Editing IPCC:AR6/SRCCL/Chapter-2 (section)

=== 2.1.1 Recap of previous IPCC and other relevant reports as baselines ===

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The evidence that land cover matters for the climate system have long been known, especially from early paleoclimate modelling studies and impacts of human-induced deforestation at the margin of deserts (de Noblet et al. 1996 <sup>[[#fn:r1|1]]</sup> ; Kageyama et al. 2004 <sup>[[#fn:r2|2]]</sup> ). The understanding of how land use activities impact climate has been put forward by the pioneering work of Charney (1975) <sup>[[#fn:r3|3]]</sup> who examined the role of overgrazing-induced desertification on the Sahelian climate.

Since then there have been many modelling studies that reported impacts of idealised or simplified land cover changes on weather patterns (e.g., Pielke et al. 2011 <sup>[[#fn:r4|4]]</sup> ). The number of studies dealing with such issues has increased significantly over the past 10 years, with more studies that address realistic past or projected land changes. However, very few studies have addressed the impacts of land cover changes on climate as very few land surface models embedded within climate models (whether global or regional), include a representation of land management. Observation-based evidence of land-induced climate impacts emerged even more recently (e.g., Alkama and Cescatti 2016 <sup>[[#fn:r5|5]]</sup> ; Bright et al. 2017 <sup>[[#fn:r6|6]]</sup> ; Lee et al. 2011 <sup>[[#fn:r7|7]]</sup> ; Li et al. 2015 <sup>[[#fn:r8|8]]</sup> ; Duveiller et al. 2018 <sup>[[#fn:r9|9]]</sup> ; Forzieri et al. 2017 <sup>[[#fn:r10|10]]</sup> ) and the literature is therefore limited.

In previous IPCC reports, the interactions between climate change and land were covered separately by three working groups. AR5 WGI assessed the role of land use change in radiative forcing, land-based GHGs source and sink, and water cycle changes that focused on changes of evapotranspiration, snow and ice, runoff and humidity. AR5 WGII examined impacts of climate change on land, including terrestrial and freshwater ecosystems, managed ecosystems, and cities and settlements. AR5 WGIII assessed land-based climate change mitigation goals and pathways related to the agriculture, forestry and other land use (AFOLU). Here, this chapter assesses land–climate interactions from all three working groups. It also builds on previous special reports such as the Special Report on Global Warming of 1.5°C (SR15). It links to the IPCC Guidelines on National Greenhouse Gas Inventories in the land sector. Importantly, this chapter assesses knowledge that has never been reported in any of those previous reports. Finally, the chapter also tries to reconcile the possible inconsistencies across the various IPCC reports.

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==== Land-based water cycle changes ====

AR5 reported an increase in global evapotranspiration from the early 1980s to 2000s, but a constraint on further increases from low soil moisture availability. Rising CO <sub>2</sub> concentration limits stomatal opening and thus also reduces transpiration, a component of evapotranspiration. Increasing aerosol levels, declining surface wind speeds and declining levels of solar radiation reaching the ground are additional regional causes of the decrease in evapotranspiration.

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==== Land area precipitation change ====

Averaged over the mid-latitude land areas of the northern hemisphere, precipitation has increased since 1901 (medium confidence before 1951 and ''high confidence'' thereafter). For other latitudes, area-averaged long-term positive or negative trends have low confidence. There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. Extreme precipitation events over most of the mid- latitude land masses and over wet tropical regions will very likely become more intense and more frequent (IPCC 2013a <sup>[[#fn:r11|11]]</sup> ).

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==== Land-based GHGs ====

AR5 reported that annual net CO <sub>2</sub> emissions from anthropogenic land use change were 0.9 [0.1–1.7] GtC yr <sup>–1</sup> on average during 2002–2011 (medium confidence). From 1750–2011, CO <sub>2</sub> emissions from fossil fuel combustion have released an estimated 375 [345–405] GtC to the atmosphere, while deforestation and other land use change have released an estimated 180 [100–260] GtC. Of these cumulative anthropogenic CO <sub>2</sub> emissions, 240 [230–250] GtC have accumulated in the atmosphere, 155 [125–185] GtC have been taken up by the ocean and 160 [70–250] GtC have accumulated in terrestrial ecosystems (i.e., the cumulative residual land sink) (Ciais et al. 2013a <sup>[[#fn:r12|12]]</sup> ). Updated assessment and knowledge gaps are covered in Section 2.3.

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==== Future terrestrial carbon source/sink ====

AR5 projected with ''high confidence'' that tropical ecosystems will uptake less carbon and with medium confidence that at high latitudes, land carbon sink will increase in a warmer climate. Thawing permafrost in the high latitudes is potentially a large carbon source in warmer climate conditions, however the magnitude of CO <sub>2</sub> and CH <sub>4</sub> emissions due to permafrost thawing is still uncertain. The SR15 further indicates that constraining warming to 1.5°C would prevent the melting of an estimated permafrost area of 2 million km <sup>2</sup> over the next centuries compared to 2°C. Updates to these assessments are found in Section 2.3.

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==== Land use change altered albedo ====

AR5 stated with ''high confidence'' that anthropogenic land use change has increased the land surface albedo, which has led to a RF of –0.15 ± 0.10 W m <sup>–2</sup> . However, it also underlined that the sources of the large spread across independent estimates were caused by differences in assumptions for the albedo of natural and managed surfaces and for the fraction of land use change before 1750. Generally, our understanding of albedo changes from land use change has been enhanced from AR4 to AR5, with a narrower range of estimates and a higher confidence level. The radiative forcing from changes in albedo induced by land use changes was estimated in AR5 at–0.15 W m <sup>–2</sup> (–0.25 to about –0.05), with medium confidence in AR5 (Myhre et al. 2013 <sup>[[#fn:r13|13]]</sup> ). This was an improvement over AR4 in which it was estimated at –0.2 W m <sup>–2</sup> (–0.4 to about 0), with ''low to medium confidence'' (Forster et al. 2007 <sup>[[#fn:r14|14]]</sup> ). Section 2.5 shows that albedo is not the only source of biophysical land-based climate forcing to be considered.

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==== Hydrological feedback to climate ====

Land use changes also affect surface temperatures through non- radiative processes, and particularly through the hydrological cycle. These processes are less well known and are difficult to quantify but tend to offset the impact of albedo changes. As a consequence, there is low agreement on the sign of the net change in global mean temperature as a result of land use change (Hartmann et al. 2013a <sup>[[#fn:r15|15]]</sup> ). An updated assessment on these points is covered in Sections 2.5 and 2.2.

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==== Climate-related extremes on land ====

AR5 reported that impacts from recent climate-related extremes reveal significant vulnerability and exposure of some ecosystems to current climate variability. Impacts of such climate-related extremes include alteration of ecosystems, disruption of food production and water supply, damage to infrastructure and settlements, morbidity and mortality, and consequences for mental health and human well- being (Burkett et al. 2014 <sup>[[#fn:r16|16]]</sup> ). The SR15 further indicates that limiting global warming to 1.5°C limits the risks of increases in heavy precipitation events in several regions ( ''high confidence'' ). In urban areas, climate change is projected to increase risks for people, assets, economies and ecosystems ( ''very high confidence'' ). These risks are amplified for those lacking essential infrastructure and services or living in exposed areas. An updated assessment and a knowledge gap for this chapter are covered in Section 2.2 and Cross-Chapter Box 4.

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==== Land-based climate change adaptation and mitigation ====

AR5 reported that adaptation and mitigation choices in the near- term will affect the risks related to climate change throughout the 21st century (Burkett et al. 2014 <sup>[[#fn:r17|17]]</sup> ). AFOLU are responsible for about 10–12 GtCO <sub>2</sub> eq yr <sup>–1</sup> anthropogenic greenhouse gas emissions, mainly from deforestation and agricultural production. Global CO <sub>2</sub> emissions from forestry and other land use have declined since AR4, largely due to increased afforestation. The SR15 further indicates that afforestation and bioenergy with carbon capture and storage (BECCS) are important land-based carbon dioxide removal (CDR) options. It also states that land use and land-use change emerge as a critical feature of virtually all mitigation pathways that seek to limit global warming to 1.5oC. The Climate Change 2014 Synthesis Report concluded that co-benefits and adverse side effects of mitigation could affect achievement of other objectives, such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development. Updated assessment and knowledge gaps are covered in Section 2.6 and Chapter 7.

Overall, sustainable land management is largely constrained by climate change and extremes, but also puts bounds on the capacity of land to effectively adapt to climate change and mitigate its impacts. Scientific knowledge has advanced on how to optimise our adaptation and mitigation efforts while coordinating sustainable land management across sectors and stakeholders. Details are assessed in subsequent sections.

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