The focus on a specific increase of the global average surface temperature has become the climate change icon that is reported on frequently in the media (e.g., see “Global warming worst in 20 000 years – report“), in government assessments and in a range of scientific papers (e.g., Jones and Moberg 2002).
The question as to what is meant by a global average surface temperature has been raised previously on the Climate Science weblog (e.g., see). As reported in the 2005 National Research Council Report (see and see ); (slightly edited for the weblog)
“According to the radiative-convective equilibrium concept, the equation for determining global average surface temperature of the planet is
dH/dt = f – T’/lamba
H = the integral of the density of air times the specific heat of the air times temperature through the depth of the heating within the climate system.
H is, therefore, the heat content of the land-ocean-atmosphere system. The variable f is the radiative forcing at the tropopause, T′ is the change in surface temperature in response to a change in heat content, and λ is the climate feedback parameter, also known as the climate sensitivity parameter, which denotes the rate at which the climate system returns the added forcing to space as infrared radiation or as reflected solar radiation (by changes in clouds, ice and snow, etc.). In essence, λ accounts for how feedbacks modify the surface temperature response to the forcing. In principle, T′ should account for changes in the temperature of the surface and the troposphere, and since the lapse rate is assumed to be known or is assumed to be a function of surface temperature, T′ can be approximated by the surface temperature.
This definition is based on a simple concept, that as heat is introduced into a system (such as heating water on a stove), the temperature of the system (in this example the water) increases until the heat lost from the system is equal to the heat being input into it. If the heat input is suddenly turned off, the temperature immediately stops rising.
The definition of T’ reported in the 2005 National Research Council Report similarly uses a simple definition when the key assumption is made that
“since the lapse rate is assumed to be known or is assumed to be a function of surface temperature, T′ can be approximated by the surface temperature. ”
This simple concept is being used to promote global climate policy to prevent elevations of the global average surface temperature above specified thresholds.
However, while on a theoretical basis a global average surface temperature trend can be diagnosed from the radiative imbalance of the Earth, the reality is that it cannot be directly measured. The use of surface temperature measurement is an arbitrary choice. Why not use the tropospheric layer averaged temperature, for example?
There is an alternative, more effective approach that was summarized in Heat storage within the Earth system .
“• A snapshot at any time documents the accumulated heat content and its change since the last assessment. Unlike temperature, at some specific level of the ocean, land, or the atmosphere, in which there is a time lag in its response to radiative forcing, there are no time lags associated with heat changes.
• Since the surface temperature is a two-dimensional global field, while heat content involves volume integrals, the utilization of surface temperature as a monitor of the earth system climate change is not particularly useful in evaluating the heat storage changes to the earth system.The heat storage changes, rather than surface temperatures, should be used to determine what fraction of the radiative fluxes at the top of the atmosphere are in radiative equilibrium.”
By using ocean heat content, including its spatial structure, as the climate metric to communicate to policymakers, we would better represent the science of climate system heat changes, including global warming.
It is time to move beyond a focus on global average surface temperature trend as the icon of climate change science.