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The statement is billed as an ‘Information Statement’ of the AMS. This statement is part of the AMS series of Policy Statements, see this link for Guidelines for Statements of the AMS. From the Guidelines:
The American Meteorological Society (AMS) promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications.
Information Statements are intended to provide a trustworthy, objective and scientifically up-to- date explanation of scientific issues of concern to the public at large. They are informational only and do not make recommendations or take positions on issues. Information Statements should use language easily understood by a lay reader and avoid technical terminology and jargon. Information statements are typically no longer than 2000 words.
The AMS wrote a previous statement on climate change in 2007 [link].
The link to the new statement can be found here [link].
Excerpts from the statement are appended below, providing a sense of the overall content:
This statement provides a brief overview of how and why global climate has changed over the past century and will continue to change in the future. It is based on the peer-reviewed scientific literature and is consistent with the vast weight of current scientific understanding as expressed in assessments and reports from the Intergovernmental Panel on Climate Change, the U.S. National Academy of Sciences, and the U.S. Global Change Research Program.
How is climate changing?
Warming of the climate system now is unequivocal, according to many different kinds of evidence. Due to natural variability, not every year is warmer than the preceding year globally. Nevertheless, all of the 10 warmest years in the global temperature records up to 2011 have occurred since 1997, with 2005 and 2010 being the warmest two years in more than a century of global records. The warming trend is greatest in northern high latitudes and over land.
Why is climate changing?
Climate is always changing. However, many of the observed changes noted above are beyond what can be explained by the natural variability of the climate. It is clear from extensive scientific evidence that the dominant cause of the rapid change in climate of the past half century is human-induced increases in the amount of atmospheric greenhouse gases, including carbon dioxide (CO2), chlorofluorocarbons, methane, and nitrous oxide. The most important of these over the long term is CO2, whose concentration in the atmosphere is rising principally as a result of fossil-fuel combustion and deforestation.
Human activity also affects climate through changes in the number and physical properties of tiny solid particles and liquid droplets in the atmosphere, known collectively as atmospheric aerosols. Examples of aerosols include dust, sea salt, and sulfates from air pollution.
Land surface changes can also affect the surface exchanges of water and energy with the atmosphere. Humans alter land surface characteristics by carrying out irrigation, removing and introducing forests, changing vegetative land cover through agriculture, and building cities and reservoirs. These changes can have significant effects on local-to-regional climate patterns, which adds up to a small impact on the global energy balance as well.
How is the climate expected to change in the future?
Future warming of the climate is inevitable for many years due to the greenhouse gases already added to the atmosphere and the heat that has been taken up by the oceans.
In general, many of the climate-system trends observed in recent decades are projected to continue. Those projections, and others in this section, are largely based on simulations conducted with climate models, and assume that the amount of greenhouse gas in the atmosphere will continue to increase due to human activity. Global efforts to slow greenhouse gas emissions have been unsuccessful so far. However, were future technologies and policies able to achieve a rapid reduction of greenhouse gas emissions — an approach termed “mitigation” — this would greatly lessen future global warming and its impacts.
Confidence in the projections is higher for temperature than for other climate elements such as precipitation, and higher at the global and continental scales than for the regional and local scales. The model projections show that the largest warming will occur in northern polar regions, over land areas, and in the winter season, consistent with observed trends.
In the 21st century, global sea level also will continue to rise although the rise will not be uniform at all locations. With its large mass and high capacity for heat storage, the ocean will continue to slowly warm and thus thermally expand for several centuries. Model simulations project about 27 cm (10 inches) to 71 cm (28 inches) of global sea level rise due to thermal expansion and melting of ice in the 21st century. Moreover, paleoclimatic observations and ice-sheet modeling indicate that melting of the Greenland and the West Antarctic ice sheets will eventually cause global sea level to rise several additional meters by 2500 if warming continues at its present rate beyond the 21st century.
Atmospheric water content will increase globally, consistent with warmer temperatures, and consequently the global hydrological cycle will continue to accelerate. For many areas, model simulations suggest there will be a tendency towards more intense rain and snow events separated by longer periods without precipitation. However, changes in precipitation patterns are expected to differ considerably by region and by season. In some regions, the accelerated hydrological cycle will likely reinforce existing patterns of precipitation, leading to more severe droughts and floods. Further poleward, the greater warming at high latitudes and over land likely will change the large-scale atmospheric circulation, leading to significant regional shifts in precipitation patterns. For example, the model simulations suggest that precipitation will increase in the far northern parts of North America, and decrease in the southwest and south-central United States where more droughts will occur.
Climate-model simulations further project that heavy precipitation events will continue to become more intense and frequent, leading to increased precipitation totals from the strongest storms. This projection has important implications for water-resource management and flood control. The simulations also indicate the likelihood of longer dry spells between precipitation events in the subtropics and lower-middle latitudes, with shorter dry spells projected for higher latitudes where mean precipitation is expected to increase. Continued warming also implies a reduction of winter snow accumulations in favor of rain in many places, and thus a reduced spring snowpack. Rivers now fed by snowmelt will experience earlier spring peaks and reduced warm-season flows. Widespread retreat of mountain glaciers is expected to eventually lead to reduced dry season flows for glacier-fed rivers. Drought is projected to increase over Africa, Europe, and much of the North American continental interior, and particularly the southwest United States. However, natural variations in world ocean conditions at decadal scale, such as those in the North Pacific and North Atlantic basins, could offset or enhance such changes in the next few decades. For the longer term, paleoclimatic observations suggest that droughts lasting decades are possible and that these prolonged droughts could occur with little warning.
Weather patterns will continue to vary from day to day and from season to season, but the frequency of particular patterns and extreme weather and climate events may change as a result of global warming. Model simulations project an increased proportion of global hurricanes that are in the strongest categories, namely 4 and 5 on the Saffir-Simpson scale, although the total counts of hurricanes may not change or may even decrease. Some regional variations in these trends are possible. Simulations also indicate that midlatitude storm tracks will shift poleward. Interannual variations of important large-scale climate conditions (such as El Niño and La Niña) will also continue to occur, but there may be changes in their intensity, frequency, and other characteristics, resulting in different responses by the atmosphere. Heat waves and cold snaps and their associated weather conditions will continue to occur, but proportionately more extreme warm periods and fewer cold periods are expected. Indeed, what many people traditionally consider a cold wave is already changing toward less severe conditions. Frost days (those with minimum temperature below freezing) will be fewer and growing seasons longer. Drier conditions in summer, such as those anticipated for the southern United States and southern Europe, are expected to contribute to more severe episodes of extreme heat. Critical thresholds of daily maximum temperature, above which ecosystems and crop systems (e.g., food crops such as rice, corn, and wheat) suffer increasingly severe damage, are likely to be exceeded more frequently.
There is unequivocal evidence that Earth’s lower atmosphere, ocean, and land surface are warming; sea level is rising; and snow cover, mountain glaciers, and Arctic sea ice are shrinking. The dominant cause of the warming since the 1950s is human activities. This scientific finding is based on a large and persuasive body of research. The observed warming will be irreversible for many years into the future, and even larger temperature increases will occur as greenhouse gases continue to accumulate in the atmosphere. Avoiding this future warming will require a large and rapid reduction in global greenhouse gas emissions. The ongoing warming will increase risks and stresses to human societies, economies, ecosystems, and wildlife through the 21st century and beyond, making it imperative that society respond to a changing climate. To inform decisions on adaptation and mitigation, it is critical that we improve our understanding of the global climate system and our ability to project future climate through continued and improved monitoring and research. This is especially true for smaller (seasonal and regional) scales and weather and climate extremes, and for important hydroclimatic variables such as precipitation and water availability.
Technological, economic, and policy choices in the near future will determine the extent of future impacts of climate change. Science-based decisions are seldom made in a context of absolute certainty. National and international policy discussions should include consideration of the best ways to both adapt to and mitigate climate change. Mitigation will reduce the amount of future climate change and the risk of impacts that are potentially large and dangerous. At the same time, some continued climate change is inevitable, and policy responses should include adaptation to climate change. Prudence dictates extreme care in accounting for our relationship with the only planet known to be capable of sustaining human life.
[This statement is considered in force until August 2017 unless superseded by a new statement issued by the AMS Council before this date.]
Update: Bill Hooke has some additional background on the AMS statement:
My strong objections to this type of statement by professional societies has been voiced previously. This statement is worse than the previous AMS statement, and much worse than the statement by the Royal Society, which is probably the most credible statement on this topic made by a professional society.
So who is responsible for this statement? Current members of the AMS Council can be found [here]. It is not clear who authored the statement, but I suspect it was the members of the AMS Committee on Climate Variability and Change (for membership list, see here). After reading this list of names, I recognize some, but less than half. Does this group of people inspire my confidence in making an assessment of climate change? In a word, NO.
Several months ago, I recall receiving an email asking for comments on the draft statement (apparently a mass mailing to the AMS membership). I tried to access it but didn’t have my site login info handy at the time. So I am seeing this statement today for the first time. I suspect that there will be a lot of AMS members that are unhappy with this statement.
Apart from the broader issue of whether or not professional societies should make such statements, the main question that I have is why write a new statement now? It appears that each statement has a life time of 5 years. Why not wait another year or two until the IPCC AR5 is out? It seems that there is little in the AMS statement that is associated with more recent publications (since the AR4). As the CMIP5 climate model simulations show a broader range of uncertainty than the simulations used in the AR4, what is the basis for making a more confident statement on attribution (which seems to be based wholly on models) than was made in the AR4?
As far as I can tell, this statement is a naive example of Michael Kelly’s invisible hand (quote from my no consensus paper):
Kelly (2005) describes an additional source of confirmation bias in the consensus building process: “As more and more peers weigh in on a given issue, the proportion of the total evidence which consists of higher order psychological evidence [of what other people believe] increases, and the proportion of the total evidence which consists of first order evidence decreases . . . At some point, when the number of peers grows large enough, the higher order psychological evidence will swamp the first order evidence into virtual insignificance.”
In other words, consensus statements get parroted without any actual intellectual examination. In this case, what is the point of the AMS statement? Apparently, to ‘inform the public’ on this controversial issue by appealing to the ‘authority’ of the society.
JC note to AMS: read my paper No consensus on consensus.
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