Robert McLeman raises a number of interesting parallels between the Dust Bowl of the 1930s and global warming in the twenty-first century (Distill wisdom from grapes of wrath, 7 August). Of equal interest are some of the differences.

Perhaps the greatest difference between the Dust Bowl and global warming is the cause and scale of the problem. The Dust Bowl was caused by abnormal variations in sea surface temperature between the Atlantic and Pacific Oceans and was felt primarily on the Great Plains of North America, whereas global warming is caused by anthropogenic emissions of heat trapping gases, such as carbon dioxide, and, as the name suggests, is being felt across the planet.

Another significant difference that has occurred over the past seventy-five years is the change in demographics and settlement patterns. In the 1930s, the planet's population was about two billion; it is now over six billion and is expected to reach between nine and ten billion by mid-century.

Today, about half the planet's population lives within 200 kilometres of a coast, many in major coastal conurbations. Global warming is expected to affect these regions in two ways: first, sea level rise, caused by the thermal expansion of the oceans and the addition of water from melting glaciers and ice sheets; and second, the pollution of underground water reserves with salt water. The flooding of low-lying, coastal areas, coupled with the loss of potable water will force millions of people to retreat inland.

Not only is the planet's population increasing and becoming more urbanized, it is also ageing. As the climate continues to warm, this will result in an increase in the number of summer-time heatwaves, especially in cities where temperatures can be 5 to 6C higher than in rural areas. Heatwaves are more than uncomfortable; they can be fatal, especially to the elderly, as the 20,000 to 30,000 or more deaths from heat-stroke in Europe showed in 2003.

Feeding the planet's growing population will also become a challenge. The suggested benefits of increased concentrations of carbon dioxide to certain plants are expected to be offset by droughts caused by changing weather patterns and increased weed growth. Insects and temperature sensitive pathogens that were once kept in check by winterkill are now surviving as global warming makes winters milder.

In the 1930s, governments were unable to mitigate the Dust Bowl because its causes were unknown. Despite this, many governments introduced policies to help people adapt to the changes.

The same cannot be said of today's governments; for almost twenty years, most have shunned the calls to reduce the anthropogenic emission of greenhouse gases, arguing that the economic impacts would outweigh the benefits. The inability to take global warming seriously also means that adaptation policies are few and far between, perhaps because this would be an admission of failure.

Submitted to Globe and Mail, 9 August 2006. Unpublished.