Editing IPCC:AR6/WGI/Chapter-1 (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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[[File:5414ad1d54dff94367e1c8c16a324ba8 IPCC_AR6_WGI_Figure_1_9.png]]

'''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).

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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[[File:c8ea2228f5e35fb4898e625752e60dbe IPCC_AR6_WGI_Figure_1_10.png]]

'''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).

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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