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14.3 Studying the future by looking at the past
(57%) are at around two centuries of another SGM, and 7 clusters of 2 or more SGM can be recognized. That is why the de Vries bicentennial cycle is so important for SGM as it is a very favored spacing. Usoskin et al. (2016) have shown that SGM have a statistically significant tendency to cluster at the lows of the 2500-yr Bray solar cycle, challenging random-probability based analyses (Lockwood 2010). If we also consider the 1000-yr Eddy solar cycle, we can see that 26 SGM (87%) fall at or right next to the periods when one of these two cycles is at its lowest 20% (colored areas in Fig. 13.7, 54% of time). The conclusion is clear, SGM tend to occur nine out of ten times when solar activity is at its lowest coinciding with the lows of the 2500 and 1000-yr solar cycles. In periods like the present, outside the lows of both solar cycles, the Sun expends only 7.5% of its time in a SGM, and with a frequency of c. 1 SGM in 1000 years. Forecasting a SGM for the mid-21st century is really a low-probability nonconservative proposition. The probability for a next SGM should become high again c. AD 2450.
Over periods of a few years climate variability appears to be dominated by El Niño/Southern Oscillation (ENSO) variability (Fig. 13.8), that so far has resisted forecasting attempts. The failure of the 2014 and 2017 El Niño forecasts (ECMWF 2021) shows the difficulty of forecasting ENSO and global temperatures even for a few months. The February 2017 El Niño forecast (ECMWF 2021) failed because it did not take into account solar activity control of ENSO (see Sect. 10.4). February 2017 belongs already to the late decline in activity towards the end of the solar cycle that corresponds to phase III in Fig. 10.12, when a La Niña is more probable than an El Niño.
The successful 2020 La Niña prediction (Vinós 2019) anticipated a continuation of the decline in global surface average temperature (GSAT) observed since February 2016. From mid-2019 the PDO returned to the negative values that prevailed during the pause (1999–2014). The combined effect of ENSO, oceanic oscillations, and low solar forcing from the Clilverd extended solar minimum suggest that the lack of significant global warming should continue perhaps until the late 2020s or early 2030s establishing a new pause. GSAT (HadCRUT4.6) is already in 2021 lower than 95% of CMIP5 models (RCP4.5 emissions scenario; Fig. 13.9) and CMIP6 models project even more warming (Meehl et al. 2020). A new pause would make the model-observations disparity even more striking, as the GSAT appears to be increasing in a linear fashion since 1950, while models project an accelerating path as prescribed by the CO2 hypothesis.
The effect of multiple solar cycles with lower than average activity on temperature is not well determined, but previous similar periods, known as solar extended minima, coincide with cool periods in the early decades of the 18th century (Maunder minimum), 19th century (Dalton minimum), and 20th century (Gleissberg minimum). A lag of c. 10–20 years has been found between the decrease in solar activity and its effect on tree-ring growth and ice core temperatures in several reconstructions (Eichler et al. 2009; Breitenmoser et al. 2012; Anchukaitis et al. 2017). A longer lag has been found on the maximum effect of solar activity reduction on the increase in heat transport, causing cooling in low latitudes and warming in high latitudes (Kobashi et al. 2015). The present solar extended minimum, named here the Clilverd minimum, includes SC24 and most likely SC25. A conservative forecast on the effect of the Clilverd minimum on temperatures indicates no additional global warming before 2035, and perhaps even a slight cooling. This forecast is also supported by the position of the 65-year oscillation in global temperature (see Fig. 12.15), that also indicates no warming for the first 3 decades of the 21st century.
It is remarkable that a knowledge-based conservative forecasting for the next 15 years as the one presented here, agrees so well with the naive no-change forecast proposed by Green and Armstrong in 2007, that has been superior to the IPCC early forecast. It is important to emphasize that although very variable in the short term in different places, Earth temperature is extraordinarily constant over the long term. 0.2 °C is a small variation in temperature at temporal scales of less than one year, but a significant variation on yearly averages, an important variation in decadal temperature scales, and a huge variation in millennial scales. The Neoglacial trend that has driven glacier expansions all over the globe, culminating at the LIA, was just –0.2 °C/ millennium or less over the past five millennia (–0.38 °C/ millennium in Greenland; Kobashi et al. 2015), due to Milankovitch forcing. The planet lost about one degree average from the Holocene Climatic Optimum to the second millennium AD, and this amount caused considerable glacier expansion and biome changes, reducing tropical forests and expanding the tundra. Higher temperature changes are observed on shorter timeframes, but they also have lower and shorter effects. From July 2013 to February 2016 the global surface average temperature anomaly increased by 0.4 °C but has since lost most of it (Fig. 13.9). Multidecadal to centennial temperature forecasting, to be conservative, must strongly constrain the amount of temperature change that it allows. That is the reason why
Fig. 13.8 ENSO-Global temperature relation
June 2013–January 2018 Niño 3.4 region sea surface temperature anomaly (black line, left scale), and monthly global surface average temperature anomaly (red line, right scale). Data from Australian Bureau of Meteorology and UK HadCRUT4.6 dataset.
Fig. 13.9 Global temperature change 1950–2021: comparing observations to models
Black curve, global surface temperature anomaly (°C; monthly HadCRUT 4.6 13-month averaged) with its linear trend (thin continuous line), and 95% confidence interval (grey area). Data from UK Met Office. Red curve, Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model mean temperature anomaly 1950–2050 under RCP 4.5 conditions (13-month averaged) with 25–75% (medium red area), and 5–95% values (light red area) for the 42 models used. Data from KNMI climate explorer, Trouet & van Oldenborgh (2013). CMIP5 models were initialized in 2006 (vertical line) and reproducing historical climate to that point was a prerequisite.
Fig. 13.10 Conservative temperature, CO2 level, and emissions forecast to AD 2200
CO2 emissions from fossil fuels forecast (brown continuous line, right outer scale) based on a peak in oil consumption by 2018–35 (dashed brown line), a second peak in coal consumption before 2050 (dotted brown line) and increasing gas consumption to 2100 (dashdotted brown line), producing a peak in CO2 emissions from fossil fuels at c. 35 Gtons by 2050. Historical CO2 emissions from fossil fuels are from Gilfilland et al. (2019), archived at CDIAC (AppState) and updated with data from BP Statistical Energy Review (2022). Atmospheric CO2 levels (blue line, right inner scale) should therefore stabilize at c. 500 ppm by 2080 before starting to decrease slowly as sinks remove more than is added. Historical atmospheric CO2 levels are from Law Dome (after Etheridge et al. 1996) to 1958 and from NOAA afterwards. An idealized millennial cycle in solar activity is represented by the orange line, peaking at 2050–2080, temporarily reduced by centennial and bicentennial lows indicated with their names. CMIP5 model-mean temperature anomaly (red line) projects reaching +1.5 °C above pre-industrial by the 2030s, and +2.0 °C by the 2050s. Temperature anomaly should stabilize until the 2030s, increasing afterwards and peaking c. the 2070s at c. +1.5 °C above pre-industrial, due to CO2, solar activity forecasts, and oceanic oscillations. Afterwards temperature anomaly could enter a slightly declining undulating plateau as both CO2 and solar activity slowly decline. Historic temperature anomaly is from UK MetOffice HadCRUT 4.6 global surface monthly dataset (13-month averaged).
