A Common-Sense Solution to Global Warming

James Hansen is director of the National Aeronautics and Space Administration's Goddard Institute for Space Studies. This article appeared in the Winter 2001 issue of NPQ.

New York--Evidence continues to build that the world is slowly but surely getting warmer. Almost all mountain glaciers are retreating. It was discovered in 2001 that even the deep ocean is warming. On the Earth's surface, where people live, the average warming is now about one degree Fahrenheit in the past 100 years.

One degree seems hardly noticeable. It's much less than weather fluctuations that occur every day. But it is a warning of possibly large climate changes as the 21st century progresses.

One worry is sea level, which will rise as glaciers melt and as ocean water expands from warming. A rise of three feet, a possibility this century, would submerge island nations such as the Maldives and the Marshall Islands, and it would be devastating to those who live in Bangladesh and on the Nile Delta.

The greatest effect of global warming for most people may be an increase in extreme weather. Global warming is expected to cause more droughts and forest fires. Yet warming also increases evaporation, which will lead, at other times and places, to heavier rainfall and floods.

The forces that drive global warming are no surprise. They are mainly the gases and fine particles that humans have been dumping into the atmosphere for many years.

The gases, especially carbon dioxide and methane, absorb the Earth's heat radiation and thus warm the surface, just as a blanket traps body heat. Fine particles of black carbon (soot) warm the air by absorbing sunlight.

Other human-made fine particles, especially sulfates, are nearly white. Sulfates come from sulfur in coal and oil, which is released to the atmosphere when these fossil fuels are burned. Sulfates cool the Earth by reflecting sunlight back to space.

The net effect of these human emissions is not accurately known, because the fine particles are not yet measured well. But it is estimated that the net heating is at least one watt, perhaps closer to two watts per square yard.

In other words, it is as if humans had placed two miniature one-watt Christmas tree bulbs over every square yard of the Earth's surface. Such a human forcing of climate is significant. It is comparable to increasing the brightness of the sun by 1 percent.

The Earth responds slowly to such forcings. The thermal inertia of the ocean, which is two to three miles deep on average, delays the response. It takes decades for most of the response to occur, and centuries for the full response.

The magnitude of the eventual response is revealed by the Earth's history. During the last major ice age the Earth was about 10 degrees Fahrenheit colder than now. An ice sheet a mile thick covered Canada and reached into parts of the United States.

This ice age cold was maintained by a reduction of climate forcing by about six watts per square yard. The reduced forcing was caused by lesser amounts of greenhouse gases in the air, and by changes on the Earth's surface that increased the reflection of sunlight.

The question we face today is how much more should we allow the human climate forcing to grow? That question was recently addressed in the Hague by the world's nations.

These deliberations were guided by climate simulations carried out by the Intergovernmental Panel on Climate Change. IPCC simulations focus on a gloomy "business-as-usual" scenario, in which it is assumed that humans will burn coal, oil and gas at faster and faster rates.

This gloomy scenario leads to an additional forcing of three watts in the next 50 years. Such a forcing, almost surely, will lead to increases in climate extremes and a rising sea level.

Some increase in the human climate forcing is inevitable. Fossil fuels are our primary source of energy. Because of the energy infrastructure, it requires decades to phase in new technologies that may produce less carbon dioxide.

However, we recently suggested a scenario that reduces the human forcing to only one watt in the next 50 years. This would yield a more moderate climate change, allowing time to understand climate change better and develop technologies and strategies to deal with the issue.

There are two elements in this common-sense solution to global warming. First, we must stop the growth of air pollution. This would eliminate any added climate forcing by constituents other than carbon dioxide.

Second, we must burn fossil fuels, and thus emit carbon dioxide, no faster than we do today. That means growing energy needs must be met by increased efficiencies in current uses and by introducing technologies that produce little or no carbon dioxide.

Both elements are achievable, but unlikely to happen by accident. Technologies that reduce air pollution must be implemented. Annual growth of carbon-dioxide emissions, which has already slowed from 4 percent to 1 percent per year, must be slowed a bit further to zero growth or a small decrease.

Many actions could reduce both air pollution and carbon dioxide emissions. We need to develop clean fuels and renewable energy sources, and remove barriers to energy efficiency. Improved technology, perhaps including fuel cells and hydrogen power, can help reverse the trend to greater gas-guzzling vehicles. Utility profits should be designed to reward improved efficiency and decreased air pollution.

Improved energy efficiency, cleaner uses of fossil fuels and development of renewable energy sources will have multiple benefits. In addition to slowing the growth of carbon dioxide, this will create jobs, improve economic competitiveness, reduce reliance on foreign sources of energy and improve public health.

Fine particles in air pollution, including soot, sulfates and organic aerosols, penetrate human tissue deeply, causing respiratory and cardiac problems. A recent study found that air pollution in France, Austria and Switzerland alone accounts for 500,000 asthma attacks and 40,000 deaths per year, with medical costs amounting to 1.7 percent of their gross domestic products. Air pollution in developing countries, such as India and China, is even more severe.

International cooperation is needed, because emissions to the atmosphere circulate worldwide. But benefits of progress, in climate stabilization and health, will be similarly widespread. Required cooperation, including technology transfer, can include incentives and economic opportunities for all parties.

The framework we have defined, which limits added climate forcing in 50 years to one watt, provides a means to assess progress year by year. The goals can be adjusted, if necessary, as observations of climate forcings and climate change advance.

The common-sense approach is to move forward by attacking air pollution, improving energy efficiency and developing renewable energy sources. This approach is economically sound and has collateral benefits. It should provide a meeting ground for persons from a wide spectrum of political viewpoints, all of whom wish to preserve the environment.