Editing IPCC:AR6/WGI/TS (section)

=== TS.2.6 Land Climate, Including Biosphere and Extremes ===

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'''Land surface air temperatures have risen faster than the global surface temperature since the 1850s, and it is ''virtually certain'' that this differential warming will persist into the future. It is ''virtually certain'' that the frequency and intensity of hot extremes and the intensity and duration of heatwaves have increased since 1950 and will further increase in the future even if global warming is stabilized at 1.5°C. The frequency and intensity of heavy precipitation events have increased over a majority of those land regions with good observational coverage (''high confidence'') and will ''extremely likely'' increase over most land regions with additional global warming.''' '''Over the past half century, key aspects of the biosphere have changed in ways that are consistent with large-scale warming: climate zones have shifted poleward, and the growing season length in the Northern Hemisphere extratropics has increased (''high confidence''). The amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> poleward of 45°N has increased since the 1960s (''very high confidence''), with increasing productivity of the land biosphere due to the increasing atmospheric CO <sub>2</sub> concentration as the main driver (''medium confidence''). Global-scale vegetation greenness has increased since the 1980s (''high confidence''). Links to chapters 2.3, 3.6, 4.3, 4.5, 5.2, 11.3, 11.4, 11.9, 12.4'''

Observed temperatures over land have increased by 1.59 [1.34–1.83] °C between the period 1850–1900 and 2011–2020. Warming of the land is about 45% larger than for global surface temperature and about 80% larger than warming of the ocean surface. Warming of the land surface during the period 1971–2018 contributed about 5% of the increase in the global energy inventory (Section TS.3.1), nearly twice the estimate in AR5 (''high confidence''). It is ''virtually certain'' that the average surface warming over land will continue to be higher than over the ocean throughout the 21st century. The warming pattern will ''likely'' vary seasonally, with northern high latitudes warming more during winter than summer (''medium confidence''). Links to chapters 2.3.1, 4.3.1, 4.5.1, 7.2.2, Box 7.2, Cross-Chapter Box 9.1, 11.3, Atlas 11.2

The frequency and intensity of hot extremes (warm days and nights) and the intensity and duration of heatwaves have increased globally and in most regions since 1950, while the frequency and intensity of cold extremes have decreased (''virtually certain''). There is ''high confidence'' that the increases in frequency and severity of hot extremes are due to human-induced climate change. Some recent extreme events would have been ''extremely unlikely'' to occur without human influence on the climate system. It is ''virtually certain'' that further changes in hot and cold extremes will occur throughout the 21st century in nearly all inhabited regions, even if global warming is stabilized at 1.5°C (Table TS.2, Figure TS.12a). Links to chapters 1.3, Cross-Chapter Box 3.2, 11.1.4, 11.3.2, 11.3.4, 11.3.5, 11.9, 12.4

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[[File:9dd167a3c0d644133e472f8dd404196f IPCC_AR6_WGI_TS_Figure_12.png]]

'''Figure TS.12 |''' '''Land-related changes relative to the 1850-1900 as a function of global warming levels.''' ''The intent of this figure is to show that extremes and mean land variables change consistently with warming levels and to show the changes with global warming levels of water cycle indicators (i.e., precipitation and runoff) over tropical and extratropical land in terms of mean and interannual variability (interannual variability increases at a faster rate than the mean)'' ''.'' (a) Changes in the frequency (left scale) and intensity (in °C, right scale) of daily hot extremes occurring every 10 and 50 years. (b) as (a), but for daily heavy precipitation extremes, with intensity change in %. (c) Changes in 10-year droughts aggregated over drought-prone regions (WNA, CNA, NCA, SCA, NSA, NES, SAM, SWS, SSA, WCE, MED, WSAF, ESAF, MDG, SAU, and EAU; for definitions of these regions, see Figure Atlas.2), with drought intensity (right scale) represented by the change of annual mean soil moisture, normalized with respect to interannual variability. Limits of the 5% − 95% confidence interval are shown in panels (a–c). (d) Changes in Northern Hemisphere spring (March–April–May) snow cover extent relative to 1850–1900; (e,f) Relative change (%) in annual mean of total precipitable water (grey line), precipitation (red solid lines), runoff (blue solid lines) and in standard deviation (i.e., variability) of precipitation (red dashed lines) and runoff (blue dashed lines) averaged over (e) tropical and (f) extratropical land as function of global warming levels. Coupled Model Intercomparison Project Phase 6 (CMIP6) models that reached a 5°C warming level above the 1850–1900 average in the 21st century in SSP5-8.5 have been used. Precipitation and runoff variability are estimated by respective standard deviation after removing linear trends. Error bars show the 17–83% confidence interval for the warmest +5°C global warming level. Links to chapters Figures 8.16, 9.24, 11.6, 11.7, 11.12, 11.15, 11.18 and Atlas.2

Greater warming over land alters key water cycle characteristics (Box TS.6). The rates of change in mean precipitation and runoff, and their variability, increase with global warming (Figure TS.12e,f). Human-induced climate change has contributed to increases in agricultural and ecological droughts in some regions due to increases in evapotranspiration (''medium confidence''). More regions are affected by increases in agricultural and ecological droughts with increasing global warming (''high confidence'' ; see also Figure TS.12c).

There is ''low confidence'' that the increase of plant water-use efficiency due to higher atmospheric CO <sub>2</sub> concentration alleviates extreme agricultural and ecological droughts in conditions characterized by limited soil moisture and increased atmospheric evaporative demand. Links to chapters 2.3.1, Cross-Chapter Box 5.1, 8.2.3, 8.4.1, 11.2.4, 11.4, 11.6, Box 11.1

Northern Hemisphere spring snow cover has decreased since at least 1978 (''very high confidence''), and there is ''high confidence'' that trends in snow cover loss extend back to 1950. It is ''very likely'' that human influence contributed to these reductions. Earlier onset of snowmelt has contributed to seasonally dependent changes in streamflow (''high confidence''). A further decrease of Northern Hemisphere seasonal snow cover extent is ''virtually certain'' under further global warming (Figure TS.12d). Links to chapters 2.3.2, 3.4.2, 8.3.2. 9.5.3, 12.4, 9.2, 11.2, [[IPCC:Wg1:Chapter:Atlas|Atlas]] 8.2

The frequency and intensity of heavy precipitation events have increased over a majority of land regions with good observational coverage since 1950 (''high confidence,'' Box TS.6, Table TS.2). Human influence is ''likely'' the main driver of this change (Table TS.2). It is ''extremely likely'' that on most land regions heavy precipitation will become more frequent and more intense with additional global warming (Table TS.2, Figure TS.12b). The projected increase in heavy precipitation extremes translates to an increase in the frequency and magnitude of pluvial floods (''high confidence'') (Table TS.2). Links to chapters Cross-Chapter Box 3.2, 8.4.1, 11.4.2, 11.4.4, 11.5.5, 12.4

Theprobability of compound extreme events has ''likely'' increased due to human-induced climate change. Concurrent heatwaves and droughts have become more frequent over the last century, and this trend will continue with higher global warming (''high confidence''). The probability of compound flooding (storm surge, extreme rainfall and/or river flow) has increased in some locations and will continue to increase due to both sea level rise and increases in heavy precipitation, including changes in precipitation intensity associated with tropical cyclones (''high confidence''). Links to chapters 11.8.1, 11.8.2, 11.8.3

Changes in key aspects of the terrestrial biosphere, such as an increase of the growing season length in much of the Northern Hemisphere extratropics since the mid-20th century (''high confidence''), are consistent with large-scale warming. At the same time an increase in the amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> poleward of 45°N since the early 1960s (''high confidence'') and a global-scale increase in vegetation greenness of the terrestrial surface since the early 1980s (''high confidence'') have been observed. Increasing atmospheric CO <sub>2</sub> , warming at high latitudes, and land management interventions have contributed to the observed greening trend, but there is ''low confidence'' in their relative roles. There is ''medium confidence'' that increased plant growth associated with CO <sub>2</sub> fertilization is the main driver of the observed increase in amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> in the Northern Hemisphere. Reactive nitrogen, ozone and aerosols affect terrestrial vegetation and carbon cycle through deposition and effects on large-scale radiation (''high confidence''), but the magnitude of these effects on the land carbon sink, ecosystem productivity and indirect CO <sub>2</sub> forcing remains uncertain. Links to chapters 2.3.4, 3.6.1, 5.2.1, 6.4.5, 12.3.7, 12.4

Over the last century, there has been a poleward and upslope shift in the distribution of many land species (''very high confidence'') as well as increases in species turnover within many ecosystems (''high confidence''). There is ''high confidence'' that the geographical distribution of climate zones has shifted in many parts of the world in the last half century. The SRCCL concluded that continued warming will exacerbate desertification processes (''medium confidence'') and that ecosystems will become increasingly exposed to climates beyond those that they are currently adapted to (''high confidence''). There is ''medium confidence'' that climate change will increase disturbance by, for example, fire and tree mortality, across several ecosystems. Increases are projected in drought, aridity and fire weather in some regions (Section TS.4.3; ''high confidence''). There is ''low confidence'' in the magnitude of these changes, but the probability of crossing uncertain regional thresholds (e.g., fires, forest dieback) increases with further warming (''high confidence''). The response of biogeochemical cycles to the anthropogenic perturbation can be abrupt at regional scales, and irreversible on decadal to century time scales (''high confidence''). Links to chapters 2.3.4, 5.4.3, 5.4.9, 11.6, 11.8, 12.5, SRCCL 2.2, SRCCL 2.5, SR1.5 3.4

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