The earth’s climate is the result of a finely balanced system, and natural events and human activities can tip the balance. When incoming radiation from the sun and infrared radiation emitted by the earth are equal, average temperatures are constant. Anything that upsets this balance results in altered temperatures around the planet.
Blast from the past
Large volcanic eruptions near the equator, carrying fine ash and acidic gas up into the atmosphere, can affect the world’s climate for several years. This became clear to scientists after the eruption of Mt Agung in Bali, Indonesia, in 1963, which affected the climate around the globe. A massive volcanic explosion in Mount Tambora, Indonesia, brought about the disastrous changes in the weather in 1816, known as ‘The Year without Summer’, causing famine throughout Asia, Europe and North America.
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 of those are increased by human activity, and others are only manufactured, 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 pre-industrial times, the atmospheric concentration of carbon dioxide has risen from around 280 parts per million (ppm) to around 410 ppm, an increase of almost 50%. Methane and nitrous oxide concentrations have also increased because of human activity.
Increasing greenhouse gas concentrations are already influencing the global climate, and these impacts are expected to increase over the coming century and beyond. The amount of water vapour in the atmosphere increases with rising temperatures, amplifying the effect of other greenhouse gases. The average temperature has already risen by about 1°C over the past century, and the face of New Zealand could change remarkably if temperatures rise by several degrees.
Other phenomena can also influence the global climate, although these usually have a temporary effect or happen much more slowly than warming due to increasing greenhouse gases.
For example, large volcanic eruptions in the tropics can deposit gases and dust particles in the stratosphere. These reflect some of the incoming solar energy and lead to worldwide cooling. Following such eruptions, temperatures in New Zealand have dropped by a few tenths of a degree Celsius for up to three years.
Solar variation and ice ages
The amount and distribution of solar radiation reaching the earth is very nearly constant, but the small variation occurs 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) varies by less than 0.1% over the approximately 11-year sunspot cycle. In contrast, reconstructions of past variations suggest that changes twice as large have occurred over the last 400 years, producing noticeable changes in the earth’s mean temperature.
Over tens of thousands of years, the well-documented glacial cycles are triggered by systematic and predictable variations in the earth’s orbit that alter the distribution of solar radiation. This effect alone is too small to cause the observed temperature changes, which are amplified by the release of greenhouse gases from oceans and land masses.
Ozone is a form of oxygen molecule produced by reactions between ultraviolet (UV) sunlight and ordinary diatomic oxygen. This occurs most efficiently in the stratosphere (at an altitude of 15–50 kilometres above the earth’s surface), creating a permanent layer of stratospheric ozone. Ozone strongly absorbs both solar UV and the earth’s infrared emissions, acting like the roof of a greenhouse and warming the stratosphere. The resulting temperature profile of the atmosphere traps most turbulence, and almost all of the water vapour, in the troposphere, the layer below an altitude of 10–15 km.
Stratospheric ozone has diminished since the 1970s as a result of the annual Antarctic ozone hole, but it is expected to recover over the coming decades thanks to concerted international action.