Changes in the Flow of Energy through the Earth’s Climate System
Kevin E. Trenberth
Weather and climate on Earth are determined by the amount and distribution of incoming radiation from the sun. For an equilibrium climate, outgoing longwave (infrared) radiation (OLR) necessarily balances the incoming absorbed solar radiation (ASR), so that the Net =ASR-OLR =0. There is a great deal of fascinating atmosphere, ocean and land phenomena that couple the ASR and OLR and the balance is only for the annual mean, not individual months or seasons. Incoming radiant energy may be scattered and reflected by clouds and aerosols, 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, latent energy (involving different water states), potential energy (involving gravity and height above the surface (or in the oceans, depth below)) and kinetic energy (involving motions) before being emitted back to space as longwave radiant energy. Energy may be stored for some time, 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 (so the Net ≠ 0), changing the climate and associated weather.
The atmosphere does not have much capability to store heat. The heat capacity of the global atmosphere corresponds to that of only a 3.5 m layer of the ocean. However, the depth of ocean actively involved in climate is much greater than that. The specific heat of dry land is roughly a factor of 4.5 less than that of sea water (for moist land the factor is probably closer to 2). Moreover, heat penetration into land is limited by the low thermal conductivity of the land surface; as a result only the top few meters of the land typically play an active role in heat storage and release (e.g., as the depth for most of the variations over annual time scales). Accordingly, land plays a much smaller role than the ocean in the storage of heat and in providing a memory for the climate system. Major ice sheets over Antarctica and Greenland have a large mass but, like land, the penetration of heat occurs primarily through conduction so that the mass experiencing temperature changes from year to year is small. Hence, ice sheets and glaciers do not play a strong role in global mean heat capacity except on greater than century time scales, while sea ice is important in those places where it forms. Unlike land, however, ice caps and ice sheets melt, altering sea level, albeit fairly slowly.
The oceans cover about 71% of the Earth’s surface and contain 97% of the Earth’s water. Through their fluid motions, their high heat capacity, and their ecosystems, the oceans play a central role in shaping the Earth’s climate and its variability. The seasonal variations in heating penetrate into the ocean through a combination of radiation, convective overturning (in which cooled surface waters sink while warmer, more buoyant waters below rise) and mechanical stirring by winds. These processes mix heat through the mixed layer. Accordingly, it is vital to monitor and understand changes in the oceans and their effects on weather and climate.
The present-day climate is changing mainly in response to human-induced variations in the composition of the atmosphere as increases in greenhouse gases, such as carbon dioxide from burning of fossil fuels, promote warming. In contrast, changes in visible pollution (particulate aerosols) add many complications regionally and can add to or subtract from any warming depending on the nature of the aerosols and their interactions with clouds. The normal flow of energy through the climate system is about 122 PW (1 Petawatt =1015 watts) (see Fig. 2 presented later below). Human activities also contribute directly to local warming through burning of fossil fuels, thereby adding heat, estimated globally to be about 1/9000 (0.01%) of the normal flow of energy (Karl and Trenberth, 2003), while radiative forcing from increased greenhouse gases (IPCC, 2007) is estimated to be about 1.3% (1.6 PW), and the total net anthropogenic radiative forcing once aerosol cooling is factored in is estimated to be about 0.7%. [Radiative forcing is the change without factoring in the effects of the response and feedbacks]. The main negative feedback is from radiation: warming promotes higher temperatures and thus more longwave cooling. The actual imbalance at the top-of-atmosphere (TOA) would increase to about 1.5% once water vapor and ice-albedo feedbacks are included, but the total is reduced and is estimated to be about 0.5 PW (0.4%) owing to the other responses of the climate system; by increasing temperatures, outgoing longwave radiation (OLR) is increased as partial compensation. Unfortunately, these values are too small to yet be directly measured from space, but their consequences can be seen and measured, at least in principle.
Fig. 1. The global annual mean Earth’s energy budget for the March 2000 to May 2004 period in W m-2. The broad arrows indicate the schematic flow of energy in proportion to their importance. From Trenberth et al. (2009). http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/10.1175_2008BAM2634.1.pdf
Figure 2: CERES-period March 2000 to May 2004 mean best-estimate TOA fluxes [PW] globally (center grey) and for globalland (right, light grey) and global-ocean (left) regions. SI is the solar irradiance and the net downward radiation RT =ASROLR. The arrows show the direction of the flow. ∇.FA is the divergence of the atmospheric energy transport and the center arrow indicates the energy flow from ocean to land. The net surface flux is also given. Adapted from Fasullo and Trenberth (2008a).
Understanding and tracking the changes in the flow of energy through the climate system as the climate changes are important for assessments of what is happening to the climate and what the prospects are in the future. Here we comment on our ability to track the energy flow changes.
2. Global mean energy flows
Since about 2000, measurements from instruments on satellite platforms have provided new estimates of global radiation from the Clouds and the Earth’s Radiant Energy System (CERES) instrument. A summary of the overall energy balance for the global atmosphere for the recent period (about 2000 to 2004) (Fig. 1) has the units of Watts per unit area. The global flows in Fig. 1 include reflection by clouds and the surface of solar radiation, and absorption by water vapor and aerosols. The energy balance at the surface is achieved through the incoming solar being mainly compensated by evaporative cooling (which drives the hydrological cycle), longwave radiation, and direct sensible heating. The very large surface longwave emissions are compensated by large back radiation by greenhouse gases and clouds, such that the evaporative cooling is larger as a whole. The global net imbalance is estimated to be 0.9 W m-2.
Fig. 2 shows the flows for the atmosphere in the ocean and land domains. Here the areas are accounted for and the units are Petawatts. Plus and minus twice the standard deviation of the interannual variability is given in the figure as an error bar. The net imbalance in the top of the atmosphere (TOA) radiation is 0.5±0.3 PW (0.9 W -2) out of a net flow through the climate system of about 122 PW of energy (as given by the ASR and OLR). The fossil fuel consumption term is too small to enter into this figure. Hence the imbalance is about 0.4%. Most of this goes into the oceans, and about 0.01 PW goes into land and melting of ice. However, 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.
When all information is combined, there are residuals that indicate errors, which can be traced to ocean heat content in the historical record, and in particular to insufficient or no sampling of the ocean in the southern hemisphere in their winter. This situation has been alleviated since about 2002 when new ARGO floats (see http://www.argo.ucsd.edu/) have been deployed that drift freely at a depth of about 2000 m, and about once per 5 days, pop up to the surface using an ingenious small pump to change the float’s volume, making a sounding of temperature and salinity along the way. The soundings are transmitted via satellite to land stations and processed to provide a comprehensive view of the ocean.
Figure 3: Zonal mean meridional energy transport by total (solid), the atmosphere (dashed), and by the ocean (dotted) accompanied with the associated ±2σ range (shaded). Adapted from Fasullo and Trenberth (2008b)
In the tropical ocean, the surface flux of energy is balanced principally by the transport of ocean energy (mainly heat), while in mid-latitudes surface fluxes are largely balanced locally by changes in ocean heat storage. The annual and zonal mean meridional energy transport by the atmosphere and ocean, and their sum (Fig. 3) show that the atmospheric transports dominate except in the tropics. There is a pronounced annual cycle of poleward ocean heat transport into the winter hemisphere exceeding 4 PW in the tropics, but the annual mean value across the equator is near zero. For the annual mean, the poleward transport by the ocean peaks at 11°S at 1.2 PW and 15°N at 1.7 PW.
3. Changes in energy and sea level rise
As noted above, there is a current radiative imbalance at the top-of-the-atmosphere of about 0.9 W -2 owing to increases of greenhouse gases, notably carbon dioxide, in the atmosphere. This has increased from a very small imbalance only 40 years ago when carbon dioxide increases and radiative forcing were less than half of those today. Where is this heat going? Some heat melts glaciers and ice, contributing mass to the ocean which is called eustatic sea level rise. Some heat enters the ocean and increases temperatures and ocean heat content, leading to expansion of the ocean which is called thermosteric sea level rise. Only very small amounts of heat enter the land. Hence the main candidate for a heat sink is the oceans, and sea level rise synthesizes both expansion and added mass from melting of ice elements. Accordingly, it is an excellent indicator of warming.
To be more concrete, a 1 mm rise in sea level requires melting of 360 Gt of ice which takes 1.2×1020 J. Because the ice is cold, warming of the melted waters to ambient temperatures can account for perhaps another 12.5% of the energy (total 1.35×1020 J). Sea level rise from thermal expansion depends greatly on where the heat is deposited as the coefficient of thermal expansion varies with temperature and pressure (the saline ocean does not have a maximum in density at 4°C as fresh water does). The amount of warming required to produce 1 mm sea level rise due to expansion if the heat is deposited in the top 700 m of the ocean can take from 50 to 75×1020 J, or ~110×1020 J if deposited below 700 m depth. Hence melting ice is a factor of about 40 to 70 times more effective than thermal expansion in raising sea level when heat is deposited in upper 700 m; the factor is ~90 when heat is deposited below 700 m depth. For comparison, 0.9 W -2 integrated globally is equivalent to about 1.4×1022 J/yr, which is a sea level equivalent of ~84 mm from ice melt or 1.3 to 2.7 mm from thermosteric ocean expansion. Note however that ice-laden land occupies only a few percent of the globe, which reduces the potential ice melt to only 1 to 2 mm/yr. Accordingly, for sea level rise to relate to energy budgets it is essential to know the eustatic and thermosteric components.
Fig. 4. Global sea level since August 1992. The TOPEX/ Poseidon satellite mission provided observations of sea level change from 1992 until 2005. Jason-1, launched in late 2001 continues this record by providing an estimate of global mean sea level every 10 days with an uncertainty of 3-4 mm. The seasonal cycle has been removed and an atmospheric pressure correction has been applied. http:// sealevel.colorado.edu/ Courtesy Steve Nerem (reproduced with permission).
Sea level is estimated to have risen throughout the 20th century by 1.8±0.5 mm/yr. The rate of sea level rise from 1993 to 2007, when accurate satellite-based global measurements of sea level from TOPEX/Poseidon and Jason altimetry are available, average about 3.1 mm/year (Fig. 4). For 1993 to 2003, there is a reasonable accounting for how this comes about. Contributions from glaciers and small ice caps and from the ice sheets of Antarctica and Greenland add mass to the oceans and eustatic rise of about 1.2 mm/yr. Contributions from changes in storage of water on land in reservoirs and dams may account for –0.55 mm/yr sea level equivalent, but these are compensated for by ground water mining, urbanization, and deforestation effects. Direct temperature measurements within the ocean show that ocean heat content increased and sea level rose from thermal expansion by 1.6 to 1.8 mm/yr. About 0.3 mm/yr is from slow isostatic rebound of the Earth’s crust.
Since 2003, however, when ARGO floats have provided better data, increase in ocean heat content has slowed, while Greenland and Antarctica melting has picked up. Whether or not the sea level budget is closed, it is not clear that the global energy budget is closed because sea level rise is much greater for land ice melt versus ocean expansion for a given amount of heat, as noted above. Accordingly, another much needed component is the TOA radiation, but CERES data are not yet processed beyond 2004 and are not yet long enough to bring to bear on this question.
4. Climate Change
A consequence of the energy imbalance at the TOA is global warming. In 2007 the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), known as AR4, clearly stated that “warming of the climate system is unequivocal” and it is “very likely” due to human activities. Since the IPCC report, nature continues to provide evidence that it is under duress with impacts affecting people and animals. Increasing rates of carbon dioxide emissions raise the specter that future climate changes could be much larger and come much quicker than IPCC suggests.
The AR4 found that warming of the climate system is unequivocal based on an increasing body of evidence showing discernible physically consistent changes. These include increases in global average surface air temperature; atmospheric temperatures above the surface, surface and sub-surface ocean water temperature; widespread melting of snow; decreases in Arctic sea-ice extent and thickness; decreases in glacier and small ice cap extent and mass; and rising global mean sea level. The observed surface warming at global and continental scales is also consistent with reduced duration of freeze seasons; increased heat waves; increased atmospheric water vapor content and heavier precipitation events; changes in patterns of precipitation; increased drought; increases in intensity of hurricane activity, and changes in atmospheric winds. This wide variety of observations gives a very high degree of confidence to the overall findings. Because these changes are now simulated in climate models for the past 100 years to a reasonable degree, there is added confidence in future projections for more warming and increased impacts. Moreover, these changes in physical variables are reflected in changes in ecosystems and human health.
Carbon dioxide concentrations are increasing at rates beyond the highest of the IPCC scenarios, suggesting even bigger and faster climate change than IPCC projected. Warming is manifested in multiple ways, not just increases in temperatures. Most dramatic is the loss of Arctic sea ice in 2007 and 2008, which affects surrounding areas, polar bears and other native species and promotes changes in permafrost. Distinctive patterns of temperature and precipitation anomalies in the winter of 2007-08 were characteristic of the strong La Niña that had a signature over most of the world. In the first 6 months of 2008, record heavy rains and flooding in Iowa, Ohio, and Missouri, led to overtopped levees along the Cedar River in Iowa and the Mississippi, and point to increases in intensity of rains associated with more water vapor in the atmosphere: a direct consequence of warming. The record-breaking numbers of tornadoes and deaths in the U.S. in 2008 probably also have a global warming component from the warm moist air coming out of the Gulf of Mexico adding to instability of the atmosphere. Longer dry spells also accompany warming, as heat goes into evaporating moisture, drying and wilting vegetation, and thus increasing the risk of wild fire enormously. Wild fires in California early in 2008 and again last summer are evidence of the impacts. Hurricanes are becoming more active. In the Atlantic in July 2008, hurricane Bertha broke several records for how early and how far east it formed, and it is the longest lasting July hurricane. Fay made landfall 4 times and hurricanes Gustav and Ike caused devastation in the U.S. in 2008. Sea level rise continues at a rate of over a foot a century. Changes in ocean acidity accompany the buildup in carbon dioxide in the atmosphere with consequences for sea creatures, and bleaching of corals occurs in association with warming oceans. Melting permafrost exposes huge potential sources of methane and carbon dioxide that can amplify future climate change. Global warming is not just a threat for the future, it is already happening, endangering the health and welfare of the planet. There is a crisis of inaction in addressing and preparing for climate change.
Dr. Kevin E. Trenberth is Head of the Climate Analysis Section at the National Center for Atmospheric Research. He was a lead author of the 1995, 2001 and 2007 Intergovernmental Panel on Climate Change (IPCC) Scientific Assessment of Climate Change
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This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.