14.7 Glacial inception in the Holocene

Fig. 13.12 Sea-level rise intermediate scenarios for 2100

Red continuous curve, sea-level rise measured since 1993 and zeroed in 2000. Data from NASA. Dashed curves, sea-level rise projections for the 2000–2100 period under intermediate emissions scenarios from different sources. 2007 IPCC AR4 B1 scenario (dashed black); 2014 IPCC AR5 RCP 4.5 scenario (dashed dark grey); Horton et al. (2014), survey intermediate scenario (average of the high and low scenarios; dashed medium grey); 2017 NOAA intermediate scenario (dashed light grey, Sweet et al. 2017). Black line, 1993-2021 linear trend extrapolated to 2100. All models were started at zero in 2000 and are shown since 2021.

only a small 0.01 mm/yr2 acceleration has been detected by most researchers (Church & White 2011; Jevrejeva et al. 2014; Hogarth 2014). The added uncertainty in future CO2 levels and emissions suggests a simple linear extrapolation might be sufficient for projecting future sea levels, that appear to be increasing quite constantly despite changing atmospheric CO2 and global temperature. Such a forecast would see a 170 mm increase every 50 years, for a total 340 mm increase in the 21st century (Fig. 13.12; table 13.1). This increase is too small to constitute a problem on a global basis but might add to the problem of local sea-level rise in areas where land subsidence or lack of sufficient sedimentation are going to require adapting measures.

13.9 Other climate change consequences for the 21st century

A conservative forecast is that most extreme weather phenomena should continue occurring in an unpredictable manner without a significant change in frequency. Storm data from the last 6500 years shows clearly that storms increase in frequency and strength with cooling, and decrease with warming (see Fig. 7.7c). The reasons are that warming reduces heat transport due to a decrease in the latitudinal temperature gradient, and that the atmospheric heat engine has a reduced ability to generate work due to an increase in the power required by the intensification of the hydrological cycle (Laliberté et al. 2015). The association of weaker storm strength to warming is supported by a global wind stilling during the 1980–2000 rapid warming period that reversed course with the pause (Zeng et al. 2019). Records (Maue 2011) and models (Sugi et al. 2015) also show a reduction in tropical cyclone activity with global warming.

The only extreme weather phenomenon that is credibly projected to increase is the frequency and intensity of heat waves. However, the change could be smaller than anticipated as Modern Global Warming is having more effect on minimum, rather than on maximum, temperatures producing generally warmer winters. This has an added benefit because a 68-authors global study of 5 million non-optimal temperature associated deaths found that 90% of them were cold-related (Zhao et al. 2021). From a societal point of view, adaptation to increased heat-waves requires cheap, abundant energy.

The effect over the biosphere is more difficult to forecast, as it has shown very high adaptability through much bigger climate changes in the past. If we accept that the world in 2021 was c. 0.95 °C above the pre-industrial average, the conservative forecast indicates it might only increase a further 0.55 °C before stabilizing. Therefore, we might have already seen over 60% of the total expected warming. The negative effects strictly from climate change over the biosphere are very limited, while the positive effects are abundant and profound. Most biomes, but particularly semi-arid ones, have responded to warming and CO2 increase through an increase in leaf area (also known as greening; Zhu et al. 2016). These three factors (temperature, CO2, and greening) appear to have caused an increase in global terrestrial net primary production of 12% between 1961–2010 (Li et al. 2017). The effect of this increased energy flux through ecosystems is beneficial to nearly all species. The net effect of the warming and increased CO2 is clearly positive for the biosphere. It is reasonable to think that too much of a good thing should reach a point when the net effect starts being negative, but

there is no evidence that we are close to that point or that it should be reached within the 21st century.

The loss of Arctic sea ice has been proposed to be a clear risk to polar bears, and the species was included in the US endangered species list solely on those grounds. However polar bears might not be very sensitive to summer ice reductions as their ice-dependent hunting takes place in spring and is negatively affected by too much ice. The species has survived very reduced or even absent summer Arctic sea ice during the Holocene Climatic Optimum and the past warmer interglacial. The main danger to polar bears has historically been human hunting, and since the international hunting limitation by the Oslo agreement of 1973, polar bear population estimates have been increasing, apparently unaffected by the loss of 30% of summer Arctic sea ice in the 1995–2007 period (Crockford 2020). At present there is no evidence that polar bears are threatened during the 21st century from climate change, even if the projected summer ice loss in the Arctic takes place (Fig. 13.11).

Regarding other possible consequences, our knowledge is too limited to say much. Claims of sinking nations, hordes of climate refugees, and a new normal every time there is an extreme weather event, are wildly exaggerated and agenda-driven. The highest return for our limited resources is very likely to come from adaptation policies, and no-regrets policies. Policies to prevent or reduce climate change are destined to be highly ineffectual given the strong natural component of climate change, as the past demonstrates.

13.10 Projections

13a. Human CO2 emissions are stabilizing. Peak coal and oil, and current trends make a decrease in emissions very likely before 2050. Atmospheric CO2 levels should reach 500 ppm but might stabilize soon afterwards. 13b. According to solar cycles, solar activity should increase after the present extended solar minimum, and 21st century solar activity should be as high or higher than 20th century. A mid-21st century solar grand minimum is highly improbable. 13c. Global warming might stall or slightly reverse for the period 2000–2035. Cyclic factors suggest renewed warming for the 2035–2065 period at a similar rate to the last half of the 20th century. Afterwards global warming could end, with temperatures stabilized around +1.5 °C above pre-industrial, and a very slow decline for the last part of 21st century and beyond. 13d. The present summer Arctic sea ice melting pause might continue until c. 2035. Renewed melting is probable afterwards, but it is unlikely that the Arctic summer will become consistently ice free even by 2100. 13e. The rate of sea-level rise can be conservatively projected to a 340 mm increase by 2100 over 2000 levels.

Most rates published are extremely non-conservative and very unlikely to take place. 13f. Climate change should remain subdued and net positive for the biosphere for the 21st century. Adaptation is likely to be the best strategy, as it has always been.

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14 THE NEXT GLACIATION

“A global deterioration of climate, by order of magnitude larger than any hitherto experienced by civilized mankind is a very real possibility and indeed may be due very soon.” George Kukla and Robert Matthews (1972)

14.1 Introduction

The expected timeframe for the next glaciation is so far in the future that traditionally it has only attracted academic interest. There was a small peak of popular interest in the early 1970s at the end of the mid-20th century cooling period. In January 1972, geologists George Kukla and Robert Matthews organized a meeting on the end of the present interglacial, and afterwards they wrote president Richard Nixon calling for federal action on the observed climate deterioration that had the potential to lead to the next glaciation. Ironically, concerns over the end of the interglacial led to the creation of NOAA's Climate Analysis Center in 1979 (Reeves & Gemmill 2004), that would substantially contribute to global warming research. Some popular magazines reported about a coming ice age at times of harsh winter weather during the early 1970s.

Current academic consensus is that a return to glacial conditions is not possible under any realistic condition for tens of thousands of years, and the IPCC expresses virtual certainty that a new glacial inception is not possible for the next 50 kyr if CO2 levels remain above 300 ppm (IPCC–AR5, Masson–Delmotte et al. 2013, p 435). This claim expressed on so certain terms is in stark contrast with the lack of precedent for any interglacial spanning over two obliquity oscillations.

14.2 Interglacial evolution

Each interglacial is different. They all have different astronomical signatures, different initial conditions, different evolution, and are subjected to non-linear chaotic climate unpredictability. But they all take place during a single obliquity oscillation. 104 marine isotope stages (MIS) have been identified over the Pleistocene's 2.6 Ma, half of them (Fig. 14.1b, odd numbers) corresponding to warm periods. On average there is one every 50,000 years, almost corresponding to the obliquity frequency (41 kyr). The match is not exact because some obliquity oscillations have failed to produce an interglacial.

For the last 800 kyr, after the Mid-Pleistocene Transition (1.5–0.7 Ma; Fig. 14.1), the planet has become so cold, and the ice-sheets grown so large, that to produce an interglacial outside the periods of high eccentricity requires the simultaneous concert of high obliquity, high northern summer insolation, and very large unstable ice sheets. This has had the effect of spacing interglacials from an obliquity-linked 41-kyr cycle to its multiples 82 or 123 kyr (see Fig. 2.11). A side effect is that after one or more obliquity oscillations without an interglacial the planet gets colder, and when an interglacial is finally produced, it reaches a warmer state. The climate of the planet has become more unstable in the Middle and Late Pleistocene, rapidly transitioning from more extreme cold to more extreme warm, and back, contributing to numerous species extinctions, and perhaps to the evolution of our species (Fig. 14.1c).

The majority of interglacials of the past 800 kyr are the product of very similar orbital and ice-volume conditions and present a common pattern (Fig. 14.2). The Holocene interglacial is the result of similar conditions, and belongs to this group. Nearly all exceptions can be explained in terms of particular orbital and ice-volume conditions that do not apply to the Holocene (see Chap. 2).

Antarctica leads the deglaciation over the Northern Hemisphere and reaches its highest temperature at the obliquity peak when the Laurentide and Fennoscandian ice-sheets are not completely melted yet. The asynchrony between a Southern Hemisphere cooling from declining obliquity and low summer insolation, and a Northern Hemisphere warming from ice-sheets melting and high summer insolation results in a global optimum that has a different span depending on latitude. Interglacial temperature decline presents a delay with respect to obliquity decline of 5,500–8,000 years (see Fig. 2.11), observed since the late Pliocene (Donders et al. 2018). This delay has been attributed to a lag in the ice volume change with respect to its rate of change (Huybers 2009). Ocean thermal inertia could also contribute to the lag. Once northern summer insolation is declining at its fastest rate, the interglacial enters a phase of slowly declining temperature (c. –0.2°C/millennium) that in the Holocene has been termed the Neoglaciation. Despite temperature decline and modest glacier expansion, sea levels are quite stable over this period, as there is no significant ice-sheet build up. The Holocene has been clearly at this stage since c. 5000 yr ago, until Modern Global Warming.

When northern summer insolation becomes low, and obliquity is at its fastest rate of decline, the interglacial reaches glacial inception. This tipping point appears to take place during a global Little Ice Age (LIA)-type cold period when due to the start of ice-sheet build up, sea-level starts dropping. The intensification of ice-albedo and vegetation feedbacks result in a point of no return. Regardless of insolation changes, once glacial inception takes place, the glaciation will continue through advances and retreats