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

== B 2.1 Processes underlying land–climate interactions ==

<div id="section-2-1-2-introduction-to-the-chapter-structure-block-1"></div>

Land continuously interacts with the atmosphere through exchanges of, for instance, GHGs (e.g., CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O), water, energy or precursors of short lived-climate forcers (e.g., biogenic volatile organic compounds, dust, black carbon). The terrestrial biosphere also interacts with oceans through processes such as the influx of freshwater, nutrients, carbon and particles. These interactions affect where and when rain falls and thus irrigation needs for crops, frequency and intensity of heatwaves, and air quality. They are modified by global and regional climate change, decadal, inter-annual and seasonal climatic variations, and weather extremes, as well as human actions on land (e.g., crop and forest management, afforestation and deforestation). This in turn affects atmospheric composition, surface temperature, hydrological cycle and thus local, regional and global climate. This box introduces some of the fundamental land processes governing biophysical and biogeochemical effects and feedbacks to the climate (Box 2.1, Figure 1).

<div id="section-2-1-2-introduction-to-the-chapter-structure-block-2"></div>

<span id="box-2.1-figure-1"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Box-2.1-Figure-1'''

<span id="the-structure-and-functioning-of-managed-and-unmanaged-ecosystems-that-affect-local-regional-and-global-climate.-land-surface-characteristics-such-as-albedo-and-emissivity-determine-the-amount-of-solar-and-long-wave-radiation-absorbed-by-land-and-reflected-or-emitted-to-the-atmosphere.-surface-roughness-influences-turbulent-exchanges-of-momentum-energy-water-and-biogeochemical-tracers.-land"></span>
<!-- IMG CAPTION -->
'''The structure and functioning of managed and unmanaged ecosystems that affect local, regional and global climate. Land surface characteristics such as albedo and emissivity determine the amount of solar and long-wave radiation absorbed by land and reflected or emitted to the atmosphere. Surface roughness influences turbulent exchanges of momentum, energy, water and biogeochemical tracers. Land […]'''
<!-- IMG FILE -->
[[File:ded85c3bcf00b5c69c6e17ee743d86d8 C2_Box-2.1-Figure-1_Final-1024x791.jpg]]

The structure and functioning of managed and unmanaged ecosystems that affect local, regional and global climate. Land surface characteristics such as albedo and emissivity determine the amount of solar and long-wave radiation absorbed by land and reflected or emitted to the atmosphere. Surface roughness influences turbulent exchanges of momentum, energy, water and biogeochemical tracers. Land ecosystems modulate the atmospheric composition through emissions and removals of many GHGs and precursors of SLCFs, including biogenic volatile organic compounds (BVOCs) and mineral dust.<br />
Atmospheric aerosols formed from these precursors affect regional climate by altering the amounts of precipitation and radiation reaching land surfaces through their role in clouds physics.

<!-- END IMG -->
<div id="section-2-1-2-introduction-to-the-chapter-structure-block-3"></div>

‘Biophysical interactions’ are exchanges of water and energy between the land and the atmosphere (Section 2.5). Land warms up from absorbing solar and long-wave radiation; it cools down through transfers of sensible heat (via conduction and convection) and latent heat (energy associated with water evapotranspiration) to the atmosphere and through long-wave radiation emission from the land surface (Box 2.1, Figure 1). These interactions between the land and the atmosphere depend on land surface characteristics, including reflectivity of shortwave radiation (albedo), emissivity of long wave radiation by vegetation and soils, surface roughness and soil water access by vegetation, which depends on both soil characteristics and amounts of roots. Over seasonal, inter-annual and decadal timescales, these characteristics vary among different land cover and land-use types and are affected by both natural processes and land management (Anderson et al. 2011 <sup>[[#fn:r18|18]]</sup> ). A dense vegetation with high leaf area index, like forests, may absorb more energy than nearby herbaceous vegetation partly due to differences in surface albedo (especially when snow is on the ground). However, denser vegetation also sends more energy back to the atmosphere in the form of evapotranspiration (Bonan, 2008 <sup>[[#fn:r19|19]]</sup> ; Burakowski et al., 2018 <sup>[[#fn:r20|20]]</sup> ; Ellison et al., 2017 <sup>[[#fn:r21|21]]</sup> ) (Section 2.5.2) and this contributes to changes in atmospheric water vapour content, and subsequently to changes in rainfall.

Particularly in extra-tropical regions, these characteristics exhibit strong seasonal patterns with the development and senescence of the vegetation (e.g., leaf colour change and drop). For example, in deciduous forests, seasonal growth increases albedo by 20–50% from the spring minima to growing season maxima, followed by rapid decrease during leaf fall, whereas in grasslands, spring greening causes albedo decreases and only increases with vegetation browning (Hollinger et al. 2010 <sup>[[#fn:r22|22]]</sup> ). The seasonal patterns of sensible and latent heat fluxes are also driven by the cycle of leaf development and senescence in temperate deciduous forests: sensible heat fluxes peak in spring and autumn and latent heat fluxes peak in mid-summer (Moore et al. 1996 <sup>[[#fn:r23|23]]</sup> ; Richardson et al. 2013 <sup>[[#fn:r24|24]]</sup> ).

Exchanges of GHGs between the land and the atmosphere are referred to as ‘biogeochemical interactions’ (Section 2.3), which are driven mainly by the balance between photosynthesis and respiration by plants, and by the decomposition of soil organic matter by microbes. The conversion of atmospheric carbon dioxide into organic compounds by plant photosynthesis, known as terrestrial net primary productivity, is the source of plant growth, food for human and other organisms, and soil organic carbon. Due to strong seasonal patterns of growth, northern hemisphere terrestrial ecosystems are largely responsible for the seasonal variations in global atmospheric CO <sub>2</sub> concentrations. In addition to CO <sub>2</sub> , soils emit methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) (Section 2.3). Soil temperature and moisture strongly affect microbial activities and resulting fluxes of these three GHGs.

Much like fossil fuel emissions, GHG emissions from anthropogenic land cover change and land management are ‘forcers’ on the climate system. Other land-based changes to climate are described as ‘feedbacks’ to the climate system – a process by which climate change influences some property of land, which in turn diminishes (negative feedback) or amplifies (positive feedback) climate change. Examples of feedbacks include the changes in the strength of land carbon sinks or sources, soil moisture and plant phenology (Section 2.5.3).

Incorporating these land–climate processes into climate projections allows for increased understanding of the land’s response to climate change (Section 2.2), and to better quantify the potential of land-based response options for climate change mitigation (Section 2.6). However, to date Earth system models (ESMs) incorporate some combined biophysical and biogeochemical processes only to limited extent and many relevant processes about how plants and soils interactively respond to climate changes are still to be included (Section 2.7). And even within this class of models, the spread in ESM projections is large, in part because of their varying ability to represent land–climate processes (Hoffman et al. 2014 <sup>[[#fn:r25|25]]</sup> ). Significant progress in understanding of these processes has nevertheless been made since AR5.

<span id="the-effect-of-climate-variability-and-change-on-land"></span>