Global Warming Science: An Annotated Bibliography
A summary of recent findings on the changing global climate.
Models Underestimate Warming, Sea Level Rise
IPCC Fourth Assessment Report (AR4), 2007 (http://www.ipcc.ch/)
Rahmstorf, S., et al. Science. May 4, 2007
Rahmstorf, S. Science. Jan. 19, 2007 (http://www.pik-potsdam.de/~stefan/)
Meier, M., et al. Science, Vol. 317: 1064. Aug. 24, 2007
2006 Annual Climate Review Summary, updated June 21, 2007, NOAA, National Climatic Data Center, available at: http://www.ncdc.noaa.gov/oa/climate/research/2006/perspectives.html
IPCC sea level rise estimates are conservative, according to Stefan Rahmstorf and other researchers. The uppermost limit of the IPCC Third Assessment Report range assumes a high emissions scenario, high climate sensitivity, and an extra amount of sea level rise that could result from "ice sheet uncertainty." Real world rates, the new studies show, have matched this upper limit since 1990.
Sea level rise so far has also been significantly faster than predicted in the 2007 IPCC reports. The rate of sea level rise for the last 20 years was 25 percent faster than in any other recent 20-year period, with ocean thermal expansion and melting from non-polar glaciers the main culprits. There was a close relationship between temperature and sea level rise throughout the 20th century, as sea levels rose at a rate of about 2 millimeters per year per degree Fahrenheit. Applying this relationship to future warming scenarios implies that sea level will rise between 50 and 140 centimeters (20 to 55 inches) by 2100. This estimate is much higher than the IPCC third Assessment Report estimates of 21 to 70 centimeters by 2100 and fourth Assessment Report estimates of 18 to 59 centimeters by 2100. The bottom line is that if the current relationship between warming and sea level rise continues, the IPCC estimates are unrealistic, and sea level rise of more than 1 meter (more than 3 feet) by 2100 is a real possibility.
A recent report on disintegrating glaciers by Mark Meier of the University of Colorado and colleagues bolsters this conclusion. They found that the discharge of glacial ice into the ocean is accelerating rapidly, due in large part to the dynamic instability of glaciers that terminate in the ocean below sea level. Once these glaciers begin to melt, their ice flows into the ocean at an accelerating rate, rather than just melting at the surface. If the current rate of acceleration continues, glaciers and ice caps alone (excluding Greenland and Antarctica) would contribute 11 to 37 centimeters to sea level rise by 2100, consistent with Rahmstorf's projection for total sea level rise, but not with the lower estimates from the IPCC.
Warming pushes the upper bound
The 2007 IPCC report is the first to give a best estimate -- 5.4 degrees Fahrenheit -- of "climate sensitivity," the global mean temperature rise that is likely to result from a doubling of carbon dioxide levels. It implies that temperatures will increase in this century by 2 to 11.5 degrees Fahrenheit, depending on how high greenhouse gas concentrations rise. However, IPCC projected temperature increases may be too low. In the last 16 years, the global mean surface temperature increase was 0.6 degrees Fahrenheit, according to NASA and the UK Met Office. This falls in the very upper part of the range projected by the IPCC in 2001, and suggests that warming may be more rapid than predicted. The average annual temperature for the contiguous United States in 2006 was the second warmest on record and within 0.1 degrees Fahrenheit of the record set in 1998. The past nine years have all been among the 25 warmest years on record in the United States -- an unprecedented streak.
It is difficult to establish exactly why the world is warming so quickly, but there are a few possibilities. The first is variability within the climate system, which is complex and goes through unpredictable ups and downs. The second possibility is aerosols. There has been less cooling from aerosol emissions than expected, potentially resulting in faster warming. A third candidate is an underestimate of how sensitive the climate is to carbon dioxide levels. The models have been right on target about carbon dioxide concentrations, with IPCC projections matching data from Mauna Loa, Hawaii and Scripps/NOAA, but they may be off target about the relationship between carbon dioxide concentrations and temperature change.
Ice Sheet Melt Concerns and Confounds Scientists
Cazenave, A. Science. Nov. 24, 2006
World Bank Policy Research Working Paper S4136: "The impact of sea level rise on developing countries: a comparative analysis." February 2007
Luthcke, S., et al. Science. Nov. 24, 2006
Chen, J., et al. Science. Sept. 29, 2006
The hardest part of predicting sea level rise is figuring out how fast land-based ice sheets will melt. So far, ice sheets have not contributed very much to sea level rise, but Greenland and West Antarctic ice sheets are now melting at an accelerated rate. Recent studies show that a large portion of the melting occurs through dynamic processes, such as rapid melt at the base of ice sheets under pressure from above, with melting at the ice surface playing a smaller role. Yet dynamic melting is not taken into account in the IPCC projections because it is too poorly understood to be modeled. This is a major omission. Even though modeling of dynamic melt is in its infancy, observations of what is occurring on the ground can be incorporated into projections, yielding more realistic estimates and giving us a better sense of what's in store.
Greenland, which holds 10 percent by weight of global ice, is losing ice at an ever-increasing rate, according to data from the GRACE gravity-measuring satellite. Throughout the 1990s, the Greenland ice sheet remained stable, but has increasingly declined in recent years. A study performed by Luthcke and colleagues found that from 2003 to 2005 the ice sheet lost more than 100 billion tons per year -- which contributes an estimated 0.3 mm per year to global sea level rise. Chen et al. used time-variable gravity measurements from GRACE to measure the weight of the ice sheet from 2002 to 2005. Consistent with Luthcke's results, the team measured a net loss of 239 cubic kilometers per year, most of which occurred in Eastern Greenland. This translates to 0.54 mm per year of global sea level rise, slightly higher than the other team's estimate.
Melting the entire Greenland ice cap would raise sea level 6.5 meters (more than 21 feet). The World Bank estimates that a much smaller 1-meter increase in sea level would displace at least 56 million people living in coastal regions, and a 5-meter rise would displace more than 240 million people. The stakes are high, and ice sheet modelers are working hard to try to better understand how these ice sheets accumulate and lose ice.
Sea Ice Is Reaching a Breaking Point
Holland, Marika, et al. Geophysical Research Letters. Dec. 12, 2006
Winton, M. Geophysical Research Letters. Dec. 13, 2006
Multiple climate models predict that sea ice will disappear as the earth warms, and recent observations provide alarming evidence that these predictions are being realized. As of September 4, 2007, Arctic sea ice extent reached a record low of 4.42 million square kilometers (1.70 million square miles), almost 20 percent lower than the previous record low of 5.32 million square kilometers (2.05 million square miles) set in September 2005, and is still falling. The monthly average Arctic sea ice extent for August 2007 was 31 percent below the average for all Augusts since 1979 and was the lowest monthly average ever recorded by satellite for any month. September typically has the lowest monthly average sea ice extent, and we can expect a new record low to be set when September 2007 data come in.
Holland et al. predict abrupt and widespread sea ice melting early in the 21st century, beginning as soon as 2015. Loss of ice spurs further melting, because sea ice reflects back more of the sun's rays than does open water, which absorbs more solar heat. As sea ice disappears, increasing amounts of open water will lead to even warmer waters and more melting, in conjunction with predictions of rapid ocean heat transport to the poles. Abrupt summer ice melting is expected to arise as the ice pack over the Arctic and in other areas thins and disappears during summer months.
Researchers analyzed multiple climate scenarios for the IPCC Fourth Assessment Report and found that abrupt summer reductions in sea ice cover occurred in more than 50 percent of the models. Examined as a group, their results suggest we may have ice-free Arctic Septembers in 30 to 50 years, and as early as 15 years from now. Reducing emissions could play a major role in mitigating this possibility. Under high-emissions scenarios, the likelihood of ice-free conditions increases sharply, while reductions in greenhouse gas emissions reduce the likelihood.
Polar Bears and Other Species Stressed from Global Warming
Parmesan, C., and G. Yohe. Nature. Jan. 2, 2003
Stirling, I., and C.L. Parkinson. Arctic. September 2006
Many plants and animals are stressed due to habitat destruction and other human-induced pressures. Global warming presents another threat, and warming is occurring at a rate that may prove too fast for species to adapt. Already, species that can migrate are moving to cooler habitats. A study of 1,700 species found poleward migration of 6 kilometers per decade and vertical migration in alpine regions of 6 meters per decade since 1950.
Global warming presents a particularly stressful change for Arctic species that cannot migrate further northward. Some populations of polar bears are already beginning to decline in numbers. Polar bears feed in the winter when the sea ice is at its greatest mass, but global warming is causing shorter winters and reduced ice cover and thickness, with the ice breaking up about three weeks earlier than it did 30 years ago. Stirling et al. performed a study in the Canadian Arctic and found that in some areas polar bear populations declined 22 percent between 1989 and 2004. If the earth continues to warm, all five of the populations they studied risk endangerment.
Carbon Sinks Reconsidered
Gullison, Raymond E., et al. Science, Vol. 316: 18. May 2007
Stephens, B.B., et al. Science. June 22, 2007
Sitch, S., et al. Nature. Published online July 25, 2007. doi:10.1038/nature06059
Science Daily article summarizing Ecological Society of America presentation. Aug. 7, 2007
Zhou, G., et al. Science, Vol. 314: 1,417. 2006
Less than half of the carbon dioxide emitted into the atmosphere from human activities, including fossil fuel burning and deforestation, actually stays in the atmosphere. The rest is absorbed by the world's oceans and land. Understanding how these carbon sinks will respond to a changing environment is important for predicting the impacts of increasing carbon dioxide concentrations.
Tropical forest sink
Conventional wisdom about land sinks has recently been challenged. Stephens et al. measured the vertical atmospheric carbon dioxide distribution at 12 sites around the globe and compared the measurements to model predictions. Their results suggest that the Northern Hemisphere plays a smaller role in carbon dioxide uptake than previously thought and that the tropics may be strong carbon sinks as opposed to a net carbon source, even with continuing high rates of deforestation. This implies that tropical forests are even more critical than previously assumed. Tropical deforestation released approximately 1.5 billion metric tons of carbon (GtC) to the atmosphere annually throughout the 1990s, accounting for almost 20 percent of anthropogenic greenhouse gas emissions. Without effective policies to slow deforestation, the clearing of tropical forests will likely release an additional 87 to 130 GtC by 2100, equivalent to more than a decade of global fossil fuel combustion at current rates. As the temperature rises, drought-induced tree mortality, logging, and fire may double these emissions, and loss of land-sink capacity as forest area decreases may further increase atmospheric carbon dioxide levels.
More bad news about smog
Climate-carbon cycle models suggest that global warming will lead to less land-carbon sequestration, but they don't include the indirect impacts of increases in smog (tropospheric ozone). A recent Nature paper estimated the impact on the land carbon sink and found significant reduction in the sink due to indirect effects on plants. Emissions from fossil fuel and biomass burning have doubled the global smog concentration, which has a damaging effect on plants. Ozone enters a plant's respiration pores (stomata) and hinders photosynthesis, resulting in weakened and smaller plants that absorb less carbon dioxide. Through this impact on plants, smog causes more carbon dioxide to build up in the atmosphere and accelerates global warming.
The study examined two scenarios on a time scale from 1901 to 2100. One was based on plants with a high sensitivity to ozone and the other on vegetation with low sensitivity. In the high-sensitivity model, ozone decreased land carbon capture by 23 percent over two centuries. In the other, land carbon capture declined 14 percent. Lead researcher Stephen Sitch of the Hadley Center warned that most climate change studies have not taken ozone's effect on carbon sinks into account -- which could indirectly add 0.9 degrees to 2.3 degrees Fahrenheit to global warming.
Old growth forests continue to accumulate carbon
As atmospheric conditions change, soils in old growth forests may keep soaking up carbon long after they reach maturity, according to recent measurements from China. Traditionally, old growth forests have been considered negligible carbon sinks (although they hold large carbon stocks), because carbon uptake and respiration rates are thought to be roughly balanced. However, a study of a forest reserve in Guangdong, southern China, showed that soil carbon increased by 68 percent in 25 years. The top soil layer accumulated atmospheric carbon at an unexpectedly high rate from 1979 to 2003. The study suggests that below-ground carbon cycle processes are changing in response to climate changes, and it challenges the belief that old growth forests are in equilibrium. Nonetheless, forests alone can't be relied on to offset carbon dioxide emissions from fossil fuel combustion. A long-term Duke University study applied higher-than-normal levels of carbon dioxide to a North Carolina loblolly pine forest. As expected, extra carbon dioxide, which stimulates plant growth, allowed the trees to grow more, but only those pines receiving the highest levels of water and nutrients were able to store significant additional carbon, scientists told a meeting of the Ecological Society of America in August.
Global Warming Affects Weather Extremes
Hansen, James, et al. Proceedings of the National Academy of Science. Sept. 26, 2006
Hoyos, C.D., et al. Science, Vol. 312: 94-97. 2006
Super El Niño
Hansen et al. suggest that the global temperature increase of about 0.4 degrees Fahrenheit per decade during the past 30 years is contributing to extreme El Niño patterns. He attributes the super El Niño episodes in 1983 and 1998 to asymmetrical warming of the Western Equatorial Pacific and the Eastern Equatorial Pacific. Warmer waters, attributed to anthropogenic global warming, will most likely increase the intensity of extreme El Niño occurrences.
Increases in hurricanes
Rising sea-surface temperatures correlate strongly with the observed increase in the number of category four and five Atlantic hurricanes between 1970 and 2004, according to scientists at Georgia Tech. Other factors that affect hurricane formation, such as wind shear and humidity levels, do not appear to have changed during that time period and are not causing the intensification of hurricanes.
Tilman, David, et al. Science, Vol. 314: 1,598-1,600. doi:10.1126/science.1133306. Dec. 8, 2006
Low-input, high-diversity (LIHD) mixtures of grassland perennials can "provide more usable energy, greater greenhouse gas reductions, and less agrichemical pollution per hectare than can corn grain ethanol or soybean biodiesel," according to Tilman and colleagues at the University of Minnesota.
Mixtures of native prairie grasses can be grown on degraded lands so they do not displace agricultural or protected lands. They require fewer inputs -- which makes them less environmentally damaging than conventional agriculture. They also have 238 percent higher bioenergy yields than monocultures after 10 years. Additionally, Tilman found that these mixtures can be used to make biofuels that have negative net lifecycle carbon dioxide emissions because they sequester more carbon dioxide than they release during production and processing. Growing LIHD perennials on the 500 million acres of agriculturally abandoned and degraded land in the world could produce 13 percent of the global petroleum consumption for transportation and 19 percent of global electricity consumption. This could remove 15 percent of current global carbon dioxide emissions and potentially more if sequestration is factored in.
Yellowstone Area Grizzly Bears Suffer as Whitebark Pines Decline
Powell, J.A., and J.A. Logan. "Insect seasonality: circle map analysis of temperature-driven life cycles." Theoretical Population Biology, Vol. 67: 161-179. 2005
Felicetti, L.A., et al. Canadian Journal of Zoology, Vol. 81: 763-770. 2003
Interagency Grizzly Bear Study Team. 2004 Annual Report. Interagency Grizzly Bear Study Team, USGS Northern Rocky Mountain Science Center, Montana State University, Bozeman.
Mattson, D.J., B.M. Blanchard and R.R. Knight. Journal of Wildlife Management, Vol. 56: 432-442. 1992
Mattson, D.J. Ursus. Vol. 10: 129-138. 1998
Schwartz, C.C., et al. Wildlife Monographs, Vol. 161. 2005
Whitebark pine seeds are a major food source for grizzly bears in the Greater Yellowstone Area, and considered important for the long-term viability of the population. During years of good whitebark pine cone production, more than 70 percent of Yellowstone area bears consume whitebark pine cones. Without rich, fatty whitebark pine seeds to help female grizzlies add weight during the fall, both the number of grizzly bear litters and the number of cubs per litter declines dramatically. This causes grizzlies to seek alternate food sources and results in increased human-bear conflicts and grizzly mortalities.
A decline in whitebark pine seed cone production is well under way in the Yellowstone area, and in many ecosystems in the western United States and Canada, as whitebark pine forests have been decimated by mountain pine beetles and by blister rust, an invasive plant disease. Whitebark pine mortality is already 40 to 100 percent in certain areas.
As the climate warms, mountain pine beetle outbreaks will spread further north and into higher elevation forests. Virtually all whitebark pine stands in the Yellowstone area, except for portions of the Absaroka Mountains and the Wind River Range, will be severely damaged during the next several decades, according to a model developed by Jesse Logan, a recently retired U.S. Forest Service scientist. The beetles can kill entire stands of whitebark pine in just one summer and have immediate impacts on grizzly bears. From 2000 to 2004, 18,000 acres of whitebark pines in Yellowstone National Park were killed by mountain pine beetle outbreaks. Once whitebark pine forests are damaged, the grizzly bears that depend on this critical food source would face food shortages for many years because it takes 60 to 80 years for the pines to begin producing large cone crops.
Warming Will Lead to Frequent and More Intense Wildfires in the Western United States
Westerling, A., et al. Science, Vol. 313: 940-943. doi:10.1126/science.1128834. Aug. 18, 2006
Following up on several previous experiments, Westerling et al. assembled data on 1,166 large wildfires (over 400 hectares) within forested areas in the Western United States since 1970. They related this data to meteorological and hydroclimatic records and found that wildfires in hotter, drier periods were more intense and more frequent and occurred over longer wildfire seasons.
Earlier snowmelt dates correspond to increased wildfire frequency because they can lead to a longer dry season, making the forests vulnerable to ignition. As a whole, longer summers result in drier vegetation and longer fire seasons. If the warming continues to exacerbate the wildfire seasons, it could be costly. Currently, Western forests sequester 20 to 40 percent of total U.S. carbon sequestration, and firefighting expenditures have consistently totaled upwards of $1 billion per year.
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