The World Meteorological Organization (WMO) reports that 2004 was the fourth warmest year since 1861. The fact that the nine warmest years in weather-keeping history have occurred in the last 10 years tells us that change in the Earth’s temperature, like that of the atmospheric concentrations of greenhouse gases, is accelerating. Over the last few years the WMO has also concluded that some of the recent increases in weather variability—intense storms, drought, and heat waves—is due to the increased heat accumulating in the atmosphere and oceans.

Satellite data indicate large warm anomalies across much of the Arctic over the past decade, with record low ice years occurring in 2002 and again in 2003. In addition, analyses of decades of North Atlantic Ocean data now show recent changes in circulation that are consistent with global warming.

The amount of insulation in Earth’s atmosphere due to greenhouse gases released from the combustion of coal, oil, and gas is well established, and it now overwhelms much of the natural decadal and centennial climate variability. This realization has led to the widely accepted scientific assessment that most of the warming over the past 50 years is due to human activity.

Over the past two million years, Earth’s climate has cycled about a dozen times between periods of cold and warm conditions. Most of the species on Earth today have thrived over this period because they can accommodate a wide range of temperatures. However, today’s temperature extremes are taking these species into uncharted territory.

Natural ecosystems are the ancestral sources of species that we now manage for crop production. These evolved under climate regimes that are now being supplanted by new conditions, mostly warmer, some wetter, and some drier, and some rapidly alternating between wetter and drier. In many areas the unusually warm conditions of the last few years are bumping up against upper temperature limits for reproductive success and yield at harvest for some of our most important grain crops.

Our social and economic systems have also evolved to accommodate a range of temperatures. Some areas, for example, are too hot in the summer for extended outdoor daytime labor, and because of this people have either adjusted their daily activity cycles accordingly or introduced cooling systems in the work and living place. But a heat wave in Europe in the summer of 2003 demonstrated just how close our current infrastructure and social systems are to critical thresholds for human habitation. There were 21,000 heat-related deaths in five European nations, and subsequent studies have found that most of the deceased were not likely to have died soon from other causes. Widespread wildfires and crop failures accompanied the human tragedy.

As Earth warms, more of its frozen water becomes liquid and more of its liquid water vaporizes, which can be carried a long way from the source since a warmer atmosphere holds more moisture. Where and how heavily this moisture falls as precipitation are other aspects of climate change that have enormous potential to tax our existing agricultural systems and urban infrastructure.

Costs associated with natural disasters (primarily extreme weather events) continue to rise. Yearly damages increased ten-fold between the 1980s and 1990s, and reached $55 billion in 2002 ($11 billion insured) and $60 billion ($15 billion insured) in 2003. The shock waves of emerging risks are reverberating through the insurance and reinsurance sector.

Factors that will influence future climate include the number of people on Earth, the state of our socioeconomic development, and, most important of all, how dependent we are on coal, oil, and gas (versus alternative sources of energy) to fuel our economies. So what are we doing to reduce the effects of humans on Earth’s climate system?

In 1992, President George Herbert Walker Bush signed, and the U.S. Senate ratified, the United Nations Framework Convention on Climate Change. This treaty, by which the ratifying nations (now numbering 188) pledge to avoid dangerous interference with the climate system, has fallen on hard times. The voluntary measures originally thought to be adequate for a return to 1990 greenhouse gas emission levels (specified by the Convention) were soon recognized as ineffective.

The Third Conference of the Parties to this treaty met in Kyoto in 1997 and produced what is known as the Kyoto Protocol, which put forth targets and a timetable for emission reductions by industrialized nations. This protocol went into effect in February 2005 following ratification by Russia. The current administration of President George W. Bush has said that it does not intend to submit this Protocol to the Senate for ratification. Obviously, the effectiveness of the Protocol will be greatly diminished without the participation of the United States, since our nation is responsible for about 25 percent of global anthropogenic greenhouse gas emissions.

Despite popular misrepresentation to the contrary, full implementation of the Kyoto Protocol was never anticipated to be sufficient to prevent potentially harmful disruption of Earth’s climate. Rather it is the first step along this path. With every passing day the challenge of meeting the objective of the Protocol is made more difficult but certainly not impossible. Consider the impressive list of major corporations, starting with British Petroleum, and more recently Shell and others, and the hundreds of cities that are demonstrating what many analyses have indicated, namely, that the Kyoto targets can be met without incurring serious economic hardship.

When the Convention on Climate Change was ratified 12 years ago, substantive adverse impacts of climate change were still largely hypothetical. This is no longer the case. Some climate experts like Jean-Marc Jancovici have come to believe that terms like global warming or climate change fail to convey the seriousness and urgency of the problem—what we are experiencing is truly climate shock. It is time to develop aggressive national energy plans that reduce our economic dependence on fossil fuel. In addition, we must begin serious planning for adaptive measures that will be needed to minimize harmful aspects of future climate change on natural and societal systems.

James J. McCarthy is the Agassiz Professor of Biological Oceanography and Head Tutor in Environmental Science and Public Policy.