People all over the world are now experiencing the early signs of what may become the greatest catastrophe in modern history: global warming and its multifaceted effects. These signs include the occurrence between 1995 and 2006 of the 11 warmest years in the worldwide climate record. It is now scientifically clear that a major cause of recent warming and associated changes in precipitation is the wanton combustion of fossil fuels, deforestation, and other malpractices that cause increases in carbon dioxide and other gases that trap heat in the atmosphere.

Also clear are the close connections between this problem and how we live, and how governments and private industries act in America and the rest of the world. What we and these institutions do now and in coming years to limit emissions of heat-trapping gases will determine the extent, duration and impact of future climate change.

The just-released report of the International Panel on Climate Change confirms that human impacts on climate are already being felt worldwide, and predicts that world temperatures will rise 3-7 degrees Fahrenheit this century, the amount varying by region. Far northern latitudes will warm the most, along with the disappearance of the polar ice cap, the accompanying loss of its light-reflecting capacity (albedo), and consequent further heating of the ocean and atmosphere.

The IPCC report predicts the economic and social effects will be broader and more diverse than previously thought, including extreme economic and social changes, impacts on land use on a vast scale, flooding of coastal cities and fertile plains near sea level, and huge displacements of populations. The most tragic impacts of climate change are likely to be felt in developing countries, where people will be least able to cope with them. Yet the implications for the developed nations, like ours, are profound.

When I ponder the effects of the predicted climate changes, I think of burning heat in summer, parched agricultural lands and crop failures on massive scales, millions of people desperate for food and water, while others are driven from lowland and coastal areas by intense storms, floods and rising sea level.

I also think about accelerated rates of plant and animal extinctions, spread of human diseases that thrive in warm climates, and unstable social conditions, lawlessness and increases in “terrorism” and wars of desperation for increasingly scarce resources. By comparison, the Katrina disaster in New Orleans, as tragic as it has been, will occupy an obscure footnote in human history. But it isn’t yet too late to greatly diminish global warming and its effects, if we act now.

So far, responses to this unprecedented threat have been pitifully inadequate. Action is needed on all levels: personal, municipal, state, national and international. In our country, Congress has taken only ministeps, and the Bush administration has been walking backward.

To emphasize the urgency for effective action, several organizations from the Bangor area have organized a rally for Saturday, April 14, called Step It Up Bangor.

Speakers from the highly respected Climate Change Institute at the University of Maine, Mayor Richard Greene of Bangor and representatives of labor and the religious community will present their views on the implications of climate change and what can be done about it. The main goal is to impress on our congressional delegation how serious we are about this problem, and to urge more effective legislation to deal with it.

The rally starts at 1 p.m., followed at 3 p.m. by free showings at the Bangor Opera House of the award winning film, “An Inconvenient Truth,” and a more recent short documentary on global warming, “Everything’s Cool.” For more information, call Leslie Dickinson at 990-1608.

Ronald B. Davis of Orono is an emeritus professor in the department of biological sciences and the Climate Change Institute at the University of Maine.