prompt governments to increase their investment in these technologies once the 1.5 C threshold is exceeded.

Recent figures indicate that, at the current rate of emissions, the remaining carbon budgets to stay below 1.6 C, 1.7 C (3.1 F), and 2 C could be exhausted within seven, twelve, and twenty-five years, respectively.

Even if 1.5 C is surpassed, limiting warming to 1.6 C is preferable to 1.7 C, and 1.7 C is preferable to 1.8 C (3.2 F). The current challenge lies in rapidly reducing carbon emissions to avert even more severe consequences.

It is important to recognise that progress is being made in emission reduction efforts.