CLIMATE CHANGE SCIENCE
Antonio J. Busalacchi, Jr., Ph.D.
Chairman, Climate Research Committee
National Academy of Sciences
Director, Earth System Science Interdisciplinary Center (ESSIC),
University of Maryland
COMMITTEE ON COMMERCE, SCIENCE, AND TRANSPORTATION
OCTOBER 1, 2003
Good morning. Thank you very much for this opportunity to testify. I am Dr. Tony Busalacchi, a professor at the University of Maryland and I serve as the chair of The National Academies’ Climate Research Committee. I will use my time this morning to summarize what most scientists agree to be true about change in the Earth’s climate.
Understanding climate and whether it is changing, and why, is one of the most crucial questions facing humankind in the twenty-first century. This question is the subject of much scientific research and, of course, policy debate, since the economic and environmental implications could be large. The National Academies have produced a number of reports focused on understanding climate in recent years and my testimony draws heavily from two of these: a February 2003 report that gives input to the Administration’s draft US Climate Change Science Program Strategic Plan (NRC 2003) and a 2001 report called “Climate Change Science” that was done at the request of the White House (NRC 2001). The latter report answered a series of specific questions designed to identify areas in climate change science where there are the greatest certainties and uncertainties. If you haven’t read this report, it is an excellent summary (only 25 pages long) written in very accessible language.
As is explained in “Climate Change Science,” there is wide scientific consensus that climate is indeed changing. Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Our confidence in this conclusion is higher today than it was ten, or even five years ago, but uncertainty remains because there is a level of natural variability inherent in the climate system on time scales of decades to centuries that can be difficult to interpret with precision because we gather this evidence from sparse observations, numerical models, and proxy records such as ice cores and tree rings. Despite the uncertainties, however, there is widespread agreement that the observed warming is real and particularly strong within the past twenty years.
As the report further explains, human-induced warming and associated sea level rises are expected to continue through the 21st century. Computer model simulations and basic physical reasoning show that there will be secondary effects from these changes. These include increases in rainfall rates and increased susceptibility of semi-arid regions to drought. The impacts of these changes will depend on the magnitude of the warming and the rate with which it occurs.
A diverse array of evidence supports the view that global air temperatures are warming. Instrumental records from land stations and ships indicate that global mean surface air temperature warmed about 0.4-0.8 degrees C (0.7-1.5 degrees F) during the 20th century. The warming trend is spatially widespread and is consistent with the global retreat of mountain glaciers, reductions in snow-cover extent, the earlier spring melting of ice on rivers and lakes, the accelerated rate of rise of sea level during the 20th century relative to the past few thousands years and the increase in upper-air water vapor and rainfall rates over many regions. A lengthening of the growing season also has been documented in many areas, along with an earlier plant flowering season and earlier arrival and breeding of migratory birds. Some species of plants, insects, birds and fish have shifted toward higher latitudes or higher elevations, often together with associated changes in disease vectors. The ocean, which represents the largest reservoir of heat in the climate system, has warmed by about 0.05 degrees C (0.09 degrees F) averaged over the layer extending from the surface down to 10,000 feet, since the 1950s.
It has been said that the Arctic will be the “canary in the coal mine” where the effects of global warming will be felt first and with the greatest magnitude. Analysis of recently declassified data from US and Russian submarines indicates that sea ice in the central Arctic has thinned since the 1970s, and satellite data indicate a 10-15 percent decrease in summer sea ice concentration over the Arctic as a whole. Satellite measurements also indicate that the time between the onset of sea-ice melting and freeze-up has increased significantly from 1978 through 1996, and the number of ice-free days have increased over much of the Arctic Ocean. A decline of about 10 percent in spring and summer continental snow cover extent over the past few decades also has been observed. Looked at in total, the evidence paints a reasonably coherent picture of change, but the conclusion that the cause is greenhouse warning is still open to debate; many of the records are either short, of uncertain quality, or provide limited special coverage.
As you may have seen in the press, a large ice shelf recently broke up along the coast of northeast Canada’s Ellesmere Island, followed by the drainage of an ice-dammed lake that had built up behind it (Disraeli Fiord). The Ward Hunt Ice Shelf was the largest remaining piece of an ice shelf that once, a century ago, rimmed the entire northern coast of Ellesmere Island. I have not studied this particular incident, nor has the Academy, but researchers working at the site had documented reductions in the freshwater volume of the lake accompanied by a rise in mean annual air temperature and have stated that they believe this change can be attributed to global warming. Other scientists have been more cautious, noting that many of the changes being seen in the Arctic could have more to do with long-term world climate patterns than with the release of carbon dioxide and other greenhouse gases.
Some of the changes being experienced at high latitudes are believed to be reflections of changes in wintertime wind patterns rather than a direct consequence of global warming per se. It is important to note that the rate of warming has not been uniform over the 20th century. Much of the warming occurred prior to 1940 and during the past few decades. The Northern Hemisphere as a whole experienced a slight cooling from 1946-1975, and the cooling during that period was quite marked over the eastern United States. The cause of this hiatus in the warming is still under debate. One possible cause might be the buildup of sulfate aerosols due to the widespread burning of high sulfur coal during the middle of the century followed by a decline; it is also possible that at least part of the rapid warming of the Northern Hemisphere during the first part of the 20th century and the subsequent cooling were of natural origin – a remote response to changes in the oceanic circulation, or variations in the frequency of major volcanic emissions or in solar luminosity.
The role that human activities have played in causing these climate changes has been a subject of debate and research for more than a decade. There is no doubt that humans have modified the abundances of key greenhouse gases in the atmosphere, in particular carbon dioxide, methane, nitrous oxide, and tropospheric ozone. These gases are at their highest recorded levels. In fact, the ice-core records of carbon dioxide and methane show their twentieth century atmospheric abundances to be significantly larger than at any period over the past 400,000 years. The increase in these greenhouse gases is primarily due to fossil fuel combustion, agriculture, and land-use changes. Recent research advances have led to widespread acceptance that the human-induced increase in greenhouse gas abundances is responsible for a significant portion of the observed climate changes. The precise size of that portion is difficult to quantify against the backdrop of natural variability and climate forcing uncertainties.
Because the Earth system responds so slowly to changes in greenhouse gas levels, and because altering established energy-use practices is difficult, changes and impacts attributable to these factors will continue during the twenty-first century and beyond. Current models indicate a large potential range for future climates, with global mean surface temperature warming by 1.4 to 5.8ºC (2.5 to 10.4 oF) by 2100 (IPCC, 2001).
Given increasing evidence of how humans have modified the Earth’s climate over the last century, it is imperative for the nation to continue directing resources toward better observing, modeling, and understanding of what form future changes in climate and climate variability may take, the potential positive and negative impacts of these changes on humans and ecosystems, and how society can best mitigate or adapt to these changes.
Thank you for this opportunity to talk about climate change. This is a problem that affects us all, and a problem the scientific community does not shy away from in terms of its responsibility to provide objective scientific assessment in support of sound policy decisions. I’d be happy to take any questions.
IPCC, 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, eds. J.T. Hought, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson. Cambridge, U.K.: Cambridge University Press.
National Research Council, 2003. Planning Climate and Global Change Research: A Review of the Draft US Climate Change Science Program Strategic Plan. The National Academies Press.
National Research Council, 2001. Climate Change Science: An Analysis of Some Key Questions. The National Academies Press.