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Climate science was probably born in 1896 with the first, and still valid, calculation of how much the earth would warm if atmospheric CO₂ content were to double from pre-industrial values (Arrhenius 2009 [1896]). Given the means of the day, without the use of electronic computers, the Swedish physicist Svante Arrhenius, famous for his contributions to thermodynamics and the understanding of chemical reactions, calculated that the earth would warm by about 4°C. His value still lies within the range of modern estimates, produced by hugely complicated computer models (Slingo 2017). Arrhenius even calculated that regions near the poles would warm much more than those near the equator, something that is still seen as a major finding of climate science.

Awareness of humanity’s vulnerability to changes in climate was high at the close of the nineteenth century. Arrhenius’s native Scandinavia and much of northern Europe had only recently come out of the Little Ice Age, a cold period that had led to frequent crop failure and starvation (Lee 2009). His hope was therefore that increasing amounts of ‘carbonic acid’ in the air – atmospheric carbon dioxide – would bring about better weather and increased crop yields for the colder regions of the earth.

Fast forward one and a quarter centuries, and the predicted climate heating has become obvious to almost everyone (WMO 2019). Even if we did not have the benefits of modern climate science, we would still be able to recognize that we are in a situation never seen by humanity since modern civilization started with the onset of agriculture. Essentially nineteenth-century technologies such as thermometers, the collection of data on emissions of carbon-containing fuels, in combination with Arrhenius’s science, already tell us a complete story: while rates of carbon dioxide emissions from human activities have been rising more or less exponentially (Global Carbon Project 2020), with no signs that this will change any time soon (Betts et al. 2020; Le Quéré et al. 2020), the earth is warming at an accelerating rate (NOAA 2020a). The climate system seems to be out of control, and human activity is the cause.

Human interest in understanding the geological past has created a different branch of climate science a long time ago, now called paleo-climatology. On its own, paleo-climatology can already tell us a compelling story of where we are and further help us understand the scale of our calamity. By drilling deep holes into Antarctic ice and analysing the composition of tiny bubbles found in them, researchers have been able to construct a continuous record of atmospheric carbon dioxide going back 800,000 years (Masson-Delmotte et al. 2010). The data show that CO₂ levels have fluctuated between around 180 parts per million (ppm) during ice ages and 280 ppm during warm periods. Only once did they briefly increase slightly above 300 ppm. At the time of writing, the last estimate of global mean CO₂ stood at 415 ppm, with an annual growth rate of almost 3 ppm over the last five years (NOAA 2020b). The last time atmospheric CO₂ was about as high as now was around three million years ago, during the Middle Pliocene (Lunt et al. 2008). Even that was only a temporary excursion, and we have to go back a staggering 25 million years to find values exceeding 500 ppm (Pagani et al. 2005), a value that at current trends we will reach in about 30 years.

When a kettle is switched on, the water in it does not heat up instantaneously, but there is a delay. The same happens with the earth’s climate: according to the International Panel on Climate Change (IPCC 2019a), 90 per cent of the heat from the enhanced greenhouse effect is absorbed by the oceans’ water. This means that the warming we have seen so far lags behind the steep rise in atmospheric carbon dioxide levels, and that even if we suddenly managed to stop the rise, warming will continue (Huntingford, Williamson and Nijsse 2020). Earth’s surface is now over 1°C warmer than during the late nineteenth century (NOAA 2020a), which is comparable to or slightly warmer than during the last geological warm period between 10,000 and 5,000 years ago (Marcott et al. 2013). But during that time the warming was caused by changes in the way the earth circles and wobbles around the sun, with the result that tropical lands were slightly cooler than today. Nowadays, the (over)heating effect is seen universally across the globe, and as such differs decidedly from any climate fluctuations since the end of the last ice age more than 10,000 years ago (Barbuzano 2019; Neukom et al. 2019).

Currently the enhanced greenhouse effect is created only to about two-thirds by CO₂ and one third by other gases, of which about half by methane.¹ Countering this warming to a certain extent are several other human-caused effects, notably from atmospheric pollution through aerosols, which cause some cooling (Myhre et al. 2013). These cooling effects probably approximately cancel out the warming effect of the non-CO₂ greenhouse gases. CO₂, however, is by far the most important greenhouse gas because its lifetime vastly exceeds almost all of the other greenhouse gases or aerosols. A question of substantial theoretical value is what would happen if we not only prevented a further rise of CO₂ but stopped all emissions of CO₂ and other greenhouse gases tomorrow. Because the oceans and land tend to take up more than half of human-made CO₂ emissions (Global Carbon Project 2020), such an instantaneous net-zero balance of man-made carbon fluxes would lead to some drawdown of CO₂ by natural sinks, and a lowering of atmospheric CO₂ levels. The extent of the drawdown, however, is by no means understood and could easily be overestimated. Forests are responsible for a large part of that sink (Global Carbon Project 2020), but are increasingly being fragmented and damaged (Grantham et al. 2020). Some recent research also suggests that we already observe a declining efficiency of the sinks (Wang et al. 2020). Furthermore, even if such lowering occurs, it may not lead to a cessation of heating: because of the possible acceleration of overheating once the ‘global dimming effect’ (from aerosols) is reduced (Xu, Ramanathan and Victor 2018); and because of vicious climate feedbacks that may already be underway (Lenton et al. 2019).

How much more overheat will result if CO₂ emissions alone stopped depends on both how rapidly the gas is removed from the atmosphere, and on the long-term climate response to the remaining CO₂ level. Model results (Matthews and Zickfeld 2012) indicate that in such a scenario, two-thirds of the human-caused excess CO₂ – i.e. above the pre-industrial level of 280 ppm – would still remain in the atmosphere after 190 years. For the case of stopping emissions in 2020 at a level of 415 ppm, this translates to more than 370 ppm by the year 2200. The earth during that time would continue to warm, but probably only by a few tenths of a degree. If all other greenhouse gas and aerosol emissions also stopped, the result might well be similar. However, the assumption that it would be excludes at least two further possibilities – possible stronger than expected carbon cycle feedback that we cannot reliably quantify, leading to higher than expected CO₂ levels (Lenton et al. 2019); and the possibility of a higher long-term sensitivity of the earth’s temperature to CO₂ (Bjordal et al. 2020).

Because of the limitations of models, a more prudent approach is to derive climate sensitivity from past climates. Most commonly, this is based on the temperature and CO₂ changes during ice-age/warm-period fluctuations (Hansen et al. 2013). Results based on this approach generally support the model results (Sherwood et al. 2020). The problem, however, is that the earth is already in a different state from any time during those glacial cycles. Climate sensitivity could be higher in a warmer state due to positive climate system feedbacks, or tipping mechanisms, not yet quantified, which is the basis of the deeply alarming ‘Hothouse Earth’ hypothesis (Steffen et al. 2018). Support for this hypothesis comes from estimates for the Pliocene warm period, when CO₂ was between 365 and 415 ppm and temperatures about 3°C warmer than during the pre-industrial era (Pagani et al. 2010; Sherwood et al. 2020). According to those data – from the last time earth was in a similar climate state to now – an immediate stop of CO₂ emissions would still lead to substantial warming after today: about another 1°C for the higher end of the CO₂ estimate for the Pliocene, and more than 2°C at the lower end.²

The mechanisms that may have led to such a high climate sensitivity are unknown, but there is some evidence that Arctic sea-ice feedback could have contributed. It is possible that even if we stopped emitting CO₂ now, we could still experience an ice-free Arctic in the near future that could lock in significant warming for decades to come because of additional energy absorbed by the ice-free ocean in the long Arctic summer days. In the latest round of climate model simulations, those models that correctly simulate past sea-ice loss tend to have a higher climate sensitivity than usually assumed. Remarkably, even models driven by an extremely low-emissions scenario, approaching a stopnow scenario, still show an ice-free Arctic before 2050 (SIMIP Community 2020). The principal mechanism here is that even at declining CO₂ concentrations, excess heat stored in the oceans will only decline very slowly (Solomon et al. 2010).

It is important to stress that the scenario just discussed is largely speculative and only serves to illustrate how far we have already proceeded on a route to irreversibly altering our planet’s climate state. Computer simulations of possible future climate states using certain scenarios of greenhouse gas emissions can be used to gain a general impression of how this trend might continue – as there is still no evidence of a lowered CO₂ level due to climate policy (Knorr 2019; Le Quéré et al. 2020).

A high-profile publication by a group of US scientists (Burke et al. 2018) confirms that we are indeed in the process of driving our climate system well into uncharted territory. Different to the approach followed by the IPCC (Hoegh-Guldberg et al. 2018), the group did not try to assess the impacts of projected changes directly by assessing impacts of past changes or using computer models. Instead, they compared expected climate warming patterns derived from model simulations with what we know from the geological past. They concluded that, at even ‘moderate’ degrees of warming, the climate in large parts of the planet will not resemble anything seen anywhere on earth since at least the onset of agricultural civilization. Instead, the combination of extreme heat and humidity due to be encountered in large parts of the world will have their closest analogue in deep time. In the case of a rapid and unprecedented decarbonization of the world economy, climate is expected to eventually stabilize at a state most closely resembling the already-discussed Mid-Pliocene warm period, some 3–5 million years ago. In a much more likely higher-emissions scenario, however, large parts of the earth will revert to a climate state last seen ‘just’ – in geological terms – after the demise of the dinosaurs: the early Eocene, some 50 million years ago.

This scary scenario is not all, because it only considers the start and end point of warming, but not the path on which we get there. If, within a few generations, we turn back the earth’s geological CO₂ levels by tens of millions of years, then the rapidity of this change must surely have an impact on the way climate heating will play out. Unfortunately, this rate exceeds anything we know of from the deep geological past (Zeebe, Ridgwell and Zachos 2016), and therefore we cannot know in what ways dangerous anthropogenic climate change will occur in the coming decades. Which in itself is worrying (Read and O’Riordan 2017a). What is known, however, is that the large swings in temperature between ice ages and warm periods, bringing about temperature changes of up to 6°C peak to peak (Hansen et al. 2013), did not happen gradually – as the climate model runs underlying the above study suggest – but in bursts and bouts (Masson-Delmotte et al. 2005). Those climate oscillations were approximately as rapid as the warming we are seeing today and were created by various climate feedbacks, or tipping points. Then, about 10,000 years ago, a much more stable climate established itself. Some scholars argue that before this point, agriculture was impossible due to rapid climate fluctuations, but afterwards more or less unavoidable (Fagan 2004; Staubwasser and Weiss 2006).

In a recent commentary, prominent scientists have warned that tipping points and feedbacks similar to those that made the climate hostile to agriculture may have already been set in motion by the rapid increase in CO₂ levels (Lenton et al. 2019). During times of change, rapid collapse rather than gradual change is quite common for both ecosystems (Cooper, Willcock and Dearing 2020; Williams and Lenton 2010) and societies (Fagan 2008; Lee 2009). This suggests a triple threat to human civilization: agriculture has never existed in a strongly fluctuating climate; it also has never existed in climate states resembling distant geological warm period; and complex systems, such as human societies, can collapse even more rapidly than the ongoing speed of climate warming.

Recognizing the severity of the threat and following on from increasingly vocal and civilly disobedient climate protesters, several countries and countless organizations – from local councils to universities – have recently declared a state of climate emergency. Among those is the European Parliament, the first parliamentary representative of a major global emitter of greenhouse gases. Unfortunately, if this state of alertness exists, it has not been followed up by actions. Human emissions of CO₂, which had just started to pick up during the time of Arrhenius, continue to rise, apart from a decline due to the recent pandemic likely to be temporary (Le Quéré et al. 2020). Continuing investments in fossil fuel exploration and production (Tong et al. 2019) and continuing subsidies for fossil fuels make it unlikely that the situation will change any time soon (Farand 2018; ODI 2019; Trinomics 2018). Using past climate records and with some minimal use of climate models, it has been inferred that if we burn all fossil fuels, most of the earth will become uninhabitable for humans (Hansen et al. 2013). That’s one reason for the name of the climate and ecological activism movement ‘Extinction Rebellion’.

The remainder of this chapter will lay out the case that, when it comes to global heating, there is still no sign of any action that would resemble a true case of emergency. It will then be devoted to the question of why the gravity of the climate threat has largely been ignored or downplayed, even by many climate scientists themselves (Spratt and Dunlop 2018).