| All major climate changes, including natural ones, are disruptive. Past climate changes led
to extinction of many species, population migrations, and pronounced changes in the land
surface and ocean circulation. The speed of the current climate change is faster than most of
the past events, making it more difficult for human societies and the natural world to adapt. The largest global-scale climate variations in Earth’s recent geological past are the ice age cycles (see
infobox, p.B4), which are cold glacial periods followed by shorter warm periods [Figure 3]. The last few
of these natural cycles have recurred roughly every 100,000 years. They are mainly paced by slow changes
in Earth’s orbit which alter the way the Sun’s energy is distributed with latitude and by season on Earth.
These changes alone are not sufficient to cause the observed magnitude of change in temperature, nor to
act on the whole Earth. Instead they lead to changes in the extent of ice sheets and in the abundance of
CO2 and other greenhouse gases which amplify the initial temperature change and complete the global
transition from warm to cold or vice versa.
Recent estimates of the increase in global average temperature since the end of the last ice age are 4 to 5
°C (7 to 9 °F). That change occurred over a period of about 7,000 years, starting 18,000 years ago. CO2 has
risen by 40% in just the past 200 years, contributing to human alteration of the planet’s energy budget
that has so far warmed Earth by about 0.8 °C (1.4 °F). If the rise in CO2 continues unchecked, warming
of the same magnitude as the increase out of the ice age can be expected by the end of this century or
soon after. This speed of warming is more than ten times that at the end of an ice age, the fastest known
natural sustained change on a global scale. |