Trenberth on Tracking Earth’s 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 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 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.
Figure 1. The global annual mean Earth’s energy budget for 2000 to 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.
The air is otherwise well mixed up to about 80 km altitude and heavier gases like carbon dioxide do not settle out owing to all the turbulent motions, convection, and so on. Also the other long lived greenhouse gases are well mixed and connect to the non-greenhouse gases with regard to temperature. Air near the surface has a temperature not much less than the surface on average, and therefore it radiates back down with almost as much energy as came up from below. But because the air gets thinner with height, its temperature falls off, and air is a lot colder at 10 km altitude where ‘planes typically fly. This air therefore radiates less both up and down, and the net loss to space is determined by the vertical temperature structure of the atmosphere and the distribution of greenhouse gases.
Changes in energy balance over the past decade
With the new measurements from space, variability in the net radiative incoming energy at the top-of-atmosphere (TOA) can now be measured very accurately. Thus a key objective is to track the flow of anomalies in energy input or output through the climate system over time in order to address the question as to how variability in energy fluxes is linked to climate variability. The main energy reservoir is the ocean (Fig. 2 below), and the exchange of energy between the atmosphere and ocean is ubiquitous, so that heat once sequestered can resurface at a later time to affect weather and climate on a global scale. Thus a change in the energy balance has consequences, sooner or later, for the climate. Moreover, we have observing systems in place that nominally can measure the major storage and flux terms, but due to errors and uncertainty, it remains a challenge to track anomalies with confidence.
Figure 2. Energy content changes in different components of the Earth system for two periods (1961–2003 and 1993–2003). Blue bars are for 1961 to 2003; burgundy bars are for 1993 to 2003. Positive energy content change means an increase in stored energy (i.e., heat content in oceans, latent heat from reduced ice or sea ice volumes, heat content in the continents excluding latent heat from permafrost changes, and latent and sensible heat and potential and kinetic energy in the atmosphere). All error estimates are 90% confidence intervals. No estimate of confidence is available for the continental heat gain. Some of the results have been scaled from published results for the two respective periods. From (IPCC 2007, Fig. TS.15 and Fig. 5.4).
A climate event, such as the drop in surface temperatures over North America in 2008, is often stated to be due to natural variability, as if this fully accounts for what has happened. Aside from weather events that primarily arise from instabilities in the atmosphere, natural climate variability has a cause. Its origins may be external to the climate system: a change in the sun, a volcanic eruption, or Earth’s orbital changes that ring in the major glacial to interglacial swings. Or its origins may be internal to the climate system and arise from interactions among the atmosphere, oceans, cryosphere and land surface, which depend on the very different thermal inertia of these components.
As an example of natural variability, the biggest El Niño in the modern record by many measures occurred in 1997-98. Successful warnings were issued a few months in advance regarding the unusual and disruptive weather across North America and around the world, and were possible in part because the energy that sustains El Niño was tracked in the ocean by a new moored buoy observing system in the Tropical Pacific. Typically prior to an El Niño, in La Niña conditions, the cold sea waters in the central and eastern tropical Pacific create high atmospheric pressure and clear skies, with plentiful sunshine heating the ocean waters. The ocean currents redistribute the ocean heat which builds up in the tropical western Pacific Warm Pool until an El Niño provides relief. The spread of warm waters across the Pacific in collaboration with changing winds in turn promotes evaporative cooling of the ocean, moistening the atmosphere and fueling tropical storms and convection over and around the anomalously warm waters. The changed atmospheric heating alters the jet streams and storm tracks, and influences weather patterns for the duration of the event.
The central tropical Pacific SSTs are used to indicate the state of El Niño, as in Fig. 3 presented below. In 2007-08 a strong La Niña event, that spilled over to the 2008-09 northern winter, had direct repercussions for cooler weather across North America and elsewhere. But by June 2009, the situation had reversed as the next El Niño emerged and grew to be a moderate event, with temperatures in the top 150 m of the ocean above normal by as much as 5°C across the equatorial Pacific in December 2009. Multiple storms barreled into Southern California in January 2010, consistent with expectations from the El Niño. The El Niño continued until May 2010, but abruptly reversed to become a strong La Niña by July 2010.
Figure 3. Recently updated net radiation (RT=ASR-OLR) from the TOA http://ceres.larc.nasa.gov/products.php?product=EBAF. Also shown is the Niño 3.4 SST index (green) (left axis); values substantially above the zero line indicate El Niño conditions while La Niña conditions correspond to the low values. The decadal low pass filter is a 13 term filter making it similar to a 12-month running mean. Units are Wm-2 for energy and deg C for SST.
We can often recognize these changes once they have occurred and they permit some level of climate forecast skill. But a major challenge is to be able to track the energy associated with such variations more thoroughly: where did the heat for the 2009-10 El Niño actually come from? Where did the heat suddenly disappear to during the La Niña? Past experience suggests that global surface temperature rises at the end of and lagging El Niño, as heat comes out of the Pacific Ocean mainly in the form of moisture that is evaporated and which subsequently rains out, releasing the latent energy.
The values and patterns of SSTs in the northern summer of 2010 undoubtedly influenced the extremes of weather, from excessive rains and flooding in China, India and Pakistan, the active hurricane season in the Atlantic, and record breaking rains in Colombia. Later the high SSTs north of Australia contributed to the Queensland flooding. The La Niña signature has also been present across the United States in the spring of 2011 with the pattern of drought in Texas and record high rains further to the north, with flooding along the Mississippi and deadly tornado outbreaks.
Anthropogenic climate change
The human influence on climate, arising mostly from the changing composition of the atmosphere, also affects energy flows. Increasing concentrations of carbon dioxide and other greenhouse gases have led to a post-2000 imbalance at the TOA of 0.9±0.5 W m-2 (Trenberth et al. 2009) (Fig. 1), that produces “global warming”, or more correctly, an energy imbalance. Tracking how much extra energy has gone back to space and where this energy has accumulated is possible, with reasonable closure for 1993 to 2003; see Fig. 2. Over the past 50 years, the oceans have absorbed about 90% of the total heat added to the climate system while the rest goes to melting sea and land ice, and warming the land surface and atmosphere. Because carbon dioxide concentrations have further increased since 2003 the amount of heat subsequently being accumulated should be even greater.
While the planetary imbalance at TOA is too small to measure directly from satellite, instruments are far more stable than they are absolutely accurate. Tracking relative changes in Earth’s energy by measuring solar radiation in and infrared radiation out to space, and thus changes in the net radiation, seems to be at hand. This includes tracking the slight decrease in solar insolation from 2000 until 2009 with the ebbing 11-year sunspot cycle; enough to offset 10 to 15% of the estimated net human induced warming.
In 2008 for the tropical Pacific during La Niña conditions, extra TOA energy absorption was observed as expected; see Fig. 3. The Niño 3.4 SST index is also plotted on this figure and the slightly delayed response of the OLR to cooler conditions in the record and especially in 2008 is clear. However, the decrease in OLR with cooler conditions is accompanied by an increase in ASR as clouds decrease in amount, leaving a pronounced net heating (>1.5 W m-2) of the planet in the cooler conditions. And so this raises the question as to whether a coherent perspective that accounts for both TOA and ocean variability can be constructed from the available observations. But ocean temperature measurements from 2004 to 2008 suggested a substantial slowing of the increase in global ocean heat content, precisely during the time when satellite estimates depict an increase in the planetary imbalance.
Since 1992, sea level observations from satellite altimeters at millimeter accuracy reveal a global increase of ~3.2 mm yr-1 as a fairly linear trend, although with two main blips corresponding to an enhanced rate of rise during the 1997-98 El Niño and a brief slowdown in the 2007-08 La Niña. Since 2003, the detailed gravity measurements from Gravity Recovery and Climate Experiment (GRACE) of the change in glacial land ice and water show an increase in mass of the ocean. This so-called eustatic component of sea level rise may have compensated for the decrease in the thermosteric (heat related expansion) component. However, for a given amount of heat, 1 mm of sea level rise can be achieved much more efficiently – by a factor of 40 to 70 typically – by melting land ice rather than expanding the ocean. So although some heat has gone into the record breaking loss of Arctic sea ice, and some has undoubtedly contributed to unprecedented melting of Greenland and Antarctica, these anomalies are unable to account for much of the measured TOA energy (Fig. 4). This gives rise to the concept of “missing energy” (Trenberth and Fasullo 2010).
Figure 4. The disposition of energy entering the climate system is estimated. The observed changes (lower panel; Trenberth and Fasullo 2010) show the 12-month running means of global mean surface temperature anomalies relative to 1901-2000 from NOAA (red (thin) and decadal (thick)) in °C (scale lower left), carbon dioxide concentrations (green) in ppmv from NOAA (scale right), and global sea level adjusted for isostatic rebound from AVISO (blue, along with linear trend of 3.2 mm/yr) relative to 1993, scale at left in millimeters). From 1992 to 2003 the decadal ocean heat content changes (blue) along with the contributions from melting glaciers, ice caps, Greenland, Antarctica and Arctic sea ice plus small contributions from land and atmosphere warming (red) suggest a total warming for the planet of 0.6±0.2 W m-2 (95% error bars). After 2000, preliminary observations from TOA (black) referenced to the 2000 values, as used in Trenberth and Fasullo (2010), show an increasing discrepancy (gold) relative to the total warming observed (red). The quiet sun changes in total solar irradiance reduce the net heating slightly but a large energy component is missing (gold). Adapted from Trenberth and Fasullo (2010). The monthly global surface temperature data are from NCDC, NOAA: http://www.ncdc.noaa.gov/oa/climate/research/anomalies/index.html ; the global mean sea level data are from AVISO satellite altimetry data: http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/ ; and the Carbon dioxide at Mauna Loa data are from NOAA http://www.esrl.noaa.gov/gmd/ccgg/trends/.
To emphasize the discrepancy, Fig. 5 presents an alternative version of Fig. 2 for 1992 to 2003, as a contrast to 2004 to 2008. The accounting for all terms and the net imbalance is compatible with physical expectations and climate model results, with the net imbalance about 0.7 W m-2 at TOA for 1992 to 2003. However, for the 2004 to 2008 period, the decrease in solar radiation associated with the sunspot cycle and the quiet sun in 2008 contributed somewhat, but the Ocean Heat Content (OHC) change is a lot less than in the previous period and a residual imbalance term: the missing energy, is required.
Figure 5. The energy entering the climate system is estimated for the various components: warming of the atmosphere and land, ocean heat content increase, melting of glaciers and ice caps (land ice), melting of the major ice sheets (Greenland and Antarctica), and changes in the sun. For 1993 to 2003 these are summed to give the total which is equivalent to about 0.7 W m-2. For 2004-2008, TOA measurements are used to provide an increment to the total based on comparisons with 2000-2003, and the quiet sun has contributed, but the sum is achieved only if a spurious residual is included. Units are 1020 Joules/year.
Further inroads into this problem will no doubt become possible as datasets are brought up to date and refined. In the meantime, we have explored the extent to which this kind of behavior occurs in the latest version of the NCAR climate model. In work yet to be published (it is submitted), we have found that energy can easily be “buried” in the deep ocean for over a decade. Further preliminary exploration of where the heat is going suggests that it is associated with the negative phase of the Pacific Decadal Oscillation and/or La Niña events.
Clearly, tracking energy and how and where it is stored, and then manifested as high SSTs which in turn affect subsequent climate is an important thing to do.