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Explainer: Will global warming ‘stop’ as soon as net-zero emissions are reached?

Explainer: Will global warming ‘stop’ as soon as net-zero emissions are reached?

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Media reports frequently claim that the world is facing “committed warming” in the future as a result of past emissions, meaning higher temperatures are “locked in”, “in the pipeline” or “inevitable”, regardless of the choices society takes today.

The best evidence suggests that warming will not stop once carbon dioxide emissions (CO2) reach zero. Humans have the power to determine their climate future.

This has been pointed out by scientists recently. has been reportedThis is a new scientific finding. However, scientists have recognized that zero CO2 emissions could mean flat future temperatures. since at least 2008. The 2018 Intergovernmental Panel on Climate Change’s (IPCC) special report on 1.5C included a focus on zero emission scenarios with similar findings. 

Committed warming is often confused because of mixing up two concepts: a world in which CO2 levels remain at current levels and a world in which emissions reach net zero and concentrations begin falling.

Even in a world with zero CO2 emissions, there are still many uncertainties surrounding what happens to non-CO2 greenhouse gas (GHGs) such as methane or nitrous oxide. sulphate aerosolsThese processes cool the planet and provide long-term feedback and natural variability in climate system. 

In addition, temperatures are likely to remain stable rather than drop for a few centuries after emissions reaches zero. This means that it will be hard to reverse the climate change that has already taken place in the absence large-scale net emissions.

Constant concentrations vs zero emissions

It is understandable that there is confusion about the impact of zero emission. Even a NASA climate FAQ – last updated in 2007 – still says that “even if we stopped emitting greenhouse gases today, global warming would continue to happen for at least several more decades, if not centuries”. (NASA’s Dr Gavin SchmidtCarbon Brief is informed that this statement does not reflect recent research, and an update is in process. 

One common cause of confusion is the mixing up of two very different scenarios: a world where CO2 concentrations remain at today’s levels; and a world where all emissions are immediately cut to zero (or net-zero, which would have the same effect; what ultimately matters is the balance of CO2 sources and sinks, though the extent to which sinks are natural or anthropogenic can be a tricky question.

Many people lived in poverty until the mid-2000s. climate modelsThey were unable to assess the impact of emissions reaching 0 on their environment. This is because they did not include modelling of biogeochemical cycles – such as the carbon cycle – and could not effectively translate emissions of CO2 into atmospheric CO2 concentrations.

Therefore, climate models were often run with scenarios that simulated the concentration of CO2 within the atmosphere. These scenarios were more important than the emissions and often examined what would occur if the atmospheric CO2 levels remained constant at their current levels.

These “constant concentration” scenarios showed that there was additional warming “in the pipeline” as the oceans slowly warmed up to reach the same temperature as the atmosphere and brought the Earth back into radiative equilibrium. This means that the Earth absorbs the same amount of energy from the sun as the radiation it sends back to space.

Models tend to suggest 0.4CTo 0.5CIf the concentrations are maintained at the same level, an additional warming of about 5% would occur over the next few decades. 

However, a world with constant concentrations is not one without emissions. To keep concentrations constant, it would be necessary to continue emitting CO2 to offset the CO2 absorbed in the oceans and on the ground. This would be around 30% of global emissions at present, but it would decrease over time.

However, if emissions were reduced to zero, atmospheric concentrations in CO2 would rapidly fall before eventually stabilising at lower levels.

The figure below was adapted from an article published in 2010. Nature GeosciencesBy Prof H Damon MatthewsAnd Prof Andrew WeaverThe following table compares temperature projections out to 2200 for scenarios with constant concentrations and zero emissions.

Projected future warming under constant concentrations and zero-emissions scenarios

Future warming projections under constant concentrations (red) and zero emission scenarios (blue). Historical warming based upon an average of NASA/NOAA, Berkeley, Cowtan, Way, Hadley/UEA records. (black). Future warming adapted to model runs in Matthews and Weaver 2010. Combining historical temperatures with model runs is possible based on a 30-year period. local regression. Use Carbon Brief to create your chart Highcharts.

Matthews and Weaver found that, in a constant concentration scenario, the world would continue to warm by around 0.3C by 2200 – with some additional warming in centuries to come as the deep oceans continued to slowly warm.

The world has warmed by about 1.3C. This means that 1.5C would be exceeded if current CO2 concentrations remain constant due to continued emissions.

By contrast, they suggested that temperatures would stabilise in a world of net-zero emissions, remaining roughly at the level they were when emissions ceased.

Warming oceans, falling atmospheric CO2

Two factors work in the opposite direction to explain why temperatures stabilize after emissions reach zero.

The Earth is currently out thermal equilibrium. This means that more energy from the sun is being trapped in the atmosphere by greenhouse gases than is escaping to space. This extra heat is heating the oceans at over 90%. As the oceans warm, they will absorb less heat from the atmosphere, and global average surface temperatures will increase.

The land and ocean absorb approximately half of the CO2 humans emit each year. If emissions go to zero, these “carbon sinks” continue to take up some of the extra CO2 that was emitted in the past – quickly at first and then more slowly over time as they move toward a new equilibrium. This reduces CO2 levels in the atmosphere and the warming that it causes. 

These two factors may be accidental cancel each other out. The additional surface warming caused by oceans continuing their heat up is offset by the cooling effects of falling atmospheric CO2.

Both of these factors are expected to show similar patterns over time. They will be larger in the first few year after net-zero emission and slowly tailing off.

In the very long run – over many hundreds to thousands of years – carbon sinks would become dominant and global temperatures would eventually fall – as long as anthropogenic CO2 emissions remained at net-zero. The lifetime of CO2The rate at which CO2 is absorbed by sinks determines how much of it stays in the atmosphere. However, about half of our emissions are absorbed fairly quickly. However, a portion of our CO2 accumulated in the atmosphere will still exist in tens or thousands of years.

The latest results from the past year are now available much stronger evidenceStudy of the impact of net-zero carbon dioxide emissions on temperatures. These results come from a set of modern climate models that include carbon cycle dynamics, called Earth system models(ESMs).

The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) used 18 different ESMs to simulate what would happen in a world where global emissions suddenly ceased after the world had emitted a total 3667 Gigatons of CO2 (or 1000 Gigatons of carbon) – which would be expected to result in warming of around 2C compared to preindustrial levels. It also examined a case when emissions gradually declined to net zero. The results were similar to the abrupt cutoff after net zero is reached.

(ZECMIP is similar in concept to model intercomparison project undertaken under the auspices CMIP6, the new generation of climate models developed in the lead-up to the IPCC’s sixth assessment report, due to be published in 2021-22. It was not intended to be part of CMIP6’s formal CMIP6 cohort. 

Below is an illustration of the results of this experiment 50 year after emissions stopped. The top panel shows the change in energy flux – the amount of energy accumulating at the Earth’s surface – from different sources. The bottom panel shows the average temperature change over 50 years of no emissions.

The red bars signify warming from reduced ocean heat absorption as the oceans warm, while the blue and yellow bars signify cooling from CO2 absorption from the oceans, and land, respectively. The net forcing shown in the top panel is not always indicative of the projected temperature impact. Natural variability, and other factors, can also have an effect on surface temperature changes in ESMs.

Energy flux and surface temperature warming 50 years after emissions reach zero

Top panel: Energy flux (top) and surface temperature rising (bottom) 50 years after emission reaches zero for ESMs that are part of the ZECMIP project. Use Carbon Brief to create your chart HighchartsAdapted from Figure 7, MacDougall, et al.2020).

The future temperature change projection 50 years after zero emissions is achieved varies between 0.3C to 0.3C, with an average cooling of 0.03C across all models that participated in the ZECMIP experiment. Ten models show surface temperature changes that are close to zero. Three models show notable cooling, and two models show notable warming.

The different meanings and uses of net-zero emissions

Climate mitigation efforts are largely focused on CO2, human emissions of other GHGs and aerosolsAlso, global surface temperatures are affected by their impact. While global temperatures will stabilize once CO2 emission drops to zero, this is not true for zero GHGs or aerosol emissions.

The IPCC’s special report on 1.5C(SR15) used an ESM simplified to evaluate the likely evolution in surface temperatures under different definitions of zero emission. The figure below shows the results of these different scenarios.

The report looked at the case examined above – zero CO2 emissions – shown by the blue line. The report also considered zero CO2 emissions and aerosol emissions (red), no GHGs (yellow), as well as zero GHGs, aerosols, and zero CO2 (purple). 

Projected global surface temperature changes under zero CO2 emissions, CO2 and aerosol emissions, GHG emissions, and GHG and aerosol emissions

Projected global temperature changes under zero CO2 emission (blue Line), zero CO2 (red), zero CO2 (yellow), zero GHG (yellow), and zero GHG (purple). Chart created by Carbon Brief HighchartsAdapted from Figure 1.5 in IPCC SR15. The methods shown in the first figure are used to estimate historical warming values (black) and their combination with model simulations.

Emissions from humans aerosols – tiny particles of sulfur or nitrogen suspended in the atmosphere that reflect incoming sunlight back to space – have a strong cooling effect on the planet, though there are large uncertaintiesIt is not clear how large this effect is. Aerosols have a very short lifetime in the atmosphere. If they stop emitting, the aerosols will quickly fall back into the atmosphere.

Accordingly, the world would be about 0.4C more warm if both CO2 and aerosol emissions are reduced to zero, as opposed to having zero CO2 emission. This scenario (red line) would see the world rise to 1.75C by 2100, exceeding the 1.5C target.

Global warming can also be driven by other GHGs. Human-caused emissions of methaneParticularly, they account for about a quarterThe historical warming the world has seen.

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Methane, unlike CO2, has a short atmospheric life span. Therefore, most methane emissions will disappear from the atmosphere within 12 years. This is why the world would cool significantly by 2100 if all GHG emission fell to zero. This would result is a cooling of approximately 0.5C, compared with a scenario in which only CO2 falls below zero.

Finally, if all human emissions that affect climate change fall to zero – including GHGs and aerosols – then the IPCC results suggest there would be a short-term 20-year bump in warming followed by a longer-term decline. This is due to the opposite effects of warming as aerosols are removed from the atmosphere and cooling from falling methane.

The cooling from non-CO2 GHG gas emissions is more effective than the warming from aerosol emissions. It will result in about 0.2C of cooling by 20100.

These are only the best estimates. As discussed earlier, even under zero-CO2 alone, models project anywhere from 0.3C of cooling to 0.3C of warming (though this is in a world where emissions reach zero after around 2C warming; immediate zero emissions in today’s 1.3C warming world would likely have a slightly smaller uncertainly range). Because of the uncertainties in aerosol effects, reducing all GHGs and aerosols by zero could result either in 0.25C or more cooling.

All of these uncertainties combined suggest that the best estimate for the effects of zero carbon dioxide is around 0C +/– 0.3C for the century following emissions go to zero. The effects of zero GHGs would be around -0.2C+/- 0.5C.

There is also the possibility that natural variability could play a part in future warming even under zero emission futures. A recent paperBy Prof Chen ZhouMy colleagues and I suggested that the warming that would have otherwise occurred due to historical emissions has been masked by natural cycles in the eastern Pacific.

Zhou and colleagues suggest between 0.2-0.5C of additional warming could occur, even in a zero emissions world, once historical patterns of cold temperatures in this part of the ocean reverse – though only a portion of this warming would likely occur by 2100.

Other researchers have been scepticalThese conclusions suggest that it is still unclear when or if these historic patterns in the Pacific ocean might change.

This article examines the effects of zero emission scenarios in the future. However, if zero emissions were to occur later on in the century, it is possible to lock in more carbon-cycle feedback processes – such as melting permafrost – than under current global temperature levels.

A world that has warmed by 3C or 4C above pre-industrial levels may lock in more committed future warming than today’s world – and more research is needed to explore these effects. 

The best current estimates suggest that temperatures will stabilize in a zero emission world. However, this does not mean all climate impacts would cease.

Melting glaciersAnd ice sheetsAnd rising sea levelsAll of these processes occur slowly and lag behind surface temperatures warming. Even if there were no emissions, rising sea levels would continue for many centuries. some estimates suggesting that at least 80cm of additional sea level rise is “locked in”.

These impacts could be prevented by reducing global temperature. net-negative global emissionsThis is more than just stopping the temperature from rising and reaching net-zero

Update: This article was revised on 30/04/2021 in order to clarify details about the ZECMIP experiment.

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