Recent polls indicate most Americans believe, and with good reason, that global warming is a fact. Data for the Northern Hemisphere over the last 1000 years show a gradual decrease in temperature from the years 1000 to 1900 and then an abrupt rise since 1900, particularly since 1970. The balance of evidence suggests humans have played a large role in the recent warming. Moreover, we are creating conditions that will almost certainly lead to substantial warming over the next 100 years.
The concern over global warming is based on well-established scientific principles. Scientists unanimously agree that naturally occurring greenhouse gases maintain the average surface temperature of our planet at about 59ºF, compared to near 0ºF in the absence of this natural greenhouse effect. Human activities of the industrial age -- the burning of fossil fuels, releasing chlorofluorocarbons, and deforestation -- have raised these gases, mainly carbon dioxide (CO2), far above natural levels. Nature takes hundreds of years to remove these excessive amounts of greenhouse gases. For instance, our colleagues at the Center for Global and Regional Environmental Research at the University of Iowa calculate that total greenhouse gas emissions for the state of Iowa by human activities amount to over 31 tons of CO2 per year per person. Of the amount each of us was responsible for in the year 2001, 13 tons will remain in the atmosphere in the year 2100, 1,000 pounds in 2500, and more than 14 pounds even in the year 3000. So the CO2 we produce today will contribute to global warming for centuries.
The current rate of increase of greenhouse gases is unprecedented. The atmosphere now contains more CO2 than at any time in the last 420,000 years and possibly the last 20 million years. CO2 has fluctuated from about 180 parts per million (ppm) to about 300 ppm until the beginning of the industrial revolution when human-generated CO2 emissions began to rise. It now stands at 370 ppm. Current and projected future emission rates allow us to predict with confidence that CO2 concentrations will be at least 500 ppm by 2050 even with severe limits on emissions and may reach 600 ppm. If we allow for rapid economic growth based on continued use of fossil fuels, CO2 will reach about 950 ppm by 2100.
Computer model results summarized by the Intergovernmental Panel on Climate Change (IPCC) estimate a global average temperature rise of 2.5-10.4ºF over the next 100 years due to expected increases in greenhouse gases. Recent scientific reports conclude there is a 40 percent chance that warming by 2100 will exceed the IPCC estimate and only a 5 percent chance that it will be below this range. There is no scientific evidence to suggest that global average temperatures will remain constant or decline in the next 100 years. The issue then is not whether warming will occur, but rather what other changes are likely to occur, and what do we do about it.
The dilemma of increasing CO2 divides into two parts: the near term (today to 2030) and the long term (2030-2100). We can say with high confidence that the earth will warm from 0.7-2.2ºF over the next 30 years, regardless of what we do to control emissions. We no longer have any control over global warming during this period. Our economic and political strategy for the near term must be one of adaptation to anticipated changes. In the long term, we do have some control, but this will be for generations far in the future who inherit a much warmer planet.
The time has come to move beyond academic debates about whether the earth is warming. We should instead take action, divide our efforts into two parts. In the short term we should focus on practical and constructive dialog about how to adapt to the change in climate over the next 30 years. In the long term we must develop emissions controls that will restrain the warming sufficiently to allow for adequate ecological and agricultural resources to meet the needs of future generations.
Note: This essay was published in the Des Moines Register as an op-ed on June 21, 2002 under the title of 'Warmer, warmer: Earth's rising temperature isn't nonsense '