Geoscience Reference
In-Depth Information
Past climate
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 50 million years the Earth's climate has been
cooling down, moving from the so-called 'greenhouse world' of the Eocene, with warm
and gentle conditions, through to the cooler and more dynamic 'ice house world' of today.
It may seem odd that in geological terms our planet is extremely cold, while this whole
topic is concerned with our rapid warming of the planet. This is because the very fact that
there are huge ice sheets on both Antarctica and Greenland, and nearly permanent sea ice in
the Arctic Ocean, makes the global climate very sensitive to changes in GHGs.
The long-term global cooling of the Earth kicked off with glaciation of Antarctica about 35
million years ago and then the great Northern Hemisphere ice ages, which began 2.5 milli-
on 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 kilometres (km) thick to form over much of North
America and Europe. Between 2.5 and 1 million years ago, these glacial-interglacial cycles
occurred every 41,000 years, and since 1 million years ago they have occurred every
100,000 years. These great ice-age cycles are driven primarily by changes in the Earth's or-
bit with respect to the Sun. In fact, the world has spent over 80 per cent of the last 2.5 milli-
on years in conditions colder than the present. Our present interglacial, the Holocene Peri-
od, started about 10,000 years ago, and is an example of the rare warm conditions that oc-
cur between each ice age. The Holocene began with the rapid and dramatic end of the last
ice age: in less than 4,000 years global temperatures increased by 6°C, relative sea level
rose by 120 metres (m), atmospheric CO
2
increased by one-third, and atmospheric CH
4
doubled. Still, this is much slower than changes we are seeing today. James Lovelock in his
topic
The Ages of Gaia
suggests that interglacials, like the Holocene, 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. These
large scale past changes in global climate are discussed in more detail in my other topic
Climate: A Very Short Introduction
.
