Trenberth on tracking the Earth’s energy: A key to climate variability and change
by Kevin Trenberth, Skeptical Science, July 12, 2011
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 Earth’s 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 Earth’s 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 3-7 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 & 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 5.1 x 1014 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 1/9,000th 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.
Figure 1. The global annual mean Earth’s energy budget for 2000-2005 (W m–2). The broad arrows indicate the schematic flow of energy in proportion to their importance. From Trenberth et al. (2009).
The net energy incoming at the surface is 161 W m-2, and this is offset by radiation (63), evaporative cooling (80), and direct heating of the atmosphere through thermals (17). Consequently, evaporative cooling and the resulting water cycle play a major role in the energy balance at the surface, and for this reason, storms are directly affected by climate change. The biggest loss at the surface is from long-wave radiation but this is offset by an almost as big downward radiation from greenhouse gases and clouds in the atmosphere to give the net of 63 units.
Updates included in this figure are revised absorption in the atmosphere by water vapor and aerosols. The direct transfer of heat has values of 17, 27 and 12 W m-2 for the globe, land and ocean, and even with uncertainties of 10%, the errors are only order 2 W m-2. There is widespread agreement that the global mean surface upward longwave (LW) radiation is about 396 W m-2, which is dependent on the skin temperature and surface emissivity.
Global precipitation should equal global evaporation for a long-term average, and estimates are likely more reliable of the former. However, there is considerable uncertainty in precipitation over both the oceans and land. The latter is mainly due to wind effects, undercatch and spatial coverage, while the former is due to shortcomings in remote sensing. The downward and net LW radiation were computed as a residual and compared to various estimates which tend to be higher but all involve assumptions and models. The correct depiction of low clouds is a continuing challenge for models and is likely to be a source of model bias in downward LW flux. For example, there are sources of error in how clouds overlap in the vertical and there is no unique way to treat the effects of overlap on the downward flux.
The new observations from space have enabled improved analyses of the energy flows, their variations throughout the annual cycle, for land versus ocean, as a function of location, and also over a number of years. There is an annual mean transport of energy by the atmosphere from ocean to land regions of 2.2 ± 0.1 PW primarily in the northern winter when the transport exceeds 5 PW. It is now possible to provide an observationally based estimate of the mean and annual cycle of ocean energy, mainly in the form of ocean heat content.
Note that the sum of all the values at the TOA and at the surface in the figure leaves an imbalance of 0.9 W m-2, which is causing global warming. As carbon dioxide and other greenhouse gases increase in the atmosphere, there is initially no change in the incoming radiation, but more energy is trapped and some is radiated back down to the surface. This decreases OLR and leads to warming. At the surface the warming raises temperatures and thus increases the surface radiation, but there is still a net amount of energy that partly goes into heating the ocean and melting ice, and some of it goes into increasing evaporation and thus rainfall. To achieve an energy balance, the vertical structure of the atmosphere changes, and the radiation to space ultimately comes from higher regions that were originally colder. In that sense, the figure is misleading because it does not show the vertical structure of the atmosphere or how it is changing.
There is often confusion about how the greenhouse effect works. Greenhouse gases are those with more than two atoms, and water vapor is most important (H2O). But water has a short lifetime in the atmosphere of 9 days on average before it is rained out. Carbon dioxide (CO2), on the other hand, has a long lifetime, over a century, and therefore plays the most important role in climate change while water vapor provides a positive feedback or amplifying effect: the warmer it gets, the more water vapor the atmosphere can hold by about 4% per degree Fahrenheit. Most of the atmosphere is nitrogen (N2) and oxygen (O2) and does not play a role in the greenhouse effect. Oxygen does play an important role through ozone (O3) though, especially in the stratosphere where an ozone layer forms from effects of ultraviolet light. Ozone is not well mixed throughout the atmosphere as it has a short lifetime in parts of the stratosphere, and in the lower atmosphere its life is measured in months as it plays a role in oxidation.
More at this link: http://www.skepticalscience.com/Tracking_Earths_Energy.html
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