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.
Source:
New York Times https://www.nytimes.com/interactive/2016/09/12/science/earth/ocean-warming-climate-change.html?module=inline
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.
Conclusion
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.
Details
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.
