Cutting emissions today limits future climate impacts, but certain changes are ‘locked in’ due to past CO2 emissions

If you were about to get in a car accident, would you leave your foot on the gas pedal or hit the brakes? Even if an accident looks unavoidable, most people would still hit the brakes to limit the damages. Should it be any different with Earth’s climate?

Climate scientists explain that our past greenhouse gas emissions will lead to inevitable future consequences^[1] – that can be a tough pill to swallow. Some may even feel apathetic towards climate action if they falsely assume that today’s mitigation efforts will have no future impact. In reality, our actions still matter. 

The reason is similar to that of the car analogy above: just because we will face some inevitable impacts from our past actions, that doesn’t mean we can’t limit future damages. 

Below, we will explore which of these changes climate scientists anticipate, and why our actions today can still help mitigate certain impacts. 

We could stop certain changes in our climate system by reducing emissions – but other changes are ‘irreversible’ on human timescales

As we noted above, some climate changes are ‘locked in’, but we can also limit certain changes with our actions today.

As explained in the Intergovernmental Panel on Climate Change (IPCC) Climate Change 2023 Synthesis Report:

“Deep, rapid, and sustained reductions in greenhouse gas emissions would lead to a discernible slowdown in global warming within around two decades, and also to discernible changes in atmospheric composition within a few years”^[2]

So, if global warming can be significantly slowed, why would certain impacts still occur?

In short, when humans perturb Earth’s climate system, things can take a while to return to their prior state. If you’ve ever jumped in a completely still pool, you’ll know the water takes a very long time to settle again because you’ve stirred up waves that take a while to dissipate. 

The climate system is similar: just as jumping in a pool disturbs the natural balance a pool achieves by settling into a flat state, greenhouse gas emissions disturb Earth’s ability to balance incoming and outgoing energy – or that which comes from the sun, and that which escapes to outer space. 

If the climate was a pool, it’s becoming increasingly choppy with these proverbial waves (climate changes) – many of which will not settle down in our lifetime, which is why scientists call them ‘irreversible’. 

For example, if we stopped emitting carbon dioxide (CO₂) today, Earth’s atmospheric CO₂ would only drop to pre-industrial levels over thousands of years or more^[1]. While it’s good news that CO₂ would eventually go away, that doesn’t help us, our children, or even our great-grandchildren. Especially because that countdown only begins once when we stop emitting CO₂ – a feat we haven’t achieved. 

Although different parts of Earth – like our oceans and forests – absorb or ‘sink’ carbon dioxide, they cannot take in all that we emit. Just as a sponge can’t endlessly absorb water as you hold it under the tap, our oceans, forests, and other vegetation can’t absorb the endless CO₂ we emit. 

In fact, humans already emit far more CO₂ than Earth’s ocean and forests can absorb, leaving the vast remainder of this CO₂ in our atmosphere^[3] which warms our planet^[2]. 

Below, Figure 1 shows the emissions and sinks/absorption of carbon on Earth. Note that the magnitude of the fossil fuel emissions of CO₂ to our atmosphere – represented by the large upward-facing gray arrow – is greater than the sinks/absorption of CO₂, represented by the downward-facing arrows. As a result, from 2014-2023, atmospheric CO₂ increased by about 5.2 billion tonnes of carbon per year on average ^[3]. 

Once emitted, the portion of CO₂ that makes it to our atmosphere stays there for a long time. Over centuries, a major fraction of atmospheric CO₂ will be absorbed by the oceans, but models show 20-60% of CO₂ from burning fossil fuels will remain in our atmosphere for thousands of years^[4]. In a PNAS paper on climate change irreversibility, the authors explain:

“climate change that takes place due to increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop. Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing*, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1,000 years.”^[1]

*Note: Radiative forcing is the process by which greenhouse gases trap excess heat on Earth; namely, by absorbing and trapping energy (heat) that would otherwise escape to outer space. 

To learn more about the irreversibility described above, Science Feedback contacted one of the paper’s authors, Dr. Pierre Friedlingstein, Professor and Chair in Mathematical Modelling of the Climate System at the University of Exeter. 

Friedlingstein, who is also the director of the Global Carbon Project – an international team of scientists who track carbon emissions and sinks/absorption on Earth (Figure 1) – explained the following to Science Feedback: 

“Assume we could stop all greenhouse gases emissions, what would happen to climate change? Essentially, our understanding of the climate system indicates that the Earth would not keep warming (which is good news) but it would not cool down (which is bad news). Hence we can say that the ongoing climate change is essentially irreversible on human time scales. In the best case scenario it would not get any worse but we won’t go back to a pre-industrial climate. This is true for the ‘fast’ components of the climate system such as temperature, precipitation, extremes, etc. For slow components of the climate system such as ice sheets or sea level rise, changes would continue for centuries, even if we stopped anthropogenic emissions.”

In the next section, we will discuss the irreversibility timelines scientists project for different climate changes and impacts. 

A timeline of the irreversible changes in our climate system

As Friedlingstein noted in the section above, some components of the climate system will respond quicker than others to emission reductions. 

Deep ocean warming and acidification, ice sheet melting, and global sea level rise are especially irreversible over timescales of hundreds to thousands of years^[2]. Whereas, cutting net emissions to zero would quickly stabilize (but not cool) global temperatures in coming decades^[1]. 

See a brief summary^† on irreversibility vs. mitigation possibilities in Figure 2, and click the drop-down boxes below it for more details. 

^†Note: there are also other changes – such as biodiversity loss – which may also become irreversible, but those outcomes depend more on future emissions. For example, coral reefs,

kelp forests and seagrass meadows will undergo irreversible changes if global temperature rise exceeds 1.5°C^[5]. The outcomes below are consequences of our past emissions.

• Global temperature rise

  Although we can stop atmospheric warming by cutting net emissions to zero, that won’t start the cooling process – at least not in our lifetimes.

  The IPCC explains, “[…] global average temperature is projected to remain approximately constant for many centuries following a complete cessation of emissions”^[6]. They explain that this is true for all cases except for one in which our net emissions are ‘negative’ for a long period of time. This means we would have to actively remove CO₂ from the atmosphere, not just bring emissions to zero. 

  But why is CO₂ the focal point of discussions on long-term climate impacts? In short, it stays in the atmosphere much longer than other greenhouse gases. 

  Although other greenhouse gases such as methane (CH₄) and nitrous oxide (NH₄) are significant contributors to climate change for the next decades to century, they naturally degrade in our atmosphere far faster than CO₂ does, and are thus less problematic over the long term if emissions are reduced^[1]. 

  In contrast, a significant portion of CO₂ from fossil-fuel burning will remain in our atmosphere for thousands of years^[1], keeping global temperatures elevated relative to pre-industrial times. 

  As explained in the PNAS paper on irreversibility we referenced earlier:

  “Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1,000 years.”^[1]

  In other words, CO₂ concentrations would start dropping if we stopped emitting, causing less warming through the greenhouse effect, but the oceans would also start absorbing heat more slowly. So the two would effectively cancel out. 

  That being said, we can limit how much warming occurs over the next century – a more practical human timescale – by reducing greenhouse gas emissions^[5]. 

• Global sea-level rise

  Sea-level rise is one of the unavoidable impacts we will face in the next century. This is well understood by climate scientists because higher temperatures lead to more ice melt^[5] – which adds more water to our oceans – and more ocean warming, which raises sea levels because water expands when heated. As the IPCC explains:

  “Sea level rise is unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years”^[2]

  They also explain that “the probability and rate of ice mass loss increase with higher global surface temperatures.”^[2]

  Sea-level rise projections are also backed by evidence from Earth’s geologic past. By analyzing geological evidence such as ice cores, sediment layers, and fossil records, scientists have reconstructed past global temperatures and sea levels to understand how they were tied. 

  The IPCC explains that roughly 3 million years ago, global temperatures were 2.5°C-4°C higher than 1850-1900, and global sea level was very likely 5-25 meters or ~16-82 feet higher than today^[2].

  Sea-levels will continue to rise due to our past emissions. However, we still have some control over the speed and amount of rise that will occur. The IPCC explains that higher emissions will lead to faster and greater sea-level rise – meaning we can help limit sea-level rise by cutting emissions. 

  Under the most optimistic emissions pathway modeled by the IPCC – with humans cutting net CO₂ emissions to zero by 2050 – we can expect 0.28-0.55 meters (0.92-1.80 feet) of sea-level rise by 2100^[7]. Or, if we follow a higher emission pathway, sea levels could be 0.61-1.10 meters (2.00-3.61 feet) higher by 2100. 

  Under all emission pathways, extreme sea-level events which have been historically rare – like 1-in-100 year coastal flood events – are expected to be common by 2100^[7]. These events can create severe hazards and cause extensive property damage^[8]. 

• ICe sheet and glacier loss

  As noted above, scientists have a well-established understanding that global warming drives ice loss at a global scale^[5]. 

  This understanding is also backed by observations of ice and snow losses over recent decades. Scientists have observed reduced snow cover, loss of mass from ice sheets and glaciers, and decreased thickness and extent of Arctic sea ice^[5,7]. The temperatures of permafrost – solid ground that remains below 0°C (32°F) year round – have increased in most regions since the 1980s^[6]. 

  Some of these changes will be irreversible, as the IPCC explains:

  “[…] ice sheet and glacier mass loss, and permafrost degradation are expected to be irreversible on time scales relevant to human societies and ecosystems. Long response times of decades to millennia mean that the ocean and cryosphere are committed to long-term change even after atmospheric greenhouse gas concentrations and radiative forcing stabilise”^[7]

  The IPCC also notes that both permafrost degradation and ice loss can speed up once they cross certain thresholds, known as tipping points, as Earth warms^[7]. In other words, these changes are not always gradual and predictable – after passing certain tipping points, degradation or loss of ice and permafrost can speed up.

• Ocean warming

  The oceans absorb a significant amount of heat. In fact, roughly 93% of the excess heat stored by the Earth is in our oceans^[6]. 

  However, this heat is not quickly distributed throughout the ocean like hot water in a bathtub. Relative to our atmosphere, our oceans circulate heat very slowly, meaning that heat takes much longer to spread out. 

  As the IPCC explains: 

  
  “Due to the long time scales of this heat transfer from the surface to depth, ocean warming will continue for centuries, even if GHG emissions are decreased or concentrations kept constant.”^[6] 

  The slow circulation in the ocean means that warming will likely continue to the year 2300^[5].

• Ocean acidification

  Roughly 30% of the CO₂ we emit is absorbed by Earth’s oceans^[6]. This CO₂ interacts with ocean water to form carbonic acid, which then breaks down and increases the ocean’s acidity. Increased acidity can affect calcification, growth, development, and survival of certain ocean species^[7].

  The IPCC explains that the ocean’s near-surface acidity can be reversed over this century if we rapidly cut emissions, but ocean acidification at greater depths is irreversible on human timescales^[6]. 

  Figure 3 below shows the different surface ocean pH levels (lower pH meaning higher acidity) under different emission scenarios from now to 2100. Note that, under the lowest emission scenario (SSP-1-1.9), surface ocean pH stabilizes around 2040 then steadily recovers over time.

  

Dr. Pierre Friedlingstein

SF:  Can you briefly explain the concept of irreversibility in climate change, and why certain changes are considered to be irreversible over certain timeframes?

PF: “A good start is the following paper (see here[1]).

Assume we could stop all greenhouse gases emissions, what would happen to climate change? Essentially, our understanding of the climate system indicates that the Earth would not keep warming (which is good news) but it would not cool down (which is bad news). Hence we can say that the ongoing climate change is essentially irreversible on human time scales. In the best case scenario it would not get any worse but we won’t go back to a pre-industrial climate. This is true for the ‘fast’ components of the climate system such as temperature, precipitation, extremes, etc. For slow components of the climate system such as ice sheets or sea level rise, changes would continue for centuries, even if we stopped anthropogenic emissions.”

SF: If some changes in our climate system are irreversible at this point, can human actions today still limit future impacts in a significant way? 

PF: “Yes we can reduce emissions and reach net zero as quickly as possible in order not to aggravate the ongoing climate change and associated impacts.”

References

• 1 – Solomon et al. (2009) Irreversible climate change due to carbon dioxide emissions. Proceedings of the National Academy of Sciences (PNAS). 
• 2 – IPCC (2023). Climate Change 2023: Synthesis Report.
• 3 – Friedlingstein et al. (2024) Global Carbon Budget 2024. Earth System Science Data.
• 4 – Archer and Brovkin (2008) The millennial atmospheric lifetime of anthropogenic CO2. Climatic Change. 
• 5 –  IPCC (2021) Sixth Assessment Report.
• 6 –  IPCC (2014) Fifth Assessment Report.
• 7 – IPCC (2019) Special Report: Special Report on the Ocean and Cryosphere in a Changing Climate.
• 8 – Tebaldi et al. (2021) Extreme sea levels at different global warming levels. Nature Climate Change.