Variation of the climate system over time

The early atmosphere of the Earth was very different from the present mixture of mainly nitrogen and oxygen. Considerable quantities of hydrogen and helium were present, which have mostly escaped into space.

Much of the present atmosphere originates from volcanic emission of gases and subsequent photo-dissociation, or reaction with solar radiation. The existence of life on Earth has also contributed towards the present atmospheric composition by providing much of the oxygen. Stronger solar radiation, and a faster rotation rate, also influenced the atmosphere during its formation.

Since the advent of the present mixture of atmospheric gases the main determinant of the basic global climate pattern has been the Earth's geography. The strength of the solar radiation has probably been essentially constant since the present atmospheric composition was attained, except for the comparatively

Fig. 1.24. Mean global climate variation over the Earth's history. The left column is mean global temperature and the right column is mean global precipitation. Note the non-linear timescale. [Fig. 9.1 of Frakes (1979), Climates through Geologic Time. Reproduced with permission of Elsevier Science Publishers.]

Fig. 1.24. Mean global climate variation over the Earth's history. The left column is mean global temperature and the right column is mean global precipitation. Note the non-linear timescale. [Fig. 9.1 of Frakes (1979), Climates through Geologic Time. Reproduced with permission of Elsevier Science Publishers.]

small variations caused by the periodicities in the Earth's orbit, discussed below. Much of the atmosphere's driving energy is provided by the underlying surface, whether land or sea. Thus, if the present continental layout was changed, either in position or the relative proportions of land to ocean, then this heating would also change. In §1.6 we saw how the plates that make up the Earth's crust collide to form mountains and oceanic troughs, or divide to form new oceans. This occurs sufficiently slowly for little variation to be observed for several million years but such movement leads to very long-term climatic evolution.

Over the past 1000 million years the Earth has experienced three prolonged periods of extensive glaciation. Two of these, around 250 and 600 million years ago, occurred at pronounced minima in the global temperature record shown in Fig. 1.24. We are currently in the third of these glacial epochs, although in an interglacial intermission in a longer period of predominantly glacial conditions. For 210 million years after the last glacial era the climate of the Earth was much warmer than today. This climatic evolution probably occurred because of tectonic plate movement. A major difference between the glacial and warm eras is whether there is land over one or both of the polar zones. If there are polar land masses there is then the opportunity for snow to accumulate during such

Fig. 1.25. Palaeogeographic reconstruction at 100 million years ago, after the

Fig. 1.25. Palaeogeographic reconstruction at 100 million years ago, after the super-continent, Pangaea, has begun to break up. Light areas on continents indicate regions flooded by shallow seas

(maximum depth 100-200 m). The narrower part of the stretching around the globe in tropical regions is sometimes called the Tethys Seaway.

[Fig. 3 of Barron et al. (1980). Reprinted with permission of Elsevier Science Publishers.]

super-continent, Pangaea, has begun to break up. Light areas on continents indicate regions flooded by shallow seas

(maximum depth 100-200 m). The narrower part of the region of continuous ocean stretching around the globe in tropical regions is sometimes called the Tethys Seaway.

[Fig. 3 of Barron et al. (1980). Reprinted with permission of Elsevier Science Publishers.]

region's sun-less winters, and the depth of polar coldness for this snow to last through the summers and eventually form ice sheets.

During the Cretaceous period, while dinosaurs still dominated the Earth, the continental plates began to 'drift' towards their present positions, from a previous, more consolidated, state. The continental geography at the beginning of the process, 100 million years ago, is shown in Fig. 1.25. A zonally averaged temperature distribution for this state is given in Fig. 1.26, with the modern distribution and a full glacial state from 21000 years ago for comparison.

As time progressed, most of the continents moved north, although at different rates. Australia and Antarctica divided rather later, about 30 million years ago, and Antarctica drifted southwards while Australia travelled northwards. As Antarctica drifted over the South Pole, and Australia moved sufficiently far north for a circumpolar current to develop and effectively isolate Antarctica from poleward oceanic heat transport, ice sheets began to form. This occurred 30-40 million years ago. The slow global cooling, quickened by the formation of the Antarctic ice cap, was coupled to the slow isolation of the Arctic Ocean by North America and Eurasia, and the raising of the Tibetan plateau by the collision of India with Asia. Eventually this Arctic isolation, in conjunction with winter cooling of extensive land areas polewards of the Arctic Circle and deflection of the global atmospheric circulation by the Himalayas, led to the development of Northern Hemisphere ice sheets.

During the last million years the Earth has experienced repeated extensive glaciation in its Northern Hemisphere. The continental geography has been essentially constant during this time. Only minor variations in the distribution of land and sea, due to sea level oscillations of up to 150 m, have occurred (this will be discussed further in §6.2.2). A reason for this glacial cycling needs to be sought elsewhere.

Fig. 1.26. Zonal mean temperature during a warm Earth climate (100 million years BP, data from Barron and Washington, 1984) — broken line; a glacial climate (21 000 years BP, data courtesy of Paul Valdes) -dotted line; and the present interglacial climate.

Fig. 1.27. Periodicities of the Milankovitch cycle in the Earth's orbit. The Earth's position corresponds to Northern Hemisphere summer. [From Bigg, 1992c]

Fig. 1.27. Periodicities of the Milankovitch cycle in the Earth's orbit. The Earth's position corresponds to Northern Hemisphere summer. [From Bigg, 1992c]

The earth's orbit about the Sun is not circular. Instead it is slightly elliptical, with the Earth being at one of the foci of the ellipse. The closest point to the Sun, or perihelion, is reached in mid-December and the furthest point in mid-June (see Fig. 1.27). In addition, the Earth is tilted at an angle of 23.5° to a line perpendicular to the plane of rotation about the Sun. This orbit is not permanent. There are slight periodicities in its components that alter the distance, or orientation, of the Earth from the Sun. The eccentricity of the ellipse varies from 0.0 (essentially circular) to 0.06, with a period of 100000 years, although there is a modulation to this cycle so that the spread of eccentricity

Fig. 1.28. Variation of Northern Hemisphere mid-latitude temperatures since the peak of the last glaciation. Note the pronounced cooling around 11 000 years BP (the Younger Dryas) and that the warmest climate was some 6000-7000 years BP.

Thousands of years ago for any particular cycle is usually well within this extreme range. The tilt of the Earth's axis varies by 2.5° over 40000 years. Finally, the position of the perihelion moves through the year (the orbit is said to precess) with a period of about 20000 years. The variation in eccentricity alters the total amount of solar radiation to reach the Earth's orbit over a year, the change in tilt changes the latitudinal variation, and the precession of the perihelion alters the seasonal distribution of radiation. All of these periods appear in sedimentary records of the past million years; the eccentricity shows up most strikingly in the recent global record as the 100 000 year oscillation in Fig. 1.24. Prior to about 470 000 years ago glacial cycling occurred with a 40 000 year periodicity, still remarkably similar to one of the orbital periods. In Chapter 6 we will consider what might have caused this change of period.

The glacial fluctuations mainly affect the Northern Hemisphere. Ice sheets have extended as far south as southern England in Europe and the central United States in North America. There is a lag between the astronomical parameters and the climate due to the strong feedback between the highly reflective ice sheets and low radiation, once the ice sheets form. There is also a link between the state of the glaciation and the concentration of key greenhouse gases, such as CO2 and CH4. Chapter 6 will discuss the strong impact these events had on the ocean circulation, and the role of the ocean in assisting, and prolonging, such events.

The peak of the last glaciation was reached about 18 000 years before present (BP) but the deglaciation has not been a monotonic process since then. After a slow warming initially, the globe rapidly shed its northern ice, predominantly in two periods of melting about 12 000 and 9000 years BP. Global climate 90006000 years BP was as warm as, and sometimes warmer than, today (Fig. 1.28). Even over the past 2000 years the global temperature has varied by up to 1° C on either side of the present value. The Vikings were able to settle Greenland, Iceland and part of North America in a regionally milder climate than at present. In contrast, during much of the period from AD 1300 to 1800 significantly colder weather than today was experienced in western Europe. The winters, in particular, were extreme, with frequent freezing of major rivers such as the Thames. These changes are poorly understood and not always global in extent. Extreme volcanic activity could be responsible for periods of cooling, but the linking evidence is not clear. The ocean could also have a role. Chapter 6 will explore these possibilities further.

Thousands of years ago

Fig. 1.29. Variation of the atmospheric concentration of CO2 since 1860 (solid line, in parts per million), plotted with the mean global temperature over the same period. Bars show the year-to-year variation in global mean temperature, relative to the 1961-90 average. Note the significant rise in global temperature from 1920 to 1940 and again from 1980 to 2000; in contrast, the rise in atmospheric CO2 occurs continuously.

Today the principal cause of any imminent climatic change is thought to be increases in atmospheric concentrations of greenhouse gases produced by man since the beginning of the Industrial Revolution in the eighteenth century. Such increases should magnify the greenhouse effect, warming the Earth.

In Fig. 1.29 the evolution of global temperature over the last century is shown. This figure also shows the change in atmospheric CO2 concentration over the same interval. The latter shows an almost smoothly increasing curve. The global warming of about 0.7°C since 1880 has, however, mainly occurred during two short periods -1920 to 1940, and 1980 to 2000. At other times the global temperature has been roughly static. Therefore any link between greenhouse gas concentration and global warming is far from linear. As we have briefly seen in this chapter, the climate system is complex and highly interactive. The amount, and rate, of any warming will therefore be dependent on the inter-relationship of the different components of the system - natural variability and feedbacks induced by greenhouse warming - and not just changes in the composition of the atmosphere itself. Once we have explored the mechanisms of interaction, focusing on those involving the ocean, we will return to the climatic future in Chapter 7.

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