Editing IPCC:AR6/WGI/Chapter-4 (section)

=== FAQ 4.2 | How Quickly Would We See the Effects of Reducing Carbon Dioxide Emissions? ===

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The effects of substantial reductions in carbon dioxide emissions would not be apparent immediately, and the time required to detect the effects would depend on the scale and pace of emissions reductions. Under the lower-emissions scenarios considered in this Report, the increase in atmospheric carbon dioxide concentrations would slow visibly after about five to ten years, while the slowing down of global surface warming would be detectable after about twenty to thirty years. The effects on regional precipitation trends would only become apparent after several decades.

Reducing emissions of carbon dioxide (CO <sub>2</sub> ) – the most important greenhouse gas emitted by human activities – would slow down the rate of increase in atmospheric CO <sub>2</sub> concentration. However, concentrations would only begin to decrease when net emissions approach zero, that is, when most or all of the CO <sub>2</sub> emitted into the atmosphere each year is removed by natural and human processes (see FAQ 5.1 and FAQ 5.3). This delay between a peak in emissions and a decrease in concentration is a manifestation of the very long lifetime of CO <sub>2</sub> in the atmosphere; part of the CO <sub>2</sub> emitted by humans remains in the atmosphere for centuries to millennia.

Reducing the rate of increase in CO <sub>2</sub> concentration would slow down global surface warming within a decade. But this reduction in the rate of warming would initially be masked by natural climate variability and might not be detected for a few decades (see FAQ 1.2, FAQ 3.2 and FAQ 4.1). Detecting whether surface warming has indeed slowed down would thus be difficult in the years right after emissions reductions begin.

The time needed to detect the effect of emissions reductions is illustrated by comparing low- and high-emissions scenarios (FAQ 4.2, Figure 1). In the low-emissions scenario (SSP1-2.6), CO <sub>2</sub> emissions level off after 2015 and begin to fall in 2020, while they keep increasing throughout the 21st century in the high-emissions scenario (SSP3-7.0). The uncertainty arising from natural internal variability in the climate system is represented by simulating each scenario ten times with the same climate model but starting from slightly different initial states back in 1850 (thin lines). For each scenario, the differences between individual simulations are caused entirely by simulated natural internal variability. The average of all simulations represents the climate response expected for a given scenario. The climate history that would actually unfold under each scenario would consist of this expected response combined with the contribution from natural internal variability and the contribution from potential future volcanic eruptions (the latter effect is not represented here).

FAQ 4.2, Figure 1 shows that the atmospheric CO <sub>2</sub> concentrations differ noticeably between the two scenarios about five to ten years after the emissions have begun to diverge in year 2015. In contrast, the difference in global surface temperatures between the two scenarios does not become apparent until later – about two to three decades after the emissions histories have begun to diverge in this example. This time would be longer if emissions were reduced more slowly than in the low-emissions scenario illustrated here and shorter in the case of stronger reductions. Detection would take longer for regional quantities and for precipitation changes, which vary more strongly from natural causes. For instance, even in the low-emissions scenario, the effect of reduced CO <sub>2</sub> emissions would not become visible in regional precipitation until late in the 21st century.

In summary, it is only after a few decades of reducing CO <sub>2</sub> emissions that we would clearly see global temperatures starting to stabilize. By contrast, short-term reductions in CO <sub>2</sub> emissions, such as during the COVID-19 pandemic, do not have detectable effects on either CO <sub>2</sub> concentration or global temperature. Only sustained emissions reductions over decades would have a widespread effect across the climate system.

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'''FAQ 4.2, Fig''' '''ure 1 |''' '''Observing the benefits of emissions reductions. (Top)''' Carbon dioxide (CO <sub>2</sub> ) emissions, '''(middle)''' CO <sub>2</sub> concentration in the atmosphere and '''(bottom)''' effect on global surface temperature for two scenarios: a low-emissions scenario (SSP1-2.6, blue) and a high-emissions scenario (SSP3-7.0). In the low-emissions scenario, CO <sub>2</sub> emissions begin to decrease in 2020 whereas they keep increasing throughout the 21st century in the high-emissions scenario. The thick lines are the average of the 10 individual simulations (thin line) for each scenario. Differences between individual simulations reflect natural variability.

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