2.8 Summer energy as the relevant insolation forcing

1 INTRODUCTION

“Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem, which it was intended to solve.” Karl R. Popper (1972)

Outstanding questions in climate change

One of the main global themes of the past decades is the global climate debate across science, policy, and advocacy. It has its root in the 1988 Toronto Conference on the Changing Atmosphere. Consensus had been building among climate scientists since 1980 that the increase in carbon dioxide (CO2) during the previous decades was finally having an effect on surface temperatures. In 1988 that concern became global, and since then it has been a central issue to the UN and many nations. The Intergovernmental Panel on Climate Change (IPCC), created in 1988, has become the world reference in climate science through its vast assessment reports on the scientific knowledge about climate change.

As the most authoritative voice of climate science, the IPCC has a great influence in shaping the political discourse. Through the development of future scenarios, the IPCC has decisively contributed to an “emergency discourse” supported by three powerful, yet unproven, scientific concepts (Asayama 2021): • The idea of a temperature threshold, a safe limit beyond which irreversible climate damage will take place. Initially set at +2 °C, this arbitrary threshold was moved to +1.5 °C in 2018 (IPCC Special Report on

Global Warming of 1.5°C; SR15). • The idea of an allowable carbon budget to stay below a given temperature, despite the lack of an established and accepted value of CO2 climate sensitivity.

This allows the translation of temperature limits into policies to restrict greenhouse gases (GHGs) emissions. • The idea of a climate deadline, derived from the calculation of when some temperature threshold will be crossed for a given level of emissions. This idea has captured the attention of climate activists and politicians. It can be illustrated by the phrase “we only have 12 years to save the planet,” (The Guardian 2018) extracted from the 2018 SR15 report that states emissions must decline by about 45% from 2010 levels by 2030 to avoid surpassing the temperature threshold.

The UN secretary general, António Guterres, urged all countries to declare a state of climate emergency until the world has reached net zero CO2 emissions (The Guardian 2020). At least 15 countries and over 2,100 local governments in 39 countries have done so, covering over 1 billion people. According to most scientists, politicians, international organizations, and the media, there is no greater pressing problem faced by humankind that only the reduction of our CO2 emissions will allow avoidance of the clear and impending danger of an irreversible climate catastrophe.

Given the stakes, it is our scientific duty to make sure we properly understand the totality of climate change. After all, climate change has always happened. In the words of Freeman Dyson: “Climate change is part of the normal order of things, and we know it was happening before humans came.” (Roychoudhuri 2007). It is clear that unusual changes are taking place in the climate system of the planet. The global glacier retreat that has been occurring since c. 1850 has no precedent for several thousand years (Solomina et al. 2015). This is happening at a time when the Milankovitch glacial cycle theory indicates glaciers should be growing, something that happened for most of the time during the last five thousand years until the 19th century. The cryosphere retreat for the past two centuries has all the marks of a strong anthropogenic contribution. However, the claim by the IPCC that the climate change since pre-industrial times shows no evidence of a significant natural contribution should be met with a dose of skepticism. After all, there is no generally agreed cause for the Little Ice Age (LIA), the early 14th century to mid-19th century cold period, well registered in human history (Zhang et al. 2007; Parker 2012). If we are uncertain of what caused the LIA, how can we be so sure about the cause of what came afterwards?

This book is the result of a profound and detailed study on natural climate change, a relatively neglected part of today's climate science. It is not a review of what is known on natural climate change, as other excellent books accomplish that. Quite the contrary, the book is a detailed examination of unanswered natural climate change questions and problems, whose discussion is usually restricted to highly specialized scientific works. The relevant evidence to these outstanding climate questions, painstakingly gathered by climate researchers over the last decades, is displayed in a multitude of custom-made illustrations. The book discusses the evidence for the presence and causes of natural climate cycles, and other natural climate events, that have had a profound effect on the past climate of the planet, and their relevance to present climate change.

Chapter 2 examines the unsettled questions in the glacial–interglacial cycle of the Pleistocene, and standing

problems with their most accepted explanation, the Milankovitch theory. The Mid-Pleistocene Transition changed the interglacial frequency from a 41-kyr obliquity cycle to a poorly defined 100-kyr cycle of uncertain origin. A drastic change with great repercussions for the climate of the planet for which no explanation can be found in a corresponding change in Milankovitch forcings, as they are currently understood. Trying to explain the 100kyr frequency in terms of eccentricity leads to the problematic weakening in Pleistocene climate records of the 125 and 405-kyr periodicities in eccentricity (Nie 2018), and to the puzzling observation that for the past 5 Myr eccentricity and its supposed climatic effect display anticorrelation (Lisiecki 2010). Two hypotheses within Milankovitch theory try to explain interglacial determination. The best well-known one focuses on 65°N peak summer insolation, a property that depends on precession changes. A competing hypothesis, that traces its origin to Milutin Milankovi# himself, focuses on a caloric summer or summer energy integral that mostly depends on obliquity changes (Huybers 2006). This hypothesis, practically unknown outside specialized circles, is the one best supported by evidence, and readily solves many of the problems detected with Milankovitch theory, including that, for some interglacials, the effect appears to precede the cause.

Chapter 3 investigates the Dansgaard–Oeschger cycle found in proxy records of the last glacial period. These events served as blueprint for the definition of abrupt climate change. Their cause is unsettled, and different hypotheses put the focus on the Atlantic Meridional Overturning Circulation, meltwater events, sea-ice processes, or abrupt changes in thermohaline water stratification. The unresolved question of their periodic occurrence is revisited, producing an interesting twist: Evidence suggests that Dansgaard–Oeschger events are triggered according to two inter-related periodicities of a suggested tidal origin.

Chapter 4 analyzes the evidence for Holocene climatic variability. The Holocene was long considered a quite stable climate period, but evidence has been uncovered in the last few decades of the occurrence of over twenty centennial periods when the rate of climate change greatly surpassed the long-term average. The most intense and best studied of these periods is the 8.2-kyr event, but the existence of so many periods of abrupt climate change at times when GHGs hardly changed reveals that our understanding of natural climate change is inadequate. The Mid-Holocene Transition that separates the Holocene Climatic Optimum from the Neoglacial was accompanied by a change in climatic frequencies that has not been properly explained (Debret et al. 2009).

Does a 2500-yr climate cycle exist? It was first proposed by Scandinavian researchers in the early 20th century that high latitude Holocene climate was comprised of four botanical periods of c. 2500 years (the BlyttSernander sequence). Their relevance is not just regional, because the high latitudes are more sensitive to climate change, amplifying its variations (e.g., Arctic amplification) and more clearly display less prominent global changes. This climatic classification was popular among researchers before the 1970s. Chapter 5 investigates the related c. 2500-yr climate cycle first proposed by Roger Bray (1968), for which abundant proxy evidence exists. He proposed a solar cause for it, and cosmogenic isotope records show a remarkable degree of agreement, showing the coincidence of Spörer-type solar grand minima with the most conspicuous periods of abrupt climate deterioration in the Holocene proxy climate evidence. The features of the proxy evidence that display this solar-climate correspondence for the Bray cycle suggest what aspects of the climate system are most affected by the persistent changes in solar activity when they last several decades.

Chapter 6 deals with the archaeological and historical evidence, in addition to climate proxy evidence, for the abrupt climatic events tied to the Bray cycle, and the mark they left in some human societies at the times they took place. There is a clear association between periods of profound climate worsening and periods of social crisis, that often coincide with important cultural transitions, lending support to the hypothesis that climate change acts as an engine for societal progress and adaptation (Roberts et al. 2011). Archaeological climate studies are increasingly important and both scientific disciplines can benefit from their interaction.

For the past two decades, since Gerard Bond et al. published their landmark article (2001), there has been a scientific debate over the existence and importance of a 1500-yr Holocene climate cycle. This unresolved discussion has greatly abated in the latest years due to contradictory evidence resulting in the cycle's waning acceptance. Chapter 7 takes a critical look at this question, and shows that when properly framed, the existence of the cycle is clearly supported by a particular subset of climate proxies. The nature of these proxies gives important clues regarding the 1500-yr cycle’s possible mode of action. Unusual features displayed by some of these proxies raise the possibility that the cycle has a different cause than generally assumed.

Holocene solar activity, as inferred from cosmogenic isotope records, displays several periodicities in frequency analysis. These controversial quasi-cycles present a variability in the period and amplitude of their oscillations similar in proportion to the substantial variability in the accepted 11-yr solar cycle. Chapter 8 examines the evidence supporting their existence and their correspondence with quasi-cyclical climate changes. The very good phase agreement between solar oscillations and climate oscillations explains why this association is so pervasive in the paleoclimatological scientific literature. This association is not often discussed outside the discipline.

Chapter 9 looks at the climatic impacts resulting from changes in the greenhouse effect. It starts with the history of how it evolved to become at present the most accepted explanation for climate change, and then it focuses mainly on the role of GHG variations as an agent for natural climate change, a perspective less explored than its anthropogenic role. The unsettled Faint Sun Paradox and the possible factors proposed as an explanation are examined. The role of CO2 in Phanerozoic climate evolution is controversial and, when closely examined, the quality of the data does little to resolve the debate. The usual explanation that long-term CO2 changes precisely compensated for long-term changes in solar brightness leads directly to the anthropic principle, i.e., if it had not happened we would not be here. A unsatisfactory, unfalsifiable answer. The Cenozoic era, with better data, displays a puzzling lack of correspondence between climate changes and CO2 changes that is seldom discussed. A role for the CO2 changes of the past 70 years in modern global warming is