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Hotter and Hotter
Hansen, J., R. Ruedy, M. Sato and K. Lo (2006) GISS Surface Temperature Analysis, Global Temperature Trends: 2005 Summation, NASA's Goddard Institute for Space Studies, available at: http://data.giss.nasa.gov/gistemp/2005/

In early 2006, NASA released data confirming that the 10 warmest years on record have all occurred since 1990, extending the warming trend of recent decades. This trend includes 2005, which NASA confirmed as tied with 1998 for the hottest year on record. Last year's warmth is particularly significant as global temperatures in 1998 received a boost of up to 0.2°C from El Niño. No such large scale ocean-atmospheric phenomenon occurred in 2005. The main reasons for the high globally-averaged temperature in 2005 were the anomalously high temperatures seen in the high northern latitudes (Figure 1). This Arctic warming is not expected from natural variability alone and supports the conclusion that the record warmth was primarily caused by a buildup of heat-trapping pollution in the atmosphere.

GISS Surface Temperature Analysis, Global Temperature Trends

Hansen, J., R. Ruedy, M. Sato and K. Lo (2006) GISS Surface Temperature Analysis, Global Temperature Trends: 2005 Summation, NASA's Goddard Institute for Space Studies, available at: http://data.giss.nasa.gov/gistemp/2005/

The Hockey Team Keeps Winning
National Research Council, The National Academies Press, (2006)
T.J. Osborn and K.R. Briffa (2006) Science, 311, 841 (February 10, 2006)
L.G. Thompson, et al., Proceedings of the National Academy of Sciences, 103(28), 10536 (May 12, 2006)

In addition to the data available from instrumental records, reconstructions of past temperature from temperature-sensitive proxies, such as tree rings, ice cores and coral reefs indicate that the globally-averaged surface temperature has been greater in the past few decades than during any comparable period during the preceding 400 years. The graph of these reconstructions resembles a hockey stick with the blade pointing up and to the right -- in other words, global temperatures were relatively steady for about 900 years and then turned up sharply during the late 20th century. After global warming deniers questioned the validity of the 'hockey stick' curve, the National Research Council (NRC) was asked to review past climate reconstructions by the House Science Committee. The NRC concluded that although uncertainties exist due to the nature of proxy records, it is plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period during the preceding millennium. Other groups have also reached similar conclusions recently. Osborn and Briffa analyzed a number of multiproxy data sets and concluded that Northern Hemispheric warming is now greater than it has been in the last 1,200 years and that the spatial extent of recent warming is of a greater significance than other periods of warmth, such as the Medieval Warm Period. And Thompson and his colleagues determined that the current warming at high elevations in the mid to low latitudes is unprecedented for at least the 2000 years.

The NRC report also notes that surface temperature reconstructions are only one of the multiple lines of evidence supporting the conclusion that heat-trapping pollution is primarily responsible for the recent warming trend. Other primary evidence includes the large increases in carbon dioxide and other global warming pollutants starting in the middle of the 19th century; energy balance calculations associated with heat-trapping gas increases and climate feedback mechanisms; numerical experiments performed by state-of-the-art climate models; and stratospheric cooling and ocean warming that match the predicted spatial and temporal pattern of human-induced warming.

Rapid Build Up
U. Siegenthaler, et al., Science, 310, 1313 (November 25, 2005)
P. Tans, NOAA/ESRL (2006)

Recent data indicate that carbon dioxide is accumulating in the atmosphere at a greater rate than in the past. In 2005 the concentration of carbon dioxide in the atmosphere increased by 2.5 parts per million (ppm), the third largest annual increase ever recorded. Although there is considerable inter-annual variability in the rate of increase in atmospheric carbon dioxide, the rise has been more than 2 ppm in 3 of the last 4 years. Prior to 1995, an annual increase of more than 2 ppm was seen only 4 times since the record began in 1959. As a result of recent jumps, the current atmospheric concentration of carbon dioxide is now over 380 ppm. This is an increase of more than 100 ppm since the start of the Industrial Revolution, and ice core records show that it is the highest concentration of atmospheric carbon dioxide for at least the last 650,000 years.

A Tipping Point at the Poles?
D. Lawrence and A. Slater, Stanford EE Computer Systems Colloquium (April 5, 2006)
B. L. Otto-Bliesner, et al., Science, 311, 1751 (March 24, 2006)
J.T. Overpeck, et al., Science, 311, 1747 (March 24, 2006)
E. Rignot and P. Kanagaratnam, Science, 311, 986 (March 24, 2006)
I. Velicogna and J. Wahr, Science, 311, 1754 (March 24, 2006)

In March 2006, Science magazine led with the cover "Climate Change -- Breaking the Ice." The edition included articles covering important new research on warming at both poles that is leading to changes in the ice system. These changes are occurring faster than previously observed or expected, therefore indicating that both the Arctic and Antarctic may be approaching a "tipping point" after which dangerous transformations will become unavoidable. Markers of such changes are visible in Greenland and in the Antarctic ice sheet, both of which are melting and thinning more rapidly than in the past. Velicogna and Wahr found that the mass of the Antarctic ice sheet has decreased significantly since 2002. A similar rapid loss of ice mass has been shown in the Arctic, where Rignot and Kanagaratnam found that the loss of mass from the Greenland ice sheet doubled between 1996 and 2005 to 224 ± 41 cubic kilometers (54 ± 10 cubic miles) per year. For comparison, the city of Los Angeles uses 1 cubic kilometers (0.23 cubic miles) of water per year.

Records of past ice-sheet melting indicate that the rate of future melting and the related sea-level rise could be faster than widely thought, according to Overpeck et al.. This recent study found that during the last interglacial period approximately 130,000 to 127,000 years ago, sea level ranged from 4 meters to more than 6 meters higher than today. Climate models project that by 2100, the high northern latitudes will be as warm, or warmer, than they were during the last interglacial period. Unless we curb heat-trapping emissions, temperatures in the late 21st century would be warm enough to melt at least large portions of Greenland and quite probably portions of West Antarctica. Millions of people would be vulnerable to flooding and displacement from the resulting sea level rise, and the economic loss associated with coastal inundation would be devastating.

Another study looked at the link between melting ice fields and rising sea levels. Otto-Bliesner et al. quantified what melting from Greenland and other Arctic ice fields contributed to sea-level rise during the last interglacial period. They evaluated ice cap retreat by analyzing results from a global climate model, a dynamic ice sheet model and paleoclimatic data. They found that melting in the western Arctic ice-fields and Greenland contributed between 2.2 meters and 3.4 meters of sea-level rise during the last interglacial period. While this was due to natural variations in global climate, human-induced global warming over the next century could lead to similar, substantial impacts on the polar environment.

But it's not just the ice system that is changing. Changes in the permafrost, or the permanently frozen ground, also reflect a warming trend in the Arctic. Recent runs of the National Center for Atmospheric Research's Community Climate System Model project that under business-as-usual greenhouse gas emission scenarios, there will be an up to 80 percent decline in near-surface permafrost by 2100. Even if the extent of permafrost melt is not as large as projected by this scenario and model, these results imply that large-scale changes in permafrost will occur in the future. If large-scale permafrost melting occurs, it may result in the rapid release of large quantities of methane, a potent global warming pollutant.

A Future without Summer Sea Ice in the Arctic?
National Snow and Ice Data Center, NASA and University of Washington Sea Ice Decline Intensifies, joint press release (September 28th, 2005)
National Snow and Ice Data Center (April 5, 2006)
J.T. Overpeck, et al., EOS, 86, 309
J.C. Stroeve, et al., Geophysical Research Letters, 32, L04501 (February 25, 2005)

At the beginning of April, scientists at the National Snow and Ice Data Center released results showing that March 2006 had the lowest Arctic wintertime sea ice coverage since 1979, the beginning of the satellite record (Figure 2). March sea ice represents the maximum cover for the year, and the record low in 2006 is particularly significant since it illustrates that for two years running, Arctic sea ice has failed to recover to its previous maximum levels during the winter months. The long-term mean March sea ice extent is 6.06 million square miles, whereas 2005 and 2006 set two new record lows at 5.72 million square miles and 5.60 million square miles, respectively. Compared to the long-term average wintertime sea ice level, the 2006 drop is approximately equivalent to three times the area of California.

March (wintertime rebound peak) mean sea ice extent

Although the decline in winter sea ice -- the annual maximum -- is not as pronounced as that of summer sea ice decline -- the annual minimum -- low winter sea ice means that the ice is freezing later in the fall and growing at a slower pace during the winter. We can expect this summer to continue the trend of all-time lows in sea ice extent. In fact, a study by Stroeve et al. recently found that four out of the five lowest years of sea ice coverage have occurred since 2002. This accelerated decline in summertime sea ice led Overpeck and his colleagues to conclude that the Arctic could be completely free of summer sea ice well before the end of this century, a state that has not occurred over at least the last million years.

Are We Underestimating Global Warming?
H.S. Brinkhuis, et al., Nature, 441 (June 1, 2006)
K.J. Moran, et al., Nature, 441 (June 1, 2006)
M.S. Torn and M.S. and J. Harte, Geophysical Research Letters, 33, L10703 (2006)
M. Scheffer, V. Brovkin and P.M. Cox, Geophysical Research Letters, 33 L10702 (2006)
A. Sluijs, et al., Nature, 441 (June 1, 2006)

New research from multiple groups suggests that current climate models may underestimate global warming projections because of a failure to account for positive feedback. For instance, higher temperatures may lead to increased releases -- or reduced uptake of -- carbon dioxide and/or methane by the ocean, forests and soils. This self-reinforcing cycle may not be fully accounted for in climate models, and until recently, few studies tried to quantify it. Torn and Harte have now found that by incorporating the carbon dioxide and methane positive feedback, the warming associated with doubling of carbon dioxide due to human activities is amplified from the range of 1.5 - 4.5°C to 1.6 - 6.0°C. Similarly, Scheffer and his colleagues found that the century-scale positive feedback of rising temperatures on atmospheric carbon dioxide concentrations will further enhance warming by an extra 15 to 78 percent. Although both groups of researchers recognize the limitations and uncertainties of their projections, their independent results, which use different methods, suggest that warming over the coming century may in fact be greater than recent trends and could be larger than that projected by the Intergovernmental Panel on Climate Change.

Additional studies found evidence that feedback mechanisms have amplified warming in the past. The Arctic Coring Expedition analyzed sediments from the Palaeocene/Eocene thermal maximum and found that polar temperatures during this period were more than 18°F warmer than those predicted by current climate models and that the Arctic is capable of warming to over 73°F and becoming ice free. This illustrates that higher-than-modern greenhouse gas concentrations must have operated in conjunction with additional feedback mechanisms, currently unaccounted for in climate models, to intensify warming. Sluijs and his colleagues suspect that polar stratospheric clouds and hurricane-induced ocean mixing could have lead to the high-latitude warming and tropical cooling found in the records.

It's Official: Satellite and Surface Temperature Records Agree
J.R. Christy and R.W. Spencer, Science, 310, 972 (November 11, 2005)
T.R. Karl, et al., eds. A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, (2006)
J. Kerr, Science, 312, 825 (May 12, 2006)

A recent report issued by the U.S. Climate Change Science Program concluded that over the 25-year satellite record, the surface and mid-troposphere have both warmed by approximately 0.15°C per decade. Global warming deniers had frequently challenged the reality of human-induced global warming and the reliability of climate models by citing previously reported discrepancies between the amount of warming at the surface compared to the amount of warming higher in the atmosphere. The original discrepancies were reported by John Christy, Roy Spencer and their team at the University of Alabama-Huntsville based on microwave emissions from the atmosphere recorded by satellites. But this argument is invalidated once errors in satellite and radiosonde data have been identified and corrected; and new temperature time series for the surface and atmosphere are consistent with each other. The University of Alabama-Huntsville team also acknowledged their previous errors in late 2005. This reconciliation of previous discrepancies led to an article in Science titled "No Doubt About It, the World is Warming."

Warmer Seas Mean More Intense Hurricanes
M.E. Mann and K.A. Emanuel, Eos, 87 (24), 233 (June 13, 2006)
R. Sriver and M. Huber, Geophysical Research Letters, 33 (June 8, 2006)
K.E. Trenberth and D.J. Shea Geophysical Research Letters, 33 (June 8, 2006)

Accumulating evidence suggests that hurricanes are becoming more intense due to global warming. Research by Michael Mann from Penn State and Kerry Emanuel from MIT suggests that warming of the tropical Atlantic due to human activity is responsible for the recent increase in tropical cyclone activity. They also concluded that there is no statistically significant evidence for natural cycles, such as the Atlantic Multidecadal Oscillation (AMO), playing a role in long-term tropical North Atlantic sea-surface-temperature variations, which are well correlated with tropical cyclone intensity. Mann and Emanuel found that the dependency of tropical Atlantic sea surface temperature on the AMO is not statistically robust, and that any trend recently accredited to the AMO may actually be a result of global warming in conjunction with cooling associated with tropospheric aerosol pollutants, such as sulfur dioxide and nitrogen oxide.

Two researchers from Purdue University also independently concluded that mean annual tropical temperatures directly regulate the total power unleashed by tropical cyclones. By using observational data from the European Centre for Medium-Range Weather Forecasts Reanalysis Project, Sriver and Huber found that the power dissipation of tropical cyclones (a measure of maximum wind speeds over the duration of the storm) correlates with both air temperature and mean annual tropical sea surface temperature, and that a substantial portion of variance in globally integrated power dissipation can be attributed to changes in mean tropical temperatures.

Analysis of the record-breaking 2005 hurricane season also reveals that higher than usual global sea surface temperatures, a signature of global warming, were responsible for the majority of the record high sea surface temperatures documented in the tropical North Atlantic during the summer of 2005. Trenberth and Shea attributed approximately half of the record warmth in the tropical North Atlantic to global sea surface temperatures, whereas only a third of the warmth was caused by the AMO and El Niño combined. As the background levels of global sea surface temperatures continue to climb, we can expect greater hurricane activity in future.

Mountain Snowpack Declines Mean Trouble for the Future of the World's Water Supply
S.J. Ghan and T. Shippert, Journal of Climate, 19, 1589 (2006)

As the world warms, we are likely to see declines in mountain snowpack, a key water source for many areas of the world, including the western United States. Two researchers from the Pacific Northwest National Laboratory recently scaled down 21st century climate change simulations from the NCAR Community Climate System Model to provide local-scale results, allowing them to determine local-scale hydrological effects based on rising global temperatures, changes in global hydrological cycle and local topography.

While Ghan and Shippert found that changes in precipitation varied from region to region, they observed profound impacts in regions with little permanent snow (such as New Zealand, Mexico and the Andes) and moderate impacts in regions with both seasonal and permanent snow (such as the western United States). Their results included a decrease in area mean snow water by over 40 percent by the end of the century for the western United States, more than a 50 percent decrease for regions such as the Andes, Mexico and Central America and over an 80 percent decrease for New Zealand. Such decreases could result in severe impacts on the world's water supply because many regions are dependent on spring and summer runoff for irrigation and drinking water.

Further Evidence for Extinctions Due to Global Warming
J.A. Pounds et al., Nature, 439, 161 (January 12, 2006)
C. Both, et al., Nature, 441, 81 (May 4, 2006)

Recent mass extinctions of the harlequin frog, endemic to Central America, have recently been linked to disease outbreaks that are tied to global warming. This gives further evidence of changes in natural ecosystems due to human-induced global warming. Pounds and his colleagues concluded with "very high confidence" (greater than 99 percent confidence, as per IPCC) that large-scale warming is a key factor in the disappearance of 67 percent of the approximately 110 species of harlequin frog (Atelopus) species. Their study illustrates that biological changes in Monteverde, a mountainous area of Costa Rica, are statistically associated with air temperatures and sea surface temperatures in the tropics, but not with sea-surface-temperature fluctuations that are controlled by El Niño alone. They also found that 80 percent of the species that have disappeared were seen for the last time after a relatively warm year, and statistical tests show that this is not due to chance, altitude, latitude or range size, but that large-scale warming is a key factor. Pounds et al. (2006) conclude that climate-driven epidemics are an immediate threat to biodiversity, and that their results indicate a chain of events whereby global warming can translate into local or microscale temperature shifts that are favorable to changes in ecosystem dynamics.

Another study, by Both and other researchers, linked population declines in nine populations of a Dutch long-distance migratory bird to the divergence of nestling hatchings and food peaks, which are well correlated with global warming. Their research found that pied flycatchers (Ficedula hypoleuca) have declined by about 90 percent in areas with the earliest peak in caterpillars, a major food source, but by only 10 percent in areas with the latest food peaks. They predict that this occurs because species higher on the food chain may not be able to adapt to changing climate conditions as easily as species lower on the chain. Although Both and his colleagues only attempted to analyze nine populations, they suggest that this warming-induced divergence in timing is probably a widespread phenomenon and that global warming can have a profound impact on population dynamics and ecosystem functioning.

Avoiding Dangerous Climate Change
M. Meinshausen, (2006) in H. Schellnhuber, et al., (eds.) Avoiding Dangerous Climate Change, Cambridge University Press (2006)
R. Warren, in H. Schellnhuber, et al., (eds.) Avoiding Dangerous Climate Change, Cambridge University Press (2006)

The United Nations Framework Convention on Climate Change (UNFCCC) has the objective of preventing "dangerous anthropogenic interference with the climate system." While a "non-dangerous" concentration level has not been defined under the UNFCCC, the European Union has set a goal of avoiding an increase of more than 2°C from pre-industrial levels, in order to avoid the most dangerous changes to climate. This 2°C target finds strong support in papers recently released in follow-up to a conference hosted by Prime Minister Tony Blair at the Hadley Center, Exeter, in February 2005. For example, Warren analyzed global warming impacts at levels at or above 2°C and, among other results, found that:

  • 1 to 2.8 billion people will experience an increase in water stress;
  • Up to 26 million people will be displaced by sea level rise and increased storm intensity;
  • Up to 220 million additional people will be at risk of hunger as agricultural yields fall;
  • There will be a total loss of summer Arctic sea ice and we will have likely committed to the complete meltdown of the Greenland Ice Sheet;
  • 97 percent of the globe's coral reefs will be lost;
  • 50 percent of major wetlands in Bangladesh and Australia will be lost;
  • And, ocean acidification may disrupt ocean ecosystem function.

A study by Meinshausen concluded that carbon dioxide concentrations need to be stabilized at 400 parts per million carbon dioxide-equivalent (after peaking at 475 parts per million carbon dioxide-equivalent) in order to provide a high level of confidence that the 2°C target will not be exceeded in this century. To achieve this goal, we need to start taking action now to reduce emissions of carbon dioxide in order to avoid having to make larger, more expensive cuts in the future.

Wildfires in the West
S.W. Running, Science, published in Science Express (July 6, 2006)
A.L. Westerling, et al., Science, published in Science Express (July 6, 2006)

Wildfires in the western United States are widely thought to have increased over the past few decades, but the extent of these changes had never been analyzed until recently. In July 2006, a team of scientists from the Scripps Institute, University of California and the University of Arizona published research which fully analyzed western fires against both hydro-climatic and land-surface data. Westerling and his colleagues found that there has been a four-fold increase in the number of western wildfires and 6.5 times as much area burned from 1987 through 2003 compared to 1970 through 1986. The length of the wildfire season has also increased by 78 days, or 64 percent, and the average burn duration of large fires has increased from 7.5 to 37.1 days over the same time period. They concluded that although land-use history is an important factor in wildfire risk, the broad-scale increase in wildfire frequency across the western United States is primarily driven by changes in climate, specifically increased spring and summer temperatures and earlier spring snowmelt.

Although Westerling and his colleagues didn't directly attribute the past increase in wildfire activity to human-induced global warming, increases in spring and summer temperatures and earlier spring snowmelt are tell-tale signs of global warming. In addition, the researchers warned that regardless of past trends, virtually all climate models project warmer springs and summers for the region in the future, thereby increasing the tendency towards earlier spring snowmelt and longer fire seasons. The team also briefly discussed how increased biomass burning in the future may result in a carbon release from forest ecosystems thereby potentially changing western forests from a sink for carbon dioxide to a source. This could have large scale implications for the terrestrial carbon cycle in the United States, since western forests are currently responsible for 20 to 40 percent of total U.S. carbon sequestration.

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