Since agriculture began, the earth's climate has been remarkably stable. Now the earth's temperature is rising, apparently due to the greenhouse effect-the warming that results from the rising concentration of heat-trapping gases, principally carbon dioxide (C02), in the atmosphere.

This rise in C02 concentration comes from two sources: the burning of fossil fuels and deforestation. Each year, more than 6 billion tons of carbon are released into the atmosphere as fossil fuels are burned. Estimates of the net release of carbon from deforestation vary widely, but they center on 1.5 billion tons per year. The release of C02 from the two sources is simply overwhelming nature's capacity to fix carbon dioxide. When the Industrial Revolution began in 1760, carbon emissions from the burning of fossil fuels were negligible. But by 1950, they had reached 1.6 billion tons per year, a quantity that was already boosting the atmospheric C02 level. In 2000, they totaled 6.3 billion tons. This fourfold increase since 1950 is at the heart of the greenhouse effect that is warming the earth.

The carbon emissions of individual fossil fuels vary. Coal burning releases more carbon per unit of energy produced than oil does, and oil more than natural gas. The global fleet of 532 million gasoline-burning automobiles, combined with thousands of coal-fired power plants, are literally the engines driving climate change.7

In addition, in recent years the world has been losing 9 million hectares of forest per year. Forests store easily 20 times as much carbon per hectare as does land in crops. If the net loss of forests can be eliminated, this source of carbon emissions will disappear.

Another set of synergies is threatening the earth's forests by fire. Intact, healthy rainforests do not burn, but forests weakened by logging or slash-and-burn farming become vulnerable to fire. The more they burn, the more vulnerable they become. The process, which feeds on itself, reinforces the global warming trend. As higher temperatures due to climate change lead to the drying out of forests and more burning, more carbon is emitted into the atmosphere. Rising atmospheric carbon dioxide levels accelerate the process of global warming. The trends of rising temperatures and burning forests begin to reinforce each other."

One consequence of many interacting changes is that they can lead to developments that surprise even the scientific community.

One such event came in August 2000, as described in Chapter 2, when the icebreaker cruise ship discovered open water at the North Pole. Yet another recent surprise is the die off of coral reefs. Again, the reasons for the coral die off are complex, but a rise in surface water temperature may be responsible. What is surprising is that a temperature rise in sea surface water of less than 1 degree Celsius can lead to reef deaths. If the reefs continue to die, oceanic' ecosystems will be altered, directly affecting the fisheries that depend on the coral reefs as nursery grounds.

These are but a few of the surprises and synergies that have been encountered in recent years. No one knows how many the new century will bring. And unfortunately, synergistic trends such as those just described are often irreversible. As Chris Bright observes, "Nature has no reset buttons".

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