FUTURE IMPLICATIONS OF BACKGROUND CLIMATE CHANGE
Causes of Climate Change on Glacial-Interglacial Timescales
Introduction to Climate
The Earth’s climate is a dynamic and variable quantity. The term ‘climate’ commonly refers to the mean and variability of temperature, precipitation and wind over a period of time. The evolution of the climate system is influenced by both internal dynamics associated with the Earth’s orbit and external factors, or ‘forcings’, including natural phenomena such as volcanic eruptions and human-induced changes (Treut et al., 2007). Investigations into the Earth’s climate over recent decades have found evidence of increasing anthropogenic influence on climate change (Treut et al., 2007), however there is comparatively less understanding of background climate change on glacial-interglacial timescales. To better understand the impact human activity has on the climate, it is important to recognise the history of climate change science.
The Earth’s Climate in Glacial-Interglacial Timescales
Over the past two million years, the Earth’s climate has naturally proceeded through glacial-interglacial timescales.
• Glacials are defined by the development of ice sheets on a sub-continental scale, as a consequence of the general cooling of Earth’s climate (Maslin, 2016), caused by variations that occur in the Earth’s orbit.
• Interglacials result from melting glaciations and are characterised by periods of reduced carbon dioxide (CO2) concentrations and cooler Antarctic temperatures (Tzedakis et al., 2012).
The Last Glacial Maximum occurred only 20,000 years ago.
Background Climate Change
In 1941, Serbian mathematician Milankovic first proposed the concept of orbital forcing – that variations in Earth’s orbit changed the distribution of solar energy on the planet’s surface, effectively controlling the occurrence of ice ages (Maslin, 2016). He suggested that insolation (incoming solar radiation) at a latitude of 65oN was critical (Maslin, 2016), leading to interglacial periods followed by the rapid growth of continental ice sheets in the Northern Hemisphere (Ganopolski et al., 2016). Hays, Imbrie and Shackleton later deigned to test Milankovic’s theory (Maslin, 2016), with their 1976 paper concluding that changes in the Earth’s orbital geometry were the “fundamental cause of the succession of Quaternary Ice Ages” (the current geological period, beginning 2.6 million years ago) (Hays et al., 1976). They also indicated that climatic variance was concentrated in three distinct periods of 23,000, 42,000 and approximately 100,000 years (Hays et al., 1976).
Earth’s Orbital Cycles
Hays, Imbrie and Shackleton showed that the intensity of solar radiation received at the top of the atmosphere, for any particular latitude, varies periodically with three aspects of the Earth’s orbit: eccentricity, obliquity and precession of the equinoxes (see Figure 1) (Hays et al., 1976).
• Eccentricity: describes the shape of the Earth’s orbit, varying from nearly circular to elliptical because of the influence of Jupiter’s gravity, with a period of approximately 100,000 years (Maslin, 2016).
• Obliquity: governs the intensity of the seasons, determined by the tilt of the Earth’s axis of rotation relative to the plane of its orbit, oscillating with a period of 42,000 years (Maslin, 2016).
• Precession: both Earth’s rotational axis and its orbital path precess (rotate) over time – the combination of these two components alters the seasonal distance between the Earth and the Sun on a cycle of approximately 23,000 years (Maslin, 2016).
These orbital variations determine the distribution of solar insolation, with obliquity having the greatest significance for insolation at high latitudes and precession affecting seasonal insolation of all latitudes. Their discoveries have since been used to prove orbital forcing of climate and thus generate Earth system model simulations, with the CLIMBER-2 model successfully simulating the last eight glacial cycles (Ganopolski et al., 2016).
Figure 1: Milankovic Cycles of eccentricity, obliquity and precession (Maslin, 2016).
Implications for Future Climate Change
Delayed Glacial Inception
Hays, Imbrie and Shackleton’s model of future climate using the observed orbital-climate relationships – but ignoring anthropogenic influences – predicted a long-term trend over the next several thousand years towards extensive Northern Hemisphere glaciation (Hays et al., 1976). Earth’s history has shown that a decrease in Northern Hemisphere insolation leads to glaciation, accompanied by a reduction in atmospheric CO2 concentration (Ganopolski et al., 2016), which plays a secondary role (Tzedakis et al., 2012). Simulations show that, without anthropogenic greenhouse gas emissions, CO2 concentration is set to gradually decline, oscillating on the orbital timescale (Ganopolski et al., 2016). However, there is presently no evidence indicating the beginning of a new ice age, despite current summer insolation at 65oN being close to its minimum (Ganopolski et al., 2016). Without a considerable drop in atmospheric CO2 concentration, this escape from glacial inception makes the commencing of a new glacial period highly unlikely (Ganopolski et al., 2016).
Impact of Atmospheric Carbon Dioxide Concentration
The relationship between CO2 concentration and insolation-CO2 explains the timing of glacial inceptions – each past glaciation has occurred when the CO2 concentration was lower than insolation-CO2 (Ganopolski et al., 2016). Due to the present low eccentricity of orbital forcing, CO2 concentration remains above insolation-CO2, and hence the conditions for glacial inception will not be met in the near future (Ganopolski et al., 2016). Furthermore, due to the particularly long lifespan of anthropogenic CO2 in the atmosphere – approximately 7% of the CO2 will remain in the atmosphere after 100,000 years (Archer and Ganopolski, 2005) – future emission will have a significant impact on the occurrence of the next glacial inception (Ganopolski et al., 2016). It is important to recognise that orbital-scale variability is no longer moderating climate change – anthropogenic activity is (Tzedakis et al., 2012).
Anthropogenic Climate Change
Small increases in greenhouse gases from the expansion of agriculture can be traced back to 8,000 years ago (Maslin, 2016). The dawn of the industrial era resulted in a 35% increase of atmospheric CO2, primarily the result of the combustion of fossil fuels and the removal of forests (Treut et al., 2007). Humankind has dramatically altered the chemical composition of the global atmosphere, with substantial implications for the climate. Evidence from climate models indicates that anthropogenic influence has delayed the next ice age, as increased greenhouse gas concentrations have countered the Earth’s natural cooling trend (Tzedakis et al., 2012). Given the long atmospheric lifetime of CO2, models predict the release of 5000 Gton of CO2 will signify the end of glacial cycles, with the stable existence of the interglacial for at least the next 500,000 years (Archer and Ganopolski, 2005).
Consequences of Divergence from Glacial-Interglacial Timescales
Deglaciation following the Last Glacial Maximum resulted in the melting of approximately 50 million km3 of ice. The successive sea level rise of 120 metres occurred between 14,000 and 8,000 years ago. Rises in sea-level are now perceptible within human lifetimes, with potentially devastating consequences. There is potential for displacement of peoples following damage to human-built environments, such as coastal flooding from rising sea levels (Shogren and Toman, 2000). Reduced productivity of natural resources that humans extract from the natural environment is also a cause for concern, resulting in damage to food sources and lower agricultural yield, and ultimately famine (Shogren and Toman, 2000). There is also a risk of destruction of the natural conditions advantageous to various landscapes, natural habitats for scarce species, and biodiversity, as rising sea levels inundate environs (Shogren and Toman, 2000). As global temperatures rise, catastrophic earthquakes can occur, caused by crustal movement following removal of an ice mass.
Future Policy Considerations
The IPCC Fourth Assessment Report on Climate Change in 2007 aimed to investigate and understand both the human and natural drivers of climate change, and project future implications for our climate (Solomon et al., 2007). In the report, they defined carbon dioxide as the “most important anthropogenic greenhouse gas”, citing fossil fuel use and land-use change as the primary sources (Solomon et al., 2007). The global atmospheric concentration of CO2 had increased from 280ppm in the pre-industrial era to 379ppm in 2005 (Solomon et al., 2007).
Global initiatives are necessary to evolve climate policy, as indicated by the 1992 United Nations Framework Convention on Climate Change. Article 4 states that nations should cooperate to improve human adaptation and mitigation of climate change (Shogren and Toman, 2000). Climate change is a challenge faced as a species, not nations, and responses must be fitted into a global framework. On a global scale, it is important to consider how the countries that inadvertently caused the problem can compensate those most suffering from the consequences. For instance, as sea levels rise, more economically developed low-lying countries will be better equipped to mitigate the effects. Global rises in CO2 concentration have evidently disrupted glacial-interglacial timescales. It is imperative that action is taken to reduce CO2 emissions and therefore, the following should be considered in climate policy making:
• The role of regulation, both public and private, with a recommended general agreement that polluters increasing atmospheric CO2 concentration should pay for the damage they may do.
• The essential investment in renewable energy – for instance, solar energy, geoengineering, power from biofuels, tidal and ocean power, and wind and hydro power – where these are suited to the climatic conditions of the local environment.
• Changing of government policy to prioritise the research, development and implementation of alternatives to finite fossil fuels.