Climate on Earth has changed on all time scales, including long before human activity could
have played a role. Great progress has been made in understanding the causes and mechanisms
of these climate changes. Changes in Earth’s radiation balance were the principal driver of
past climate changes, but the causes of such changes are varied. For each case – be it the
Ice Ages, the warmth at the time of the dinosaurs or the fluctuations of the past millennium
– the specific causes must be established individually. In many cases, this can now be done
with good confidence, and many past climate changes can be reproduced with quantitative models.
Global climate is determined by the radiation balance of the planet (see FAQ 1.1). There are three fundamental ways the Earth’s radiation
balance can change, thereby causing a climate change: (1) changing the incoming solar radiation
(e.g., by changes in the Earth’s orbit or in the Sun itself), (2) changing the fraction of solar
radiation that is reflected (this fraction is called the albedo – it can be changed, for example,
by changes in cloud cover, small particles called aerosols or land cover), and (3) altering the
longwave energy radiated back to space (e.g., by changes in greenhouse gas concentrations). In
addition, local climate also depends on how heat is distributed by winds and ocean currents. All
of these factors have played a role in past climate changes.
Starting with the ice ages that have come and gone in regular cycles for the past nearly three
million years, there is strong evidence that these are linked to regular variations in the Earth’s
orbit around the Sun, the so-called Milankovitch cycles (Figure 1). These cycles change the amount
of solar radiation received at each latitude in each season (but hardly affect the global annual
mean), and they can be calculated with astronomical precision. There is still some discussion about
how exactly this starts and ends ice ages, but many studies suggest that the amount of summer
sunshine on northern continents is crucial: if it drops below a critical value, snow from the past
winter does not melt away in summer and an ice sheet starts to grow as more and more snow accumulates.
Climate model simulations confirm that an Ice Age can indeed be started in this way, while simple
conceptual models have been used to successfully ‘hindcast’ the onset of past glaciations based on
the orbital changes. The next large reduction in northern summer insolation, similar to those that
started past Ice Ages, is due to begin in 30,000 years.
Although it is not their primary cause, atmospheric carbon dioxide (CO2)
also plays an important role in the ice ages. Antarctic ice core data show that
CO2 concentration is low in the cold glacial times (~190 ppm), and
high in the warm interglacials (~280 ppm); atmospheric CO2 follows
temperature changes in Antarctica with a lag of some hundreds of years. Because the climate changes
at the beginning and end of ice ages take several thousand years, most of these changes are affected
by a positive CO2 feedback; that is, a small initial cooling due to the
Milankovitch cycles is subsequently amplified as the CO2 concentration
falls. Model simulations of ice age climate (see discussion in Section 6.4.1) yield realistic results
only if the role of CO2 is accounted for.
During the last ice age, over 20 abrupt and dramatic climate shifts occurred that are particularly
prominent in records around the northern Atlantic (see Section 6.4). These differ from the
glacial-interglacial cycles in that they probably do not involve large changes in global mean
temperature: changes are not synchronous in Greenland and Antarctica, and they are
FAQ 6.1, Figure 1. Schematic of the Earth’s orbital changes (Milankovitch cycles) that drive the ice age
cycles. ‘T’ denotes changes in the tilt (or obliquity) of the Earth’s axis, ‘E’ denotes
changes in the eccentricity of the orbit (due to variations in the minor axis of the ellipse),
and ‘P’ denotes precession, that is, changes in the direction of the axis tilt at a given
point of the orbit. Source: Rahmstorf and Schellnhuber (2006).
in the opposite direction in the South and North Atlantic. This means that a major change in global
radiation balance would not have been needed to cause these shifts; a redistribution of heat within
the climate system would have sufficed. There is indeed strong evidence that changes in ocean
circulation and heat transport can explain many features of these abrupt events; sediment data and
model simulations show that some of these changes could have been triggered by instabilities in the
ice sheets surrounding the Atlantic at the time, and the associated freshwater release into the ocean.
Much warmer times have also occurred in climate history during most of the past 500 million years,
Earth was probably completely free of ice sheets (geologists can tell from the marks ice leaves on
rock), unlike today, when Greenland and Antarctica are ice-covered. Data on greenhouse gas abundances
going back beyond a million years, that is, beyond the reach of antarctic ice cores, are still rather
uncertain, but analysis of geological samples suggests that the warm ice-free periods coincide with
high atmospheric CO2 levels. On million-year time scales,
CO2 levels change due to tectonic activity, which affects the rates
of CO2 exchange of ocean and atmosphere with the solid Earth. See
Section 6.3 for more about these ancient climates.
Another likely cause of past climatic changes is variations in the energy output of the Sun.
Measurements over recent decades show that the solar output varies slightly (by close to 0.1%)
in an 11-year cycle. Sunspot observations (going back to the 17th century), as well as data from
isotopes generated by cosmic radiation, provide evidence for longer-term changes in solar activity.
Data correlation and model simulations indicate that solar variability and volcanic activity are
likely to be leading reasons for climate variations during the past millennium, before the start
of the industrial era.
These examples illustrate that different climate changes in the past had different causes. The fact
that natural factors caused climate changes in the past does not mean that the current climate change
is natural. By analogy, the fact that forest fires have long been caused naturally by lightning strikes
does not mean that fires cannot also be caused by a careless camper. FAQ
2.1 addresses the question of how human influences compare with natural ones in their
contributions to recent climate change.