Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

=== 1.3.6 How do Previous Climate Projections Compare with Subsequent Observations? ===

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Many different sets of climate projections have been produced over the past several decades, so it is valuable to assess how well those projections have compared against subsequent observations. Consistent findings build confidence in the process of making projections for the future. For example, [[#Stouffer--2017|Stouffer and Manabe (2017)]] compared projections made in the early 1990s with subsequent observations. They found that the projected surface pattern of warming, and the vertical structure of temperature change in both the atmosphere and ocean, were realistic. Rahmstorf et al. (2007, 2012) examined projections of global surface temperature and GMSL assessed by TAR and AR4 and found that the global surface temperature projections were in good agreement with the subsequent observations, but that sea level projections were underestimates compared to subsequent observations. The AR5 WGI also examined earlier IPCC assessment reports to evaluate their projections of how global surface temperature and GMSL would change ( [[#Cubasch--2013|Cubasch et al., 2013]] ) with similar conclusions.

Although these studies generally showed good agreement between past projections and subsequent observations, this type of analysis is complicated because the scenarios of future radiative forcing used in earlier projections do not precisely match the actual radiative forcings that subsequently occurred. Mismatches between the projections and subsequent observations could be due to incorrectly projected radiative forcings (e.g., aerosol emissions, GHG concentrations or volcanic eruptions that were not included), an incorrectly modelled response to those forcings, or both. Alternatively, agreement between projections and observations could be fortuitous due to a compensating balance of errors, for example, too low climate sensitivity but too strong radiative forcings.

One approach to partially correct for mismatches between the forcings used in the projections and the forcings that actually occurred is described by [[#Hausfather--2020|Hausfather et al. (2020)]] . Model projections of global surface temperature and estimated radiative forcings were taken from several historical studies, along with the baseline ‘no-policy’ scenarios from the first four IPCC assessment reports. These model projections of temperature and radiative forcing are then compared to (i) the observed change in temperature through time over the projection period, and (ii) the observed change in temperature relative to the observationally estimated radiative forcing over the projection period (Figure 1.9; data from [[#Hausfather--2020|Hausfather et al., 2020]] ).

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'''Figure 1.9 |''' '''Assessing past projections of global temperature change.''' '''(Top)''' Projected temperature change post-publication on a temperature vs time (1970–2020) and '''(bottom)''' temperature vs radiative forcing (1970–2017) basis for a selection of prominent climate model projections (taken from [[#Hausfather--2020|Hausfather et al., 2020]] ). Model projections (using global surface air temperature, GSAT) are compared to temperature observations (using global mean surface temperature, GMST) from HadCRUT5 (black) and anthropogenic forcings (through 2017) from [[#Dessler--2018|Dessler and Forster (2018)]] , and have a baseline generated from the first five years of the projection period. Projections shown are: [[#Manabe--1970|Manabe (1970)]] , [[#Rasool--1971|Rasool and Schneider (1971)]] , [[#Broecker--1975|Broecker (1975)]] , [[#Nordhaus--1977|Nordhaus (1977)]] , Hansen et al. (1981, H81), Hansen et al. (1988, H88), [[#Manabe--1993|Manabe and Stouffer (1993)]] , along with the Energy Balance Model (EBM) projections from FAR, SAR and TAR, and the multi-model mean projection using CMIP3 simulations of the Special Report on Emissions Scenarios (SRES) A1B scenario from AR4. H81 and H88 show most expected scenarios 1 and B, respectively. See [[#Hausfather--2020|Hausfather et al. (2020)]] for more details of the projections. Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).
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Although this approach has limitations when the modelled forcings differ greatly from the forcings subsequently experienced, they were generally able to project actual future global warming when the mismatches between forecast and observed radiative forcings are accounted for. For example, Scenario B presented in [[#Hansen--1988|Hansen et al. (1988)]] projected around 50% more warming than has been observed during the 1988–2017 period, but this is largely because it overestimated subsequent radiative forcings. Similarly, while FAR ( [[#IPCC--1990a|IPCC, 1990a]] ) projected a higher rate of global surface temperature warming than has been observed, this is largely because it overestimated future GHG concentrations: FAR’s projected increase in total anthropogenic forcing between 1990 and 2017 was 1.6 W m <sup>–2</sup> , while the observational estimate of actual forcing during that period is 1.1 W m <sup>–2</sup> ( [[#Dessler--2018|Dessler and Forster, 2018]] ). Under these actual forcings, the change in temperature in FAR aligns with observations ( [[#Hausfather--2020|Hausfather et al., 2020]] ).

Inaddition to global surface temperature, past regional projections can be evaluated. For example, FAR ( [[#IPCC--1990a|IPCC, 1990a]] ) presented a series of temperature projections for 1990–2030 for several regions around the world. Regional projections were given for the best estimate of 1.8°C of global warming by 2030, compared to a baseline of 1850–1900, and were assigned ''low confidence'' . The FAR also suggested that regional temperature changes should be scaled by –30% to +50% to account for the uncertainty in projected global warming.

The regional projections presented in FAR are compared to the observed temperature change in the period since 1990 (Figure 1.10), following Groseet al. (2017). Subsequent observed temperature change has tracked within the FAR projected range for the best estimate of regional warming in the Sahel, South Asia and southern Europe. Temperature change has tracked at or below this range for the central North America and Australia regions, yet remains within the range reduced by 30% to generate FAR’s lower global warming estimate. This is consistent with the smaller observed estimate of radiative forcing compared to the FAR central estimate. Note that the projections assessed in [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] of this Report suggest that global temperatures will be around 1.2°C–1.8°C above 1850–1900 levels by 2030, a range which is also lower than the FAR central estimate.

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'''Figure 1.10 |''' '''Range of projected temperature change for 1990–2030 for various regions defined in IPCC First Assessment Report (FAR).''' The '''left-hand''' panel shows the FAR projections ( [[#IPCC--1990a|IPCC, 1990a]] ) for southern Europe, with the darker blue shade representing the range of projected change given for the best estimate of 1.8°C global warming by 2030 compared with pre-industrial levels, and the fainter blue shade showing the range scaled by '''–''' 30% to +50% for lower and higher estimates of global warming. Blue lines show the regionally averaged observations from five global temperature gridded datasets, and blue dashed lines show the linear trends in those datasets for 1990–2020 extrapolated to 2030. Observed datasets are: HadCRUT5, Cowtan and Way, GISTEMP, Berkeley Earth and NOAA GlobalTemp. The inset map shows the definition of the FAR regions used. The '''right-hand''' panel shows projected temperature changes by 2030 for the various FAR regions, compared to the extrapolated observational trends, following [[#Grose--2017|Grose et al. (2017)]] . Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).
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Overall, there is ''medium confidence'' that past projections of global temperature are consistent with subsequent observations, especially when accounting for the difference in radiative forcings used and those which actually occurred ( ''limited evidence, high agreement'' ). The FAR regional projections are broadly consistent with subsequent observations, allowing for regional-scale climate variability and differences in projected and actual forcings. There is ''medium confidence'' that the spatial warming pattern has been reliably projected in past IPCC reports ( ''limited evidence, h'' ''igh agreement'' ).

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'''Box 1.2 | Special Reports in the IPCC Sixth Assessment Cycl''' '''e: Key Findings'''

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The Sixth Assessment Cycle started with three Special Reports. The Special Report on Global Warming of 1.5°C (SR1.5, [[#IPCC--2018|IPCC, 2018]] ), invited by the Parties to the UNFCCC in the context of the Paris Agreement, assessed current knowledge on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas (GHG) emissions pathways. The Special Report on Climate Change and Land (SRCCL, [[#IPCC--2019a|IPCC, 2019a]] ) addressed GHG fluxes in land-based ecosystems, land use and sustainable land management in relation to climate change adaptation and mitigation, desertification, land degradation and food security. The Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC, [[#IPCC--2019b|IPCC, 2019b]] ) assessed new literature on observed and projected changes of the ocean and the cryosphere, and their associated impacts, risks and responses.

The SR1.5 and SRCCL were produced through a collaboration between the three IPCC Working Groups, SROCC by only Working Groups I and II. Here we focus on key findings relevant to the physical science basis covered by WGI.

''''''Observations of''' '''climate change''''''
The SR1.5 estimated with ''high confidence'' that human activities caused a global warming of approximately 1°C between the 1850–1900 period and 2017. For the period 2006–2015, observed global mean surface temperature (GMST <sup>[[#footnote-001|7]]</sup> ) was 0.87°C ± 0.12°C higher than the average over the 1850–1900 period ( ''very high confidence'' ). Anthropogenic global warming was estimated to be increasing at 0.2 ± 0.1°C per decade ( ''high confidence'' ) and ''likely'' matches the level of observed warming to within ±20%. The SRCCL found with ''high confidence'' that over land, mean surface air temperature increased by 1.53°C ± 0.15°C between 1850–1900 and 2006–2015, or nearly twice as much as the global average. This observed warming has already led to increases in the frequency and intensity of climate and weather extremes in many regions and seasons, including heat waves in most land regions ( ''high confidence'' ), increased droughts in some regions ( ''medium confidence'' ), and increases in the intensity of heavy precipitation events at the global scale ( ''medium confidence'' ). These climate changes have contributed to desertification and land degradation in many regions ( ''high confidence'' ). Increased urbanization can enhance warming in cities and their surroundings (heat island effect), especially during heat waves ( ''high confidence'' ) '','' and intensify extreme rainfall ( ''medi'' ''um confidence'' ).

With respect to the ocean, SROCC assessed that it is ''virtually certain'' that the ocean has warmed unabated since 1970 and has taken up more than 90% of the excess heat contributed by global warming. The rate of ocean warming has ''likely'' more than doubled since 1993. Over the period 1982–2016, marine heatwaves have ''very likely'' doubled in frequency and are increasing in intensity ( ''very high confidence'' ). In addition, the surface ocean acidified further ( ''virtually certain'' ) and loss of oxygen occurred from the surface to a depth of 1000 m ( ''medium confidence'' ). The Report expressed ''medium confidence'' that the Atlantic Meridional Overturning Circulation (AMOC) weakened in 2004–2017 relative to 1850–1900.

Concerning the cryosphere, SROCC reported widespread continued shrinking of nearly all components. Mass loss from the Antarctic Ice Sheet tripled over the period 2007–2016 relative to 1997–2006, while mass loss doubled for the Greenland Ice Sheet ( ''likely'' , ''medium confidence'' ). The Report concludes with ''very high confidence'' that due to the combined increased loss from the ice sheets, global mean sea level (GMSL) rise has accelerated ( ''extremely likely'' ) . The rate of recent GMSL rise (3.6 ± 0.5 mm yr <sup>–1</sup> for 2006–2015) is about 2.5 times larger than for 1901–1990. The report also found that Arctic sea ice extent has ''very likely'' decreased for all months of the year since 1979 and that September sea ice reductions of 12.8 ± 2.3% per decade are ''likely'' unprecedented for at least 1000 years. Feedbacks from the loss of summer sea ice and spring snow cover on land have contributed to amplified warming in the Arctic ( ''high confidence'' ), where surface air temperature ''likely'' increased by more than double the global average over the last two decades. By contrast, Antarctic sea ice extent overall saw no statistically significant trend for the period 1979–2018 ( ''hi'' ''gh confidence'' ).

Box 1.2

The SROCC assessed that anthropogenic climate change has increased observed precipitation ( ''medium confidence'' ), winds ( ''low confidence'' ), and extreme sea level events ( ''high confidence'' ) associated with some tropical cyclones. It also found evidence for an increase in the annual global proportion of Category 4 or 5 tropical cyclones in recent decades ( ''l'' ''ow confidence'' ).

''''''Drivers of''' '''climate change''''''
The SRCCL stated that the land is simultaneously a source and sink of CO <sub>2</sub> , due to both anthropogenic and natural drivers. It estimates with ''medium confidence'' that agriculture, forestry and other land use (AFOLU) activities accounted for around 13% of CO <sub>2</sub> , 44% of CH <sub>4</sub> , and 82% of N <sub>2</sub> O emissions from human activities during 2007–2016, representing 23% (12.0 ± 3.0 GtCO <sub>2</sub> equivalent yr <sup>–1</sup> ) of the total net anthropogenic emissions of GHGs. The natural response of land to human-induced environmental change – such as increasing atmospheric CO <sub>2</sub> concentration, nitrogen deposition and climate change – caused a net CO <sub>2</sub> sink equivalent of around 29% of total CO <sub>2</sub> emissions ( ''medium confidence'' ); however, the persistence of the sink is uncertain due to climate change ( ''hi'' ''gh confidence'' ).

The SRCCL also assessed how changes in land conditions affect global and regional climate. It found that changes in land cover have led to both a net release of CO <sub>2</sub> , contributing to global warming, and an increase in global land albedo, causing surface cooling. However, the report estimated that the resulting net effect on globally averaged surface temperature was small over the historical period ( ''medi'' ''um confidence'' ).

The SROCC found that the carbon content of Arctic and boreal permafrost is almost twice that of the atmosphere ( ''medium confidence'' ), and assessed ''medium evidence'' with ''low agreement'' that thawing northern permafrost regions are currently releasing additional net CH <sub>4</sub> and CO <sub>2</sub> .

''''''Projections of''' '''climate change''''''
The SR1.5 concluded that global warming is ''likely'' to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate ( ''high confidence'' ). However, even though warming from anthropogenic emissions will persist for centuries to millennia and will cause ongoing long-term changes, past emissions alone are ''unlikely'' to raise global surface temperature to 1.5°C above 1850–1900 levels.

The SR1.5 also found that reaching and sustaining net zero anthropogenic CO <sub>2</sub> emissions and reducing net non-CO <sub>2</sub> radiative forcing would halt anthropogenic global warming on multi-decadal time scales ( ''high confidence'' ). The maximum temperature reached is then determined by (i) cumulative net global anthropogenic CO <sub>2</sub> emissions up to the time of net zero CO <sub>2</sub> emissions ( ''high confidence'' ) and (ii) the level of non-CO <sub>2</sub> radiative forcing in the decades prior to the time that maximum temperatures are reached ( ''medi'' ''um confidence'' ).

Furthermore, climate models project robust differences in regional climate characteristics between the present day and a global warming of 1.5°C, and between 1.5°C and 2°C, including mean temperature in most land and ocean regions and hot extremes in most inhabited regions ( ''high confidence'' ). There is ''medium confidence'' in robust differences in heavy precipitation events in several regions and the probability of droughts in some regions.

The SROCC projected that global-scale glacier mass loss, permafrost thaw, and decline in snow cover and Arctic sea ice extent will continue in the period 2031–2050 due to surface air temperature increases ( ''high confidence'' ). The Greenland and Antarctic ice sheets are projected to lose mass at an increasing rate throughout the 21st century and beyond ( ''high confidence'' ). Sea level rise will also continue at an increasing rate. For the period 2081–2100 with respect to 1986–2005, the ''likely'' ranges of GMSL rise are projected at 0.26–0.53 m for RCP2.6 and 0.51–0.92 m for RCP8.5. For the RCP8.5 scenario, projections of GMSL rise by 2100 are higher by 0.1 m than in AR5 due to a larger contribution from the Antarctic Ice Sheet ( ''medium confidence'' ). Extreme sea level events that occurred once per hundred years in the recent past are projected to occur at least once per year at many locations by 2050, especially in tropical regions, under all RCP scenarios ( ''high confidence'' ). According to SR1.5, by 2100 GMSL rise would be around 0.1 m lower with 1.5°C global warming compared to 2°C ( ''medium confidence'' ). If warming is held to 1.5°C, GMSL will still continue to rise well beyond 2100, but at a slower rate and a lower magnitude. However, instability and/or irreversible loss of the Greenland and Antarctic ice sheets, resulting in a multi-metre rise in sea level over hundreds to thousands of years, could be triggered at 1.5°C–2°C of global warming ( ''medium confidence'' ). According to SROCC, sea level rise in an extended RCP2.6 scenario would be limited to around 1 m in 2300 ( ''low confidence'' ) while under RCP8.5 multi-metre sea level rise is projected by then ( ''medi'' ''um confidence'' ).

The SROCC projected that over the 21st century, the ocean will transition to unprecedented conditions, with increased temperatures ( ''virtually certain'' ), further acidification ( ''virtually certain'' ), and oxygen decline ( ''medium confidence'' ). Marine heatwaves are projected to become more frequent ( ''very high confidence'' ) as are extreme El Niño and La Niña events ( ''medium confidence'' ). The AMOC is projected to weaken during the 21st century ( ''very likely'' ) , but a collapse is deemed ''very unlikely'' (albeit with ''medium confidence'' due to known biases in the climate models used for the assessment).

''''''Emissions pathways to limit''' '''global warming''''''
The SR1.5 focused on emissions pathways and system transitions consistent with 1.5°C global warming over the 21st century. Building upon the understanding from AR5 WGI of the quasi-linear relationship between cumulative net anthropogenic CO <sub>2</sub> emissions since 1850–1900 and maximum global mean temperature, the Report assessed the remaining carbon budgets compatible with the 1.5°C or 2°C warming goals of the Paris Agreement. Starting from year 2018, the remaining carbon budget for a one-in-two (50%) chance of limiting global warming to 1.5°C is about 580 GtCO <sub>2</sub> , and about 420 GtCO <sub>2</sub> for a two-in-three (66%) chance ( ''medium confidence).''

At constant 2017 emissions, these budgets would be depleted by about the years 2032 and 2028, respectively. Using GMST instead of GSAT gives estimates of 770 GtCO <sub>2</sub> and 570 GtCO <sub>2</sub> , respectively ( ''medium confidence'' ). Each budget is further reduced by approximately 100 GtCO <sub>2</sub> over the course of this century when permafrost and other less well represented Earth system feedbacks are taken into account.

It is concluded that all emissions pathways with no or limited overshoot of 1.5°C imply that global net anthropogenic CO <sub>2</sub> emissions would need to decline by about 45% from 2010 levels by 2030, reaching net zero around 2050, together with deep reductions in other anthropogenic emissions, such as methane and black carbon. To limit global warming to below 2°C, CO <sub>2</sub> emissions would have to decline by about 25% by 2030 and reach net zero around 2070.

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