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

==== 2.5.1.2 Impacts of future global land cover changes on climate ====

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'''''At the global level'''''

The most extreme CMIP5 emissions scenario, RCP8.5, has received the most attention in the literature with respect to how projected future anthropogenic land use land cover changes (Hurtt et al. 2011 <sup>[[#fn:r1048|1048]]</sup> ) will affect the highest levels of global warming.

Seven model-based studies have examined both the biophysical and biogeochemical effects of anthropogenic changes in land cover, as projected in RCP8.5, on future climate change (Simmons and Matthews 2016 <sup>[[#fn:r1049|1049]]</sup> ; Davies-Barnard et al. 2014 <sup>[[#fn:r1050|1050]]</sup> ; Boysen et al. 2014 <sup>[[#fn:r1051|1051]]</sup> ) (Table 2.5). They all agree on a biogeochemical warming, ranging from +0.04°C to +0.35°C, in response to land cover change. Two models predict an additional biophysical warming, while the others agree on a biophysical cooling that dampens (or overrules) the biogeochemical warming. Using a wider range of global

climate models, the biogeochemical warming ( ''high confidence'' ) is +0.20 ± 0.15°C whereas it is +0.28 ± 0.11°C when estimated from DGVMs (Pugh et al. 2015 <sup>[[#fn:r1052|1052]]</sup> ; Stocker et al. 2014 <sup>[[#fn:r1053|1053]]</sup> ). This biogeochemical warming is compensated for by a biophysical cooling ( ''medium confidence'' ) of –0.10 ± 0.14°C (Quesada et al. 2017a <sup>[[#fn:r1054|1054]]</sup> ; Davies-Barnard et al. 2015 <sup>[[#fn:r1055|1055]]</sup> ; Boysen et al. 2014 <sup>[[#fn:r1056|1056]]</sup> ). The estimates of temperature changes resulting from anthropogenic land cover changes alone remain very small compared to the projected mean warming of +3.7°C by the end of the 21st century (ranging from 2.6°C–4.8°C depending on the model and compared to 1986–2005; Figure 2.14).

Two other projected land cover change scenarios have been examined (RCP4.5 and RCP2.6; Table 2.5; Figure 2.14) but only one climate modelling experiment has been carried out for each, to estimate the biophysical impacts on climate of those changes (Davies-Barnard et al. 2015 <sup>[[#fn:r1057|1057]]</sup> ). For RCP2.6, ESMs and DGVMs agree on a systematic biogeochemical warming resulting from the imposed land cover changes, ranging from +0.03 to +0.28°C (Brovkin et al. 2013 <sup>[[#fn:r1058|1058]]</sup> ), which is significant compared to the projected mean climate warming of +1°C by the end of the 21st century (ranging from 0.3°C–1.7°C depending on the models, compared to 1986–2005). A very small amount of biophysical cooling is expected from the one estimate. For RCP4.5, biophysical warming is expected from only one estimate, and results from a projected large forestation in the temperate and high latitudes. There is no agreement on the sign of the biogeochemical effect: there are as many studies predicting cooling as warming, whichever the method to compute those effects (ESMs or DGVMs).

Previous scenarios – Special Report on Emission Scenarios (SRES) results of climate studies using those scenarios were reported in AR4 – displayed larger land use changes than the more recent ones (RCP,AR5). There is ''low confidence'' from some of those previous scenarios (SRES A2 and B1) of a small warming effect (+0.2 to +0.3°C) of anthropogenic land cover change on mean global climate, this being dominated by the release of CO <sub>2</sub> in the atmosphere from land conversions (Sitch et al. 2005 <sup>[[#fn:r1059|1059]]</sup> ). This additional warming remains quite small when compared to the one resulting from the combined anthropogenic influences (+1.7°C for SRES B1 and +2.7°C for SRES A2). A global biophysical cooling of –0.14°C is estimated in response to the extreme land cover change projected in SRES A2, a value that far exceeds the impacts of historical land use changes (–0.05°C) calculated using the same climate model (Davin et al. 2007 <sup>[[#fn:r1060|1060]]</sup> ). The authors derived a biophysical climatic sensitivity to land use change of about –0.3°C W.m <sup>–2</sup> for their model, whereas a warming of about 1°C W.m <sup>–2</sup> is obtained in response to changes in atmospheric CO <sub>2</sub> concentration.

Those studies generally do not report on changes in atmospheric variables other than surface air temperature, thereby limiting our ability to assess the effects of anthropogenic land cover changes on regional climate (Sitch et al. 2005 <sup>[[#fn:r1061|1061]]</sup> ). However, small reductions reported in rainfall via changes in biophysical properties of the land, following the massive tropical deforestation in SRES A2 (+0.5 and +0.25 mm day <sup>–1</sup> respectively in the Amazon and Central Africa). They also report opposite changes – that is, increased rainfall of about 0.25 mm day <sup>–1</sup> across the entire tropics and subtropics, triggered by biogeochemical effects of this same deforestation.

'''''At the regional level'''''

In regions that will undergo land cover changes, dampening of the future anthropogenic warming can be as large as –26% while enhancement is always smaller than 9% within RCP8.5 by the end of the 21st century (Boysen et al. 2014 <sup>[[#fn:r1062|1062]]</sup> ). Voldoire (2006) <sup>[[#fn:r1063|1063]]</sup> shows that, by 2050, and following the SRES B2 scenario, the contribution of land cover changes to the total temperature change can be as large as 15% in many boreal regions, and as large as 40% in south-western tropical Africa. Feddema et al. (2005) <sup>[[#fn:r1064|1064]]</sup> simulate large decreases in the diurnal temperature range in the future (2050 and 2100 in SRES B1 and A2) following tropical deforestation in both scenarios. In the Amazon, for example, the diurnal temperature range is lowered by 2.5°C due to increases in minimum temperature, while little change is obtained for the maximum value.

There is thus ''medium evidence'' that future anthropogenic land cover change will have a significant effect on regional temperature via biophysical effects in many regions of the world. There is, however, ''no agreement'' on whether warming will be dampened or enhanced, and there is ''no agreement'' on the sign of the contribution across regions.

There are very few studies that go beyond analysing the changes in mean surface air temperature. Some studies attempted to look at global changes in rainfall and found no significant influence of future land cover changes (Brovkin et al. 2013 <sup>[[#fn:r1065|1065]]</sup> ; Sitch et al. 2005 <sup>[[#fn:r1066|1066]]</sup> ; Feddema et al. 2005 <sup>[[#fn:r1067|1067]]</sup> ). Quesada et al. (2017a <sup>[[#fn:r1068|1068]]</sup> , b <sup>[[#fn:r1069|1069]]</sup> ) however carried out a systematic multi-model analysis of the response of a number of atmospheric, radiative and hydrological variables (e.g., rainfall, sea level pressure, geopotential height, wind speed, soil-moisture, turbulent heat fluxes, shortwave and longwave radiation, cloudiness) to RCP8.5 land cover scenario. In particular, they found a significant reduction of rainfall in six out of eight monsoon regions studied (Figure 2.16) of about 1.9–3% (which means more than –0.5 mm day <sup>–1</sup> in some areas) in response to future anthropogenic land cover changes. Including those changes in global climate models reduces the projected increase in rainfall by about 9–41% in those same regions, when all anthropogenic forcings are accounted for (30% in the global monsoon region as defined by Wang and Ding (2008 <sup>[[#fn:r1070|1070]]</sup> )). In addition, they found a shortening of the monsoon season of one to four days. They conclude that the projected future increase in monsoon rains may be overestimated by those models that do not yet include biophysical effects of land cover changes. Overall, the regional hydrological cycle was found to be substantially reduced and wind speed significantly strengthened in response to regional deforestation within the tropics, with magnitude comparable to projected changes with all forcings (Quesada et al. 2017b <sup>[[#fn:r1071|1071]]</sup> ).

'''''Effects on extremes'''''

Results from a set of climate models have shown that the impact of future anthropogenic land cover change on extreme temperatures can be of similar magnitude as the changes arising from half a degree global mean annual surface temperature change (Hirsch et al. 2018 <sup>[[#fn:r1072|1072]]</sup> ). However, this study also found a lack of agreement between models with respect to the magnitude and sign of changes, thus making land cover change a factor of uncertainty in future climate projections.

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'''Table 2.5'''

<span id="change-in-mean-global-annual-surface-air-temperature-resulting-from-anthropogenic-land-cover-changes-projected-for-the-future-according-to-three-different-scenarios-rcp8.5-rcp4.5-and-rcp2.6."></span>
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 '''Change in mean global annual surface air temperature resulting from anthropogenic land cover changes projected for the future, according to three different scenarios: RCP8.5, RCP4.5 and RCP2.6.'''

Temperature changes resulting from biophysical and biogeochemical effects of land cover change are examined.
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[[File:06f33ed15db05e99f31259a8e97d2ea1 table-2.5.png]]

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'''Figure 2.16'''

<span id="changes-in-monsoon-rainfall-in-rcp8.5-scenario-resulting-from-projected-changes-in-anthropogenic-land-cover-in-eight-monsoon-regions-blue-bars.-differences-are-calculated-between-the-end-of-the-21st-century-20712100-and-the-end-of-the-20th-century-19762005-and-the-percent-change-is-calculated-with-reference-to-19762005.-grey-bars-refer-to"></span>
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'''Changes in monsoon rainfall in RCP8.5 scenario resulting from projected changes in anthropogenic land cover, in eight monsoon regions (%, blue bars). Differences are calculated between the end of the 21st century (2071–2100) and the end of the 20th century (1976–2005), and the percent change is calculated with reference to 1976–2005. Grey bars refer to […]'''
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[[File:b7f35600beedd6a93242d3be3b32c1ff Figure-2.16-1024x659.jpg]]

Changes in monsoon rainfall in RCP8.5 scenario resulting from projected changes in anthropogenic land cover, in eight monsoon regions (%, blue bars). Differences are calculated between the end of the 21st century (2071–2100) and the end of the 20th century (1976–2005), and the percent change is calculated with reference to 1976–2005. Grey bars refer to the relative contribution of land-cover changes (in %) to future rainfall projections: it is the ratio between the change in rainfall responding to land cover changes and the one responding to all anthropogenic changes (Quesada et al. 2017b <sup>[[#fn:r1073|1073]]</sup> ). Negative values mean that changes in land cover have an opposite effect (dampening) on rainfall compared to the effects of all anthropogenic changes. Monsoon regions have been defined following Yim et al. (2014) <sup>[[#fn:r1074|1074]]</sup> . The changes have been simulated by five climate models (Brovkin et al. 2013) <sup>[[#fn:r1075|1075]]</sup> . Results are shown for December-January-February for southern hemisphere regions, and for June-July-August for northern hemisphere regions. Statistical significance is given by green tick marks and circles: one, two and three blue tick marks are displayed for the regions where at least 80% of the climate models have regional changes significant at the 66th, 75th and 80th confidence level, respectively; green circles are added when the regional values are also significant at 90th confidence level. Note that future land cover change impacts on South American monsoon are neither significant nor robust among models, along with very small future projected changes in South American monsoon rainfall.

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