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

=== 2.2.4 Climate-driven changes in terrestrial ecosystems ===

<div id="section-2-2-4-climate-driven-changes-in-terrestrial-ecosystems-block-1"></div>

Previously, the IPCC AR5 reported ''high confidence'' that the Earth’s biota composition and ecosystem processes have been strongly affected by past changes in global climate and that the magnitudes of projected changes for the 21st century under high warming scenarios (for example, RCP8.5) are higher than those under historic climate change (Settele et al. 2014 <sup>[[#fn:r95|95]]</sup> ). There is ''high confidence'' that as a

result of climate changes over recent decades many plant and animal species have experienced range size and location changes, altered abundances and shifts in seasonal activities (Urban 2015 <sup>[[#fn:r96|96]]</sup> ; Ernakovich et al. 2014 <sup>[[#fn:r97|97]]</sup> ; Elsen and Tingley 2015 <sup>[[#fn:r98|98]]</sup> ; Hatfield and Prueger 2015 <sup>[[#fn:r99|99]]</sup> ; Savage and Vellend 2015 <sup>[[#fn:r100|100]]</sup> ; Yin et al. 2016 <sup>[[#fn:r101|101]]</sup> ; Pecl et al. 2017 <sup>[[#fn:r102|102]]</sup> ; Gonsamo et al. 2017 <sup>[[#fn:r103|103]]</sup> ; Fadrique et al. 2018 <sup>[[#fn:r104|104]]</sup> ; Laurance et al. 2018 <sup>[[#fn:r105|105]]</sup> ). There is ''high confidence'' that climate zones have already shifted in many parts of the world, primarily as an increase of dry, arid climates accompanied by a decrease of polar climates (Chan and Wu 2015 <sup>[[#fn:r106|106]]</sup> ; Chen and Chen 2013 <sup>[[#fn:r107|107]]</sup> ; Spinoni et al. 2015b <sup>[[#fn:r108|108]]</sup> ). Regional climate zones shifts have been observed over the Asian monsoon region (Son and Bae 2015 <sup>[[#fn:r109|109]]</sup> ), Europe (Jylhä et al. 2010 <sup>[[#fn:r110|110]]</sup> ), China (Yin et al. 2019 <sup>[[#fn:r111|111]]</sup> ), Pakistan (Adnan et al. 2017 <sup>[[#fn:r112|112]]</sup> ), the Alps (Rubel et al. 2017 <sup>[[#fn:r113|113]]</sup> ) and north-eastern Brazil, southern Argentina, the Sahel, Zambia and Zimbabwe, the Mediterranean area, Alaska, Canada and north-eastern Russia (Spinoni et al. 2015b <sup>[[#fn:r114|114]]</sup> ).

There is ''high confidence'' that bioclimates zones will further shift as the climate warms (Williams et al. 2007 <sup>[[#fn:r115|115]]</sup> ; Rubel and Kottek 2010 <sup>[[#fn:r116|116]]</sup> ; Garcia et al. 2016 <sup>[[#fn:r117|117]]</sup> ; Mahony et al. 2017 <sup>[[#fn:r118|118]]</sup> ; Law et al. 2018 <sup>[[#fn:r120|120]]</sup> ). There is also ''high confidence'' that novel, unprecedented climates (climate conditions with no analogue in the observational record) will emerge, particularly in the tropics (Williams and Jackson 2007 <sup>[[#fn:r121|121]]</sup> ; Colwell et al. 2008a <sup>[[#fn:r122|122]]</sup> ; Mora et al. 2013 <sup>[[#fn:r123|123]]</sup> , 2014 <sup>[[#fn:r124|124]]</sup> ; Hawkins et al. 2014 <sup>[[#fn:r126|126]]</sup> ; Mahony et al. 2017 <sup>[[#fn:r127|127]]</sup> ; Maule et al. 2017 <sup>[[#fn:r128|128]]</sup> ). It is ''very likely'' that terrestrial ecosystems and land processes will be exposed to disturbances beyond the range of current natural variability as a result of global warming, even under low- to medium-range warming scenarios, and that these disturbances will alter the structure, composition and functioning of the system (Settele et al. 2014 <sup>[[#fn:r129|129]]</sup> ; Gauthier et al. 2015; Seddon et al. 2016 <sup>[[#fn:r130|130]]</sup> ).

In a warming climate, many species will be unable to track their climate niche as it moves, especially those in extensive flat landscapes with low dispersal capacity and in the tropics whose thermal optimum is already near current temperature (Diffenbaugh and Field 2013 <sup>[[#fn:r131|131]]</sup> ; Warszawski et al. 2013 <sup>[[#fn:r132|132]]</sup> ). Range expansion in higher latitudes and elevations as a result of warming often, but not exclusively, occurs in abandoned lands (Harsch et al. 2009 <sup>[[#fn:r133|133]]</sup> ; Landhäusser et al. 2010 <sup>[[#fn:r134|134]]</sup> ; Gottfried et al. 2012 <sup>[[#fn:r135|135]]</sup> ; Boisvert-Marsh et al. 2014 <sup>[[#fn:r136|136]]</sup> ; Bryn and Potthoff 2018 <sup>[[#fn:r137|137]]</sup> ; Rumpf et al. 2018 <sup>[[#fn:r138|138]]</sup> ; Buitenwerf et al. 2018 <sup>[[#fn:r139|139]]</sup> ; Steinbauer et al. 2018 <sup>[[#fn:r140|140]]</sup> ). This expansion typically favours thermophilic species at the expense of cold adapted species as the climate becomes suitable for lower latitude/altitude species (Rumpf et al. 2018 <sup>[[#fn:r141|141]]</sup> ). In temperate drylands, however, range expansion can be countered by intense and frequent drought conditions which result in accelerated rates of taxonomic change and spatial heterogeneity in an ecotone (Tietjen et al. 2017 <sup>[[#fn:r142|142]]</sup> ).

Since the advent of satellite observation platforms, a global increase in vegetation photosynthetic activity (i.e., greening) as evidenced through remotely sensed indices such as leaf area index (LAI) and normalised difference vegetation index (NDVI). Three satellite-based leaf area index records (GIMMS3g, GLASS and GLOMAP) imply increased growing season LAI (greening) over 25–50% and browning over less than 4% of the global vegetated area, resulting in greening trend of 0.068 ± 0.045 m <sup>2</sup> m <sup>−2</sup> yr <sup>−1</sup> over 1982–2009 (Zhu et al. 2016 <sup>[[#fn:r143|143]]</sup> ). Greening has been observed in southern Amazonia, southern Australia, the Sahel and central Africa, India, eastern China and the northern extratropical latitudes (Myneni et al. 1997 <sup>[[#fn:r144|144]]</sup> ; de Jong et al. 2012 <sup>[[#fn:r145|145]]</sup> ; Los 2013 <sup>[[#fn:r146|146]]</sup> ; Piao et al. 2015 <sup>[[#fn:r147|147]]</sup> ; Mao et al. 2016 <sup>[[#fn:r148|148]]</sup> ; Zhu et al. 2016 <sup>[[#fn:r149|149]]</sup> ; Carlson et al. 2017 <sup>[[#fn:r150|150]]</sup> ; Forzieri et al. 2017 <sup>[[#fn:r151|151]]</sup> ; Pan et al. 2018 <sup>[[#fn:r152|152]]</sup> ; Chen et al. 2019 <sup>[[#fn:r153|153]]</sup> ). Greening has been attributed to direct factors, namely human land use management and indirect factors such as CO <sub>2</sub> fertilisation, climate change, and nitrogen deposition (Donohue et al. 2013 <sup>[[#fn:r154|154]]</sup> ; Keenan et al. 2016 <sup>[[#fn:r155|155]]</sup> ; Zhu et al. 2016 <sup>[[#fn:r156|156]]</sup> ). Indirect factors have been used to explain most greening trends primarily through CO <sub>2</sub> fertilisation in the tropics and through an extended growing season and increased growing season temperatures as a result of climate change in the high latitudes (Fensholt et al. 2012 <sup>[[#fn:r157|157]]</sup> ; Zhu et al. 2016 <sup>[[#fn:r158|158]]</sup> ). The extension of the growing season in high latitudes has occurred together with an earlier spring greenup (the time at which plants begin to produce leaves in northern mid- and high-latitude ecosystems) (Goetz et al. 2015 <sup>[[#fn:r159|159]]</sup> ; Xu et al. 2016a <sup>[[#fn:r160|160]]</sup> , 2018 <sup>[[#fn:r161|161]]</sup> ) with subsequent earlier spring carbon uptake (2.3 days per decade) and gross primary productivity (GPP) (Pulliainen et al. 2017 <sup>[[#fn:r162|162]]</sup> ). The role of direct factors of greening are being increasingly investigated and a recent study has attributed over a third of observed global greening between 2000 and 2017 to direct factors, namely afforestation and croplands, in China and India (Chen et al. 2019 <sup>[[#fn:r163|163]]</sup> ).

It should be noted that measured greening is a product of satellite- derived radiance data and, as such, does not provide information on ecosystem health indicators such as species composition and richness, homeostasis, absence of disease, vigour, system resilience and the different components of ecosystems (Jørgensen et al. 2016 <sup>[[#fn:r164|164]]</sup> ). For example, a regional greening attributable to croplands expansion or intensification might occur at the expense of ecosystem biodiversity.

Within the global greening trend are also detected regional decreases in vegetation photosynthetic activity (i.e., browning) in northern Eurasia, the southwestern USA, boreal forests in North America, inner Asia and the Congo Basin, largely as a result of intensified drought stress. Since the late 1990s rates and extents of browning have exceeded those of greening in some regions, the collective result of which has been a slowdown of the global greening rate (de Jong et al. 2012 <sup>[[#fn:r165|165]]</sup> ; Pan et al. 2018 <sup>[[#fn:r166|166]]</sup> ). Within these long-term trends, inter-annual variability of regional greening and browning is attributable to regional climate variability, responses to extremes such as drought, disease and insect infestation and large- scale tele-connective controls such as ENSO and the Atlantic Multi- decadal Organization (Verbyla 2008 <sup>[[#fn:r167|167]]</sup> ; Revadekar et al. 2012 <sup>[[#fn:r168|168]]</sup> ; Epstein et al. 2018 <sup>[[#fn:r169|169]]</sup> ; Zhao et al. 2018 <sup>[[#fn:r170|170]]</sup> ).

Projected increases in drought conditions in many regions suggest long-term global vegetation greening trends are at risk of reversal to browning in a warmer climate (de Jong et al. 2012 <sup>[[#fn:r171|171]]</sup> ; Pan et al. 2018 <sup>[[#fn:r172|172]]</sup> ; Pausas and Millán 2018 <sup>[[#fn:r173|173]]</sup> ). On the other hand, in higher latitudes vegetation productivity is projected to increase as a result of higher atmospheric CO <sub>2</sub> concentrations and longer growing periods as a result of warming (Ito et al. 2016) (Section 2.3 and Box 2.3). Additionally, climate-driven transitions of ecosystems, particularly range changes, can take years to decades for the equilibrium state to be realised and the rates of these ‘committed ecosystem changes’ (Jones et al. 2009 <sup>[[#fn:r174|174]]</sup> ) vary between low and high latitudes (Jones et al. 2010 <sup>[[#fn:r175|175]]</sup> ). Furthermore, as direct factors are poorly integrated into Earth systems models (ESMs) uncertainties in projected trends of greening and browning are further compounded (Buitenwerf et al. 2018 <sup>[[#fn:r176|176]]</sup> ; Chen et al. 2019 <sup>[[#fn:r177|177]]</sup> ). Therefore, there is ''low confidence'' in the projection of global greening and browning trends.

Increased atmospheric CO <sub>2</sub> concentrations have both direct and indirect effects on terrestrial ecosystems (Sections 2.2.2 and 2.2.3, and Box 2.3). The direct effect is primarily through increased vegetation photosynthetic activity as described above. Indirect effects include decreased evapotranspiration that may offset the projected impact of drought in some water-stressed plants through improved water use efficiency in temperate regions, suggesting that some rain-fed cropping systems and grasslands will benefit from elevated atmospheric CO <sub>2</sub> concentrations (Roy et al. 2016 <sup>[[#fn:r178|178]]</sup> ; Milly and Dunne 2016 <sup>[[#fn:r179|179]]</sup> ; Swann et al. 2016 <sup>[[#fn:r180|180]]</sup> ; Chang et al. 2017 <sup>[[#fn:r181|181]]</sup> ; Zhu et al. 2017 <sup>[[#fn:r182|182]]</sup> ). In tropical regions, increased flowering activity is associated primarily with increasing atmospheric CO <sub>2</sub> , suggesting that a long- term increase in flowering activity may persist in some vegetation, particularly mid-story trees and tropical shrubs, and may enhance reproduction levels until limited by nutrient availability or climate factors such as drought frequency, rising temperatures, and reduced insolation (Pau et al. 2018 <sup>[[#fn:r183|183]]</sup> ).

<span id="climate-extremes-and-their-impact-on-land-functioning"></span>