In 2010, the United States emitted over 6.8 billion metric tons of greenhouse gases (CO2e). Carbon dioxide accounted for the largest percentage of greenhouse gases (84%), followed by methane (10%), nitrous oxide (4%), and other greenhouse gases (2%). Total U.S. emissions for 2010 totaled 6,821 million metric tons of CO2e and net emissions, taking sinks into account, totaled 5,747 tons CO2e.
Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2010 (EPA 2012)
Friday February 9, 2007
2325 Rayburn House Office Building
Sea level rise is one of the most widespread climate impacts expected to result from human-induced global warming. New evidence from modern satellite observations on the one hand, and from the study of how large polar ice sheets responded to ancient global warming events on the other, suggests that global warming is already causing sea level to rise and that it could rise faster and to a greater extent this century—and beyond—than previously estimated. This briefing will help congressional staff understand recent scientific progress and current scientific thought on sea level rise.
Following a brief introduction to global climate change by Dr. Jay Gulledge, two leading sea level experts, Dr. Steve Nerem and Dr. Jonathan Overpeck, will describe the present state of the science on global sea level rise, with emphasis on state-of-the-art satellite measurements of contemporary sea level change, the various climate processes that contribute to sea level rise, and lessons learned from studying ancient climate–sea level relationships. Following short scientific presentations from each scientist, there will be ample time for the audience to interact directly with these internationally recognized experts.
R. Steven Nerem, Ph.D.
University of Colorado
Dr. Steve Nerem is Professor of Aerospace Engineering Sciences at the University of Colorado at Boulder and a fellow of the Cooperative Institute for Research in Environmental Sciences. Prior to joining the CU faculty in 2000, he was Assistant Professor and then Associate Professor of Aerospace Engineering for four years at the University of Texas at Austin. Prior to that he was a geophysicist with NASA/Goddard Space Flight Center for six years. He earned his Ph.D. in Aerospace Engineering from The University of Texas at Austin. Dr. Nerem has authored approximately 60 peer-reviewed journal publications covering a variety of topics related to his specialty, which involves satellite orbit determination, remote sensing, and measuring the Earth's shape, gravity field, and sea level from space. He is a Contributing Author for the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Dr. Nerem has received more than a dozen awards for his work, including NASA's Exceptional Scientific Achievement Medal for his research in the area of gravity field determination.
Jonathan T. Overpeck, Ph.D.
University of Arizona
Dr. Overpeck is Director of the Institute for the Study of Planet Earth and professor of Geosciences at the University of Arizona, Tucson. Prior to joining the faculty in 1999 he was head of the NOAA Paleoclimatology Program at the National Geophysical Data Center in Boulder, Colorado for nine years. He earned a Ph.D. in geological sciences from Brown University. Dr. Overpeck has authored over 100 papers that focus on global change dynamics, with a major focus on how and why climate systems vary on timescales of decades and longer. Current work focuses on the Asian and West African Monsoon systems, tropical Atlantic variability, El Niño-Southern Oscillation dynamics, Arctic environmental change, and reconstruction of ancient environments. He is a Coordinating Lead Author for the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Dr. Overpeck has received numerous awards recognizing his climate research, including the U.S. Department of Commerce Gold Medal and the American Meteorological Society Walter Orr Roberts Award.
Jay Gulledge, Ph.D.
Pew Center on Global Climate Change
Dr. Gulledge is Senior Research Fellow for Science and Impacts at the Pew Center on Global Climate Change. He serves as the Center’s in-house scientist and coordinates its work to communicate the state of knowledge on the science and environmental impacts of global climate change to policy-makers and the public. He is also an adjunct Associate Professor at the University of Wyoming, home to his academic research on biological cycling of atmospheric greenhouse gases, which he publishes regularly in peer-reviewed journals. Prior to joining the Pew Center, he served on the faculties of Tulane University and University of Louisville. Dr. Gulledge earned a PhD in ecosystem sciences from the University of Alaska Fairbanks. He currently serves as an associate editor of Ecological Applications, a peer-reviewed journal published by the Ecological Society of America.
- Two primary processes contribute to sea level rise (SLR):
- Thermal expansion—the increase in water volume resulting from heat uptake;
- Mass inputs—the transfer of freshwater from land to the ocean. Large ice sheets on Greenland and Antarctica represent the largest potential source of additional water mass (#2 below).
- There are two general mechanisms by which such ice sheets transfer water to the ocean:
- Surface melting of ice on land with subsequent runoff into the ocean;
- Ice dynamics, wherein ice actually flows from land into the ocean
- New evidence indicates that ice flow is accelerating around Greenland and Antarctica, making this process the greatest source of uncertainty for predicting future SLR as warming proceeds.
Ancient Sea Level Change
- Over the past four cycles, each lasting about 100,000 years, sea level rose and fell by about 400 feet, with ice ages having lower sea level and warm interglacial periods having higher sea level. Contributions from land-based ice sheets dominated SLR during interglacial warming.
- During the warmest part of the last interglacial period (about 130,000 years ago), global average temperature was 2-3 °F warmer than today and global sea level was 13-20 feet higher. During the Middle Pliocene (3 million years ago), global temperature was 3.5-5.5 °F warmer than today and sea level was 80-115 feet higher.
- The complete loss of large ice sheets would take from several centuries to millennia, but warming could cross a threshold within decades that could permanently destabilize large ice sheets.
Recent Sea Level Rise
- Based on tide gauges, sea level rose by an average of 0.7 inches per decade and accelerated by 0.05 inches per decade over the 20th century.
- More accurate satellite measurements indicate that global sea level has risen by 1.2 inches over the past decade, about 70% faster than the 20th century average.
- Scientists assumed that thermal expansion dominated contemporary SLR, but recent progress reveals that freshwater contributions from land dominate, consistent with recent acceleration of ice loss from glaciers.
Projections of Future SLR
- Because the ocean has an enormous thermal inertia, it takes many decades for sea level to adjust to a quantity of heat that it absorbs. This delay means that even if man-made greenhouse gas emissions were halted today, sea level rise would be committed to an additional 0.4-1.0 foot by the end of this century.
- In 2001 the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) projected that sea level would rise by 0.3-2.9 feet by the end of the 21st century.
- In 2007 the Fourth Assessment Report of the IPCC projected that sea level would rise by 0.6-1.9 feet by the end of the 21st century.
- The lower ends of the projected SLR ranges from the Third and Fourth IPCC reports are similar to the sea level rise commitment described above, leaving little or no room for additional SLR generated by continued greenhouse gas emissions. The upper end of both omits the uncertainty associated with the future ice dynamical changes in Greenland and West Antarctica.
- Current SLR models undershoot the sea level change observed during the 20th century, possibly because they do not adequately simulate freshwater mass contributions from land. As an alternative, a recent study extended the statistical relationship between temperature rise and SLR observed during the 20th century, using models of future temperature change to drive SLR. This empirical approach estimated 21st century SLR to be in the range of 1.6-4.5 feet, if manmade greenhouse gas emissions continue to grow.
- Both long-term tide gauge measurements and recent satellite measurements suggest that SLR accelerated during the 20th century, concomitant with increasing global temperature.
- One of the most significant developments of recent years is the realization that mass contributions from land ice have dominated contemporary SLR.
- Large polar ice sheets are more sensitive to surface warming than previously realized, and large dynamical changes are now being observed on the Greenland and West Antarctic ice sheets.
- New understanding of how these ice sheets behaved in the past suggests that they could add water mass to the oceans much more quickly than previously assumed.
- The Fourth Assessment Report of the IPCC projects a 21st century sea level rise of 0.6-1.9 feet by 2100, but notes that accelerated glacial melt could contribute another 0.7 feet, raising the upper end to 2.6 feet.
- A new empirical projection of SLR for the 21st century indicates that sea level could rise by 1.6-4.5 feet this century.
- The potential for the rapid collapse of large ice sheets seems more plausible than in the recent past, and some scientists therefore warn that continued anthropogenic warming could result in the triggering of abrupt sea level rise within the current century.
Temperatures have increased both over land and water. Some of the heat from global warming gets stored in the world’s oceans causing higher water temperatures near the surface. This figure shows the trend in the heat content of the ocean between the surface and 700 meters deep. Warming of the oceans has many consequences, including sea level rise (warmer water expands), coral bleaching, loss of sea ice, and intensification of hurricanes.
Source: NOAA/NESDIS/NODC Ocean Climate Laboratory, Updated from Levitus et al. (2009)
One of the impacts of climate change is an increase in sea level. This figure shows the results of satellite measurements of the change in average global sea level over time. Sea level rise is caused by the expansion of water as it warms up and by melting land ice from glaciers and ice caps.
Source: University of Colorado (Seasonal signals removed)
This figure compares measurements of the Earth’s past temperature variations (shown by the black line) with computer model simulations of past temperature variations (shown by the red and blue lines) in order to determine whether the major changes in temperature were caused by natural or human-caused factors.
All lines are shown as variations from the average temperature. Natural drivers include solar radiation and volcanic emissions, while anthropogenic (man-made) drivers include human emitted greenhouse gases and sulfate aerosols. The blue line shows variation when only natural drivers are included in the calculations, while the red line shows variation when both natural and anthropogenic drivers are included.
This figure shows that the combination of natural and anthropogenic drivers (the red line) provides a better match to the observed temperatures (black line) than only natural drivers (the blue line). Natural drivers alone can explain much of the temperature change in the first half of the century, as demonstrated by the similarity between the black and blue lines during that time period. As can be seen with the close match between the red and black lines, human-produced drivers strongly dominated the temperature change in the latter part of the 20th century.
As can been seen in this figure, throughout the millennia, there has been a clear correlation between carbon dioxide levels and average global surface temperatures. This provides strong evidence that CO2 is a major driver of global temperatures.
Scientists say the world is entering largely uncharted territory as atmospheric levels of greenhouse gases continue to rise. Today’s carbon dioxide levels are substantially higher than anything that has occurred for more than 400,000 years.
The recent increase in concentration of carbon dioxide in the atmosphere is the result of human activities, mainly the burning of fossil fuels. As the concentration of CO2 in the atmosphere has increased, so has the average surface temperature of the Earth.
The relationship between atmospheric CO2 concentration and surface temperature is shown here for the past 130 years.
Source: NASA-GISS, CDIAC, NOAA ESRL