Extreme Rainfall with Flooding in the U. S. South

Summary.  Two extreme rainfall events with catastrophic flooding occurred in the U. S. recently.  The first was in the Baton Rouge, Louisiana, area in August 2016, and the second is ongoing at this writing in August 2017 in southeastern Texas including Houston.

Attribution of extreme events to global warming has become more reliable as a result of increased capabilities built into the statistical procedures employed in such analyses.  Global warming likely contributed about 20% to the rainfall experienced in the Baton Rouge flooding event of 2016. 

Global warming is now recognized to be due largely to emissions of greenhouse gases by humans.  It is projected to grow worse in coming decades if stringent efforts are not made to reduce these emissions.  In that case it is foreseen that extreme weather events may become more frequent and more severe.

 

Introduction.  The southern United States has suffered two episodes of unprecedented rainfall and flooding in the past year.  In August 2016 Baton Rouge, Louisiana and the surrounding area experienced torrential rain and rapid, extreme flooding beginning August 11 and extending beyond August 16.  Major damage and human dislocations resulted from this catastrophe.

In 2017 Hurricane Harvey left the Gulf of Mexico and made landfall near Corpus Christi, Texas on August 25.  Contrary to the paths of many hurricanes, Harvey degenerated into a tropical depression and stalled over southern Texas for days; as of this writing on August 29 it has drifted slowly to the northeast, hovering over Houston, Texas.  At various locations it has drenched the land with 20-40 inches (50-100 cm) of rain over this time (accessed August 29, 2017), causing extreme flooding, especially in the Houston area.  It is projected to continue northeastward toward Louisiana in the next day or more.

Flooding in Baton Rouge arose as an unusual weather pattern leading to excessive rainy conditions slowed considerably over the region for several days .  In the most severe case rain fell at a rate of 2–3 inches (5.1–7.6 cm) per hour, and produced a total of 24 inches (61 cm) of rainfall, with a maximum recorded as 31.4 inches (79.7 cm) in Watson, Louisiana.  The National Weather Service estimated the likelihood of such an event as 0.1%.  Flooding of eight rivers in the area led to major disruptions and damage, including damage to 146,000 homes, with tens of thousands of people relocated to emergency shelters.  About 265,000 children, or one-third of Louisiana’s school pupils, were prevented from attending school.  The economic impact has been estimated at between $10-$15 billion.

Rainfall and flooding in southern Texas is continuing at the time of this writing, and is expected to migrate east toward Louisiana in the coming days.  The amount of rainfall to date is extremely high; an interactive display of rainfall rates and total accumulated rainfall at various locations is available online (based on the National Weather Service; accessed August 29, 2017).  As of this writing, the total for the Corpus Christi area is 20 inches (50 cm), with a maximum rate of almost 3 inches per hour (7.5 cm per hour) on August 26.  The Houston area is far more seriously affected, according to the interactive map.  One location northeast of Houston shows a total rainfall to date of 52 inches (130 cm) with a maximum rate of about 10 inches per hour (25 cm per hour).  (The normal annual rainfall in Houston  is 49.8 inches (126 cm).  Images and videos of the flooding, its damage and human tragedy can be seen currently on news sources and the internet.  The economic impacts will certainly be extremely high.

Reports such as the Fourth National Climate Assessment draft (NCA) foresee worsening catastrophes such as those described here.   The draft NCA was prepared by climate scientists and related specialists drawn from thirteen U. S. government departments and agencies, as well as a large number of scientists in nongovernmental research facilities. They critically assessed peer-reviewed research and similar public sources, including primary datasets and widely-recognized climate modeling frameworks.  These standards assure that the findings of the report are objectively accurate, avoiding bias toward any unsubstantiated point of view.  By law the NCA cannot make any policy recommendations.

Among its conclusions, the NCA finds it is “extremely likely” that activities by humans have been the “dominant” cause of the warming observed since the middle of the 20^th century.  It states with “very high confidence” that no alternatives, such as cyclical changes in solar energy reaching the Earth or variations in natural planetary factors, can explain the observed climate changes.

The NCA projects with “high confidence” that heavy precipitation events will continue increasing over the 21^st century.  As noted, these trends are attributed to human activity.  They will likely worsen considerably as the climate warms.

Global warming contributes to the severity of extreme weather events.  Of the excess heat retained by the earth, i.e., the land, air and sea, as a result of man-made global warming, 90% enters the waters of the ocean.  The U. S. National Oceanographic and Atmospheric Administration finds that the sea surface temperature of the Gulf of Mexico in the early months of 2017 exceeded the 35-year average for 1981-2016 by about 0.75°C (1.3°F), and about equaled the record for that period.  Since the amount of water vapor that air can hold increases by about 7% per °C (about 4% per °F), the warmer Gulf surface temperature increased the water vapor capacity of the air by about 5% compared to earlier years. 

Since the complete weather system defined as hurricane/depression Harvey is spending a large fraction of its time over the Gulf, it recharges its moisture content continuously, indefinitely.  Over land, much of this added moisture in the system falls as additional amounts of rain, compared to earlier years.  Similar considerations hold for the Baton Rouge extreme event of 2016.  The physical damage and human harm inflicted by such calamities is costly.  Ultimately much of the burden becomes added expenditures imposed as taxes on the population at large.

Conclusion

Attribution of specific events to the general finding that global temperatures are rising has become far more reliable in recent years.  The procedures use advanced statistical measures to assess whether the extent by which the extreme event exceeds historical records has explanations other than global warming.  If not, a proportion of the overall extreme event may be attributed to the excess effect provided by global warming.

Since the Houston extreme rainfall and flooding event is still in progress, it is too early to attempt attribution of its causes.  The Baton Rouge event, however, has been assessed by attribution methods.  Wang and coworkers identified atmospheric weather patterns that promoted the catastrophic rainfall of this episode.  Regional model simulations lead to an estimate that global warming since 1985 likely increased the observed rainfall by 20%.   

Authoritative analyses of the earth’s climate show that the warming experienced to date is primarily due to man-made additions of greenhouse gases to the atmosphere.  This enhances retention of heat within the earth system rather than radiating excess heat to space.  Continued human activity that produces more greenhouse gases in the future is expected to worsen this effect, according to climate models, leading to excessive warming of the planet’s air, land and oceans.  In such a case, one consequence is expected to be more severe, and more frequent, extreme weather events such as the Baton Rouge intense rain and flooding, and hurricane/tropical depression Harvey currently wreaking havoc in Texas and Louisiana. 

Stringent reductions in further emissions of greenhouse gases are called for in order to lessen the impact of future extreme weather events. 

© 2016 Henry Auer

The Earth Continues Warming: The Fourth National Climate Assessment Draft Report

Summary

A draft of the Fourth National Climate Assessment reports that the global average temperature for the 30-year period from 1986 to 2016 rose by 1.2°F (0.7°C). It is extremely likely that activities by humans have been the principal cause of this warming. Extreme temperature and rainfall events have increased over this time, as have forest wildfires.
 

Arctic land-based ice has been lost to melting, and the extent and thickness of sea ice has decreased.  The mean sea level has risen about 7-8 in (about 26-21 cm) since 1900.  Ocean waters have taken up 93% of the excess heat of the Earth system due to global warming since the 1950s.
 

Global greenhouse gas emissions are projected to continue and consequently global temperatures will continue increasing and related trends will continue.  Limits to the intended increase require that humanity reduce annual emissions to zero by 2100. 
 

The draft states “Choices made today will determine the magnitude of climate change risks beyond the next few decades.”

The Fourth National Climate Assessment (NCA) is due in 2018 (See Background at the end of this post).  However the U.S. Global Change Research Program (USGCRP), which oversees preparation of the NCA, has prepared a Final Draft of a Climate Science Special Report (CSSR; see Note 1 at the end of the post) that has become publicly available as a freestanding document on which the actual NCA will be based.
 

This post is based on the CSSR Executive Summary (ES).  Confidence levels and likelihoods given here in italics are taken directly from the ES.  They are carefully defined in the CSSR.  Phrases in quotes are taken verbatim from the CSSR text.

The Historical Record

The global average temperature has risen above the average for the six decades 1901-1960 by 1.2°F (0.7°C) for the recent period from 1986 to 2016 (very high confidence). The map below shows temperature increases gridded across the globe.

Color-coded global map grid of historical changes in the average temperature for the period 1986-2016 relative to the average from the six-decade reference period 1901-1960, in °F. No data are available at the poles, indicated by gray.
Source: CSSR; https://www.nytimes.com/interactive/2017/08/07/climate/document-Draft-of-the-Climate-Science-Special-Report.html.

 

The map shows that since the reference decades the entire surface of the planet, both land and sea, has increased in regional average temperature.  The greatest increase has occurred in Canada (especially in the northern and Arctic regions), Alaska, Siberia, northeastern China and eastern Brazil. Indeed, the Arctic is warming about twice as fast as the global average.

It is extremely likely that activities by humans have been the “dominant” cause of the warming observed since the middle of the 20^th century.  No alternatives, such as the cyclical changes in solar energy reaching the Earth or variations in natural planetary factors, can explain the observed climate changes (very high confidence).

Extreme climate-related weather events have increased in number and severity.  Since 1980 the cost of such calamities in the U. S. is over US$1 trillion. Extreme events can impact water quality, agriculture, human health, infrastructure, and lead to disaster events.  In the U. S. the number of high temperature records in the past 20 years is much higher than the number of low temperature records (very high confidence).

Heavy precipitation events in most regions of the U. S. have increased in intensity and frequency since 1901, especially in the northeast. (high confidence). 

The occurrence of large forest wildfires has increased in the U. S. West and Alaska since the early 1980s (high confidence). 

The waters of the oceans have absorbed about 93% of the heat accumulating in the Earth system due to global warming since the 1950s (very high confidence).  This affects climate patterns around the world.

In the Arctic, ice sheets overlaying land have been melting for at least the last three decades; in some locations the rate of loss is accelerating (very high confidence).    The rate of melting of ice sheets over Greenland has accelerated in the last few years (high confidence).  As this ice melts the water flows to the ocean, resulting in a net increase of sea level.

Arctic sea ice has been imaged since satellite flights permitted. The sea ice floats on the Arctic Ocean; its area expands and contracts in freeze-thaw seasonal cycles without any net change to global sea levels.  Rather, the extent responds to changes in air and sea temperatures.  The least extent, i.e., the most melting, occurs typically in September.  Striking images showing the loss of September sea ice from 1984 to 2016, both in thickness (color coded white as having been formed at least four years earlier) and in overall surface area, are shown in the images below:

Satellite images of Arctic sea ice extent and thickness in September, for 1984 (top) and 2016 (bottom).  The color bar shows the local age of the ice in years, a proxy for its thickness, from recent (dark gray) to more than 4 years (white).
Source: Adapted from CSSR; https://www.nytimes.com/interactive/2017/08/07/climate/document-Draft-of-the-Climate-Science-Special-Report.html.

 

Sea ice thickness has decreased by between 4.3 and 7.5 feet.  September sea ice extent has decreased by 10.7% to 15.9% per decade (very high confidence).  These changes reflect warming of the Arctic region over this time frame.  It is virtually certain that human activity has contributed to Arctic surface temperature increases, sea ice loss, glacier mass loss and snow extent decline seen across the Arctic (very high confidence).

The mean sea level has risen about 7-8 in (about 26-21 cm) since 1900 (very high confidence).  This is attributed “substantially” to human-induced climate change (high confidence).  The rate of sea level rise is greater than any found in the last 2,800 years (medium confidence).

Ocean waters are absorbing more than 25% of the carbon dioxide emitted into the atmosphere by burning fossil fuels.  Carbon dioxide is weakly acidic when dissolved in water, increasing its acidity (very high confidence).  This negatively impacts marine ecosystems in many important ways.

Projected Future Climate Trends

 

Extreme climate-related weather events will continue for many decades.

By the end of the 21^st century if the world generates significant reductions in greenhouse gases the global average temperature increase could be limited to 3.6°F (2.0°C) or less. This would require a pathway of annual GHG emissions reaching near zero by then.  In contrast, minimal constraints on the annual emissions rate could result in a rise of 5.8-11.9°F (3.2-6.6°C) (high confidence).

Even if the annual rate of greenhouse gas (GHG) emissions were to fall to zero, the high burden of GHGs already accumulated in the atmosphere would persist for a long time.  The CSSR foresees that even in this event the global average temperature would rise further (high confidence), perhaps by an additional 1.1°F (0.6°C) (medium confidence).

But realistic projections all foresee continued GHG emissions into the future.  U. S. temperatures will continue to rise (very high confidence); new records for high temperatures will be frequent (virtually certain).  Temperatures by the end of this century will be much higher than the present (high confidence).
Heavy precipitation events are projected to continue increasing over the 21^st century (high confidence).  In the western U. S., large reductions in mountain snowpack, and more precipitation falling as rain rather than snow, are projected as the climate warms (high confidence). These trends are attributed to human activity (high confidence).  They will likely worsen considerably as the climate warms (very high confidence).  In the absence of reductions in emission rates long-duration hydrological drought, due to decreased retention of soil moisture, becomes more likely by the end of the century (very high confidence).

Further warming is projected to lead to increases in wildfires (medium confidence).

 

If GHG emission rates continue unconstrained, the average sea surface temperature is projected to increase about 4.9°F (about 2.7°C) by 2100 (very high confidence).

The mean sea level will continue increasing, to varying extents depending on future emission rates, by at least 1 ft (30 cm; very high confidence) and as much as 4 ft (130 cm; low confidence) by 2100.  If the Antarctic ice shelf is lost due to high emission rates the upper bound could be as high as 8 ft (260 cm).  It is extremely likely that sea level will continue rising beyond 2100 (high confidence) as ice continues melting.

Further loss in Arctic sea ice will continue throughout the 21^st century, very likely resulting in a virtually ice-free ocean by the 2040s (very high confidence).

Conclusions of the CSSR

 

Limiting the total global average temperature increase to 3.6°F (2.0°C), or less, from a 19^th century baseline will require significant constraints on future GHG emission rates. Even though annual emission rates decreased slightly in 2014 and 2015, they are still too high to meet commitments that nations made upon entering the 2015 Paris Agreement (high confidence).  Indeed, present and projected emission rates would bring the atmospheric level of GHGs to levels so high that they have not occurred for at least the last 50 million years (medium confidence).

New carbon dioxide released “today” is long-lived, persisting in the atmosphere for decades to thousands of years. Therefore it’s important to note that the relationship between total atmospheric CO₂ concentration and the increase in global temperature is a linear one. 

The ES states “Choices made today will determine the magnitude of climate change risks beyond the next few decades.  Stabilizing global mean temperature below 3.6°F (2°C) or lower relative to preindustrial levels requires significant reductions in …CO₂ emissions…before 2040 and likely requires net emissions to become zero….”  If humanity continues emitting GHGs at rates higher than called for here we would reach the 3.6°F limit only two decades from now, with further temperature increases later.

Finally, changes that are unanticipated or difficult or impossible to manage, may arise during the next century.  Examples are complex (or simultaneously occurring) phenomena, and self-reinforcing changes (positive feedbacks).  Such occurrences would accelerate the world’s changes to points beyond the accepted CO₂ temperature limits.

Analysis

Issuance of this NCA is mandated by an act of Congress.  It is important that this Final Draft, the CSSR, continue on its bureaucratic trajectory and be issued on schedule in 2018.  Yet some of the scientists contributing to the Draft hold positions in departments or agencies whose heads have expressed disdain or opposition to the phenomenon of global warming, the Paris Climate Agreement, have acted to reverse federal policies that limit extraction and use of fossil fuels, or have deleted pages concerning global warming from their agency’s websites.  They all work in an administration whose head has declared global warming to be a hoax. These situations potentially place the contributing scientists in conflicting positions.  Their work is commendable and should be supported.  The NCA should be issued without being altered, nor should it be suppressed.

[Update August 17, 2017: The science journal Nature has published a news article that discusses the CSSR and the political considerations facing the U. S. administration as it weighs issuing the Fourth NCA. Climate scientists are concerned about the fate of the Report. Nature notes that the Heartland Institute, a conservative think tank that promotes skepticism about global warming, is consulting with the Environmental Protection Agency on this issue.]
 

This NCA is only the latest in a long series of reports detailing the reality of warming and specifying the harms that global warming and climate change cause to our planet.  In particular, it attributes the cause to human activity, including the burning of fossil fuels. 

We must all undertake to reduce emissions of GHGs in our personal lives, and support policies promoting reductions at the state, national and international levels.
 

Background

The U. S. Global Change Research Act of 1990 mandates preparation of assessments of global change every four years to “assist the nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change”.  It assesses the current state of scientific understanding of global change on the natural and human environments. Its tasks, however, do not include formulation of policies to address global warming.

Climate scientists and related specialists drawn from thirteen U. S. government departments and agencies (see Note 2 at the end of this post), as well as a large number of scientists in nongovernmental research facilities, prepared the CSSR and the NCA. They critically assessed peer-reviewed research and similar public sources, including primary datasets and recognized climate modeling frameworks.

 Notes

1.    USGCRP, 2017: Climate Science Special Report: A Sustained Assessment Activity of the U.S. Global Change Research Program [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 669 pp.

 

2.    The federal scientists involved in preparing the NCA and the CSSR are drawn from the:  

Department of Agriculture,

Department of Commerce (National Oceanic and Atmospheric Administration),

Department of Defense,

Department of Energy,

Department of Health and Human Services,

Department of the Interior,

Department of State,

Department of Transportation,

Environmental Protection Agency,

National Aeronautics and Space Administration,

National Science Foundation,

Smithsonian Institution, and

U.S. Agency for International Development.      

© 2017 Henry Auer

The U. S. National Climate Assessment: Effects of Warming and Measures to Counteract Them

Summary. The Third U. S. National Climate Assessment, a federally mandated project, reports on the current status of global warming in the U. S. and its harmful effects on the population and the environment.  As a result of manmade emissions of greenhouse gases, damages resulting from harsh warming events and extreme weather events have been worse than in earlier periods.  They are projected to be become even more severe by the end of the present century in the absence of meaningful action to combat them. 

Mitigation and adaptation measures are available and need to be implemented to limit further warming and its harmful effects by 2100. 

Important procedures involved in combating further warming include risk assessment and evaluation, and iterative (cyclical) sequences of planning, decision-making and implementation of projects, coupled with critical analysis of steps in these processes along the way.  Applying the results of such analysis back to optimize the steps should be done repetitively in order to achieve desired goals.

 

Introduction.  The Third U. S. National Climate Assessment(NCA) was issued in May 2014 as mandated under the Global Change Research Act of 1990.  It discusses the man-made origins of contemporary global warming and the detrimental effects it is having on the U. S., presents projected future trends of warming and its effects, and discusses how the U. S. can embark on measures to combat these phenomena. Its tasks, however, do not include formulation of specific policies to address global warming.

An earlier post briefly presented important climatic changes observed around the world, as documented in the NCA.  It then rigorously developed the scientific findings that support the conclusions that a) humanity’s burning of fossil fuels (and deforestation activities) has caused, and continue to be responsible for, the increase by 40% of the atmospheric concentration of the greenhouse gas (GHG) carbon dioxide (CO₂) since the industrial revolution; and b) the increased CO₂ causes the global warming we are now experiencing.  This post summarizes some of the findings of the NCA on current and projected effects of global warming in the U. S., and how to address its damaging effects. 

This post and the earlier one are based on a Fact Sheet distributed by email by Bess Evans in the White House Office of Science and Technology Policy, and on the NCA Highlights (Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Highlights of Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program). 

Significance of the NCA.  The NCA was prepared by hundreds of scientists and experts, overseen by federal officials from thirteen departments and agencies (see the earlier post).  Its procedures assure that the results reflect the highest scientific standards and are presented in an objective, unbiased manner.

Effects of Global Warming Already Under Way.  The NCA presents the following graphic as one way of summarizing harmful effects already occurring worldwide as a result of global warming.

 

Ten atmospheric and environmental properties that are changing as a result of global warming already under way.  Upward white arrows show increasing trends, and downward black arrows show decreasing trends.

Source: Third National Climate Assessment; http://nca2014.globalchange.gov/.

 

 

Significant changes are occurring regionally in the U. S. and are projected to get worse as warming intensifies.  The NCA presented the summary table shown below to give examples of significant impacts; many more were described in the report.

 

Source: Third National Climate Assessment; http://nca2014.globalchange.gov/. 

 

 

The NCA finds important climatic extremes are occurring with higher frequency than in earlier times.  The number of heat waves in 2011 and 2012 was almost three times the long-term average.  Record heat reduced soil moisture drastically, such that in 2011 and 2012 severe drought impacted Oklahoma, Texas, Kansas, Arkansas and Missouri.  Elevated temperature in other regions increases the moisture content of the atmosphere.  This resulted in exceptionally heavy precipitation in the Northeast, Midwest, and upper Great Plains.  Heavy rainfall results in more, and more damaging, river flooding.  Coastal flooding has worsened, due to ocean storm surges which are made worse by higher average sea levels.

 

Projected future changes by the end of the 21^st century.  Under a favorable scenario of “substantial” reductions in worldwide emissions of GHGs, the continental U. S. is expected to experience temperature increases of 3-5ºF, varying by region, and much more in northern Alaska, by the last three decades of this century.  In a harsher scenario of “continued increases” in emissions, this increase ranges about 7-9ºF over most of the country, and much higher in most of Alaska.  In the “continued” scenario, the entire southern portion of the U. S. will have less precipitation, as will the northwest during summer, while the northern portion will have more. 

 

By 2100 the sea level is expected to rise 1 foot (substantial reductions) to 4 feet (continued increases) above the level of 2000.  (A tutorial on why sea level continues rising is given here.)  Over the continental U. S. summertime precipitation is projected to be 10-30% reduced in Texas, Oklahoma and neighboring regions, and Florida, and reduced even more over in the Pacific Northwest.  The NCA concludes for the remainder of this century “aggressive and sustained GHG emission reductions by the U. S. and by other nations would be needed to reduce global emissions to a level consistent with the lower [“substantial” reductions] scenario…”

 

Worsening warming will have widespread impacts on the American economic, social and natural environments.  Using the effects of Hurricane Katrina as an example, the NCA points out that extreme events can adversely affect interconnected facets of urban life in ways that worsen the effects of the calamity.  A second example is drawn from the Midwestern heat wave and drought of 2011-2012.  Agriculture suffered because water became less available than usual, since it was drawn to cool electricity generating plants called on to supply more electricity for air conditioning, rather than being available to operate irrigation pumps.

 

Coral reefs are already suffering destruction caused by higher ocean temperature and acidification due to increased dissolution of carbon dioxide, an acidic compound.

 

Warming adversely affects human health because it worsens air quality, increases heat stress and worsens the spread of food-borne, insect-borne and airborne diseases.  Additional stress from more intense heat waves will adversely impact mental health and physical wellbeing.  The NCA points out that measures to reduce GHG emissions will have “co-benefits” on health because of lower levels of particulates and sulfur dioxide as fossil fuel use is reduced.

 

About 80% of the U. S. population lives in cities and surrounding metropolitan areas.  This heightens our sensitivity to any extreme weather- or climate-related damage affecting our daily lives that may occur. 

 

As already noted, components of the various systems of the infrastructure are interconnected, so that disruption in one segment leads to loss of service in another.  Such systems include water supply and sewage treatment, electricity and fuel supplies, highways and train service for transportation, and the like.  Coping with disruptions in these systems by preventive adaptation will be costly; most coastal cities, and others further inland, are already at various stages of planning and implementing such strategies at local and regional levels.

 

Precipitation patterns and availability of water are projected to change considerably across the U. S.  Heavy precipitation events are increasing, even in regions with lower overall projected precipitation.  This can exacerbate flash flooding and river floods.  Increased drought conditions will put stress on water supplies, while sea level rise will affect coasts and adjacent aquifers.  Regionally, the Midwest and Southwest will experience lower precipitation, including reduced water from thawed snowpacks.  Water demand by 2060 will increase 25-50% or more across the Midwest and Great Plains, the Southeast, the Southwest and the Northwest coastal region.  The NCA emphasizes the interconnections among water resources, land use and provision of energy.  In the future these sectors will compete for availability and disposition of their respective resources in ways that may constrain use of each of them.

 

Agricultural productivity will be subjected to stress by the projected effects of global warming.  Agriculture currently produces crops valued at almost US$330 billion.  Compared to the last three decades of the last century, projections for the period 100 years later include a nationwide increase in the number of frost-free days which might benefit productivity and diversity; increased lengths of consecutive dry day periods by as much as 9-12 days or more, especially across the southern Midwest, the Southwest, and the Northwest; and increased occurrence of hot nights.  Both of the last two changes would adversely affect grain yields and increase stress on livestock.

 

The NCA presents overviews of adaptation strategies and mitigation measures, without espousing any particular proposal.  Adaptation relates to measures taken, either in response to a damaging event or in anticipation of a potential threat, to minimize their impacts.  In the absence of a national policy implementing adaptive measures, most activity is occurring at the state and local levels.  Significant barriers exist for adaptation, including political lethargy, reluctance to commit needed resources, and the difficulty of mobilizing action in the face of an indefinite and unpredictable future need.  Sharing of planning and execution experiences among locales should ease the way forward; to date most adaptation measures in the U. S. are in the planning stage.  In addition private corporations are implementing adaptation plans to protect business models and physical assets.

 

Mitigation refers to measures intended to reduce or minimize the emission of GHGs into the atmosphere.  In reducing further emissions, the NCA recognizes that the accumulated atmospheric concentration of GHGs is not reduced, but only stabilized at a new, higher level.  The “substantial” scenario used in the NCA would require reaching a peak emissions rate within 25 years, with reduced rates thereafter.  The NCA notes, however, that current emission trends are higher than required for this to occur, making the “substantial” scenario unattainable.  It recognizes that reforestation contributes importantly to mitigation, but realizes that this strategy may not be sustainable indefinitely. 

 

The NCA reports that the U. S. has no “comprehensive national climate legislation” in place, and that progress in the U. S. has been made as a result of regional and local efforts, and voluntary actions.  “Over the remainder of this century,” it states, “aggressive and sustained GHG emission reductions by the U.S. and by other nations will be needed to reduce global emissions to a level consistent with the [‘substantial’ scenario].”  While the NCA focuses on the U. S., this statement refers crucially to the important role that other nations, some with higher emissions than those of the U. S., must play in resolving the global warming issue.

 

The NCA summarizes possible measures available for mitigation, recognizing that it would be required significantly to decarbonize the global energy economy by 2100.  These include a) putting a price on carbon (i.e. on fossil fuels), b) regulating emission-producing activities, c) changing energy subsidy policies, and d) direct federal expenditures.  Other governmental policies include energy efficiency measures, migrating from burning fossil fuels to expansion of renewable energy sources, and lowering emission of non-CO₂ GHGs.

 

Making decisions to address global warming requires use of sophisticated analytical tools to assess risk and evaluate alternatives.  Several interacting factors are involved.  Assessing the likelihood of future damage, and valuation of likely harms, is critical in proceeding.  Interdisciplinary approaches to this undertaking are important, because making decisions involves not only scientific input, but also the frameworks used in the political and business world for program adoption in the face of incomplete knowledge.  For example, in an environment of limited resources, choices must be made about allocation of projects between mitigation of future emissions and adaptation to present and projected damages.  Public opinion is important, because such decisions cannot be sustained in the absence of support by stakeholders.  The NCA recommends an iterative process for proceeding, in which information and intermediate results are repeatedly subjected to assessment and evaluation in order to provide updated information on project development and implementation. 

 

Analysis

 

The National Climate Assessment is but one of many recent reports addressing global warming.  Another significant contribution was the Fifth Assessment Report issued by the United Nations-sponsored Intergovernmental Panel on Climate Change over the period from Sept. 2013 to April 2014.  These reports conclude that manmade emissions of GHGs supporting industrialization around the world cause higher long-term global average temperature, and its consequent harms and damages to society and the environment.  The NCA focuses on effects on the United States, whereas most other reports cover global effects broadly.

 

The NCA provides details on warming-related changes to weather patterns and their harmful social and economic effects already being felt, and projects even more severe future changes as emissions continue to accumulate and the global temperature continues to increase.  It enumerates various measures that can help mitigate future emissions, and notes that mitigation and adaptation strategies are closely interrelated.  The report points out that major, sustained efforts will be required in the U. S. and worldwide to reduce global GHG emissions sufficiently to keep additional warming low.

 

Importantly, it stresses the importance of risk analysis and interdisciplinary approaches that need to be invoked in order to combat global warming.  A significant aspect of this strategy should include repetitive cycles of planning, decision-making and action combined with assessment of interim processes and results.  The results obtained by weighing which processes work and which do not would provide useful inputs to the successive cycles of project development and implementation.

 

Finally, the NCA recognizes that global warming is truly a worldwide problem requiring global cooperation in arriving at an international agreement addressing both mitigation and adaptation.  GHGs, once emitted from a point source, are dispersed throughout the atmosphere around the world.  Accordingly, all source nations have to coalesce around the common objective of minimizing further emissions.

 

© 2014 Henry Auer