Past climate change

Climate change in the geological past has been reconstructed using a number of key archives, including marine and lake sediments, ice cores, cave deposits, and tree rings. These various records reveal that over the last 100 million years the Earth's climate has been cooling down, moving from the so-called 'Greenhouse World' of the Cretaceous Period, when dinosaurs enjoyed warm and gentle conditions, through to the cooler and more dynamic 'Ice House World' of today (see Figure 12). It may seem odd that in geological terms our planet is relatively cold, while this whole book is concerned with our great fears of global warming. This is because even today we have huge ice sheets on both Antarctica and Greenland and nearly permanent sea ice in the Arctic Ocean. So, compared to the time of the dinosaurs, when there were no massive ice sheets, we live in chilly times.

This long-term, 100-million-year transition to cooler global climate conditions was driven mainly by tectonic changes, such as the opening of the Tasmanian-Antarctic gateway and the Drake passage, which isolated Antarctica from the rest of the world, the uplift of the Himalayas, and the closure of the Panama ocean gateway. There is also geological evidence that levels of atmospheric carbon dioxide have become significantly lower over the last 100 million years. These changes culminated in the glaciation of

12. The anatomy of past climatic changes

Antarctica about 35 million years ago and then the great northern hemisphere ice ages, which began 2.5 million years ago. Since the beginning of the great northern ice ages the global climate has cycled from conditions that were similar or even slightly warmer than today, to full ice ages, which caused ice sheets over 3 km thick to form over much of North America and Europe. Between 2.5 and 0.9 million years ago these glacial-interglacial cycles occurred every 41,000 years and since 0.9 million years ago they have occurred every 100,000 years. These great ice-age cycles are driven primarily by changes in the Earth's orbit with respect to the sun. In fact the world has spent over 80% of the last 2.5 million years in conditions colder than the present. Our present interglacial, the Holocene Period, started about 10,000 years ago and is an example of the rare warm conditions that occur between each ice age. The Holocene began with the rapid and dramatic end of the last ice age; in less e than 4,000 years global temperatures increased by 6°C, relative sea I. level rose by 120 m, atmospheric carbon dioxide increased by a = third, and atmospheric methane doubled. ?

It may seem strange in a book about global warming to suggest that e we are currently in a geological 'Ice House World'. This is, however, n an important point when we look at the consequences of the world e warming up, because, despite being in a relatively warm interglacial period, both poles are still glaciated, which is a rare occurrence in the geological history of our planet. Antarctica and Greenland are covered by ice sheets, and the majority of the Arctic Ocean is covered with sea ice. This means that there is a lot of ice that could melt in a warmer world, and, as we will see, this is one of the biggest unknowns that the future holds for our planet. The two glaciated poles also make the temperature gradient or difference between the poles and the Equator extremely large, from an average of about + 30°C at the Equator down to -35°C or colder at the poles. This temperature gradient is one of the main reasons that we have a climate system, as excess heat from the tropics is exported both via the oceans and the atmosphere to the poles, which causes our weather. Geologically, we currently have one of the largest Equator-

pole temperature gradients, which leads to a very dynamic climate system. So our 'Ice House' conditions cause our very energetic weather system, which is characterized by hurricanes, tornadoes, extra-tropical (temperate) winter storms, and monsoons. James Lovelock in his book 'The Ages of Gaia' (New edition, 1995 p. 227) suggests that interglacials, like the Holocene Period, are the fevered state of our planet, which clearly over the last 2.5 million years prefers a colder average global temperature. Lovelock sees global warming as humanity just adding to the fever.

Climate, however, has not been constant during our interglacial, i.e. the last 10,000 years. Palaeoclimate evidence suggests that the early Holocene was warmer than the 20th century. Throughout the Holocene there have been millennial-scale climate events, called Dansgaard-Oeschger cycles, which involve a local cooling of 2°C. These events have had a significant influence on classical og civilizations; for example, the cold arid event about 4,000 years ago | coincides with the collapse of many classical civilizations, such as j| the Old Kingdom in Egypt (see discussion in Chapter 9). The last of 3 these millennial climate cycles was the Little Ice Age. This event is really two cold periods; the first follows the Medieval Warm Period which ended a thousand years ago, and is often referred to as the Medieval Cold Period. The Medieval Cold Period played a role in extinguishing Norse colonies on Greenland and caused famine and mass migration in Europe. It started gradually before ad 1200 and ended at about ad 1650. The second cold period, more classically referred to as the Little Ice Age, may have been the most rapid and largest change in the North Atlantic region during the late Holocene, as suggested by ice-core and deep-sea sediment records. The reconstruction of temperature records for the last thousand years includes the Little Ice Age and is essential data for demonstrating that the last two centuries are very different from the preceding eight (Figure 13). There are four main data sets which have attempted to reconstruct temperatures for the northern hemisphere over the last millennium: tree rings, corals, ice cores, and/or the direct measurement of past temperatures from

1000

1200

1600

1800

1400 Year

13. Northern Hemisphere temperature reconstruction for the last thousand years

2000

1000

1200

1600

1800

1400 Year

13. Northern Hemisphere temperature reconstruction for the last thousand years

2000

boreholes. First, it should be noted that the different data sets compare well with each other, which gives added confidence that we are seeing real temperature variations in these reconstructions. Second, the data show that the centuries before 1900 were much colder. They also show that the Medieval Warm Period and the Little Ice Age did occur, but that in much of the northern hemisphere the climate changes seen are only small, with the exception of northern Europe. Without this data the instrumental temperature data set for the last 150 years would have no context. As it is, it can now be clearly shown that temperatures, at least for the northern hemisphere, have been warmer in the 20th century than at any other time during the last thousand years.

Was this article helpful?

0 0

Post a comment