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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'''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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'''Box TS.6 | Water Cycle'''

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'''Human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century ( ''high confidence'' ), and it is projected to cause substantial further changes at both global and regional scales ( ''high confidence'' ).''' '''Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s ( ''medium confidence'' ). Atmospheric water vapour has increased throughout the troposphere since at least the 1980s ( ''likely'' ) . Annual global land precipitation will increase over the 21st century as global surface temperature increases ( ''high confidence'' ). Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P–E) patterns over the ocean ( ''high confidence'' ).''' '''The severity of very wet and very dry events increase in a warming climate ( ''high confidence'' ), but changes in atmospheric circulation patterns affect where and how often these extremes occur. Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emissions scenarios ( ''high confidence'' ).''' '''Over the 21st century, the total land area subject to drought will increase and droughts will become more frequent and severe ( ''high confidence'' ). Near-term projected changes in precipitation are uncertain mainly because of internal variability, model uncertainty and uncertainty in forcings from natural and anthropogenic aerosols ( ''medium confidence'' ).''' '''Over the 21st century and beyond, abrupt human-caused changes to the water cycle cannot be excluded ( ''medium confidence'' ). Links to chapters 2.3, 3.3, 4.3, 4.4, 4.5, 4.6, 8.2, 8.3, 8.4, 8.5, 8.6, 11.4, 11.6, 11.9'''
There is ''high confidence'' that the global water cycle has intensified since at least 1980 expressed by, for example, increased atmospheric moisture fluxes and amplified precipitation minus evaporation patterns. Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s ( ''medium confidence'' ), and a ''likely'' human contribution to patterns of change, particularly for increases in high-latitude precipitation over the Northern Hemisphere. Increases in global mean precipitation are determined by a robust response to global surface temperature ( ''very likely'' 2–3% per °C) that is partly offset by fast atmospheric adjustments to atmospheric heating by greenhouse gases (GHGs) and aerosols (Section TS.3.2.2). The overall effect of anthropogenic aerosols is to reduce global precipitation through surface radiative cooling effects ( ''high confidence'' ). Over much of the 20th century, opposing effects of GHGs and aerosols on precipitation have been observed for some regional monsoons ( ''high confidence'' ) (Box TS.13). Global annual precipitation over land is projected to increase on average by 2.4% (–0.2% to +4.7% ''likely'' range) under SSP1-1.9, 4.6% (1.5% to 8.3% ''likely'' range) under SSP2-4.5, and 8.3% (0.9% to 12.9% ''likely'' range) under SSP5-8.5 by 2081–2100 relative to 1995–2014 (Box TS.6, Figure 1). Inter-model differences and internal variability contribute to a substantial range in projections of large-scale and regional water cycle changes ( ''high confidence'' ). The occurrence of volcanic eruptions can alter the water cycle for several years ( ''high confidence'' ). Projected patterns of precipitation change exhibit substantial regional differences and seasonal contrast as global surface temperature increases over the 21st century (Box TS.6, Figure 1). Links to chapters 2.3.1, 3.3.2, 3.3.3, 3.5.2, 4.3.1, 4.4.1, 4.5.1, 4.6.1, Cross-Chapter Box 4.1, 8.2.1, 8.2.2, 8.2.3, Box 8.1, 8.3.2.4, 8.4.1, 8.5.2, 10.4.2

Global total column water vapour content has ''very likely'' increased since the 1980s, and it is ''likely'' that human influence has contributed to tropical upper tropospheric moistening. Near-surface specific humidity has increased over the ocean ( ''likely'' ) and land ( ''very likely'' ) since at least the 1970s, with a detectable human influence ( ''medium confidence'' ). Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P–E) patterns over the ocean ( ''high confidence'' ). It is ''virtually certain'' that evaporation will increase over the ocean and ''very likely'' that evapotranspiration will increase over land, with regional variations under future surface warming (Box TS.6, Figure 1). There is ''high confidence'' that projected increases in precipitation amount and intensity will be associated with increased runoff in northern high latitudes (Box TS.6, Figure 1). In response to cryosphere changes (Section TS.2.5), there have been changes in streamflow seasonality, including an earlier occurrence of peak streamflow in high-latitude and mountain catchments ( ''high confidence'' ). Projected runoff (Box TS.6, Figure 1c) is typically decreased by contributions from small glaciers because of glacier mass loss, while runoff from larger glaciers will generally increase with increasing global warming levels until their mass becomes depleted ( ''high confidence'' ). Links to chapters 2.3.1, 3.3.2, 3.3.3, 3.5.2, 8.2.3, 8.4.1, 11.5

Warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events ( ''high confidence'' ). Greater warming over land than over the ocean alters atmospheric circulation patterns and reduces continental near-surface relative humidity, which contributes to regional drying ( ''high confidence'' ). A ''very likely'' decrease in relative humidity has occurred over much of the global land area since 2000. Projected increases in evapotranspiration due to growing atmospheric water demand will decrease soil moisture over the Mediterranean region, south-western North America, South Africa, South-Western South America and south-western Australia ( ''high confidence'' ) (Box TS.6, Figure 1). Some tropical regions are also projected to experience enhanced aridity, including the Amazon basin and Central America ( ''high confidence'' ). The total land area subject to increasing drought frequency and severity will expand ( ''high confidence'' ), and in the Mediterranean, South-Western South America, and Western North America, future aridification will far exceed the magnitude of change seen in the last millennium ( ''high confidence'' ). Links to chapters 4.5.1, 8.2.2, 8.2.3, 8.4.1, Box 8.2, 11.6, 11.9

Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle ( ''high confidence'' ). Large-scale deforestation ''likely'' decreases evapotranspiration and precipitation and increases runoff over the deforested regions relative to the regional effects of climate change ( ''medium confidence'' ). Urbanization increases local precipitation ( ''medium confidence'' ) and runoff intensity ( ''high confidence'' ) (Box TS.14). Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions ( ''medium confidence'' ). There is ''high confidence'' that groundwater depletion has occurred since at least the start of the 21st century, as a consequence of groundwater withdrawals for irrigation in agricultural areas in drylands. Links to chapters 8.2.3, 8.3.1, 11.1.6, 11.4, 11.6, FAQ 8.1

Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emissions scenarios ( ''high confidence'' ). A warmer climate increases moisture transport into weather systems, which intensifies wet seasons and events ( ''high confidence'' ). The magnitudes of projected precipitation increases and related extreme events depend on model resolution and the representation of convective processes ( ''high confidence'' ). Increases in near-surface atmospheric moisture capacity of about 7% per 1ºC of warming lead to a similar response in the intensification of heavy precipitation from sub-daily up to seasonal time scales, increasing the severity of flood hazards ( ''high confidence'' ). The average and maximum rain-rates associated with tropical and extratropical cyclones, atmospheric rivers and severe convective storms will therefore also increase with future warming ( ''high confidence'' ). For some regions, there is ''medium confidence'' that peak tropical cyclone rain-rates will increase by more than 7% per 1°C of warming due to increased low-level moisture convergence caused by increases in wind intensity. In the tropics year-round and in the summer season elsewhere, interannual variability of precipitation and runoff over land is projected to increase at a faster rate than changes in seasonal mean precipitation (Figure TS.12e,f) ( ''medium confidence'' ). Sub-seasonal precipitation variability is also projected to increase, with fewer rainy days but increased daily mean precipitation intensity over many land regions ( ''high confidence'' ). Links to chapters 4.5.3, 8.2.3, 8.4.1, 8.4.2, 8.5.1, 8.5.2, 11.4, 11.5, 11.7, 11.9

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'''Box TS.6, Figure 1 |'''

'''Projected water cycle changes.''' ''The intent of this figure is to give a geographical overview of changes in multiple components of the global water cycle using an intermediate emissions scenario. Important key message: without drastic reductions in greenhouse gas emissions, human-induced global warming will be associated with widespread changes in all components of the water cycle.'' Long-term (2081–2100) projected annual mean changes (%) relative to present-day (1995–2014) in the SSP2-4.5 emissions scenario for '''(a)''' precipitation, '''(b)''' surface evapotranspiration, '''(c)''' total runoff and '''(d)''' surface soil moisture. Numbers in top right of each panel indicate indicate the number of Coupled Model Intercomparison Project Phase 6 (CMIP6) models used for estimating the ensemble mean. For other scenarios, please refer to relevant figures in Chapter 8. Uncertainty is represented using the simple approach: No overlay indicates regions with high model agreement, where ≥80% of models agree on sign of change; diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Links to chapters 8.4.1; Figures 8.14, 8.17, 8.18, and 8.19

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'''Infographic TS.1 |''' '''Climate Futures'''

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'''Infographic TS.1 |''' '''Climate Futures.''' ''The intent of this figure is to show possible climate futures: The climate change that people will experience this century and beyond depends on our greenhouse gas emissions, how much global warming this will cause and the response of the climate system to this warming.''

'''(top left)''' Annual emissions of CO <sub>2</sub> for the five core Shared Socio-economic Pathway (SSP) scenarios (very low: SSP1-1.9, low: SSP1-2.6, intermediate: SSP2-4.5, high: SSP3-7.0, very high: SSP5-8.5). '''(bottom left)''' Projected warming for each of these emissions scenarios. '''(top right)''' Response of some selected climate variables to four levels of global warming (°C). Changes in the ‘Today’ column are based on a global warming level of 1°C. '''(bottom right)''' The long-term effect of each global warming level on sea level. See Section TS.1.3.1 for more detail on the SSP climate change scenarios. This infographic builds from several figures in the Technical Summary: Figure TS.4 (for top left panel), Figure TS.6 (bottom left), Figure TS.12 (top right) and Box TS.4, Figure 1b (bottom right).

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