Role of Global Warming in Present and Future Hurricanes

Summary.  The day that Hurricane Harvey made landfall in Texas the water temperature in the Gulf of Mexico was about 3-7°F higher than the average for 1961-1990.  This is important, because warmer water releases more moisture into the air than cooler water, feeding heavier rainfall.  This contributed to the extreme, unprecedented flooding in the Houston area caused by Harvey.  More moisture also leads to stronger winds in storms.

Climate models project that if humanity continues to burn fossil fuels without restraint the added carbon dioxide produced will lead to sharply higher global average temperatures.  These will produce more frequent and intense extreme weather and climate events, which bring serious socioeconomic harms to society.  One model study of storm activity along the Texas coast finds that the probability of an event will triple, from 6% per year to 18% per year, by the end of this century if emissions continue unabated.

Climate scientists have been warning of major climate consequences from man-made greenhouse gas emissions for almost three decades.  Those predictions have not changed, indeed have only improved, as scientific capabilities grew.  If humanity had responded earlier, the costs of action would have been lower or spread over longer times.  In the absence of past action at the scale needed, now is the time to act.
 

Introduction.  Climate scientists understand that the long-term global average temperature will continue to increase largely in response to the increased concentration of carbon dioxide (CO₂) and other greenhouse gases (GHGs) in the atmosphere.  CO₂ is increasing since it is the combustion product of humanity’s burning of fossil fuels (i.e., fuels based on carbon: coal, petroleum products and natural gas).  Other GHGs likewise arise from human activity.  CO₂ is especially significant since, once emitted into the air, it resides there for centuries; it continues accumulating without balancing effects that remove it from the atmosphere (after about one-third of it dissolves into the ocean).

The greenhouse effect originating from these excess GHGs raises the global average temperature.  The temperature will remain elevated in coming centuries as the excess GHGs continue residing in the atmosphere. 

One effect of higher temperatures at the surface of lakes and oceans is that more water evaporates as the water temperature rises, by about 4% per degree F (about 7% per degree C).  In addition, evaporation of water vapor requires the input of heat; as a result the surrounding air momentarily cools off.  Conversely, as water vapor condenses, such as in cloud and raindrop formation, heat is released, warming the surrounding air momentarily.  These temperature changes lead to local winds. 

Storms such as hurricanes sweep over ocean water and entrap large amounts of water vapor.  When the vapor condenses the liquid falls to the ground as rain.  As this activity intensifies strong winds result.  Climate scientists foresee that as the earth warms, storms such as hurricanes will potentially carry more water vapor and generate stronger winds than in earlier decades.

Continued GHG emissions will lead to a higher incidence of extreme hurricanes.

The United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) pubished its Fifth Assessment Report, Part 1, in 2013.  It includes climate model projections of the relationship between the excess CO₂ accumulated in the atmosphere from human activity and the predicted increase in the global average temperature resulting from the added CO₂. 

The models were run by assuming four CO₂ emission scenarios up to the year 2100: the most stringent ends GHG emissions beyond 2050, while the least stringent continues current use and unabated future growth in use of fossil fuels.  The results are shown in the graphic below, showing the dependence of the global average temperature on the atmospheric CO₂ level, including the historical record of global average temperature from 1880 to 2010 in the lower left of the image.

 

Historical record of global annual temperature increase above the average for 1861-1880 (vertical axis) as a function of historical atmospheric accumulated weight of excess carbon dioxide, due to human use of fossil fuels, above the level in 1870 (black; lower left).  The circles mark decades from 1870 to 2100.  Future model projections of the same temperature-carbon dioxide dependencies are shown from 2010 to 2100, based on four scenarios describing the stringency of policy used to limit future emissions (dark blue, most stringent; light blue, next less stringent; orange, weak limits on emissions; red, continued emissions from unabated use of fossil fuels).
Source: Intergovernmental Panel on Climate Change, Fifth Assessment Report, Working Group 1, Summary for Policymakers. http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf .

The historical data show that in 2010 the global average temperature was about 0.9°C (1.6°F) higher than in 1870.  The modeling shows that the most stringent scenario (dark blue) projects a temperature increase above the 1870 level of about 1.8°C (3.2°F) by 2050-2100.  On the other hand, the scenario based on unconstrained continued use of carbon-containing fuels (red) foresees that the global average temperature in 2100 will be about 4.7°C (8.5°F) above the 1870 temperature.  Such a drastic increase in global temperature will lead to periods of time, and/or regions of the earth’s surface, experiencing one or more of fierce heat waves; extreme storms that may be more frequent, or have more intense rainfall and winds; droughts; wildfires; and pronounced increases in sea levels.

Hurricane Harvey pummeled Houston and neighboring regions with torrential rainfall in August 2017.  While the hurricane would likely have happened anyway, rainfall was more intense because the water of the Gulf of Mexico was warmer than in the past.  This is seen in heat map for the Gulf, shown below for the day that Harvey made landfall.

               Source: http://www.climatesignals.org/node/7158 (accessed late August 2017).

 

(The legend under the heat map is the one appearing on the web site from which the map was copied.)  The map makes clear that the excess heat in the Gulf of Mexico abutting the Texas coast, shown by the color code bar at the right, was a factor in the extreme rainfall and flooding generated by the storm. As explained in the Introduction, warmer water leads to more moisture evaporating into the storm.   A second factor was that the hurricane lingered over the Houston area for several days.  The total rainfall from the storm at Cedar Bayou was 51.88 in (1318 mm), perhaps the highest in the region.

The likelihood of a return event of a hurricane like Harvey increases 3-fold in the unrestrained emission scenario described above for the first graphic.  K. Emanuel published an analysis analysis of hurricane rainfall properties by modeling previous storms impacting Texas.  This was carried out historically for the period 1980-2016, and with the unrestrained scenario for 2081-2100.  He developed results assuming a storm with 500 mm (19.7 in) of rainfall, much less than the local maximum cited above for Cedar Bayou.  The likelihood of such precipitation is evaluated at 6% per year for 2017, and increases three-fold to 18% per year by the final decades of this century if fossil fuel use remains unconstrained. Emanuel also found that for the period 1981-2000 the historical likelihood is modeled as 1% per year.  Thus, his modeling shows that global warming has already increased hurricane/storm likelihoods in recent decades by a factor of six, and for the century-long interval from the end of the 20^th century to the end of 21^st century by 18-fold. 

Conclusion

Climate scientists have been warning for almost three decades of the hazards arising from increased levels of greenhouse gases in the atmosphere.  The increase in atmospheric CO₂ from 1958 to the present is shown below.

Atmospheric concentration of CO₂ in parts of CO₂ per million parts of air; ppm).

Source: www.CO2.earth.

 

From the first IPCC Assessment Report in 1990 to the fifth in 2013-2014, the forecasts of future climate trends and harmful events have not changed.  It was already understood in1990 that manmade global warming imperiled our society’s wellbeing.  What has changed is first, an increase in the CO₂ level from about 355 ppm in 1990 to 402 ppm in August 2016 (see the graphic just above); and second, increased certainty in the predictions of the effects of higher levels of greenhouse gases on the earth’s climate, arising from dramatic increases in data available, sophistication of climate models employed, and the computational power of modern supercomputers.

The flooding from the hurricanes that struck the Caribbean and southeastern U. S. in the summer of 2017 is just one example of the types of extreme events that climate scientists have foreseen over the past decades.  Others include heat waves, droughts, forest wildfires and sea level rise.  In the earlier decades they were only predictions, dismissed by many.  But by today the warnings have come to pass; extreme events will increase in occurrence and severity as warming worsens.

The harms and damages inflicted by extreme events have major economic and societal consequences.  The need arises to reconstruct damaged homes and facilities and to undertake projects that increase resiliency in the face of future climate threats.  These costs ultimately fall on the population at large, for example from increased insurance premiums and higher taxes.  Had earlier action been undertaken it is likely that such societal costs would have been lower, or at least spread out over longer time frames. 

As of today, however, much of the response is on an emergency basis, i.e. as the response to unforeseen disasters.  The U.S. in particular, as well as the world at large, should accept the reality of the climate change threat.  We must make the investments now that are needed to minimize further greenhouse gas emissions and to adapt to the threats already with us.



© 2017 Henry Auer

Politics Trumps Science at the Environmental Protection Agency

Summary.  The Environmental Protection Agency cancelled presentations by three EPA scientists at a conference about the effects of climate change on Narragansett Bay, Rhode Island.  Their work focused on the effects of climate change on the ecology of the Bay.  EPA gave no reason.  It has been supporting this research for decades, but in its draft budget for the coming year the agency has zeroed out support for all 28 of its estuary projects.

Administrator Pruitt has publicly rejected the role of humans in causing climate change, suggesting there remains some disagreement over the issue.  In fact, 99.99% of climate scientists affirm the reality that humans cause global warming.  Scientists the world over agree fossil fuel use emits excess carbon dioxide which retains extra heat in the earth system, leading to warming and its harmful consequences.

Politicization of science harms the public because political considerations supersede scientific reality in developing policy.  Here EPA suppressed research findings characterizing effects of worsening climate change.  A professor of oceanography at the University of Rhode Island considers the muffling of his colleagues a deliberate act of scientific censorship.  We must all resist further efforts at stifling research.  We must reinstate bona fide science as the guide for our policies.

 

EPA Scientists Prevented from Speaking.  The Environmental Protection Agency (EPA) abruptly cancelled the speaking presentations of three EPA scientists at a conference about the effects of climate change on Narragansett Bay, in Rhode Island, held on Oct. 23, 2017.   One of them, a research ecologist at the EPA’s National Health and Environmental Effects Research Laboratory Atlantic Ecology Division in Rhode Island was to give the keynote address.  The other two, a postdoctoral researcher at the same EPA facility and a scientific contractor for EPA, were to be on a panel addressing the topic “The Present and Future Biological Implications of Climate Change.”  EPA’s decision was relayed to the meeting organizers just one business day before the conference.  No substantive reason was provided for the prohibition. 

The NBE Program was the host for the conference.  It issued a 500-page Technical Report, entitled “The State of Narragansett Bay and Its Watershed”, on the day of the conference.  Of three effects stressing the condition of the bay, climate change was identified as one.  This in turn was broken down to effects of temperature change, precipitation intensity and frequency, and sea level rise.

The Narragansett Bay Estuary Program (NBE Program), long supported by EPA, has been studying the ecological health of the bay since 1985.  The NBE was recognized as an “estuary of national significance” by the EPA’s National Estuary Program in 1988.  For reasons such as these colleagues were surprised at the EPA’s prohibition.

Research support of US$26 million for all the 28 Estuary Programs, including the NBE Program, has been dropped from EPA’s proposed budget for 2018. 

Politicization of science harms the public because political considerations supersede scientific reality in developing policy.  Here EPA suppressed research findings characterizing effects of worsening climate change.  EPA’s interference with its NBE Program employees stifles scientific study related to climate change. “It’s definitely a blatant example of the scientific censorship we all suspected was going to start being enforced at EPA,” said John King, professor of oceanography at the University of Rhode Island, the head of the science advisory committee of the NBE Program. He continued “[t]hey don’t believe in climate change, so I think what they’re trying to do is stifle discussions of the impacts of climate change.”

Adminstrator Pruitt rejects human-caused climate change.  The NBE Program case is but one of the agency’s many political actions.  In March 2017 the Administrator, Scott Pruitt, said “…there’s tremendous disagreement about the degree of impact [of ‘human activity on the climate’], so no, I would not agree that it’s a primary contributor to the global warming that we see”.  Pruitt’s assertion conflicts directly with EPA’s own statement, reported by The Guardian on March 9, 2017, “carbon dioxide is the ‘primary greenhouse gas that is contributing to recent climate change’”. (This statement could not be accessed on Oct. 26, 2017 using the Guardian’s link to the EPA page.)   Pruitt’s statement is also contradicted by James Lawrence Powell’s journal article (Bulletin of Science, Technology & Society 1–4, 2016; DOI:10.1177/0270467616634958).  He found that during 2013 and 2014 only 4 of 69,406 (0.0058%) authors of peer-reviewed journal articles dealing with climate change rejected the reality of man-made global warming.  That is, there is no disagreement on impact of human activity on the climate.

EPA is cleansing its sites of references to climate change.  EPA has purged most content dealing with climate change from its web pages related to helping state and local governments deal with climate change.  The site, previously called “Climate and Energy Resources for State, Local and Tribal Governments” is now titled “Energy Resources for State, Local and Tribal Governments,” dropping the lead word “Climate”.  The original 375 web pages now are reduced to 175, with changes in content that an outside group terms “substantial”.  Looking to the future, a draft outline of EPA’s plans for the next four years omits mention of climate change.

EPA is undertaking a review of automobile Corporate Average Fuel Economy standards that were intended to increase efficiency and reduce fuel use by about half by 2025, the Portland Press Herald reports.  “Administrator Scott Pruitt is intent on subverting that agency’s mission. At the behest of automakers, he is now reconsidering vehicular emission standards that help protect public health, save consumers money, and guard against further climate disruption”, the newspaper writes.  It reports that William D. Ruckelshaus, former EPA director under two Republican presidents, says that Pruitt’s approach appears more like “taking a meat ax to the protections of public health and the environment and then hiding [the ax].” 

EPA recently announced a draft rule overturning the Clean Power Plan (CPP), the Obama administration’s detailed program to reduce carbon dioxide emissions from large electric generating plants.  While Scott Pruitt was Attorney General of Oklahoma he helped lead more than 24 states in suing to overturn the CPP.  Gina McCarthy, EPA Administrator under President Obama, said the proposal “is a wholesale retreat from EPA’s legal, scientific and moral obligation to address the threats of climate change.”

Discussion

Carbon dioxide was identified as a greenhouse gas in the middle of the nineteenth century.  A warning that the gas would contribute to warming of the atmosphere was first made in 1896.  More recently the work of hundreds of climate scientists from countries all around the world have been researching this field for decades.  As noted above, essentially all agree: Man-made emissions of carbon dioxide from burning fossil fuels, and other greenhouse gases, are warming the earth system at geologically unprecedented speed.  The effects of fossil fuel use lead to weather extremes, wildfires, sea level rise and changing habitats of pests and disease carriers.  Toxins released from fossil fuels cause illnesses among the public.  The costs of future mitigation of, and adaptation to, global warming keep rising, as the threats become more severe. 

The United States is the only country of the more than 190 nations that joined the Paris Agreement to withdraw from it.  Of the other nations, only Syria never acceded to the Agreement.  (In October 2017 the only other holdout, Nicaragua, joined the Agreement.)  It is unconscionable that a nation as respected as the United States has consistently refused to join the other nations of the world in recognizing the irrefutable scientific evidence, and acted accordingly.  The present U. S. administration, instead of mitigating emission rates of greenhouse gases, is consciously reversing previously enacted policies.  The result can only be accelerated emissions of greenhouse gases, with the consequent worsening of all the effects of warming.  This will be a legacy for all our children and further progeny, one our leaders cannot be proud of.

For these reasons we here in the U.S. must act to restrain Pruitt’s EPA policies and the framework envisioned by the Trump administration.  We must reinstate bona fide science as the guide for our actions.

© 2017 Henry Auer

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