Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.3.4 Changes in ecosystem function, biomass and carbon stocks ====

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Working Group II of AR5 (Settele et al., 2014) <sup>[[#fn:r490|490]]</sup> concluded that there is ''high confidence'' that net terrestrial ecosystem productivity at the global scale has increased relative to the pre-industrial era and that rising CO <sub>2</sub> concentrations are contributing to this trend through stimulation of photosynthesis. There is, however, no clear and consistent signal of a climate change contribution. In northern latitudes, the change in productivity has a lower velocity than the warming, possibly because of a lack of resource and vegetation acclimation mechanisms (M. Huang et al., 2017) <sup>[[#fn:r491|491]]</sup> . Biomass and soil carbon stocks in terrestrial ecosystems are currently increasing ( ''high confidence'' ), but they are vulnerable to loss of carbon to the atmosphere as a result of projected increases in the intensity of storms, wildfires, land degradation and pest outbreaks (Settele et al., 2014; Seidl et al., 2017) <sup>[[#fn:r492|492]]</sup> . These losses are expected to contribute to a decrease in the terrestrial carbon sink. Anderegg et al. (2015) <sup>[[#fn:r493|493]]</sup> demonstrated that total ecosystem respiration at the global scale has increased in response to increases in night-time temperature (1 PgC yr <sup>–1</sup> °C <sup>–1</sup> , ''P'' =0.02).

The increase in total ecosystem respiration in spring and autumn, associated with higher temperatures, may convert boreal forests from carbon sinks to carbon sources (Hadden and Grelle, 2016) <sup>[[#fn:r494|494]]</sup> . In boreal peatlands, for example, increased temperature may diminish carbon storage and compromise the stability of the peat (Dieleman et al., 2016) <sup>[[#fn:r495|495]]</sup> . In addition, J. Yang et al. (2015) <sup>[[#fn:r496|496]]</sup> showed that fires reduce the carbon sink of global terrestrial ecosystems by 0.57 PgC yr <sup>–1</sup> in ecosystems with large carbon stores, such as peatlands and tropical forests. Consequently, for adaptation purposes, it is necessary to enhance carbon sinks, especially in forests which are prime regulators within the water, energy and carbon cycles (Ellison et al., 2017) <sup>[[#fn:r497|497]]</sup> . Soil can also be a key compartment for substantial carbon sequestration (Lal, 2014; Minasny et al., 2017) <sup>[[#fn:r498|498]]</sup> , depending on the net biome productivity and the soil quality (Bispo et al., 2017) <sup>[[#fn:r499|499]]</sup> .

AR5 assessed that large uncertainty remains regarding the land carbon cycle behaviour of the future (Ciais et al., 2013) <sup>[[#fn:r500|500]]</sup> , with most, but not all, CMIP5 models simulating continued terrestrial carbon uptake under all four RCP scenarios (Jones et al., 2013) <sup>[[#fn:r501|501]]</sup> . Disagreement between models outweighs differences between scenarios even up to the year 2100 (Hewitt et al., 2016; Lovenduski and Bonan, 2017) <sup>[[#fn:r502|502]]</sup> . Increased atmospheric CO <sub>2</sub> concentrations are expected to drive further increases in the land carbon sink (Ciais et al., 2013; Schimel et al., 2015) <sup>[[#fn:r503|503]]</sup> , which could persist for centuries (Pugh et al., 2016) <sup>[[#fn:r504|504]]</sup> . Nitrogen, phosphorus and other nutrients will limit the terrestrial carbon cycle response to both elevated CO <sub>2</sub> and altered climate (Goll et al., 2012; Yang et al., 2014; Wieder et al., 2015; Zaehle et al., 2015; Ellsworth et al., 2017) <sup>[[#fn:r505|505]]</sup> . Climate change may accelerate plant uptake of carbon (Gang et al., 2015) <sup>[[#fn:r506|506]]</sup> but also increase the rate of decomposition (Todd-Brown et al., 2014; Koven et al., 2015; Crowther et al., 2016) <sup>[[#fn:r507|507]]</sup> . Ahlström et al. (2012) <sup>[[#fn:r508|508]]</sup> found a net loss of carbon in extra-tropical regions and the largest spread across model results in the tropics. The projected net effect of climate change is to reduce the carbon sink expected under CO <sub>2</sub> increase alone (Settele et al., 2014) <sup>[[#fn:r509|509]]</sup> . Friend et al. (2014) <sup>[[#fn:r510|510]]</sup> found substantial uptake of carbon by vegetation under future scenarios when considering the effects of both climate change and elevated CO <sub>2</sub> .

There is limited published literature examining modelled land carbon changes specifically under 1.5°C of warming, but existing CMIP5 models and published data are used in this report to draw some conclusions. For systems with significant inertia, such as vegetation or soil carbon stores, changes in carbon storage will depend on the rate of change of forcing and thus depend on the choice of scenario (Jones et al., 2009; Ciais et al., 2013; Sihi et al., 2017) <sup>[[#fn:r511|511]]</sup> . To avoid legacy effects of the choice of scenario, this report focuses on the response of gross primary productivity (GPP) – the rate of photosynthetic carbon uptake – by the models, rather than by changes in their carbon store.

Figure 3.17 shows different responses of the terrestrial carbon cycle to climate change in different regions. The models show a consistent response of increased GPP in temperate latitudes of approximately 2 GtC yr <sup>–1 °</sup> C <sup>–1</sup> . Similarly, Gang et al. (2015) <sup>[[#fn:r512|512]]</sup> projected a robust increase in the net primary productivity (NPP) of temperate forests. However, Ahlström et al. (2012) <sup>[[#fn:r513|513]]</sup> showed that this effect could be offset or reversed by increases in decomposition. Globally, most models project that GPP will increase or remain approximately unchanged (Hashimoto et al., 2013) <sup>[[#fn:r514|514]]</sup> . This projection is supported by findings by Sakalli et al. (2017) <sup>[[#fn:r515|515]]</sup> for Europe using Euro-CORDEX regional models under a 2°C global warming for the period 2034–2063, which indicated that storage will increase by 5% in soil and by 20% in vegetation. However, using the same models Jacob et al. (2018) <sup>[[#fn:r516|516]]</sup> showed that limiting warming to 1.5°C instead of 2°C avoids an increase in ecosystem vulnerability (compared to a no-climate change scenario) of 40–50%.

At the global level, linear scaling is acceptable for net primary production, biomass burning and surface runoff, and impacts on terrestrial carbon storage are projected to be greater at 2°C than at 1.5°C (Tanaka et al., 2017) <sup>[[#fn:r517|517]]</sup> . If global CO <sub>2</sub> concentrations and temperatures stabilize, or peak and decline, then both land and ocean carbon sinks – which are primarily driven by the continued increase in atmospheric CO <sub>2</sub> – will also decline and may even become carbon sources (Jones et al., 2016) <sup>[[#fn:r518|518]]</sup> . Consequently, if a given amount of anthropogenic CO <sub>2</sub> is removed from the atmosphere, an equivalent amount of land and ocean anthropogenic CO <sub>2</sub> will be released to the atmosphere (Cao and Caldeira, 2010) <sup>[[#fn:r519|519]]</sup> .

In conclusion ''',''' ecosystem respiration is expected to increase with increasing temperature, thus reducing soil carbon storage. Soil carbon storage is expected to be larger if global warming is restricted to 1.5°C, although some of the associated changes will be countered by enhanced gross primary production due to elevated CO <sub>2</sub> concentrations (i.e., the ‘fertilization effect’) and higher temperatures, especially at mid-and high latitudes ( ''medium confidence'' ).

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'''Figure 3.17'''

<span id="the-response-of-terrestrial-productivity-gross-primary-productivity-gpp-to-climate-change-globally-top-left-and-for-three-latitudinal-regions-30s30n-3060n-and-6090n."></span>
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'''The response of terrestrial productivity (gross primary productivity, GPP) to climate change, globally (top left) and for three latitudinal regions: 30°S–30°N; 30–60°N and 60–90°N.'''
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[[File:eed1efa6770815d3006b426ffc7e5322 figure-3.17-1024x755.jpg]]

Data come from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive (http://cmip-pcmdi.llnl.gov/cmip5/). Seven Earth System Models were used: Norwegian Earth System Model (NorESM-ME, yellow); Community Earth System Model (CESM, red); Institute Pierre Simon Laplace (IPLS)-CM5-LR (dark blue); Geophysical Fluid Dynamics Laboratory (GFDL, pale blue); Max Plank Institute-Earth System Model (MPI-ESM, pink); Hadley Centre New Global Environmental Model 2-Earth System (HadGEM2-ES, orange); and Canadian Earth System Model 2 (CanESM2, green). Differences in GPP between model simulations with (‘1pctCO <sub>2</sub> ’) and without (‘esmfixclim1’) the effects of climate change are shown. Data are plotted against the global mean temperature increase above pre-industrial levels from simulations with a 1% per year increase in CO <sub>2</sub> (‘1pctCO <sub>2</sub> ’).

Original Creation for this Report using CMIP5

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