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Trenberth on Tracking Earths energy: A key to climate variability and change
Posted on 12 July 2011 by Kevin Trenberth
Energy and Climate
Climate change is very much involved with energy, most commonly in the form of heat but other forms of energy are also important. Radiation comes in from the sun (solar radiation at short wavelengths), and every body radiates according to its temperature (proportional to the fourth power of absolute temperature), so that on Earth we, and the surface and atmosphere radiate at infrared wavelengths.
Weather and climate on Earth are determined by the amount and distribution of incoming radiation from the sun. For an equilibrium climate, global mean outgoing longwave radiation (OLR) necessarily balances the incoming absorbed solar radiation (ASR), but with redistributions of energy within the climate system to enable this to happen on a global basis. Incoming radiant energy may be scattered and reflected by clouds and aerosols (dust and pollution) or absorbed in the atmosphere. The transmitted radiation is then either absorbed or reflected at the Earths surface. Radiant solar (shortwave) energy is transformed into sensible heat (related to temperature), latent energy (involving different water states), potential energy (involving gravity and altitude) and kinetic energy (involving motion) before being emitted as longwave infrared radiant energy. Energy may be stored, transported in various forms, and converted among the different types, giving rise to a rich variety of weather or turbulent phenomena in the atmosphere and ocean. Moreover the energy balance can be upset in various ways, changing the climate and associated weather
Hence the incoming radiation may warm up the ground or any object it hits, or it may just go into drying up surface water. After it rains and the sun comes out, the puddles largely dry up before the temperature goes up. If energy is absorbed it raises the temperature. The surface of the body then radiates but also loses heat by transfer through cooler winds or by evaporative cooling. Some energy gets converted into motion as warm air rises and cold air sinks, and this creates winds and thus kinetic energy, which gets dissipated by friction. Over oceans the winds drive ocean currents.
The differential between incoming and outgoing radiation: the net radiation is generally balanced by moving air of different temperature and moisture content around. Air temperature affects density as warmer air expands and thus it takes up more room, displacing cooler air, thereby changing the air in a column whose weight determines the surface pressure. Consequently, this sets up pressure differences that in turn cause winds, which tend to blow in such a way as to try to offset the temperature differences. The Earths rotation modifies this simple picture. A result is that southerlies are warm in the northern hemisphere and northerlies are cold. And so we get weather with clouds and rain in all of its wondrous complexity.
The changing seasons illustrate what happens as the sun apparently moves across the equator into the other hemisphere. In summer some excess heat goes into the ocean, which warms up reaching peak values about the equinox, and in winter the land cools off but heat comes out of the oceans and this is carried onto land, and so oceans moderate the seasonal climate variations. Much of the exchange involves water evaporating and precipitating out, and thus the hydrological cycle.
The same can happen from year to year: heat can accumulate in the ocean and then later be released, leading to warmer spells and cooler spells. This commonly happens in the tropical Pacific and gives rise to the El Niño phenomenon. El Niño is the warm phase in the tropical Pacific while La Niña is the cool phase. During and following an El Niño there is a mini global warming as heat comes out of the ocean, while during La Niña, heat tends to get stored in the ocean. The El Niño cycle is irregular but has a preferred time scale of 3 to 7 years.
Ocean heat storage can last longer: for decades or centuries and inevitably involves ocean currents and the much deeper ocean. In the North Atlantic, cold waters sink and move equatorward at depth while the Gulf Stream at the surface takes warmer waters polewards, creating an overturning circulation that can also involve density changes in the ocean associated with both temperature and salt (the thermohaline circulation). Salty water is denser. Nonetheless, much of the ocean overturning circulation is wind driven. The overturning may involve the ocean down to several kilometers and can take many centuries to complete a cycle.
As well as the ocean taking up heat, heat can be lost by forming ice, as glaciers, ice caps, or major ice sheets (Greenland and Antarctica) on land, or as sea ice. Extra heat can melt this ice and may contribute to sea level rise if land ice melts. Surface land can also absorb a small amount of heat but not much and not to great depths as it relies on conduction to move heat through the land unless water is flowing. Land energy variations occur mostly in the form of water or its absence, as heat goes to evaporate surface water. Highest temperatures and heat waves typically occur in droughts or deserts.
The atmosphere can not hold much heat and is dependent for its temperature on links to the underlying surface through conduction and thermals, convection, and radiation, as well as the wind in moving it around.
The global energy budget
In the past, we (Kiehl and Trenberth 1997) provided estimates of the global mean flow of energy through the climate system and presented a best-estimate of the energy budget based on various measurements and models, by taking advantage of the fact that energy is conserved. We also performed a number of radiative computations to examine the spectral features of the incoming and outgoing radiation and determined the role of clouds and various greenhouse gases in the overall radiative energy flows. At the top-of-atmosphere (TOA) values relied heavily on observations from the Earth Radiation Budget Experiment (ERBE) from 1985 to 1989, when the TOA values were approximately in balance.
Values are given in terms of Watts per square meter. The incoming radiation is about 342 W m-2. But there are about 5.1x1014 square meters for the surface area and so the total incoming energy is about 174 PetaWatts (=1015 watts, and so 174 with 15 zeros after it or 174 million billion). About 30% is reflected back to space and so about 122 PW flows through the climate system. For comparison, the biggest electric power plants are of order 1000 MegaWatts, and so the natural flow of energy is 122 million of these power plants. If we add up all of the electric energy generated and add in the other energy used by humans through burning etc, it comes to about 1/9000th of the natural energy flow. Hence the direct effects of human space heating and energy use are small compared with the sun, although they can become important very locally in cities where they contribute to the urban heat island effect.
New observations from space have enabled improved analyses of the energy flows. Trenberth et al. (2009) have updated the earlier global energy flow diagram (Fig. 1) based on measurements from March 2000 to November 2005, which include a number of improvements. We deduced the TOA energy imbalance to be 0.9 W m-2, where the error bars are ±0.5 W m-2 based on a number of estimates from both observations and models.
Read more-> Trenberth on Tracking Earth
Posted on 12 July 2011 by Kevin Trenberth
Energy and Climate
Climate change is very much involved with energy, most commonly in the form of heat but other forms of energy are also important. Radiation comes in from the sun (solar radiation at short wavelengths), and every body radiates according to its temperature (proportional to the fourth power of absolute temperature), so that on Earth we, and the surface and atmosphere radiate at infrared wavelengths.
Weather and climate on Earth are determined by the amount and distribution of incoming radiation from the sun. For an equilibrium climate, global mean outgoing longwave radiation (OLR) necessarily balances the incoming absorbed solar radiation (ASR), but with redistributions of energy within the climate system to enable this to happen on a global basis. Incoming radiant energy may be scattered and reflected by clouds and aerosols (dust and pollution) or absorbed in the atmosphere. The transmitted radiation is then either absorbed or reflected at the Earths surface. Radiant solar (shortwave) energy is transformed into sensible heat (related to temperature), latent energy (involving different water states), potential energy (involving gravity and altitude) and kinetic energy (involving motion) before being emitted as longwave infrared radiant energy. Energy may be stored, transported in various forms, and converted among the different types, giving rise to a rich variety of weather or turbulent phenomena in the atmosphere and ocean. Moreover the energy balance can be upset in various ways, changing the climate and associated weather
Hence the incoming radiation may warm up the ground or any object it hits, or it may just go into drying up surface water. After it rains and the sun comes out, the puddles largely dry up before the temperature goes up. If energy is absorbed it raises the temperature. The surface of the body then radiates but also loses heat by transfer through cooler winds or by evaporative cooling. Some energy gets converted into motion as warm air rises and cold air sinks, and this creates winds and thus kinetic energy, which gets dissipated by friction. Over oceans the winds drive ocean currents.
The differential between incoming and outgoing radiation: the net radiation is generally balanced by moving air of different temperature and moisture content around. Air temperature affects density as warmer air expands and thus it takes up more room, displacing cooler air, thereby changing the air in a column whose weight determines the surface pressure. Consequently, this sets up pressure differences that in turn cause winds, which tend to blow in such a way as to try to offset the temperature differences. The Earths rotation modifies this simple picture. A result is that southerlies are warm in the northern hemisphere and northerlies are cold. And so we get weather with clouds and rain in all of its wondrous complexity.
The changing seasons illustrate what happens as the sun apparently moves across the equator into the other hemisphere. In summer some excess heat goes into the ocean, which warms up reaching peak values about the equinox, and in winter the land cools off but heat comes out of the oceans and this is carried onto land, and so oceans moderate the seasonal climate variations. Much of the exchange involves water evaporating and precipitating out, and thus the hydrological cycle.
The same can happen from year to year: heat can accumulate in the ocean and then later be released, leading to warmer spells and cooler spells. This commonly happens in the tropical Pacific and gives rise to the El Niño phenomenon. El Niño is the warm phase in the tropical Pacific while La Niña is the cool phase. During and following an El Niño there is a mini global warming as heat comes out of the ocean, while during La Niña, heat tends to get stored in the ocean. The El Niño cycle is irregular but has a preferred time scale of 3 to 7 years.
Ocean heat storage can last longer: for decades or centuries and inevitably involves ocean currents and the much deeper ocean. In the North Atlantic, cold waters sink and move equatorward at depth while the Gulf Stream at the surface takes warmer waters polewards, creating an overturning circulation that can also involve density changes in the ocean associated with both temperature and salt (the thermohaline circulation). Salty water is denser. Nonetheless, much of the ocean overturning circulation is wind driven. The overturning may involve the ocean down to several kilometers and can take many centuries to complete a cycle.
As well as the ocean taking up heat, heat can be lost by forming ice, as glaciers, ice caps, or major ice sheets (Greenland and Antarctica) on land, or as sea ice. Extra heat can melt this ice and may contribute to sea level rise if land ice melts. Surface land can also absorb a small amount of heat but not much and not to great depths as it relies on conduction to move heat through the land unless water is flowing. Land energy variations occur mostly in the form of water or its absence, as heat goes to evaporate surface water. Highest temperatures and heat waves typically occur in droughts or deserts.
The atmosphere can not hold much heat and is dependent for its temperature on links to the underlying surface through conduction and thermals, convection, and radiation, as well as the wind in moving it around.
The global energy budget
In the past, we (Kiehl and Trenberth 1997) provided estimates of the global mean flow of energy through the climate system and presented a best-estimate of the energy budget based on various measurements and models, by taking advantage of the fact that energy is conserved. We also performed a number of radiative computations to examine the spectral features of the incoming and outgoing radiation and determined the role of clouds and various greenhouse gases in the overall radiative energy flows. At the top-of-atmosphere (TOA) values relied heavily on observations from the Earth Radiation Budget Experiment (ERBE) from 1985 to 1989, when the TOA values were approximately in balance.
Values are given in terms of Watts per square meter. The incoming radiation is about 342 W m-2. But there are about 5.1x1014 square meters for the surface area and so the total incoming energy is about 174 PetaWatts (=1015 watts, and so 174 with 15 zeros after it or 174 million billion). About 30% is reflected back to space and so about 122 PW flows through the climate system. For comparison, the biggest electric power plants are of order 1000 MegaWatts, and so the natural flow of energy is 122 million of these power plants. If we add up all of the electric energy generated and add in the other energy used by humans through burning etc, it comes to about 1/9000th of the natural energy flow. Hence the direct effects of human space heating and energy use are small compared with the sun, although they can become important very locally in cities where they contribute to the urban heat island effect.
New observations from space have enabled improved analyses of the energy flows. Trenberth et al. (2009) have updated the earlier global energy flow diagram (Fig. 1) based on measurements from March 2000 to November 2005, which include a number of improvements. We deduced the TOA energy imbalance to be 0.9 W m-2, where the error bars are ±0.5 W m-2 based on a number of estimates from both observations and models.
Read more-> Trenberth on Tracking Earth
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