Over the past year, the public has been confronted, month after month, with reports that the globally averaged temperature for the previous month was a record high. Now there is a report that 1998 is the warmest year on record. Several questions undoubtedly arise: Is this real? What is this due to, "global warming" or some other cause? What will the consequences be? Research in CGD is helping to answer these questions.
Is this real? For several years there has been a scientific controversy about the available temperature records. The traditional temperature record is far from perfect. Individual measurements in this record have serious problems. The number and location of temperature measurements have changed over the past century. Also, the environment of some of the sites has changed, for example, due to urbanization. In spite of these problems, scientists believe that the record shows warming over the 1970s and record high globally averaged temperatures over the past few years.
In contrast, temperature measurements for the lower troposphere using satellite data over the past two decades show no rising trend and perhaps a decreasing trend for globally averaged temperature. Scientists at NCAR have examined the satellite record and have found problems with the analysis of the satellite data (see the Climate Analysis Section for more details). At present, it looks as if the satellite record has been brought into better agreement with the traditional measurements, and the upward trend of temperatures is real.
What is this due to? Scientists in CGD are investigating many aspects of natural climate variability, such as the El Niño/Southern Oscillation (ENSO) phenomenon. We know that the temperature goes up during and following a warm ENSO event, and a large warm event occurred during 1997 and early 1998. Since the summer, however, the warm event has turned into a cold event, but the warm records have continued. CGD scientists are also studying variability in the North Atlantic Oscillation and its effects and changes in the ocean's thermohaline circulation and its effects, including the change in the North Atlantic sea ice cover. The consensus is that the observed changes in the variability are consistent with the observed temperature rise, but we cannot explain all of the observed rise or be the cause of the observed rise.
What about "global warming?" Scientists in CGD, together with colleagues from other institutions, have performed an experiment with NCAR's Climate System Model (CSM) in which they have attempted to simulate the observed changes in the climate over the 20th Century. In this experiment, the past century's observed history of greenhouse gases, together with the inferred changes in sulfate aerosols, is used as forcing functions for the model. The globally averaged temperature for the simulated 20th Century shows little change from 1870 through 1970; but from 1970 to the present, the simulated temperature increases closely in rate and magnitude to that that has been observed. There are some problems with this experiment that are being addressed, but the strong suggestion from this experiment is that we are indeed beginning to see evidence of "global warming."
What are the likely consequences? A few consequences are obvious. The largest temperature increases are in the Arctic and northern high latitudes. The land surface temperatures are rising more than ocean temperatures. The sea ice extent and snow cover are decreasing. However, other changes are not so clear. More research is necessary.
CGD scientists are working to provide some of our model data to a wide variety of other researchers who are contributing to the U.S. National Climate Assessment. This report is due in January, 2000, and will be a wide ranging, multi-faceted look at how climate change may affect the U.S. We are pleased to be a part of this assessment work because we want to help ensure that political decisions that will affect all of us are based on the best possible scientific information.
CGD scientists have also performed two experiments in which the climate of the 21st Century has been simulated, based on two different assumptions about the future emissions of greenhouse gases. Analysis of these experiments will help us understand the magnitude of "global warming," the timescale of its development, and the impact of mitigation efforts. More about this will be included in next year's Annual Scientific Report, after we have had time to analyze the results. We fully expect that these experiments will be submitted for consideration by the authors of the next report of the Intergovernmental Panel on Climate Change (IPCC), due sometime after 2000.