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

New Analysis Does Not Support a Warming “Hiatus”

The “Hiatus”.  Annual results for the global average temperature show a break from an earlier high rate of increase, beginning about 1998.  Compared with the preceding three decades, recently recorded global average temperatures have shown only a slight increase.  The reduction in the rate of warming of the globe may be termed a pause, and is generally referred to as a "hiatus".  This difference in warming trends was noted in the Fifth Assessment Report (AR5; Summary for Policymakers and Working Group I: The Physical Science Basis Ch. 2, Observations) of the Intergovernmental Panel on Climate Change (IPCC).  This break has been seized upon by those doubting or denying the reality of global warming as evidence that warming has effectively ceased since about 1998.

A new analysis of global temperature data extending from 1880 to the present was published by Thomas Karl and coworkers, from the National Oceanographic and Atmospheric Administration, the National Centers for Environmental Information, and LMI (McLean, VA), on June 4, 2015.  Climate scientists recognized problems in the ways temperature data have been collected.  Karl and coworkers sought to reevaluate historical data to account for these problems (see Details section at the end of this post).  The scientists conclude “based on our new analysis, the IPCC’s … statement of two years ago – that the global surface temperature ‘has shown a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years’ – is no longer valid.”  Rather, the rate of warming of the world's atmosphere during 1998 to 2014, averaged over its entire surface, has continued unabated compared to the warming experienced in earlier decades (see Details).

Warming Skeptics.  Almost immediately upon the publication of this report organizations known to be skeptical about global warming, or to deny that it is occurring and/or that humanity is causing it, issued statements questioning the validity of the report.  However it is essentially impossible for these groups to draw such conclusions without actually taking the time to review the data and methods used in the reanalysis.  After all, the authors devoted many months or more to analyze the information available.  The data sets themselves are available to the public, and the report sets out in detail how the analysis was done.  Detractors can only question the validity of the conclusions reached by Karl and coworkers after critically reviewing this information and objectively pointing to any perceived faults in the analysis.  Anything short of such assessment is mere speculation.

Ocean Heat Content.  The reanalysis conducted by Karl and coworkers importantly shows that air temperatures averaged over the surface of the entire globe have continued to increase without any pause from about 1950 to the present.  In addition, the oceans absorb about 90% of the excess heat accumulated by the complete earth system.  Measurements collected over the past decades show that the heat content contained in the oceans has continued increasing without cease during this period.

Melting of Glacial Ice.  The excess heat added to the earth system, both air-based and ocean-based, has resulted in increased rates of loss of land-based ice in both the Arctic and Antarctica in recent years.  This is but one example of the effects of global warming on the Earth.  Ice masses will suffer net melting  if the temperature, averaged over the full year, is higher than the melting point and the water lost to melting is not replaced by new precipitation frozen into place. 

 

New reports published in April and May 2015 exemplify this.  Whereas melting of the Antarctic ice shelves was very low between 1994 and 2003, the rate of loss increased more than 12-fold over 2003-2012, especially in West Antarctica.  Loss of ice mass from glaciers in the Southern Antarctic Peninsula that end in ocean-floating ice shelves has rapidly increased from near none in the 2000s to high rates since 2009.  Adding this ice or its melted equivalent as liquid water to the oceans contributes significantly to rising sea levels around the world.  It is important that both these reports describe increased melting only in the recent years that coincide with the continued increase in global temperatures characterized by Karl and coworkers.

Since warming will continue for decades to come, land-based ice at the poles and in mountain glaciers will continue melting, indeed will do so at increasing rates.  So sea level will continue rising into the indefinite future.  This threatens coastal regions around the world, including many of the world’s cities.  Defensive measures needed to avert the damage wrought by flooding and ocean storm surges will require large investments of money, derived from public sources.

Global agreement is needed.  This post reports that global warming has continued unabated for at least the last fifty years, raising average temperatures of the atmosphere and the ocean.  Since this is truly a global problem it is necessary that all the nations of the world come together to implement meaningful reductions in annual rates of emission of greenhouse gases.  The United Nations-sponsored negotiations involving all nations of the world are under way now, with the goal of reaching agreement on the way forward at a meeting to be held at the end of 2015.  The harms from global warming already underway will only grow worse by 2100 and beyond if an agreement is not reached.  All nations need to agree on limiting emissions, with the goal of transitioning to a decarbonized energy economy by about 2050.  Major efforts by all nations will be needed to reach this goal.

Details

Karl and coworkers examined three sources by which temperature averages could have provided erroneous results in AR5.  An important feature of this reassessment is the use of new data sets for temperatures that were not available when AR5 was prepared.  First, by far the largest number of observation stations in the world is land-based.  But over the last one and one-half centuries their number has grown, and the physical settings of older stations have changed.  Karl and coworkers reassessed land-based measurements accordingly, including incorporating new data sets not previously used.  This process roughly doubled the number of reporting stations.  An important feature of this improvement is far more authoritative reporting from the Arctic, which has in fact undergone a high extent of warming that was not fully accounted for in AR5.

Additionally, the ways of gathering sea surface temperature have changed.  A second factor recognized that historically these data were obtained primarily by ocean-going vessels.  Their numbers likewise have grown, but more importantly, the way in which they routinely measured ocean temperature has changed.  A third factor has been that, over the last 15-20 years, buoys have been deployed across the ocean, one of whose capabilities is real time measurement of air temperature.  Karl and coworkers harmonized the old and new ship-based measurements, and applied a correction to all those values to make them consistent with the buoy-based observations.

The earlier data, such as those presented in AR5 as given by Karl and coworkers in their Supplementary Materials appendix, show a reduction in the rate of increase in global average temperature between 1950-1999 and the interval 1998-2012, from 0.101 ± 0.026 ºC/decade, to 0.039 ± 0.082 ºC/decade.  It is important to note, as climate scientists have recognized, that the single temperature value recorded for 1998 was exceptionally high (see second graphic below) because it was affected by an unusually intense El Nino event.  This has the effect of artificially elevating the starting point for the 1998-2012 data range, thus lowering the steepness of the trend line for this period.

The reevaluation of Karl and coworkers shows that these two temperature rates are now very similar, namely 0.113 ± 0.027 ºC/decade and 0.086 ± 0.075 ºC/decade, respectively.  Thus their reevaluation shows that for the recent interval thought to experience the “hiatus”, the rate of increase of global temperature is more than double than that found earlier in AR5. 

When temperature data extending to the most recent period, up to 2014, are considered, the rate of increase in temperature, instead of decreasing from 1950-1999 to 2000-2014 as AR5 shows up to 2012, actually increases slightly between these two time periods.  This is illustrated in the following graphic:

 

Comparison of the warming trends in ºF/decade for the old analysis (methods as used in AR5) but including data up to 2014, and the new analysis (data and methods used by Karl and coworkers).

Source: Los Angeles Times based on the report by Karl and coworkers;

http://www.latimes.com/science/sciencenow/la-sci-sn-no-global-warming-hiatus-noaa-20150603-story.html#page=1&lightbox=83703423

 

 

The near identity of temperature change rates before and after 1998 is seen in the following graphic showing annual temperatures from 1880 to 2014, with a single statistics-derived line drawn based on analysis of data from 1950 to 2014.

Global average temperature difference in ºF from 1880 to 2014, with the orange Trend line evaluated by Karl and coworkers.  The base line representing 0ºF is the average temperature over 1961 to 1990.

Source: Los Angeles Times based on the report by Karl and coworkers;

http://www.latimes.com/science/sciencenow/la-sci-sn-no-global-warming-hiatus-noaa-20150603-story.html#page=1&lightbox=83703423

  

These two graphics make clear that there has been no “hiatus” in warming after 1998.

 

 

© 2015 Henry Auer