The earth’s climate is the result of a finely balanced system. Incoming radiation from the sun and infrared radiation emitted by the earth are approximately equal. Anything that upsets this exchange results in altered temperatures around the planet. For example, large volcanic eruptions in the tropics deposit gases and dust particles in the stratosphere, reflecting some of the incoming solar energy and leading to worldwide cooling. Following such eruptions, temperatures in New Zealand can drop by a few tenths of a degree Celsius for up to three years.
Blast from the past
Large volcanic eruptions near the equator can affect the world’s climate for several years. The impact of the blast on the Indonesian island of Krakatoa, which occurred around 536 AD, was still being referred to by European historians many centuries later. According to the 11th-century scholar Michael the Syrian, ‘The sun became dark and its darkness lasted for eighteen months ... The fruits did not ripen, and the wine tasted like sour grapes’. 1
Solar variation and ice ages
The amount and distribution of solar radiation reaching the earth can vary over a wide range of timescales.
Measurements taken from space since the late 1970s, for example, show that the solar constant (the average amount of solar radiation that reaches the earth's upper atmosphere) varied by less than 0.1% over the approximately 11-year sunspot cycle. Reconstructions of past variations, however, suggest that changes 5–10 times larger have occurred over the last 400 years, sufficient to produce noticeable changes in the earth’s mean temperature.
Over tens of thousands of years the well-documented ice age cycles, caused by systematic and predictable variations in the earth’s orbit, alter the distribution of solar radiation at polar latitudes in summer.
Ozone is a form of oxygen produced by reactions between sunlight and oxygen. This occurs most efficiently in the stratosphere (at an altitude of 15–50 kilometres). Since the 1970s there has been a long-term trend towards a stronger Antarctic Oscillation (stronger westerly winds at 50° south), which has been attributed to both an increase in greenhouse gases and a depletion of stratospheric ozone.
Increasing concentrations of greenhouse gases have serious consequences for the future climate. Most of the gases occur naturally – water vapour, carbon dioxide, ozone, methane and nitrous oxide – but some are man-made, such as chlorofluorocarbons (CFCs). A build-up of these gases traps infrared radiation in the lower atmosphere, leading to a warming of the earth’s surface. This is called the enhanced greenhouse effect, since it is an amplification of a natural process that has operated for billions of years and kept the planet habitable.
Since 1750, atmospheric concentrations of carbon dioxide have increased by 31%, methane by 151%, nitrous oxide by 17%, and tropospheric ozone by 36%. There has been a reduction in stratospheric ozone since 1979, partly because of the ozone hole. However, it is expected to recover during the 21st century if countries comply with international targets on CFC emissions.