Chapter 1
Climate Change Through Earth's History
Jan Zalasiewicz, and Mark Williams Department of Geology, University of Leicester, Leicester, UK
Abstract
For at least 3.8billionyears, the Earth has possessed a climate system that continuously maintained a surface environment conducive to life, with liquid water at the surface, interchanging with various amounts of polar ice. The early Earth was warm, perhaps with higher concentrations of greenhouse gases compensating for a solar output smaller than present. Intermittent ice ages began ~2.5billion years ago, perhaps associated with oxygenation of the Earth's atmosphere and reduction in methane levels; these included more or less worldwide ‘Snowball Earth’ events, with ice extending to low latitudes. From the beginning of the Phanerozoic, the Earth has alternated between ‘icehouse states’ – albeit of lesser severity than the ‘Snowball’ events – and ‘greenhouse states’, such as that of the Mesozoic Era. The Earth is currently in an icehouse state marked by geologically closely spaced fluctuations of climate, largely paced by astronomical variations and amplified by changes in greenhouse gas levels. The present interglacial state of the Holocene is very likely to soon attain levels of climatic warmth not seen for several million years, because of human modification of climate drivers, notably greenhouse gases.
Keywords
Anthropocene; Climate; Glaciations; Palaeoclimate; Quaternary
Chapter Outline
1. Introduction
Earth's climate is now changing in response to an array of anthropogenic perturbations, notably the release of greenhouse gases; an understanding of the rate, mode and scale of this change is now of literally vital importance to society. There is currently intense study of current and historical (i.e. measured) changes in both perceived climate drivers and the Earth system response. Such studies typically lead to climate models that, in linking proposed causes and effects, are aimed at allowing prediction of climate evolution over an annual to centennial scale.
However, the Earth system is complex and imperfectly understood, not least as regards resolving the effect of multiple feedbacks in the system and of assessing the scale and importance of leads, lags and thresholds (‘tipping points’) in climate change. There is thus a need to set modern climate studies within a realistic context by examining the preserved history of the Earth's climate in the rock succession. Such study cannot provide precise replicas of the unplanned global experiment that is now underway (for the sum of human actions represents a geological novelty). However, it is providing an increasingly detailed picture of the nature, scale, rate and causes of past climate change and of its wider effects
[1], regarding, for instance, sea level and biota. Imperfect as it is, it provides an indispensable context for modern climate studies, not least as a provision of ground truth for computer models (see below) of former and present climate.
Aspects of climate that are recorded in strata include temperature and seasonality
[2,
3], humidity/aridity
[4], and wind direction and intensity
[5]. Classical palaeoenvironmental indicators such as glacial tills, reef limestones and desert dune sandstones have in recent years been joined by a plethora of other proxy indicators. These include many biological (fossilized pollen, insects, marine algae) and chemical proxies (e.g. Mg/Ca ratio in biogenic carbonates). Others are isotopic: oxygen isotopes provide information on temperature and ice volume; carbon isotopes reflect global biomass and inputs (of methane or carbon dioxide) into the ocean/atmosphere system; strontium and osmium are proxies for weathering, and the latter, with molybdenum also, for oceanic oxygenation levels. Other proxies include recalcitrant organic molecules: long-chain algal-derived alkenones as sea temperature indicators
[6] and isorenieratane as a specific indicator of photic zone anoxia
[7]. These and many other proxies are summarized in
[8]. Levels of greenhouse gases such as carbon dioxide and methane going back to 800ka can be measured in ice cores
[9]. For older periods, indirect measurements are made, based on proxies such as leaf stomata densities
[10], palaeosol chemistry
[11], boron isotopes
[12] and alkenones
[13]; estimates of greenhouse gas concentrations have also been arrived at by modelling
[14,
15].
2. Climate Models
Since the 1960s, computer models of climate have been developed that provide global and regional projections of future climate and reconstructions of deep time climate. Some of these models are used to simulate conditions during ancient icehouse climates, whilst others examine warm intervals of global climate, such as during the Mesozoic and Early Cenozoic greenhouse
[16]. The most widely applied computer simulations of palaeoclimate are general circulation models (GCMs). The increasing complexity of these models has followed the exponential growth in computer power.
GCMs divide the Earth into a series of grid boxes. Within each of box variables important for the prediction of climate are calculated, based upon the laws of thermodynamics and Newton's laws of motion. At progressive time steps of the model, the reaction between the individual grid boxes is calculated. GCM simulations rely on establishing key boundary conditions. These conditions include solar intensity, atmospheric composition (e.g. level of greenhouse gases), surface albedo, ocean heat transport, geography, orography, vegetation cover and orbital parameters. In general, the boundary conditions are more difficult to establish for increasingly older time periods. Thus, orbital parameters may be established with high precision in a computer model for a short time interval of the Pliocene
[17]. But for much older time periods, for example the glacial world of the late Ordovician, the rock record is much less complete, and it is difficult to constrain most of the boundary conditions with a reasonable degree of precision
[18].
Geological data (e.g. sedimentology, palaeontology) are essential to ‘ground truth’ climate models, to establish whether they are providing a realistic reconstruction of the ancient world and also to provide data for calibrating boundary conditions for the models. Of major importance for GCM palaeoclimate reconstructions is accurate information about sea surface temperatures (SSTs), as these provide a strong indication of how ocean circulation is working. The most extensive deep time (pre-Quaternary) reconstruction of SSTs is that of the United States Geological Survey PRISM Group
[19]. This global dataset has been used for calibrating a range of climate model scenarios for the ‘mid Pliocene warm period’ (a.k.a. ‘Mid Piacenzian Warm Period’) and also includes an extensive catalogue of terrestrial data
[20]. Warm periods of the Pliocene are often cited as a useful comparison (though definitely not an analogue) for the path of late twenty-first century climate
[16].
3. Long-Term Climate Trends
Earth's known climate history, as decipherable through forensic examination of sedimentary strata, spans some 3.8billionyears (3.8×109a), to the beginning of the Archaean (
Fig. 1). The previous history, now generally assigned to the Hadean Eon, is only fragmentarily recorded as occasional ancient mineral fragments contained within younger rocks, particularly of highly resistant zircon dated to nearly 4.4billion years (4.4×109 a) ago
[21] and thus stretching back to very nearly the beginning of the Earth at 4.56billionyears ago
[22]. The chemistry of these very ancient fragments hints at the presence of a hydrosphere even at that early date, though one almost certainly disrupted by massive meteorite impacts
[23]. Certainly, by the beginning of the Archaean, oceans had developed, and there was an atmosphere sufficiently reducing to allow the preservation of detrital minerals such as pyrite and uraninite in river deposits that would not survive in the presence of free oxygen
[24].
Figure 1 Global climate variation at six different timescales.
(Data adapted from sources including [8,
28,
56,
74,
88,
117].) On the left side of the figure, the figure ‘T’ denotes relative temperature. Note that...
