Editing IPCC:AR6/SRCCL/TS (section)

=== Biophysical and biogeochemical land forcing and feedbacks to the climate system ===

'''Changes in land conditions from human use or climate change in turn affect regional and global climate (''high confidence'').''' On the global scale, this is driven by changes in emissions or removals of CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O by land (biogeochemical effects) and by changes in the surface albedo (''very high confidence''). Any local land changes that redistribute energy and water vapour between the land and the atmosphere influence regional climate (biophysical effects; ''high confidence''). However, there is ''no confidence ''in whether such biophysical effects influence global climate. {2.1, 2.3, 2.5.1, 2.5.2}

'''Changes in land conditions modulate the likelihood, intensity and duration of many extreme events including heatwaves (''high confidence'') and heavy precipitation events (''medium confidence'').''' Dry soil conditions favour or strengthen summer heatwave conditions through reduced evapotranspiration and increased sensible heat. By contrast wet soil conditions, for example from irrigation or crop management practices that maintain a cover crop all year round, can dampen extreme warm events through increased evapotranspiration and reduced sensible heat. Droughts can be intensified by poor land management. Urbanisation increases extreme rainfall events over or downwind of cities (''medium confidence''). {2.5.1, 2.5.2, 2.5.3}

'''Historical changes in anthropogenic land cover have resulted in a mean annual global warming of surface air from biogeochemical effects (''very high confidence''), dampened by a cooling from biophysical effects (''medium confidence'').''' Biogeochemical warming results from increased emissions of GHGs by land, with model-based estimates of +0.20 ± 0.05°C (global climate models) and +0.24 ± 0.12°C – dynamic global vegetation models (DGVMs) as well as an observation-based estimate of +0.25 ± 0.10°C. A net biophysical cooling of –0.10 ± 0.14°C has been derived from global climate models in response to the increased surface albedo and decreased turbulent heat fluxes, but it is smaller than the warming effect from land-based emissions. However, when both biogeochemical and biophysical effects are accounted for within the same global climate model, the models do not agree on the sign of the net change in mean annual surface air temperature. {2.3, 2.5.1, Box 2.1}

'''The future projected changes in anthropogenic land cover that have been examined for AR5 would result in a biogeochemical warming and a biophysical cooling whose magnitudes depend on the scenario (''high confidence'').''' Biogeochemical warming has been projected for RCP8.5 by both global climate models (+0.20 ± 0.15°C) and DGVMs (+0.28 ± 0.11°C) (''high confidence''). A global biophysical cooling of 0.10 ± 0.14°C is estimated from global climate models and is projected to dampen the land-based warming (''low confidence''). For RCP4.5, the biogeochemical warming estimated from global climate models (+0.12 ± 0.17°C) is stronger than the warming estimated by DGVMs (+0.01 ± 0.04°C) but based on limited evidence, as is the biophysical cooling (–0.10 ± 0.21°C). {2.5.2}

'''Regional climate change can be dampened or enhanced by changes in local land cover and land use (''high confidence'') but this depends on the location and the season (''high confidence'').''' In boreal regions, for example, where projected climate change will migrate the treeline northward, increase the growing season length and thaw permafrost, regional winter warming will be enhanced by decreased surface albedo and snow, whereas warming will be dampened during the growing season due to larger evapotranspiration (''high confidence''). In the tropics, wherever climate change will increase rainfall, vegetation growth and associated increase in evapotranspiration will result in a dampening effect on regional warming (''medium confidence''). {2.5.2, 2.5.3}

'''According to model-based studies, changes in local land cover or available water from irrigation will affect climate in regions as far as few hundreds of kilometres downwind (''high confidence'').''' The local redistribution of water and energy following the changes on land affect the horizontal and vertical gradients of temperature, pressure and moisture, thus altering regional winds and consequently moisture and temperature advection and convection and subsequently, precipitation. {2.5.2, 2.5.4, Cross-Chapter Box 4 in Chapter 2}

'''Future increases in both climate change and urbanisation will enhance warming in cities and their surroundings (urban heat island), especially during heatwaves (''high confidence'').''' Urban and peri-urban agriculture, and more generally urban greening, can contribute to mitigation (''medium confidence'') as well as to adaptation (''high confidence''), with co-benefits for food security and reduced soil-water-air pollution. {Cross-Chapter Box 4 in Chapter 2}

'''Regional climate is strongly affected by natural land aerosols (''medium confidence'') (e.g., mineral dust, black, brown and organic carbon), but there is ''low confidence'' in historical trends, inter-annual and decadal variability and future changes.''' Forest cover affects climate through emissions of biogenic volatile organic compounds (BVOC) and aerosols (''low confidence''). The decrease in the emissions of BVOC resulting from the historical conversion of forests to cropland has resulted in a positive radiative forcing through direct and indirect aerosol effects, a negative radiative forcing through the reduction in the atmospheric lifetime of methane and it has contributed to increased ozone concentrations in different regions (''low confidence''). {2.4, 2.5}