Editing IPCC:AR6/WGI/Chapter-5 (section)

=== FAQ 5.3 | Could Climate Change Be Reversed By Removing Carbon Dioxide From the Atmosphere? ===

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''Deliberate removal of carbon dioxide (CO'' 2 '') from the atmosphere'' ''could reverse (i.e., change the direction of) some aspects of climate change. However, this will only happen if it results in a net reduction in the total amount of CO'' 2 ''in the atmosphere, that is, if deliberate removals are larger than emissions. Some climate change trends, such as the increase in global surface temperature, would start to reverse within a few years. Other aspects of climate change would take decades (e.g., permafrost thawing) or centuries (e.g., acidification of the deep ocean) to reverse, and some, such as sea level rise, would take centuries to millennia to ch'' ''ange direction.''

The term ''negative carbon dioxide (CO'' 2 '') emissions'' refers to the removal of CO <sub>2</sub> from the atmosphere by deliberate human activities, in addition to removals that occur naturally, and is often used as synonymous with ''carbon dioxide remova'' ''l'' . Negative CO <sub>2</sub> emissions can compensate for the release of CO <sub>2</sub> into the atmosphere by human activities. They could be achieved by strengthening natural CO <sub>2</sub> sequestration processes on land (e.g., by planting trees or through agricultural practices that increase the carbon content of soils) and/or in the ocean (e.g., by restoration of coastal ecosystems) or by removing CO <sub>2</sub> directly from the atmosphere. If CO <sub>2</sub> removals are greater than human-caused CO <sub>2</sub> emissions globally, emissions are said to be ''net negative'' . It should be noted that CO <sub>2</sub> removal technologies are unable, or not yet ready, to achieve the scale of removal that would be required to compensate for current levels of emissions, and most have undesired side effects.

In the absence of deliberate CO <sub>2</sub> removal, the CO <sub>2</sub> concentration in the atmosphere (a measure of the amount of CO <sub>2</sub> in the atmosphere) results from a balance between human-caused CO <sub>2</sub> release and the removal of CO <sub>2</sub> by natural processes on land and in the ocean (natural ‘carbon sinks’; see FAQ 5.1). If CO <sub>2</sub> release exceeds removal by carbon sinks, the CO <sub>2</sub> concentration in the atmosphere would increase; if CO <sub>2</sub> release equals removal, the atmospheric CO <sub>2</sub> concentration would stabilize; and if CO <sub>2</sub> removal exceeds release, the CO <sub>2</sub> concentration would decline. This applies in the same way to ''net'' CO <sub>2</sub> emissions – that is, the sum of human-caused releases and deliberate removals.

If the CO <sub>2</sub> concentration in the atmosphere starts to go down, the Earth’s climate would respond to this change (FAQ 5.3, Figure 1). Some parts of the climate system take time to react to a change in CO <sub>2</sub> concentration, so a decline in atmospheric CO <sub>2</sub> as a result of net negative emissions would not lead to immediate reversal of all climate change trends. Recent studies have shown that global surface temperature starts to decline within a few years following a decline in atmospheric CO <sub>2</sub> , although the decline would not be detectable for decades due to natural climate variability (see FAQ 4.2). Other consequences of human-induced climate change, such as reduction in permafrost area, would take decades; yet others, such as warming, acidification and oxygen loss of the deep ocean, would take centuries to reverse following a decline in the atmospheric CO <sub>2</sub> concentration. Sea level would continue to rise for many centuries to millennia, even if large deliberate CO <sub>2</sub> removals were successfully implemented.

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'''FAQ 5.3, Figure 1 |''' '''Changes in aspects of climate change in response to a peak and decline in the atmospheric CO''' <sub>2</sub> '''concentration''' '''(top panel).''' The vertical grey dashed line indicates the time of peak CO <sub>2</sub> concentration in all panels. It shows that the reversal of global surface warming lags the decrease in the atmospheric CO <sub>2</sub> concentration by a few years, the reversal of permafrost area decline lags the decrease in atmospheric CO <sub>2</sub> by decades, and ocean thermal expansion continues for several centuries. The quantitative information in the figure (i.e., numbers on vertical axes) is not to be emphasized as it results from simulations with just one model and will be different for other models. The qualitative behaviour, however, can be expected to be largely model independent.

‘Overshoot’ scenarios are a class of future scenarios that are receiving increasing attention, particularly in the context of ambitious climate goals, such as the global warming limits of 1.5°C or 2°C included in the Paris Agreement. In these scenarios, a slow rate of reduction in emissions in the near term is compensated by net negative CO <sub>2</sub> emissions in the later part of this century, which results in a temporary breach or ‘overshoot’ of a given warming level. Due to the delayed reaction of several climate system components, it follows that the temporary overshoot would result in additional climate changes compared to a scenario that reaches the goal without overshoot. These changes would take decades to many centuries to reverse, with the reversal taking longer for scenarios with larger overshoot.

Removing more CO <sub>2</sub> from the atmosphere than is emitted into it would indeed begin to reverse some aspects of climate change, but some changes would still continue in their current direction for decades to millennia. Approaches capable of large-scale removal of CO <sub>2</sub> are still in the state of research and development or unproven at the scales of deployment necessary to achieve a net reduction in atmospheric CO <sub>2</sub> levels. CO <sub>2</sub> removal approaches, particularly those deployed on land, can have undesired side effects on water, food production and biodiversity.

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