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This blog is expressly directed to readers who do not have strong training or backgrounds in science, with the intent of helping them grasp the underpinnings of this important issue. I'm going to present an ongoing series of posts that will develop various aspects of the science of global warming, its causes and possible methods for minimizing its advance and overcoming at least partially its detrimental effects.

Each post will begin with a capsule summary. It will then proceed with captioned sections to amplify and justify the statements and conclusions of the summary. I'll present images and tables where helpful to develop a point, since "a picture is worth a thousand words".

Friday, October 17, 2014

Deniers Mistakenly Say that Global Warming Has Ended


Global Average Temperatures and CO2.  The global average temperature has increased by about 0.7-0.8ºC (1.3-1.4ºF) over pre-industrial values.  Humanity’s use of fossil fuels to power industrialization emits carbon dioxide (CO2), a greenhouse gas, leading to warmer average temperatures.  Analysis of the increased CO2 levels shows that they arise directly from burning fossil fuels, and not from natural causes (see also the U. S. National Climate Assessment).

Climate deniers do not accept that our planet is warming, and/or that human activity is its cause.  Climate skeptics may question that human actions are responsible for warming, or that warming is harmful to human populations and other life forms.  Here both groups will be called “deniers”.

Climate deniers claim that global warming has ended.  They selectively display global temperature data for, say, the period 1980 to the present, as shown in this graphic:
 
 
Yearly values of the global average temperature selected for the interval 1980-2013, shown as their difference from the average temperature for the entire 20th century.
Source: Data table from National Aeronautics and Space Administration;
 
 
Other more biased presenters don’t show any data before 1997.  These deniers point to the interval after 1997 as showing that the temperature has remained essentially unchanged (here called the “pause”), breaking with the upward trend from 1980 to 1997.  Since atmospheric CO2 concentrations continued to increase during the pause period (see below), deniers state that increasing atmospheric concentrations of CO2 cannot be the cause for global warming.

 
Deniers  cannot selectively choose the data they wish to use while rejecting the entire data set from consideration.  It is unacceptable to focus arbitrarily on only the period supporting their view while ignoring the extended global temperature record.  Data covering most of the industrial era, 1880-present, are shown below. 
 

Yearly values of the global average temperature for 1880-2013, shown as the differences from the average temperature for the entire 20th century.  Black points and line, annual average temperature differences; Red line, smoothing obtained as a 5-year running window centered at each data point; Green, error bars showing estimates of uncertainty in the measurements.
Source: National Aeronautics and Space Administration.  http://data.giss.nasa.gov/gistemp/graphs_v3/
 
 
The overall trend shows a clear, if uneven, rise in the global average temperature beginning at about 1910, coinciding with increasing atmospheric levels of CO2 (see below and this post).  Importantly, a seemingly long pause also occurred from about 1950-1975, followed by more than 20 years in which the temperature rose sharply.  It is noteworthy that deniers fail to mention this earlier pause as evidence that warming has ceased.
 
A Simple Inert Earth Model.  Deniers are incorrectly assuming that in the Earth system, the only factor affecting the air temperature around the globe is the amount of CO2 in the atmosphere.  Such a simple model, featuring an inert Earth, may be illustrated using the following graphic.  

Model for a simplified inert Earth system close to radiation balance.  It re-emits much of the sun’s energy back into space as heat (infrared) radiation.  In this model only the atmosphere retains excess heat.  © Henry Auer

 
In essence, deniers ignore any additional components in the Earth system that affect the energy balance.

A Complex Earth System Model.  Why is the Earth’s temperature record so erratic?  Why do these pauses occur?  The answer to these questions is that the Earth is not a simple object inert to the effects of the sun’s energy.  Rather, the Earth is a complex system that responds to inputs of excess energy from the sun in many ways.  This can be modeled by a complex Earth system in the image below.
Model of the Earth system, including CO2 in its atmosphere and potential reservoirs of heat in the land, the oceans and the polar ice caps.  This Earth is not in energy equilibrium; less energy is radiated back into space than the energy falling on it from sunlight.  The extra energy heats the entire earth system, with most of the heat being stored in the ocean rather than in the atmosphere. © Henry Auer

 
This more realistic model for Earth is not in energy balance.  Direct satellite measurements of radiation leaving Earth are compared with sunlight energy reaching the Earth.  Because of the greenhouse effect the Earth retains excess heat, rather than re-emitting it back into space.  

Most of the retained heat is stored in the oceans, and not in the atmosphere.  This is why deniers are mistaken by speaking in terms of an inert Earth model, i.e., in assuming that the temperature in the atmosphere is determined only by the atmospheric CO2 concentration.  This is shown in the following graphic.
 

Top panel: Total heat energy stored in the top half-mile of Earth’s oceans compared to the average from 1955-2006.  Middle panel: Yearly global average temperature compared to the average value for the full 20th century (repeating the pattern shown in the earlier graphic).  Bottom panel: Direct measurement of atmospheric CO2 from 1958 in parts per million (ppm).
Source: National Oceanic and Atmospheric Administration;
 
 
It is seen from the lower panel that the CO2 concentration has been rising steadily since 1958; indeed a smooth curve such as seen here extends back to pre-industrial times, when the concentration was 280 ppm.  The pronounced variability in the temperature data (middle panel) contrasts with the smooth, steady increasing trend seen  for CO2.  This suggests, as indicated above, that factors other than only the atmospheric CO2 concentration are at play. 
 
90% of the excess heat retained in the Earth system is stored in the oceans.  The data in the top panel show that heat energy absorbed by the oceans has been steadily increasing since at least about 1970, including the prior pause of global atmospheric temperature, and has continued to increase even during the current pause.  Instead of ending up warming the atmosphere, excess heat has been absorbed into the oceans, warming them (see the Details section at the end of this post).  Since oceans have decade-long cycles of vertical as well as lateral currents, this heat remains latent in the oceans, but will eventually be transferred back to the atmosphere, renewing the trend of increasing global atmospheric temperature.  
 
Conclusion
 
The long-term global average temperature has increased by about 0.7-0.8ºC over pre-industrial temperatures.  A current pause of annual global temperatures began after 1997 even though the atmospheric concentration of CO2 continued to increase during this period.  Global warming deniers have seized on this pause to say that warming of the Earth has ended, since the air temperature has not responded to the increased CO2 concentration on a year-by-year basis. 
 
In fact direct measurements of the Earth’s energy balance show that it does retain excess heat, but does not store it in the atmosphere.  Rather, the excess heat enters the oceans.  It is stored there as deep as 1,500 m (4,920 ft) in slow-moving ocean currents, both lateral and vertical.  As the warmer water is lifted to the surface again, it will exchange this stored heat with the atmosphere, resuming the warming of the air.  Similar processes happened in an earlier pause event.  Global (atmospheric) warming continues on the extended time scales dictated by Earth system processes.  Global warming deniers are mistaken in saying that global warming has ended.
 
Details
 
Guemas and coworkers (Nature Climate Change vol. 3, pp. 649–653 (2013); doi:10.1038/nclimate1863)  examined the current pause in global warming.  They used earlier data as a baseline to project sea surface temperatures forward up to 2010 using a coupled ocean-atmosphere climate model.  From their results they “attribute the onset of [the pause] to an increase in ocean heat uptake.”  They verify that no reduction in the sun’s radiation can explain the pause.
 
Loeb and coworkers (Nature Geoscience, vol. 5, pp. 110–113 (2012); doi:10.1038/ngeo1375) compared the energy imbalance of the Earth system with ocean heat content.  They measured radiated heat energy and sea temperatures.  They found that the energy imbalance of the Earth system and the increase in the upper-ocean heat content are similar in magnitude.  Combining satellite temperature measurements and ocean heat measurements to 1,800 m (5,900 ft) they found “between January 2001 and December 2010, Earth has been steadily accumulating energy at a [significant rate]. We conclude that energy storage is continuing to increase in the sub-surface ocean.”
 
Chen and Tung (Science Vol. 345, pp. 897-903 (2014)  DOI: 10.1126/science.1254937) analyzed earlier data as well as more extensive newer observations gathered by buoys disposed worldwide at various ocean depths.  They found that “the [pause] is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans….Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.”  They further conclude “because the planetary heat [reservoirs] in the Atlantic and the Southern Oceans remain intact, the [pause] should continue on a decadal time scale. When the internal variability that is responsible for the current [pause] switches sign, as it inevitably will, another episode of accelerated global warming should ensue.”
 
© 2014 Henry Auer