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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".

Tuesday, November 6, 2018

Faster Ocean Warming Means We Need Net Zero Emissions Sooner

The total amount of heat added to the Earth’s oceans since 1991 is about 60% higher than mean values previously measured, according to a new study published in the journal Nature on October 31, 2018.  The authors, Laure Resplandy and a team of coworkers from the U. S., China, France and Germany, made this evaluation using a completely alternative way of measuring how much heat is retained by the oceans (see Details below).

It has been understood for many years that about 90% of the heat retained by the Earth system (ocean, melting glaciers, land and air) is absorbed by the oceans.  This is graphically depicted in the following image, which shows the excess amounts of heat from each of the four main contributors to the total increase in heat content of the entire Earth system since 1971.
Accumulation of heat in the Earth system’s components over 1970-2011, referenced to assigning zero for 1971, in zettajoules (where “zetta” signifies 1021, i.e., 1 followed by 21 zeros, or one trillion billion joules of heat).  One joule is the amount of heat needed to warm 1 gram of water by 0.24°C (0.43°F).  Following the color code scheme shown at the top, it is seen that the ocean (light gray) accounts for more than 90% of the total incremental heat and that the air (deep red), our atmosphere, accounts for only about 1-2% of the total in 2011.
Now the authors conclude that the rate at which the Earth system has been warming, i.e., the excess rate at which the Earth retains heat from the sun’s radiation instead of re-radiating that heat out into space, is about 60% higher than previously evaluated; a recent such estimate appears in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), issued in 2013-2014.
To help understand this result, Dr. Resplandy says “[i]magine if the ocean was only 30 feet deep. Our data shows that it would have warmed by 6.5°C (11.7°F) every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4°C (7.2°F) every decade.  This means that the amount of excess heat absorbed by the Earth system because of man-made global warming increases the global average temperature more than earlier estimated. This finding is a warning that Earth’s temperature is more sensitive to absorbed heat, so that humanity has less leeway remaining for limiting the increase in the global average temperature.
The latter global average, the surface air global average, is the value commonly broadcast to the public about global warming, not the increasing oceanic heat content.  The Paris Agreement of 2015 set as its goal limiting the increase of the global average air temperature to less than 2.0°C (3.6°F) above the pre-industrial value by the end of this century, with a strong recommendation to be more ambitious by aspiring to limit warming to 1.5°C (2.7°F).   
This new finding reported by Resplandy and coworkers provides strong support for the IPCC’s most recent warning to humanity.  In October 2018 it separately reported that our annual rates of emitting greenhouse gases (those that contribute to warming of the planet such as carbon dioxide) are increasing even more rapidly than thought as recently as 2015 when the Paris Agreement was reached.  The IPCC now declares that it is very urgent that, to avoid worse consequences, the countries of the world have to work toward limiting the total global temperature increase to 1.5°C by 2050, which is lower and sooner than the 2.0°C limit originally set as the goal for 2100 in the Paris Agreement.  This has to be accomplished by worldwide reduction of annual emission rates toward zero by that date. 
The reason for this urgency is that the global average temperature depends, in almost a straight-line fashion, on the total accumulated atmospheric burden of greenhouse gases.  It does not depend on annual emission rates.  To keep the total greenhouse gas level as low as possible – thus limiting temperature rise – annual emission rates have to fall toward zero as soon as possible.  
The authors’ results support their conclusion that “ocean warming is at the high end of previous estimates, with implications for policy-relevant measurements of the Earth response to climate change, such as [how rapidly the Earth responds] to greenhouse gases and the thermal component of sea-level rise” (recognizing that part of sea-level rise is due to expansion of the water in the ocean as its temperature increases).   
A main policy consideration would be that humanity has to drastically reduce the annual rate at which we emit greenhouse gases, by as much as 25% faster than previously recommended.  Many groups are working to achieve a net emission rate of 0% by 2050, i.e., transforming the world’s entire energy economy to use of only renewable energy (including also other contributions such as increasing forestation and permanently injecting carbon dioxide deep underground).  We all have to transform our individual actions, and subscribe to dramatically different policies, in order to establish this radical new energy economy. 
The method for obtaining ocean temperature measurements since 2007 uses robotic buoys dispersed across the oceans that directly measure the temperature; they automatically descend to various depths in the ocean for their measurements, then resurface to relay their data to land databases.  Earlier data were gathered from fewer buoys on the surface or at only shallow depths.  Such measurements provide results such as shown in the image above. 
Resplandy and coworkers instead relied on measurements of the concentrations of oxygen and carbon dioxide gases in the air around the globe, evaluating data that had already been collected back to 1991.  Over 78,000 data points were available for use.  These measurements serve as a substitute, or proxy, for ocean temperature because the amount of these gases that can dissolve in water depends on the temperature of the water: the warmer the water, the lower the amount of gases the water can hold and the more the gases are returned to the air.  Measuring the gas amounts in air provides direct information about the temperature of the Earth’s oceans.
© 2018 Henry Auer

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