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‘has shown a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years’ – is no longer valid.”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

 

How Sea Level Rises: A Tutorial

Summary.  Among the documented effects of global warming has been a rise of the average sea level around the globe since 1900.  This post explains, in tutorial fashion, how this happens.

First, the water in the oceans expands in volume as its temperature increases.  Expansion is constrained to occur only in the upward direction, leading to sea level rise.  Second, land-based glaciers and ice sheets melt from their upper surfaces as the air they contact warms above the melting point of water.  Third, ice shelves buoyed by the ocean in Antarctica melt from their lower surfaces as the ocean circulating under them warms.  All these processes contribute to observed sea level rise.  Both melting processes are expected to continue indefinitely as long as warming produces temperatures in the air and in the underlying ocean that remain above the melting point.

 

Introduction.  One of the consequences of global warming identified by climate scientists is sea level rise.  Higher levels of the oceans’ waters potentially affect shorelines and low-lying islands all around the world.

The United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC), in its Fifth Assessment Report (5AR), includes the following graphic showing the extent to which the global average sea level has risen in past decades up to the present.

Global average sea level change from 1900 to the present.  Each color presents a different data set.  The red line gives satellite measurements beginning in 1993.  Shadings, when present, characterize statistical variability in the data. mm, millimeters.

Source: IPCC 5AR Working Group I; http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf.

 

The graphic shows that since 1900 global average sea level has risen by about 200 millimeters (about 7.9 in.).  It is expected to continue rising indefinitely into the future (see below).

This post describes two main factors contributing to rising sea level, expansion due to heating and melting.

Expansion of water upon heating.  Like all forms of matter, water expands when it is heated and contracts when it is cooled.  At a temperature of 20ºC (68ºF) water expands by a factor of about 0.00020 per ºC (0.00011 per ºF).  We may think that the expansion occurs in all directions, as if the water of the ocean were in an elastic balloon.  This would have the effect of minimizing expansion in the vertical direction.  But in fact, the waters of the oceans are constrained on the bottom by the ocean floor and on the sides by shorelines, so that all the expansion occurs only upwards. 

Oceanographers have been measuring temperature changes in the ocean by depth, and find that the ocean temperature has increased in the last several decades down to depths of several hundred meters (see below).  For the sake of this discussion, if the temperature increased on average from 20ºC to 21ºC down to a depth of 700 meters (2,275 feet; defined as the “upper ocean”) expansion would cause the surface of the water to rise by 140 mm, or 5.6 in.  This simple calculation shows that an increase in surface temperature of the ocean is a contributing factor for sea level rise.

Melting of land-based ice.  Some sea ice arises by freezing of the ocean water.  This process transfers some water from liquid to solid, say as the polar winter arrives, which then melts back to the liquid during the polar summer.  Such cyclical changes in state have no net effect on sea level.

Transfer of land-based ice to the oceans, however, a one-time process, represents a net addition of water to the sea, raising its level.  The new water was not part of the ocean system before melting.   There are several sources of new ocean water.  Mountain glaciers at high elevations are melting around the world as a result of global warming.  The water courses through streams and rivers, and ultimately reaches the sea. 

Ice sheets over land masses, such as the Greenland ice sheet, melt from their upper surfaces when the air is above the freezing point.  This water penetrates gaps in the ice sheets and finds its way to the ocean.  Additionally, land mass glaciers at the interface with the sea calve icebergs as the glacier flows downhill toward the sea.  The solid ice in the icebergs and the water that it gives rise to as it melts contribute to raising the sea level.

This process can be diagrammed using the following simplified graphic:

Ice cube model for melting glaciers and ice sheets.  The ice cube melts at exactly 0ºC.  Ice cube image from www.dreamstime.com.

 

At the left in the diagram, the air temperature is below the melting point of the ice cube, so it stays solid and does not lose any mass.  The second frame shows the case for the air temperature being exactly 0ºC.  Under these conditions solid ice and liquid water, shown as the tiny white puddle at the base of the ice cube, are in equilibrium with each other.  Again the ice cube essentially remains unchanged, losing no mass. 

In the third frame the air temperature is 1ºC (33.8ºF).  Ice melts because heat contained in the air is transferred to the solid ice, providing the energy needed to melt it (see here  for further explanation of this notion).  The ice cube melts relatively slowly at this moderate air temperature, creating the small water puddle around its base and making the ice cube slightly smaller.  The ice cube will continue to melt slowly as long as the air temperature stays about 1ºC.

The fourth frame shows the case in which the air temperature is 2ºC (35.6ºF).  The ice cube melts more rapidly, because the rate of heat transfer from the air to the ice is higher.  Now the water puddle is quite large, and the ice cube has shrunk considerably in size.  The ice cube will continue to melt rapidly as long as the air temperature remains about 2ºC. 

The ice cube model can be taken to represent the melting of high-altitude mountain glaciers, land-mass ice sheets such as the Greenland ice sheet, and, indirectly, the calving of icebergs from glaciers moving into the sea; the latter movement is accelerated by global warming.  In addition some glaciers that were earlier in contact with the ocean have melted so fast that their leading edges have receded from the ocean and are now found at some considerable distances from the shoreline.

Melting of Antarctic Ice Shelves.  Ice shelves, such as are found in Antarctica, are large areas of ice that are the oceanic ends of land-based ice sheets that flow over the ocean and float on its surface.  Ice shelves are distinguished from ice sheets by the fact that they cover ocean water, rather than land.  An ice shelf is diagrammed in the graphic below:

 

                            Simplified model of an ice shelf extending over the ocean.

 

An ice shelf does not primarily melt from the upper surface.  The Antarctic region is sufficiently cold that surface melting does not occur to a significant extent.  Rather the ice shelf melts from below, by contacting the liquid ocean, whenever the water temperature is above the equilibrium melting temperature of the ocean, about -2°C (28.4°F; this lower melting point is due to the dissolved salts present in ocean water).

Melting of the ice shelf eats away at its substance from its lower surface, as shown in the following graphic:

 

Mechanism of melting of an Antarctic ice shelf from its lower surface.  Warm ocean water flows toward the shore over the ocean floor (orange arrows).  It transfers its heat to the undersurface of the ice shelf, melting it.  The water containing the melted ice remains near the upper surface because, having a lower salt content, it is less dense than the ocean water flowing in.  This newly-melted water flows back toward the bulk ocean (orange arrows).

 

The result of this melting process is to add water substance to the ocean that was not present before, raising the level of the ocean.  In addition, the ice shelf thins and recedes as melting proceeds, including breaking off of ice floes that will continue to melt.  The rate of melting gets greater as the ocean temperature becomes increasingly warmer than the melting point of ocean water. 

Ocean warming is in fact happening.  5AR estimates  that 90% of the excess heat arising from global warming is stored in the oceans.  The historical trend of the total amount of heat contained in the oceans has been rising from 1950 (the time when these measurements began) to 2010, as shown in the graphic below:

Change in the global mean upper ocean (0–700 m) heat content in joules (a unit of energy) from 1950 to 2010.  The data in different colors come from different data sets, and the shadings in the same colors represent estimates of statistical variability for the given data set.  The values along the vertical axis show the changes from a zero point assigned relative to the mean of all datasets for 1971, and have been computationally adjusted to overlap for the period 2006-2010.

Source: IPCC 5AR Working Group I; http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf.  

 

As the heat content increases the ice shelf will melt more rapidly and more extensively.  Climate scientists expect the global ocean heat content to continue increasing, so that ice shelf melting will continue indefinitely.  As noted above, ice shelf melting can only stop if the ocean temperature remains at or below the ocean melting point for ice. 

 

Conclusion

 

There are two processes contributing to rising sea levels due to global warming, expansion of the volume of water contained in the oceans and net melting of ice mass to become liquid water.

 

Thermal expansion is a natural property of water and other liquids.  As water warms it occupies more volume.  This expansion probably occurs for several hundred meters of depth, raising the level of the surface of the ocean. 

 

Melting of glaciers and land-based ice sheets occurs primarily from their upper surfaces, as heat is transferred from the air to the ice solid, liquefying it.  Antarctic ice shelves, on the other hand, melt from below due to contact with ocean water whose temperature is above the ocean’s freezing point.  In both cases, the rate of melting increases as the temperature of the air, or the liquid ocean, respectively, becomes warmer.  Melting increases the total volume of the earth’s oceans, leading to a rise in the global average sea level.

 

Thermal expansion will cease if and when the global average temperature stops increasing, reaching a new, higher plateau value.  Enhanced melting of ice mass will continue indefinitely, however, as long as the global average temperature remains above the freezing point of ice or of ocean water.  Unfortunately, since carbon dioxide, the principal greenhouse gas, remains in the atmosphere for several centuries, even achieving near-zero annual rates of emission will only stabilize the global average temperature at some new, higher value; with current technology carbon dioxide cannot be removed from the atmosphere.  This means the average temperature of the atmosphere and of the oceans will not fall, and will likely continue to rise.  For this reason land-based ice sheets and Antarctic ice shelves will continue melting indefinitely for generations to come.  The effects on ocean shorelines around the world will be considerable and essentially permanent.

 

 

© 2014 Henry Auer