A few days ago I received a letter from one of my students expressing concern over massive snowfalls which had hit parts of Iran that normally never see snow from one decade to the next. Was this, she asked, proof of climate change?

My response was that 'freak' weather events occur from time-to-time because of local atmospheric conditions, and climate change was not responsible for every unexpected snowstorm or flood. Years of data are necessary to determine if trends are occurring, and if they are, to determine if the trend is related in any way to anthropogenic (that is, human) activities.

Fortunately, data is being collected from many sources and from it we can tell whether the planets climate is changing.

Perhaps the best known of the data comes from the Mauna Loa Observatory in Hawaii, where atmospheric concentrations of carbon dioxide have been recorded since 1958. The first observations showed an atmospheric concentration of 316 ppmv (parts per million by volume); by 2003 the concentration had increased by almost 60 ppmv to 375.6 ppmv.

Further data has been obtained from the Vostok ice cores, drilled 3,300 metres into the Antarctic ice and giving a view of the concentrations of carbon dioxide over the past 420,000 years. During this time, concentrations varied between a minimum of about 182 ppmv to a maximum of almost 300 ppmv, with an average of 232 ppmv.

The variations in the Vostok ice core data have been found to correspond to periods of planetary cooling (less carbon dioxide, resulting in glaciation) and heating (more carbon dioxide, during the interglacial periods). These episodes have been attributed to changes in the Sun's output and the perturbations of the Earth's rotation.

The earliest Vostok ice core data (about 2,300 years old) shows carbon dioxide concentrations of about 285 ppmv which stayed relatively stable until the beginning of the Industrial Revolution in the mid-18th century. Since then, the world has seen a massive growth in industrialization, powered by coal, oil, and now, natural gas.

The growth in atmospheric carbon dioxide concentrations shown in the Mauna Loa data has two causes. The first, and most obvious, is the sources of carbon dioxide: burning fossil fuels (coal, oil, and natural gas), deforestation (notably in the tropics), and the production of cement. The second is that the sinks of carbon dioxide (the oceans and land) are limited in the amount of carbon dioxide that they can remove from the atmosphere. With limited sinks and continuing growth in the sources, any carbon dioxide that cannot be removed by the sinks is left in the atmosphere.

This uncontrolled growth of atmospheric carbon dioxide, reaching levels that have not been seen in over 400,000 years, is beginning to make an observable impact on the planet.

First, terrestrial and satellite data suggests that the planet is warming. In the Arctic, sea-ice thickness has declined by about 40 percent in late summer to early autumn, while there has been a slower decline in sea-ice thickness in the winter. In the Antarctic, parts of the Larson ice-sheet and the Western Antarctic Ice Sheet (WAIS) are breaking up; in the case of WAIS, melting ice is apparently speeding the flow of glaciers towards the ocean. Non-polar glaciers are retreating in most areas of the world.

Second, recent data suggests that the oceans are becoming gradually more acidic because of the reaction of atmospheric carbon dioxide with sea-water, producing carbonic acid. The acidification, coupled with warming, appears to be responsible for the coral die-back being observed in many of the planet's oceans.

Each year, as more observations are made and more data is collected and analyzed, more is being learned about the planet's climate. The link between the biological and physical changes that are being observed and the increased levels of atmospheric carbon dioxide emissions is also becoming better understood.

Whether we are willing to act on limited knowledge and stop the uncontrolled emissions of carbon dioxide before these changes become irreversible is another issue altogether.

An abridged version of this was published in the Daily News - 1 March 2005