Cicerone Verbal  Testimony to Congress on Climate Change

By Ralph J. Cicerone, Ph.D.

Dr. Ralph Cicerone is the President of the National Academy of Sciences. Prior to that he served as Chancellor of the 
University of California at Irvine, where he also held the Daniel G. Aldrich Chair in Earth System Science. In addition, 
in 2001 he chaired the National Academies committee that wrote the report, Climate Change Science: An 
Analysis of Some Key Questions, at the request of the White House.
 
Here is his verbal abbreviated testimony to congress describing the current state of scientific understanding of 
climate change, based largely on findings and recommendations from a number of recent National Academies' reports.

 

I will summarize briefly the current state of scientific understanding on climate change, based largely on the findings and recommendations in recent National Academies' reports. These reports are the products of a study process that brings together leading scientists, engineers, public health officials and other experts to provide consensus advice to the nation on specific scientific and technical questions.

The Earth is warming. Weather station records and ship-based observations indicate that global mean surface air temperature increased about 0.7  F (0.4  C) since the early 1970's.

Although the magnitude of warming varies locally, the warming trend is spatially widespread, and is consistent with an array of other evidence (including melting glaciers and ice caps, sea level rise, extended growing seasons, and changes in the geographical distributions of plant and animal species).

The ocean, which represents the largest reservoir of heat in the climate system, has warmed by about 0.12  F (0.06  C) averaged over the layer extending from the surface down to 750 feet, since 1993. Recent studies have shown that the observed heat storage in the oceans is consistent with expected impacts of a human-enhanced greenhouse effect.

The observed warming has not proceeded at a uniform rate. Virtually all the 20th century warming in global surface air temperature occurred between the early 1900s and the 1940s and from the 1970s until today, with a slight cooling of the Northern Hemisphere during the interim decades. The causes of these irregularities and the disparities in the timing are not completely understood, but the warming trend in global-average surface temperature observations during the past 30 years is undoubtedly real, and is substantially greater than the average rate of warming during the 20th century.

Laboratory measurements of gases trapped in dated ice cores have shown that for hundreds of thousands of years, changes in temperature have closely tracked atmospheric carbon dioxide concentrations. Burning fossil fuel for energy, industrial processes, and transportation releases carbon dioxide to the atmosphere. Carbon dioxide in the atmosphere is now at its highest level in 400,000 years, and continues to rise.

Nearly all climate scientists today believe that much of Earth's current warming has been caused by increases in the amount of greenhouse gases in the atmosphere, mostly from the burning of fossil fuels. The degree of confidence in this conclusion is higher today than it was 10, or even 5 years ago, but uncertainties remain. As stated in the Academies 2001 report, ``the changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.''

One area of debate has been the extent to which variations in the Sun might contribute to recent observed warming trends. The Sun's total brightness has been measured by a series of satellite-based instruments for more than two complete 11-year solar cycles. Recent analyses of these measurements argue against any detectable long-term trend in the observed brightness to date. Thus, it is difficult to conclude that the Sun has been responsible for the warming observed over the past 25 years.

Carbon dioxide can remain in the atmosphere for many decades and major parts of the climate system respond slowly to changes in greenhouse gas concentrations. The slow response of the climate system to increasing greenhouse gases also means that changes and impacts will continue during the 21st century and beyond even if emissions were to be stabilized or reduced in the near future.

Simulations of future climate change project that, by 2100, global surface temperatures will be from 2.5 to 10.4  F (1.4 to 5.8  C) above 1990 levels. Similar projections of temperature increases, based on rough calculations and nascent theory, were made in the Academies first report on climate change published in the late 1970s. Since then, significant advances in our knowledge of the climate system and our ability to model and observe it have yielded consistent estimates. Pinpointing the magnitude of future warming is hindered both by remaining gaps in understanding the science, and by the fact that it is difficult to predict society's future actions, particularly in the areas of population growth, economic growth, and energy use practices.

Other scientific uncertainties about future climate change relate to the regional effects of climate change and how climate change will affect the frequency and severity of weather events. Although scientists are starting to forecast regional weather impacts, the level of confidence is less than it is for global climate projections. In general, temperature is easier to predict than changes such as rainfall, storm patterns, and ecosystem impacts.

It is important to recognize however, that while future climate change and its impacts are inherently uncertain, they are far from unknown. The combined effects of ice melting and sea water expansion from ocean warming will likely cause the global average sea-level to rise by between 0.1 and 0.9 meters between 1990 and 2100. In colder climates, such warming could bring longer growing seasons and less severe winters. Those in coastal communities, many in developing nations, will experience increased flooding due to sea level rise, and are likely to experience more severe storms and surges. In the Arctic regions, where temperatures have risen more than the global average, the landscape and ecosystems are being altered rapidly.

The task of mitigating and preparing for the impacts of climate change will require worldwide collaborative inputs from a wide range of experts, including natural scientists, engineers, social scientists, medical scientists, those in government at all levels, business leaders, and economists. Although the scientific understanding of climate change has advanced significantly in the last several decades, there are still many unanswered questions. Society faces increasing pressure to decide how best to respond to climate change, associated global changes, and applied research in direct support of decision making is needed.

 

2007 Greenspan
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