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Record-breaking heat and fire
National Climatic Data Center, NOAA. (2007), The Climate of 2007, (http://www.ncdc.noaa.gov/oa/climate/research/2007/perspectives.html)
National Interagency Fire Center, (http://www.nifc.gov/fire_info/fire_stats.htm)

In 2007, global land surface temperature was the warmest on record, while total surface temperature (including oceans) was the fifth-warmest since recordkeeping began in 1880. In the United States, 2007 was the 10th-warmest year, and six of the 10 warmest years on record for this country have occurred since 1998. A severe heat wave affected large parts of the central and southeastern United States in August 2007, setting more than 2,500 new daily record highs. The seven months from January through July 2008 ranked as the ninth-warmest seven-month period on record for total surface temperature.

More acres burned in the United States in 2006 and in 2007 than in any other year since 1960 (9.9 and 9.3 million acres, respectively).

Droughts, water crises and fires
Barnett, T.P., et al. (2008) Science 319: 1080-1083.
Barnett, T.P., and Pierce, D.W. (2008) Water Resources Research, 44: doi:10.1029/2007WR006704.
Saunders, S., et al. (2008) Hotter and Drier: The West's Changed Climate. (http://www.nrdc.org/globalWarming/west/west.pdf)

In 2007, severe to exceptional drought affected the southeastern and western United States. More than three-quarters of the Southeast experienced drought conditions from midsummer into December 2007. As of July 2008, about 28 percent of the contiguous United States was experiencing moderate to extreme drought. The American West, in particular, has been suffering the harsh reality of global warming over the past decade, with increased temperature (an increase 70 percent greater than the global average), more frequent and severe heat waves, more intense and longer droughts, increased incidence of wildfires and a variety of other damages to ecosystems (Saunders et al., 2008).

A powerful example of the severity of the water crisis is the falling water level in both Lake Mead and Lake Powell, which supply drinking water and power to millions in the American Southwest. It has been predicted that Lake Mead and Lake Powell stand a 50 percent chance of running dry by 2021, due to a combination of rising water demand and global warming, unless dramatic changes take place in how the region uses water.

But are these water crises due to global warming? A recent study in the journal Science investigated just that question and found that up to 60 percent of the climate-related trends in river flow, winter air temperatures and snowpack between 1950 and 1999 in the western United States are human-induced through greenhouse gas and aerosol emissions. The study team used a multivariable detection and attribution methodology to statistically demonstrate that recent hydrologic changes differ significantly from those of the past, and in ways specifically expected from human emissions. This study, in combination with previous work, predicts "a coming crisis in water supply in the western United States' caused by greenhouse gas emissions.

Extreme Weather
Ackerman, F., et al. (2008) The Cost of Climate Change. (http://www.nrdc.org/globalwarming/cost/contents.asp)
Emanuel, K.A., et al. (2008) Bulletin of the American Meteorological Society, 89(3): 347-367.
Emanuel, K.A. (2007) Journal of Climate, 20(2): 5497-5509.
Karl, T.R., et al. (2008) Weather and Climate Extremes in a Changing Climate. (http://www.climatescience.gov/Library/sap/sap3-3/final-report/default.htm)

The U.S. Climate Change Science Program recently produced a comprehensive assessment of how global warming has transformed -- and will continue to transform -- the climate of the United States and Canada, with a special focus on extreme events. Karl et al. (2008) find that gGreenhouse gas emissions likely will cause North America to experience more droughts and excessive heat in some regions even as more intense downpours, stronger winds and higher storm surges pound other regions or the same region in a different season or year. Climate models indicate that currently rare extreme events will become commonplace. The report warns that extreme weather events "are among the most serious challenges to society in coping with a changing climate.' The dramatic increase in extreme events is likely to have significant economic consequences on the order of hundreds of billions of dollars annually, as estimated by Ackerman et al. (2008).

Hurricanes: Legitimate uncertainty continues about how climate change might impact hurricane intensity and frequency, because the creation and development of individual hurricanes occur at such a small scale relative to climate models. Studies seem to suggest that hurricane frequency probably will not increase and may, in fact, decrease with global warming. Several studies predict that hurricane intensity will increase with global warming, as has been observed in the increase of Category 4 and 5 Atlantic hurricanes. Hurricane projections contrast with the better-understood and more certain impact studies discussed in this update. (Karl et al. 2008; Emanuel et al. 2008, 2007; Knutson et al. 2008)

Higher emissions and declining natural carbon sinks
Canadell, J.G., et al. (2007) PNAS, 104(47): 18866-18870.
Raupach, M. R., et al. (2007) PNAS, 104(24): 10288-10293.
Schuster, U., and A. J. Watson. (2007) Journal of Geophyiscal Research, 112: doi:10.1029/2006JC003941.

Global warming impact projections are dependent upon emissions scenarios for their calculations. Emissions scenarios are descriptions of possible future levels of greenhouse gas emissions based on different assumptions about economic and social conditions, population growth and technology. If emissions are higher than the scenario used, then the impacts will be underestimated. Rapauch et al. (2007) finds that global carbon dioxide emissions are now higher than in any of the emissions projections used in the IPCC reports. If these high emissions continue, then the IPCC projections will have significantly underestimated the climate change and damages that the world will face.

Atmospheric greenhouse gas concentrations are a function not only of emissions but also of sinks. The oceans and land vegetation currently soak up and store about half of the carbon dioxide added to the atmosphere by humans. However, recent studies find that we have been losing our natural sinks at an alarming rate in the past decade, far more quickly than had been predicted by global climate models (Canadell et al. 2007; Schuster & Watson 2007).

The figure below shows that the fraction of total fossil fuel emissions that absorbed into the ocean each year has significantly decreased. (Canadell et al., 2007)

Public health damage from climate change
Gamble, J.L., et al. (2008) Analyses of the Effects of Global Change on Human Health and Welfare and Human Systems. U.S.C.C.S.P. (http://www.climatescience.gov/Library/sap/sap4-6/final-report/default.htm)
Jacobson, M.Z. (2008) Geophysical Research Letters 35, doi:10.1029/2007GL031101.
Knowlton, K., et al. (2007) American Journal of Public Health 97: 2028-2034, doi:10.2105/AJPH.2006.102947.

A recent national assessment coordinated by the EPA (Gamble et al., 2008), provides a rigorous analysis of the variety of mechanisms by which global warming might negatively impact human health in the United States. Heat-related morbidity and mortality very likely will increase over the coming decades, as will illnesses and deaths related to ozone and particulate matter air pollution. For example, Knowlton et al. (2007) found that under changing climate conditions, heat-related premature mortality could nearly double by the 2050s in the New York City metropolitan region. Several food- and water-borne pathogens are likely to increasingly spread among susceptible populations in a changing climate, such as increased salmonella outbreaks with rising temperatures. Extreme events also can impact health. For instance, floods could contaminate drinking water with pathogens, and wildfires could increase air pollution. Health burdens will vary by region and likely will accentuate the disparities already evident in American health care.

Jacobson (2008) specifically investigated the effect of climate change on human health through increased ozone and particulate matter. Jacobson developed a novel model that combines a high-resolution global-regional model with an atmospheric pollution model and mortality studies. He found that climate change increases both water vapor and temperatures and that both then separately increase ozone, particularly in locations with preexisting higher ozone. Climate change also increases particulate matter by enhancing stability, humidity and biogenic particle mass. Jacobson projected that ozone will cause 40 percent and particulate matter 60 percent of the additional deaths expected from air pollution brought about by climate change. For each 1° C. (1.8° F.) of CO2-induced warming, annual air pollution deaths in the United States will likely rise by about 1,000 (350–1,800), and cancers will rise by 20–30 people per year. He finds that globally more people are expected to die from CO2-induced air pollution than by CO2-enhanced storminess.

Arctic sea ice decline: faster than forecast
National Snow and Ice Data Center. (http://nsidc.org/arcticseaicenews/index.html)
Stroeve, J., et al. (2007) Geophysical Research Letters 34, doi:10.1029/2007GL029703

Arctic sea ice reflects sunlight and thus plays a crucial role in keeping polar regions cool and moderating the global climate. As sea ice melts, a much darker ocean is exposed, and the darker ocean then absorbs more radiation, causing an amplification of polar warming. In September 2007, the amount of Arctic sea ice reached its lowest point since satellite measurements began in 1979, shattering the 2005 record by 23 percent. The rate of decline in September sea ice is now about 10 percent per decade, or 28,000 square miles per year (as shown by the trend line below). The Northwest Passage completely opened for the first time in human memory in 2007 and opened again in 2008. Scientists are projecting that the Arctic could be ice-free in the summer within five to ten years. The rate of Arctic sea ice decline has outpaced any prediction by the IPCC models, implying that the Arctic region is even more sensitive to global warming than we had thought.

Accelerated Artic warming and melting permafrost
National Oceanic and Atmospheric Administration (NOAA). (April 23, 2008) (http://www.noaanews.noaa.gov/stories2008/20080423_methane.html)
Lawrence, D. M., et al. (2008) Geophysical Research Letters, 35: doi:10.1029/2008GL033985.
Levinson, D. H., et al. (2008) State of the Climate in 2007. American Meteorological Society.

Polar regions in 2007 were the warmest on record for the Arctic. In the past year, global methane in the atmosphere rose sharply, by 27 million tons, after nearly a decade with little or no increase (see below). It is too soon to tell whether this sudden jump in methane is a sign of methane release from Arctic permafrost, a positive warming feedback that was thought to be a concern only on a scale of hundreds of years. Methane is 25 times more potent a greenhouse gas than carbon dioxide.

Lawrence et al. (2008) used a climate model to explore the relationship between low sea ice extent, increased air temperatures and permafrost thawing. He found that extended episodes of rapid sea ice loss can cause the Arctic to warm 3.5 times more quickly than the secular 21st century climate-change trends, and that rapid warming could trigger the thawing of permafrost and the release of methane. In addition to contributing substantial amounts of heat-trapping gases to the atmosphere, the melting of permafrost can have a range of impacts on sensitive ecosystems and human infrastructure in the Arctic, such as buckling highways and destabilizing houses.

Killing our oceans
Carpenter, K.E., et al. (2008) Science, 321: 560-63.
Feely, R.A., et al. (2008) Science, 329: 1490-1492.
Hoegh-Guldberg, O., et al. (2008) Science, 318: 1737-1742.
Zeebe, R.E., et al. (2008) Science, 321: 51-52.

One-third of the world's reef-building corals currently face elevated risks of extinction due to climate change and local impacts, a dramatic increase over the past decade. Carpenter et al. (2008) investigated hundreds of coral species as part of the International Union for Conservation of Nature (IUCN) Red List Categories and Criteria assessment. His team found that if corals cannot adapt to climate change, "the cascading effects of the functional loss of reef ecosystems will threaten the geologic structure of reefs and their coastal protection function and have huge economic effects on food security for hundreds of millions of people dependent on reef fish. Our consensus view is that the loss of reef ecosystems would lead to large-scale loss of global biodiversity.'

Another recent study investigated the impacts of different CO2 emissions scenarios on the world's coral reefs. Hoegh-Guldberg et al. (2008) found that any concentration of atmospheric CO2 above 500 ppm is "extremely risky' for coral reefs due to the combined effects of ocean acidification and coral bleaching from rising temperatures, even under the most optimistic circumstances. Off the west coast of the United States, Feely .et al. (2008) have discovered that ocean pH and carbonate availability have declined enough to result in episodes of corrosive waters (able to dissolve aragonite-based shells) being upwelled onto the continental shelf of the United States. Others have found that avoiding environmental damage to the ocean from acidification requires reductions in carbon dioxide emissions, regardless of changes in the climate (Zeebe et al., 2008).

Global Food Security Concerns
Lobell, D.L., et al. (2008) Science 319, 607-610. Policy summary: (http://iis-db.stanford.edu/pubs/22099/science_policy_brief.pdf)
Backlund, P., et al. (2008) The Effects of Climate Change on Agriculture, Land Resources, Water Resources, and Biodiversity in the United States. U.S. Climate Change Science Program. (http://www.climatescience.gov/Library/sap/sap4-3/final-report/default.htm)

Food security will be threatened by climate change in the next two decades in several regions around the world. Lobell et al. (2008) analyzed the climate risks for crops in 12 food-insecure regions using statistical crop models and climate projections for 2030 from 20 general-circulation models. They found that South Asia and southern Africa will likely suffer negative impacts on several crops that are important to large food-insecure human populations, unless sufficient mitigation and/or adaptation measures are achieved. These findings are all the more concerning in light of today's record high food prices.

A recent USDA report (produced by more than a dozen agencies and reflecting the findings of more than 1,000 scientific studies) similarly projects a grim future for American agriculture due to the negative impacts of climate change. These impacts are far-reaching: reduced yields for some crops, higher water requirements, faster-growing and faster-spreading weeds, greater livestock mortality and more frequent insect and crop disease outbreaks. (Backlund et al., 2008)

The Earth is nearing tipping points
Lenton, T. M., et al. (2008) PNAS, 105(6): 1786-1793.
Weitzman, M. (2008) On Modeling and Interpreting the Economics of Catastrophic Climate Change. (http://www.economics.harvard.edu/faculty/weitzman/files/REStatModeling.pdf).

Lenton et al. (2008) brought together an international group of scientists to critically evaluate potential policy-relevant "tipping elements': large-scale components of the earth system currently in danger of coming to a critical point where a small perturbation qualitatively alters the future fate of that system, with large-scale impacts on ecology and human life. The study adds value to the previous IPCC overview by using more precise definitions, encompassing some more-recent studies, and undertaking a more in-depth evaluation of the policy relevance, sensitivity and uncertainty of the various tipping elements. The group's article in Proceedings of the National Academy of Sciences provides an excellent review and summary of the most up-to-date literature on the various tipping elements.

Weitzman has demonstrated the great importance of directly considering low-probability, high-consequence impacts in economic analyses. He finds that the potential for extreme-impact events, such as those from tipping elements, can outweigh discounting in climate-change economics and can radically increase the benefits and economic rationale of immediate and strong emissions reduction policies.

National security threats
The CNA Corporation. (2007)National Security and the Threat of Climate Change. (http://securityandclimate.cna.org/)
National Intelligence Council. (2008) National Intelligence Assessment on the National Security Implications of Global Climate Change to 2030. Presented in the testimony of Dr. Thomas Fingar to the Permanent Select Committee on Intelligence and the Select Committee on Energy Independence and Global Warming, U.S. House of Representatives on 25 June 2008. (http://www.dni.gov/testimonies/20080625_testimony.pdf)

The national security impacts of climate change on the United States are potentially highly significant, according to recent research by 11 retired U.S. generals and admirals and separate research by the National Intelligence Council. These reports find that climate change "poses a serious threat to America's national security' and that "climate change will act as a threat multiplier for instability in some of the most volatile regions of the world."

Stabilizing climate requires near-zero emissions
Matthews, H.D., and Caldeira, K. (2008) Geophysical Research Letters, 35: doi:10.1029/2007GL032388.

Stabilizing CO2 levels in the atmosphere is insufficient to stabilize the world's climate. Near-zero future CO2 emissions are necessary to hold the climate constant at a given global temperature. Matthews and Caldeira (2008) explored the long-term implications of CO2 emissions, using a climate model of intermediate complexity (UVic ESCM 2.8). Their study found that human-induced climate warming will continue for many centuries even after atmospheric CO2 concentrations are stabilized, due to the ocean's large heat capacity and high thermal inertia. These results differ importantly from previous analyses, which have neglected the heat capacity of the deep ocean and thus have concluded that global temperature would stabilize more rapidly after CO2 concentrations level off. The new findings imply that in order to stabilize the climate, CO2 emissions need to be nearly eliminated so that atmospheric CO2 concentrations begin to fall. They further imply that consideration of these emissions will be very important for developing policy to meet temperature targets, such as the European Union's goal of limiting global warming to 2° C.

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