Our planet is changing fast. In recent decades many environmental indicators have moved outside the range in which they have varied for the past half-million years. We are altering our life support system and potentially pushing the planet into a far less hospitable state.
Such large-scale and long-term changes present major policy challenges. The Kyoto Protocol is important as an international framework for combating climate change, and yet its targets can only ever be a small first step. If we cannot develop policies to cope with the uncertainty, complexity and magnitude of global change, the consequences for society may be huge.
We have made impressive progress in the last century. Major diseases have been eradicated and life expectancy and standards of living have increased for many. But the global population has tripled since 1930 to more than six billion and will continue to grow for several decades, and the global economy has increased more than 15-fold since 1950. This progress has had a wide-ranging impact on the environment. Our activities have begun to significantly affect the planet and how it functions. Atmospheric composition, land cover, marine ecosystems, coastal zones, freshwater systems and global biological diversity have all been substantially affected.
Yet it is the magnitude and rate of human-driven change that are most alarming. For example, the human-driven increase in atmospheric carbon dioxide is nearly 100 parts per million and still growing - already equal to the entire range experienced between an ice age and a warm period such as the present. And this human-driven increase has occurred at least 10 times faster than any natural increase in the last half-million years.
Evidence of our influence extends far beyond atmospheric carbon dioxide levels and the well-documented increases in global mean temperature. During the 1990's, the average area of humid tropical forest cleared each year was equivalent to nearly half the area of England, and at current extinction rates we may well be on the way to the Earth's sixth great extinction event.
The Earth is a well-connected system. Carbon dioxide emitted in one country is rapidly mixed throughout the atmosphere, and pollutants released into the ocean in one location are transported to distant parts of the planet. Local and regional emissions create global environmental problems.
The impacts of global change are equally complex, as they combine with local and regional environmental stresses in unexpected ways. Coral reefs, for example, which were already under stress from fishing, tourism and agricultural pollutants, are now under additional pressure from changing carbonate chemistry in ocean surface waters, a result of the increase in atmospheric carbon dioxide.
Similarly, the wildfires that hit southern Europe, western Canada, California and southeastern Australia last year were a result of many factors, including land management, ignition sources and extreme local weather. However, prevailing warm and dry conditions - probably linked to climate change - amplified fire intensity and extent.
Poor access to fresh water means that more than two billion people currently live under what experts call "severe water stress." With population growth and economic expansion, this figure is expected to nearly double by 2025. Climate change would further exacerbate this situation.
Biodiversity losses, currently driven by habitat destruction associated with land-cover change, will be further exacerbated by future climate change. Beyond 2050, rapid regional climate change, as would be caused by changes in ocean circulation in the North Atlantic, and irreversible changes, such as the melting of the Greenland ice sheet and the accompanying rise in sea levels of 6 meters, or 20 feet, could have huge economic and societal consequences.
It is now clear that the Earth has entered the so-called Anthropocene Era - the geological era in which humans are a significant and sometimes dominating environmental force. Records from the geological past indicate that never before has the Earth experienced the current suite of simultaneous changes: we are sailing into planetary terra incognita.
Global environmental change challenges the political decision-making process by its uncertainty, its complexity and its magnitudes and rates of change.
Because of the uncertainties involved, decision-making will have to be based on risks that particular events will happen, or that possible scenarios will unfold. A lack of certainty does not justify inaction - the precautionary principle must be applied.
Because of its complexity, global environmental change is often gradual until critical thresholds are passed, and then far more rapid change ensues, as seen in the growth of the ozone hole. Some rapid changes - such as the potential melting of the Greenland ice sheet - would also be irreversible in any meaningful human timescale, while other changes may be unstoppable, and indeed may have already been set in motion.
Because of the magnitudes and rates of change, we are unsure of just how serious our interference with the dynamics of the Earth's system will prove to be, but we do know that there are significant risks of rapid and irreversible changes to which it would be very difficult to adapt.
The first step toward meeting the challenge presented by global change is to appreciate the complex nature of the Earth's system, the ways in which we are affecting the system, and the economic and societal consequences. Scientists and policy-makers must establish a dialogue to communicate current knowledge and to guide future research.
Real policy progress must address the need for large-scale change, technological advances and global cooperation. Incremental change will not prevent, or even significantly slow, climate change, water depletion, deforestation or biodiversity loss. Breakthroughs in technologies and natural resource management that will affect all economic sectors and the lifestyles of people are required.
Although action at local, regional and national levels is important, international frameworks are essential for addressing global change. We must develop new approaches that consider the diversity of national circumstances and interests, based on a shared political will for action. Never before has an effective multilateral system been more necessary.
The evidence of our impact on our own life-support system is growing rapidly. Will we accept the challenge to respond in a precautionary manner, or wait until a catastrophic, irreversible change is upon us?
Margot Wallström is the European Commissioner for the environment. Bert Bolin is the founding chair of the Intergovernmental Panel on Climate Change. Paul Crutzen was awarded the 1995 Nobel Prize in Chemistry. Will Steffen is executive director of the International Geosphere-Biosphere Program. This comment is based on "Global Change and the Earth System: A Planet Under Pressure," which looks at the findings of the International Geosphere-Biosphere Program.
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